d61 evaluation of existing test methods...(hbm) esd test [1]. and the number of ultrasensitive...

GROWTH PROJECT G6RD-CT-2001-00615DELIVERABLE REPORT D6.1

10.9.2003

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VTT INDUSTRIAL SYSTEMS

Evaluation of existingtest methods

G6RD-CT-2001-00615DELIVERABLE REPORT D6.1

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VTT INDUSTRIAL SYSTEMSTekniikankatu 1, TampereP.O. Box 1306, FIN–33101 TampereFINLAND

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Title

Evaluation of existing test methodsCustomer or financing body and order date/No. Research report No.

EU FP5 Growth project G6RD-CT-2001-00615 D6.1Project Project No. at VTT

ESTAT-Garments G2SU00220Author(s) No. of pages/appendices

Jaakko Paasi, Tuija Luoma, Mervi Soininen, Tapio Kalliohaka,Christian Vogel, Jürgen Haase, Jeremy Smallwood, andPhilippe Lemaire

80

Keywords

Electrostatic discharge, ESD, protective glothing, ESD garments, measurement methodsSummary

In this report we have evaluated existing test methods for the assessment of ESD protectivegarments used in electronics manufacturing industry. The evaluation is based on the realfailure mechanisms of ESD sensitive devives. The following parameters were identified asthe key parameters to control in order to minimise ESD failures with reference to garments:peak ESD current, charge transfer in a discharge, and device charging due to electric fieldexternal to the garment.

Current standard test methods for full garments do not characterise satisfactorily well theprotective performance of modern ESD garments used in electronics manufacturing. Noneof the laboratory test methods for full garments selected for the study could assess theprotective performance of an ESD garment satisfactorily. Completely new methods ormodifications for the existing test methods are required. From the existing test methods forfull garments, the STFI method PS07 and the SP method 2175 seem to be the mostpromising tests for further modification. Confirmation of conclusions, however, cannot bedone before the assessment of risks with reference to charged clothing is completed.

Fabrics tests, which were not considered in this report, form an important part of thegarment performance evaluation. A general view is obtained only after the results fromgarment fabric and full garment tests are combined.

Date Tampere, 10 September, 2003

Jaakko PaasiSenior Research Scientist

Distribution (customers and VTT):VTT, University of Genova, SP, Centexbel, STFI, Nokia, Celestica, Electrostatic Solutions Ltd.,European Commission (RDG


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ForewordSome types of everyday clothing are known to generate high electrostatic charge levels thatcould put ESD sensitive components at risk of damage. It has been common practice forpersonnel within the EPA to avoid this problem by wearing over their clothes an ESDprotective cover garment that covers the torso and arms. In cleanroom productionenvironment the ESD protective garment is of overall type. However the evaluation of theprotection offered by these garments has been in doubt. It is this question that is addressed bythe ESTAT Garments GROWTH Project G6RD-CT-2001-00615.

This report focuses on the evaluation of exiting test methods for ESD protective garmentsused in electronics industry. The aim has been to evaluate the methods with respect to theirability to assess the ESD protective performance of the tested garments. The report is themain output of Workpackage 6 “Test methods and recommendations” for the project mid-term. In the report we♦ review the main risks of ESD damage to sensitive devices with reference to garments,♦ present criteria for the evaluation of the protective performance of the garments as well as

their test methods,♦ show results of full garment tests with selected existing test methods♦ make conclusions on the ability of the selected test methods to characterise the ESD

protective performance of the tested garments.The report is closely linked to Deliverable report D5.2 “Report on the evaluation of existingtest methods for fabrics” by C. Vogel, J. Haase and J. Paasi, where existing methods testingthe electrostatic performance of garment material are evaluated. Together these two reports(D6.1 Evaluation of existing test methods and D5.2 Report on the evaluation of existing testmethods for fabrics) form a full description of existing test methods for the characterisation ofelectrostatic performance of ESD garments. The report has also a strong connection toDeliverable report D4.2 "Results on physical tests” by Philippe Lemaire where the garmentsand garment materials used in the measurements were characterised by infrared spectroscopyand optical microscopy.

The authors wish to thank other Project Management Committee members of the ESTAT-Garments project and their deputies (Gianfranco Coletti and Francesco Guastavino at UGDIE,Lars Fast and Anders Nilson at SP, Jan Laperre at Centexbel, Giuseppe Reina at Celestica,and Terttu Peltoniemi at Nokia) for their comments during the work. Special thanks are givenfor Arne Börjesson at Agb-konsult for his input.

Authors


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Table of contents

1 Introduction .....................................................................................................4

2 Literature and interview survey .....................................................................8

2.1 ESD damage prevention standards with reference to garments .......................82.1.1 ESD control program..................................................................................92.1.2 A comparison of S20.20 and 61340-5-1...................................................10

2.2 Research on electrostatic discharges from garments .....................................142.2.1 The evaluation of the electrostatic safety of personal protective clothing for

use in flammable atmospheres ................................................................142.2.2 The nature of electrostatic discharges from textile surfaces and their

damaging effect on electronic components..............................................162.3 Considerations for ESD garment tests ............................................................17

2.3.1 Current standard test methods.................................................................172.3.2 Considerations for ESD garment tests according to Baumgartner ...........182.3.3 Considerations for ESD garment tests according to other researchers....19

2.4 IEC Garment Survey .......................................................................................20

3 Risk of damage to electronics with reference to garments.......................25

3.1 Risk from direct discharges.............................................................................253.2 Device charging and CDM damage risks ........................................................273.3 Risk from indirect discharges ..........................................................................28

4 Evaluation criteria of test methods .............................................................29

4.1 Characteristics of good ESD garments ...........................................................294.2 Evaluation criteria for fabric and full garment test methods.............................31

5 Selected existing methods for the tests .....................................................33

5.1 List of selected test methods...........................................................................335.2 Description of the selected test methods for full garments..............................34

5.2.1 Resistive methods of IEC 61340-5-1 and ESD STM2.1...........................345.2.2 VTT’s point-to-point method for full garments ..........................................355.2.3 SP Method 2175 ......................................................................................355.2.4 STFI test method No. PS 07 ....................................................................375.2.5 Shirley method 202 ..................................................................................405.2.6 JIS L 1094:1997 “Frictionally charged electricity-amount measuring

method” ....................................................................................................41

6 Test results for full garments.......................................................................43

6.1 Test samples...................................................................................................436.2 Test results .....................................................................................................45


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6.2.1 Point-to-point resistance (Resistive IEC 61340-5-1 and ESD STM2.1methods) ..................................................................................................45

6.2.2 Charge decay time from point-to-point (VTT’s method for garments) ......486.2.3 SP Method 2175 (System measurement) ................................................546.2.4 Charge transfer - garment test, STFI-Reference No. PS 07 ....................576.2.5 Static electricity generated when removing garment, Shirley Method 202

626.2.6 Frictionally charged electricity, JIS L 1094 ...............................................66

7 Evaluation of the test methods for full garments.......................................70

7.1 Resistive methods of IEC 61340-5-1 and ESD STM2.1..................................707.2 VTT’s point-to-point method for full garments .................................................717.3 SP Method 2175 .............................................................................................727.4 STFI test method No. PS 07 ...........................................................................737.5 Shirley method 202 .........................................................................................737.6 JIS L 1094:1997 Frictionally charged electricity-amount measuring method ..747.7 Discussion.......................................................................................................74

8 Conclusions...................................................................................................76

References ...............................................................................................................79


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1 IntroductionThe evolution of electronics has led to the development of products in which increasingnumber of operations concerning an increasing amount of data can be performed faster andfaster per each generation of devices. The improved properties and the increased functionalityhave been rendered possible by the achievements in semiconductors science. The latterachievements mirrored an ongoing reduction of the semiconductors elements dimensions.Unfortunately, this evolution lead to devices which are more sensitive to electricaldisturbances than ever before. One such disturbance is caused by electrostatic discharges(ESD). Most ICs can withstand ESD in kV-range but, on the other hand, many discretecomponents are in the range of 100 – 150 V according to the standard Human Body Model(HBM) ESD test [1]. And the number of ultrasensitive devices with ESD withstand voltagesbelow 100 V is increasing (including magnetoresistive (MR) recording heads, special rf-devices, flat panel displays and CCD devices, etc.). Overview of device technology HBMwithstand voltages (sensitivities) is given in Table 1.

Investigations performed in different parts of the world show that about 30-50 % of allfailures in electronic products detected during manufacturing can be attributed to some kindof electrical overstress, of which ESD is one type, see e.g. [2]. Electric charges present on theoperators clothing are a source of ESD in the manufacturing environment. These charges aretypically accumulated when the operator is moving, that is, by tribo-electric effects (rubbingor separation of two different materials). Specially designed protective clothing is used toavoid accumulation and the retention of the latter charges. This clothing, called ESD-garment,is worn over the ordinary clothing of the operator.

The present standards for the evaluation of the ESD-garments protective performance [3,4]are mainly based on the results of researches performed in the 70-80’s. Such methods, as wellas the garments, satisfied the requirements of that time. Since then the electronics industrydemanded increasing performances from the ESD-protective clothing.

In some cases the ESD-garments are not used just to prevent ESD-damage to electronics butalso to prevent the electronics from being damaged by dust particles (clean room clothing). Atthe same time there has been much progress in the textile industry. As a result the ESD-garments in use today are made of composite fabrics where a grid or stripes of conductivethreads are present inside a matrix of cotton, polyester or mixtures of these materials.Furthermore, the conductive threads are more and more frequently made by composites, thatis by a mixture of conductive and insulating fibres (core conductive fibres, sandwich typefibres etc.), see Fig. 1 [5]. All the latter elements lead to very heterogeneous fabrics forgarments. ESD risks of modern charged fabrics were investigated in a European project “Theevaluation of the electrostatic safety of personal protective clothing for use in flammableatmospheres (SMT 4962079-1998)” with reference to flammable atmospheres [6-7]. Inelectronics industry the ESD risk levels are much lower than in flammable atmospheres, so


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the knowledge from that project cannot be directly transferred to ESD-protective clothingused in electronics industry.

Table 1 Overview of device technology sensitivities for the Human Body Model (HBM)type of ESD and corresponding ESD sensitivity classification according to the IEC standard61340-3-1.

Technology Typical HBMsensitivity (V)

Approximate HBMESD sensitivity class

MR heads, RF FETs, SAW devices 10 - 100 0power MOSFETs, PIN diodes 100 - 300 0 - 1Alaser diodes 200 - 1500 0 – 1CMMICs 100 - >2000 0 - 2Flat panel displays and CCDs 50 - 150 0LEDs 500 - 8000 1B - 3pre-1990 MOS VLSI 400 - 1000 1A - 1Bmodern VLSI 1000 - 3000 1C - 2HC and similar families 1500 - 5000 1C - 3ACMOS B series 2000 - 5000 1C - 3ACMOS A series 1000 - 2500 1C-2MOS linear 800 - 4000 1A-2small geometry old generation bipolar 600 - 6000 1A - 3Asmall geometry modern bipolar 2000 - 8000 2 - 3Apower bipolar 7000 - 25000 3A - 3Bfilm resistor 450 - 5000 1B - 3A


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Figure 1 (a) Structures of homogeneous and heterogeneous textiles,(b) structures of some commonly used conductive fibres. [5]

The presently available standard test methods for garments used in electronics industry [3,4]have been developed for homogeneous materials, thus, they do not allow a propercharacterisation of the modern garments performances. Furthermore, it is not certain that theyindicate how much the garments will protect the electronics from ESD.

In this report we have evaluated existing test methods for ESD protective garments used inelectronics industry. The aim has been to study the methods with respect to their ability toevaluate the ESD protective performance of the tested garments. In Chapter 2 we presentsome key findings of literature and interview survey done for the study, including conclusionsof a previous European research project “The evaluation of the electrostatic safety of personalprotective clothing for use in flammable atmospheres (SMT 4962079-1998)” [6-7] and anIEC TC 101 WG5.2 survey done for ESD garments end-users [8]. In Chapter 3 we review themain risks of ESD damage to sensitive devices with reference to garments. The chapter is asummary of the key results of Deliverable report D1.2 “Preliminary evaluation of the risk ofESD damage to sensitive devices arising from ESD from garment surfaces” by J: Smallwoodand J. Paasi [9]. In Chapter 4 we present criteria for the evaluation of the protectiveperformance of the garments as well as for the evaluation of their test methods. The criteriaare based on the real threats described in Chapter 3. The evaluation criteria are the same forboth full garments (studied in this work) and for garment materials studied in Workpackage 5


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“Electrostatic characterisation of fabrics” and reported in D5.2 “Report on the evaluation ofexisting test methods for fabrics” [10]. At the beginning of the work in Workpackage 6 “Testmethods and recommendations” we made a survey to existing international and nationalstandard methods as well as to public laboratory methods. A few methods were selected forfurther studies, the methods presented in Chapter 5. In Chapter 6 we show results of fullgarment tests with selected existing test methods for garments. The measurements were doneusing garments selected and supplied in Workpackage 4 “Material and constructioncharacterisation”. In Chapter 7 we make conclusions on the ability of the selected testmethods to characterise the ESD protective performance of the tested garments. Finally, themain results are summarised in Chapter 8.


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2 Literature and interview survey

2.1 ESD damage prevention standards with reference togarments

ESD damage prevention standards are widely used in the electronics manufacturing industry.These give requirements for procedures and equipment used in protection of ESD sensitivecomponents from ESD damage during manufacture, handling and transportation. Some of thestandards in current use with reference to garments are listed in Table 2. In addition, manymanufacturers use in-house ESD Programs based on these standards.

Table 2. Commonly used ESD Program standards

Standard Main application areaEN 61340-5-1: 2001. Electrostatics Part 5.1: Protection ofelectronic devices from electrostatic phenomena - Generalrequirements

Europe. Identical IECTechnical Reportapplicable to rest of worldbut does not have standardstatus.

EN100015-1: 1992 Basic specification: Protection ofelectrostatic sensitive devices Part 1. General requirements

Europe, now supersededby EN61340-5-1, but stillin use.

ANSI/ESD S20.20-1999. ESD Association Standard for thedevelopment of an electrostatic discharge control program for –Protection of electrical and electronic parts, assemblies andequipment (excluding electrically initiated explosive devices);ESD STM2.1-1997: ESD Association standard test method forthe protection of electrostatic discharge susceptible items –Garments.

US. Often used in rest ofworld

EN 1149: Protective clothing – Electrostatic properties.-Part 1: Test methods for measurement of surface resistivity-Part 2: Test method for measurement of the electrical resistancethrough a material (vertical resistance)-Part 3: Test methods for measurement of charge decay (the part3 has still prEN-status).

Europe. Not directed toelectronics industry, buteven then sometimes used

JIS L 1094:1997. Japanese Industrial Standard: Testing methodsfor electrostatic propensity of woven and knitted fabrics

Japan

Compliance with an ESD standard gives some assurance to a manufacturer and their clientsthat good practice is in use, and the risk of ESD damage compromising the reliability of themanufactured product is minimised. Electronics manufacture is a global industry with manymultinational companies, and the development and harmonisation of ESD standards world-


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wide is sought in order to promote and facilitate international trade in electronic and ESDrelated products.

In this chapter we consider the main current standards related to ESD protective clothing usedin electronics industry. We will here neither describe the test methods described in thestandards nor the requirements in detail. Here we will just shortly review selected generalpoints and discuss them in order to give background for the following chapters.

The main ESD Program standards considered here are ANSI/ESD S20.20 and EN61340-5-1.The latter is identical to the IEC61340-5-1 Technical Report, which is under development as aworld ESD Program standard, planned to emerge in 2004/5. It is likely that the new version ofIEC 61340-5-1 will be harmonised with ESD S20.20 to achieve world-wide consensus.ANSI/ESD S20.20 is an ESD Program standard covering several standard test methods. Forthe ESTAT-Garments the most important one is ESD STM 2.1 “ESD Association standardtest method for the protection of electrostatic discharge susceptible items – Garments”.

2.1.1 ESD control program

The key elements of an ESD control program are common to ESD S20.20 and 61340-5-1.These are;• Establishment of an ESD control program document. This specifies procedures and

requirements for equipment, materials and packaging to be used as well as for the ESDprotection of personnel. Test methods for ESD garments and recommendations for the useand selection of the garments belong to the ESD control program document. They may bedirectly included in the main document or, alternatively, being an integral part as areferenced document.

• Establishment of a training program for all staff involved with ESD sensitive partsincluding purchase, handling or assembly, management, and design

• Establishment of a verification program that monitors the status of the ESD controlprogram measures

Key strategies in ESD control are common to ESD S20.20 and 61340-5-1, and most otherESD program standards. These include:• An ESD protected area (EPA) is established. Within the EPA the risk of static damage

during sensitive device handling and assembly is minimised by carefully controlling staticlevels

• Outside the EPA, the sensitive parts are protected by ESD protective packaging

Within the EPA certain practices are adopted to reduce the risk of static damage to anegligible level. These are;• All conductors (including personnel) are equipotential bonded (and preferably grounded)

• All non-essential insulating materials are excluded

• Materials and equipment designed for use in the EPA have carefully controlled chargegeneration and dissipation properties


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• Where insulating materials are necessarily present, the charge on these is minimised bymeasures such as ion neutralisation

The objective of these strategies is to keep electrostatic charge build-up below a level atwhich damage to sensitive devices may occur. Often a minimum device ESD withstandvoltage is specified, below which the standard procedures and requirements may not besufficient to ensure ESD protection of components.

2.1.2 A comparison of S20.20 and 61340-5-1

Both 61340-5-1 and S20.20 aim to protect ESDS to a minimum ESD withstand level of 100 VHBM. Devices having a lower ESD withstand may require additional protection measures. Amajor difference between the two standards is in their approach. S20.20 emphasises theprovision of an ESD Control Program in which the organisation is obliged to prepare an ESDProgram Plan document, covering the ESD Control Program Plan, Training Plan, andCompliance Verification Plan. This is necessary as S20.20 does not specify in detail how ESDdamage prevention measures are to be implemented – it is left to the user to define. Howeverthe standard does give certain technical requirements, some of which are mandatory, andothers are optional.

ESD S20.20 encourages the user to tailor their ESD Control program. Tailoring is a processof

“…evaluating the applicability of each requirement for the specific application. Uponcompletion of the evaluation, requirements may be added, modified or deleted. Tailoringdecisions, including rationale, shall be documented in the ESD Control Program Plan..”(ANSI/ESD S20.20 - 1999)

61340-5-1, however, does not require the user to write an ESD program document – this iseffectively the standard itself, which specifies requirements for ESD Control, training andaudit programs, which are expected to be implemented in full in many cases. However theUser Guide (EN61340-5-2: 2001) notes

“There will however, be some cases where parts of it are not relevant to, or needed by,the processes carried out. Where this is the case then only the relevant parts of thetechnical specification need be followed.” (EN61340-5-2: 2001)

It is thus recognised that in particular certain equipment may not be needed, and omission ofthese is not an issue of non-compliance – however if they are included they must comply withthe standard requirements.

It is likely that the next version of 61340-5-1 will be harmonised with ESD S20.20, and theapproach may well be more like that of the ESD S20.20 standard.Table 3 shows a summary of the technical requirements of ANSI/ESD S20.20 and EN 61340-5-1 related to personnel protection.

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2.2 Research on electrostatic discharges from garments

In this section we present some key results of two relevant major studies: a European Project SMT4-CT96-2079 “The evaluation of the electrostatic safety of personal protective clothing for use inflammable atmospheres” and the PhD thesis of Dr. Paul Holdstock [11]. The results of these studieshave been reviewed in more detail in Report D1.2 [9].

2.2.1 The evaluation of the electrostatic safety of personal protective clothing foruse in flammable atmospheres

The final report of Project SMT4-CT96-2079 “The evaluation of the electrostatic safety of personalprotective clothing for use in flammable atmospheres” [6] and a related summary report [7] cover theinvestigation of whether a range of fabrics were safe for use in flammable atmospheres by usingignition tests using flammable gas mixtures. The summary given here is concerned primarily withevaluating the project results for additional information relating to the evaluation of existing testmethods for garments used in electronics manufacturing industry. The input of the results for theassessment of ESD risk in electronic components has been considered further in Chapter 3.

Forty four samples of material were tested. Eleven of these had no static reducing technology, 5 had atopical antistatic agent or melt additive, 7 had metallic fibres in the weave, 6 had surface conductingfibres, 10 had trilobal carbon core fibres, and 5 had other carbon core fibre technology. The projectexamined material using several ways of charging samples and charge decay time measurements, using;

♦ triboelectric charging with rollers♦ manual triboelectric charging♦ full body charging of garments using a “body voltage chair”♦ Induction charging♦ corona charging using fixed points and a moving sample♦ corona charging using a fixed sample and removable moving corona points (JCI 155)♦ corona charging using a fixed sample and scanning corona points

Full review of the triboelectrification results is beyond the scope of this report. However it is worthnoting that the final electrostatic field measured from materials varied from less than 1 kV/m to over200 kV/m. Ignitions obtained while the sample was earthed were always associated with high levels ofelectrostatic field over 200 kV/m, but in one case a high initial field was not associated with ignitionsfrom the material1. In this case the field strength after 30 seconds had reduced from >200 kV/m to23 kV/m.

Ignitions were also associated with long charge decay times (> 0.1 s) although several materials hadlong decay time but did not give ignitions. Ignitions were also associated with high surface voltages(>750V) after corona charging, although some materials gave this level of surface voltage but did notgive ignitions.

Laundering the fabrics would be expected to remove some of the static dissipative treatment andincrease triboelectrification. In practice this was so for most fabrics. For eight of the materialslaundering had little effect – most of these were had metallic or surface conductive carbon threads. Fivefabric had reduced charge generation after laundering – these were cellulosic (cotton or viscose) or hada hygroscopic coating. Some constituents of the detergents used may have an affinity for hygroscopic

1 The relevance of ignition data to risk of damage to electronic devices will be discussed later.

G6RD-CT-2001-00615


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materials and may improve static performance. In corona deposited charge decay tests, laundering onlyincreased the decay time of about half the samples.

Some materials apparently showed static dissipative performance even when isolated from earth. Thisincluded two homogenous materials as well as some with conducting threads. The hyphothesis givenwas that the low surface resistivity of the materials allowed charge to spread evenly across the samples,giving short charge decay times.

“Induction charging” tests were performed in which a field probe electrode on one side of the sample of the sample. The sample in between the electrodes acted as a screen against the electrostatic fields dueto the pulsed electrodes.

Figure 1. "Induction charging" profiles showing metallic, core-like and homogenous material decayprofiles [7]

The screening effect of the sample was not immediate and the responses were classified as 3 types –Metallic, core like and homogenous. Materials that had a fast response with no initial peak wereclassified as metallic-like. Core-like materials showed an initial peak Ep that quickly decayed within 30- ER. A “field penetration” figure of merit ER/E100 was defined, where E100 is theinitial field strength. Field penetration was found to depend on the grid density of conductive fibres inthe fabric. An “electrostatic shielding value” was defined, that approaches 1 when ER approaches 0

“Homogenous” materials had long slow decay times that could range from milliseconds to greater than50 seconds. Ignitions were always associated with high initial peak field values in this test that had notdecayed after 30 seconds, and with long decay times (> 30 seconds). However some materials had longdecay times but did not give ignitions.

The project found the following parameters to be indicators of a risk of ignition:♦ high surface voltage after charging♦ long charge decay times♦ long induction decay times

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These will be familiar concerns in the study of risk of ESD damage. A key point, perhaps, was thatBTTG [6,7] found that none of these parameters was found to be reliable predictors of ignition risk. It isperhaps likely that the same may be found in ESD risks to sensitive devices.

As a conclusion, the project recommended two new methods as standard test methods for themeasurement of charge decay of protective clothing materials used to avoid incendiary discharges. Nowthe methods are known as the tribocharging and induction charging methods of prEN 1149-3 [12]. Inboth methods charge is monitored by observation of the electrostatic field it generates and this is doneusing non-contacting field measuring instruments. The principal difference between the methods is thetechnique used to generate the electrostatic charge. Triboelectric charging relies on the charge generatedas two materials come into contact, rub together and subsequently separate. Induction charging involvesan electrode placed beneath the test surface and is raised to a defined potential. Induced charge on thetest material influences the net field that is observed by a field-measuring probe positioned above thetest surface. The two methods are recommended because, although charge decay for both test methodsis monitored by observation of the change in measuring field, the physical mechanisms involved aredifferent in each case and so there is not necessarily a correlation between the results of the twomethods or other charge decay test methods.

For the acceptance criteria the project recommended that the triboelectric charging test should notgenerate charge resulting in maximum electric field of 80 kV/m or more. The charge decay should be sothat after 30 s the measured electric field should be less than 50% of the initial value. For the inductioncharging test the recommendations are: charge decay to 50% of the initial value in less than 4 s or theshielding factor (defined above) S > 0.2.

2.2.2 The nature of electrostatic discharges from textile surfaces and theirdamaging effect on electronic components

Holdstock (1999) has provided an in-depth investigation of damage by ESD from fabric surfaces tosome electronic components. His study shows a wide range of discharge waveforms obtained fromtextile surfaces. The test load included a 4 MB DRAM integrated circuit. Voltage at the device pin,ESD current and power waveforms for five ESD events were recorded from each fabric.

Holdstock concluded that the HBM made the closest match to the waveform of a discharge frompolyester, with the time characteristics being slightly different. HBM has a risetime of the order 5 nscompared to 40 ns for polyester. The fall time (to 1/e) is 100 ns for polyester compared to 150 ns forHBM. Comparison of discharges from other base materials show similarity to HBM, with differences inthe rise and fall times. The findings are in accordance with the results of the ESTAT-Garments project,reported in Deliverable D1.3 “Results from tests on electrostatic behaviour on worn garments” [13].

Holdstock subjected 1 MB CMOS DRAM integrated circuits from 2 manufacturers (Mitsubishi &Samsung) to ESD from charged fabrics. Damage was assessed in terms of 86 measurements of outputvoltage, leakage currents, standby current levels and average operating currents. The devices hadinternal ESD protection circuits. In preliminary tests after being subjected to ESD from polyester fabric,9 out of 20 Mitsubishi devices failed functional checks whereas all Samsung devices passed functionalchecks. 64% of failed devices failed open circuit, and 36% failed short circuit. The open circuit devicesmay have failed due to metallization overcurrent stress, whereas the short circuit failures are likely to be !"#$%&"'() *$!!+#,-./packages) were subjected to ESD from polyester fabric. Failures were classified as

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♦ Catastrophic failures – total lack of functionality♦ Degradation or parametric failure – at least one operating parameter is outside the acceptable

range♦ Latent failure – damage hidden to functional or parametric tests that later develops into

degradation or catastrophic failureAbout 25% of devices failed, half of these with latent failures and half with degradation. No devicesfailed catastrophically.

Although no ESD damage tests were done with fabrics containing conductive threads Holdstockcommented that the ESD waveforms obtained from these fabrics were at least an order of magnitudeless than the DRAM IC damage threshold levels.2 He also noted that the three sample fabrics tested hadfailed EN100015 charge decay tests and suggested that either the charge decay tests might beinappropriate or the charge decay value requirements too stringent.

As a result of the study Holdstock has found thresholds of ESD peak current, voltage at the device pins,and charge transferred in the discharge for different levels of damage to a CMOS IC. This seems toconfirm that these parameters should be investigated further as indicators of ESD damage risk.

2.3 Considerations for ESD garment tests

2.3.1 Current standard test methods

The principal standard test methods for ESD garments both in EN 61340-5-1 and ANSI/ESD S20.20 areresistive. EN 61340-5-1 includes also a charge decay measurement technique using corona charging. Inaddition to the EN 61340-5-1 and ANSI/ESD S20.20 methods, there are many test methods applied togarments, some are applicable others are not. Some of the test methods being used today for ESDgarments and fabrics were designed for other specific materials. As an example, the resistive methods(surface resistance, point-to-point resistance) were developed for homogeneous materials. Presumablythey characterise well the ESD protective performance of garments made of homogeneous materialssuch as 100% cotton. But the state-of-the-art garments used in electronics industry are not homogeneousbut very heterogeneous. They are made of composite fabrics where a grid or stripes of conductivethreads are present inside a matrix of cotton, polyester or mixtures of these materials. Furthermore, theconductive threads are made by composites, that is by a mixture of conductive and insulating fibres(core conductive fibres, sandwich type fibres, surface conductive fibres, etc.).

Standard EN 1149 “Protective clothing – Electrostatic properties” present two resistive methods for thecharacterisation of electrostatic properties of protective clothing to avoid incendiary discharges:-Part 1: Test methods for measurement of surface resistivity-Part 2: Test method for measurement of the electrical resistance through a material (vertical resistance).

2 Details of the conductive threads used in Holdstock’s studies are not available. His results are not consistentwith the results of D1.3 of the ESTAT-Garments, where direct ESD of destructive level is easily obtained fromunearthed ESD fabrics having metallic conductive threads. Direct ESD from ESD fabrics having conductivethreads with resistivity in the electrostatic dissipative range, however, would cause only small or negligible risk ofESD damage to sensitive devices [viite].

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In both standards it is stated that the method is directly applicable to homogeneous materials3. It is saidthat the test methods are not applicaple for fabrics with core conducting fibres. Considering surfaceresistivity, performance requirements for homogeneous materials it is said that the surface resistivityshall be less than 5x1010 Ω, after testing according to the standard. For inhomogeneous materials4

containing conducting threads it is stated for the performance requirements that they should have:a) resistances none of which exceeds 109 Ω on at least one surface when measured according to the

standard;b) a grid pattern of conducting threads;c) a maximum spacing of 10 mm between the conducting threads.

When considering EN 1149 and its relevance to electronics industry, we should keep in mind that themain purpose of ESD garments in electronics industry is to protect sensitive devices from ESD damagecaused by a charged person. The purpose is very different to protective clothing in general. For examplethe purpose of ESD garments used in flammable atmospheres is to protect the operator from externalthreats (explosion or fire due to ESD). Any good test method for ESD protective garments used inelectronics industry should assess the garments ability to provide ESD protection.

2.3.2 Considerations for ESD garment tests according to Baumgartner

Baumgartner has considered ESD garment specifications for electronics industry and proper testmethods for the ESD garments and ESD fabrics [14]. He points out that many test methods that are inuse do not address ESD protection of a device. At most, the test methods only measure garmentspecimen properties. At best the measurements are a “Figure of Merit.” Garment fabric properties havebeen and are presently measured with improper instrumentation or test methods. This results inincorrect assumptions of ESD garment properties. Just because data can be taken, does not prove thevalidity of test methods or mean that the value recorded is applicable for determine good and badgarments. No models of garment or test methods have been proposed to allow validation of ESDprotection.

Baumgartner also comments [14] that there are many test methods used in facilities. Tests such as staticdecay, tribocharging, and shielding have not been generally accepted. Instrumentation, such as handheld electrostatic field meters and other similar devices are often used incorrectly and in inappropriateapplications. Some measurements have been proposed as the basis for a new garment standard inaddition to the present (ESD Association) standard. The effort has not progressed since the GarmentStandard Working Group of the ESD Association has not presented evidence substantiating the need foradditional test procedures. Therefore, according to Baumgartner, the present standard describes aresistance measurement.

The aim of Baumgartner’s report is to show the enormous need for a good test method or a serious oftest methods for ESD protective clothing used in electronics industry that actually validates theprotective performance of the garment. For developers of such methods he gives valuable advice: Anyproposed test methods must show that they evaluate the ESD garment’s ability to provide ESD

3 Homogeneous material is here defined as a material, where the electrical properties of the components(threads, layers) do not differ substantially from each other, or a material which contains an intimate blend ofconductive fibres.4 Inhomogeneous material is here defined as a material that contains small quantities of conducting threads,which are distributed discretely in a grid pattern throughout the material; or material that is coated or laminatedwith polymeric or metallic materials where the electrical properties of the material components differ substantially(e.g. more than a factor of 10) from each other.

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protection. Garment test methods, or system tests5, need to clearly state what the test methods evaluate,the objective and usefulness of the test method. Garment test methods should be considered anddeveloped for three different functions;

♦ evaluation6,♦ acceptance7, and♦ periodic8 test.

It is necessary to define what a garment is supposed to do for the protection of sensitive electronicsbefore a test method is developed. Baumgartner comments also ESD fabrics tests by saying “Tests onfabric are generally a ‘Figure of Merit’ test. If a small sample is used for such a test, a relationship to acomplete garment must be demonstrated”.

2.3.3 Considerations for ESD garment tests according to other researchers

Validity of current standard test methods and other common test methods has been considered also byseveral other researchers. Börjesson has done a comparative test series for several test methods used forfull garments tests [15]. All the garments used in the tests were so called clean room ESD garments withcore conductive fibres. It is generally accepted that the standard resistive methods completely fail withthese clean room garments in assessing the garments ESD protective performance. It seems also that thecharge decay test of EN 61340-5-1 fails with clean room garments [16]. Therefore the decision to useclean room garments with core conducting fibres in the test was well justified. Börjesson analysed theresults with reference to the correlation of the measurement results between the different methodsincluded in the study. He did not evaluate the garments ability to protect sensitive electronics fromESD. The correlation between the results from different test methods (i.e whether a tested ESD garmentis ‘acceptable’ or ‘not’) was not good. The conclusion of Börjesson was that further studies are needed.

An extensive test programme has been carried out also by Chubb, Holdstock and Dyer [17]. Theymeasured different kinds of clean room garments using a tribocharging method of J. Chubb, coronacharging charge decay method of EN 61340-5-1, and the capacitance loading method of J. Chubb.

Referring to Baumgartner [14], all these measurements and methods (induction, tribocharging, chargedecay, capacitance loading, etc.) give results that are ‘Figures of Merit”, which ranks the material moreor less arbitrarily and are specific to a configuration of the test method. The measurement result may beuseful for many cases, but it is so far not evident how well these measurements characterise thegarments ability to protect sensitive devices from ESD. It is the task of this report to find answers tothese questions.

An additional important issue is whether the electrical integrity of seams is important or not whenassessing the ESD protective performance of an ESD garment. Current standards (IEC 61340-51 andESD STM2.1) expect that a measurement is done over a seam structure from sleeve-to-sleeve or fromsleeve-to-hem. Chubb has expressed that fabrics test would be sufficient and no measurement over a 5 A system test is defined as a test of many items functioning together to determine their ability to meet a specificrequirement or for evaluation purposes.6 Evaluation test is a test or series of tests that determine the properties of interest. The test method may be anaccepted standard or any other technically applicable and valid test or measurement.7 Acceptance test is a test that is used on the first article or for incoming material to determine if the measuredvalues or other requirement specified by the inspection order or that the requirements are within limits. A singlemeasurement or test method may be sufficient. A series of tests may be required if several properties are to beverified.8 Periodic test is a measurement that is performed in the manufacturing area or in the field on a specific scheduleto check the property of an item.

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seam is necessary [18]. He argued that charge generated on a garment surface would be rapidlydistributed over a wide area, if the resistivity of the garment is not too high. When distributed to widearea, the charge could be sufficiently fast transferred from the garment surface into operator’s body. Hisopinion is that undergarments will not be sufficiently insulating to isolate the garment from the body -even if a wrist connection is not used. With footwear there is no restriction on the socks that may beworn! Body perspiration will surely be quite effective to keep undergarments dissipative inside acleanroom garment -and hence able to dissipate charge from the outer garment by local area contact. Ifthere is adequate dissipation to the body for each garment section, then there is no need to worry aboutseams. Large area of overlap contact between garment sections should give adequate dissipation. Paasihas replied to Chubb [19] by saying that, while the charge may transfer sufficiently well from the outergarment surface through undergarments to operator’s body, the charge transfer may not happen whenthere are two layers of normal clothing under the ESD garment. Such cases are not unusual in wintertime, when the ESD risks are, in general, the highest.

The need to include seams for the study of the protective performance of a garment (i.e. garment tests)is, however, an important question which needs to be studied in depth in the ESTAT-Garments project.If it appears that electrical conductivity of seams is of high importance for the ESD protectiveperformance of the garment, then garment testing must include a test where charge is forced to flowover a seam or seams. On the other hand, if it appears that seam conductivity is not important, thenfabric tests would be sufficient to characterise the protective performance of a garment.

2.4 IEC Garment Survey

The IEC working group IEC TC101 WG 5.2 “Test methods for garments” made in 2002 an interviewfor the end-users of ESD garments. The chairman of the working group, Mr. Giuseppe Reina, presentedthe results of the survey [8] for the ESTAT-Garments project in the 5th Project Management Committeemeeting. The end-users came from Finland, Sweden, Netherlands, UK, Turkey, Germany and USA.Most of the answers came from Finland and Sweden. About 80% of the end-users representedelectronics industry.

According to the survey, 80 % of the companies consider that the present standard test methods as wellas the requirements for ESD garments are insufficient. In Figure 2 it has been specified that in whichway the standards are lacking, according to the interviewed companies. In Figs 3-5 we show otherresults of the study. Special attention should be paid to the answers of Question 16 (Fig. 5) “Whatproperties do you recommend in the production of new standards for ESD protective garments?” Thetop 4 properties in the answers were:

1) Shielding of electrostatic field2) Triboelectric chargeability3) Resistance of full ESD-protective clothing4) Decay time of full ESD-protective clothing.

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Figure 2 Answers to the IEC Survey question Nro 15 “Do you feel that standards in the area of ESD-protective garments are lacking?” Answer alternatives: a) Yes, requirements for fabric, b) Yes,requirements for full ESD-protective clothing, c) Yes, requirements for worn (including person) ESD-protective clothing, d) Yes, test methods for fabric, e) Yes, test methods for full ESD-protective clothing,f) Yes, test methods for worn (including person) ESD-protective clothing, g) No.

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Figure 3 Answers to the IEC Survey question Nro 13 “Which properties of ESD-protective clothing doyou feel should be specified to assure avoidance of the risks you have defined?” Answer alternatives:a) Shielding of an electrostatic field, b) Shielding of electromagnetic transients caused by dischargesbetween different parts of the system: person - ordinary clothing - ESD protective clothing, c)Triboelectric chargeability, d) Decay time of fabric, e) Decay time of full ESD-protective clothing, f)Decay time of worn ESD-protective clothing (system), g) Resistance of fabric, h) Resistance of full ESD-protective clothing, i) Resistance to ground(-able point) of ESD-protective clothing, j) Resistance toground of worn ESD-protective clothing, k) ignitability of fabric, l) Ignitability of clothing, m)Ignitability of worn ESD-protective clothing (system), n) other.

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Figure 4 Answers to the IEC Survey question Nro 14 “Which properties do you specify/consider atpurchase?” Answer alternatives: a) Shielding of electrostatic field, b)Shielding of electromagnetictransients, c) Triboelectric chargeability, d) Decay time of fabric, e) Decay time of full ESD-protectiveclothing, f) Resistance of fabric, g) Resistance of full ESD-protective clothing, h) Ignitability of fabric, i)Ignitability of full clothing, j) Other.

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Figure 5 Answers to the IEC Survey question Nro 16 “Which priority would you recommend for theproduction of new standards?” Answer alternatives: a) Shielding of electrostatic field, b) Shielding ofelectromagnetic transients, c) Triboelectric chargeability, d) Decay time of fabric, e) Decay time of fullESD-protective clothing, f) Surface resistance of fabric, g) Resistance of full ESD-protective clothing,h) Ignitability of fabric, i) Ignitability of full clothing, j) Other.

The results of the IEC Survey give valuable input for the ESTAT-Garments project. They reflect theopinions and practices of ESD garments end-users in electronics manufacturing industry. On the otherhand, they may be influenced more by general beliefs and myths than by researched evidence.

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3 Risk of damage to electronics with reference togarments

There are two main failure mechanisms for ESD sensitive devices. Most of the devices are sensitive tothe energy of the discharge. Some devices are more sensitive to internal overvoltage due to ESD.Energy sensitive devices fail by the high discharge current heating a small volume of material to afailure temperature. The failure temperature is often the melt temperature of the material but may bedetermined by changes in other characteristics such as magnetic properties in MR heads. In the case ofdischarges having long duration and significant heat transfer from the damage region, the key parameterfor energy sensitive devices is the discharge power instead of discharge energy. In voltage sensitivedevices, the voltage sensitive part fails when a breakdown voltage or field strength is reached. This mayhappen due to charge accumulation on an isolated part, or by the voltage drop due to a passing highcurrent ESD impulse. The peak ESD current and charge are likely to be fundamentally important inboth energy and voltage sensitive device damage. The failure mechanisms are presented in detail inDeliverable report D1.2 [9]. Here is sufficient to recognise the division to energy and voltage sensitivedevices.

ESD-garments should protect sensitive electronics from ESD damage caused by a charged person(operator). But in addition to the charged operator, also improperly designed or working, charged ESD-garment itself can be a potential source of ESD failures. In the ESTAT-Garments project we havespecified possible risks of damage to ESDS with reference to garments [9]. An ESD failure caused bycharged operator or charged clothing can potentially happen, at least, in three different ways:

1) by a direct discharge to a device,2) by a discharge from a charged device, and3) by radiation, i.e. by an induced EMI (electromagnetic interference) pulse due to ESD.

3.1 Risk from direct discharges

Direct discharges from the body of the operator, from unearthed conducting threads of the garment, orfrom insulating surfaces of the garment fabric are related to improperly worn, working or designed ESDgarments, respectively. Direct discharges may cause damage by charge injection to the device. Thiscould occur whether or not the device is grounded. Every conducting object, including an isolatedobject, has an equivalent capacitance and voltage. A discharge will occur when another object at adifferent voltage comes sufficiently close.

A discharge between an object and a device may occur when:♦ A charged device approaches a garment part, which may be grounded in the case of a

conducting part♦ A charged garment fibre approaches a grounded or non-grounded device

Garment characteristics should be chosen so that neither of these possibilities can give significant ESDdamage risk.

The exact discharge waveform will depend on the electrical characteristics of the ESD source andvictim device. Examples of ESD waveforms from charged, unearthed fabrics commonly used in ESD

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garments are given in Fig. 6, where the discharges are taken from the conducting threads of the fabrics.The measurements were done using a transmission line probe described in ref. [20]. Fig. 6a representsan ESD from stainless steel threads charged to 2 kV and Fig. 6b an ESD from surface conducting,carbon fibre based threads. ESD peak current from surface conducting carbon threads is only about onehundredth of the peak current from stainless steel threads. ESD peak current from core conductivecarbon threads (not shown in the figure) is even slightly smaller than that from surface conductingthreads. An ESD from charged insulating base fabric (polyester) was so weak at 2 kV that the probecould not measure that (discharge current below 1 mA). So it seems that direct discharges frominsulating surfaces of modern ESD garment fabric is not an ESD risk for ESD sensitive electronics.Charged, unearthed conducting threads, however, form a risk for ESD damage of devices: unearthedstainless steel threads are a risk for all ESD sensitive devices and carbon fibre threads at least for Class0 devices with HBM ESD withstand voltages below 250 V. More detailed study of direct dischargesrelated to ESD garment fabrics are given in Deliverable report D1.3 [13].

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Figure 6 ESD current waveforms from unearthed ESD fabric charged to 2 kV: (a) fabric with stainlesssteel threads, (b) fabric with surface conducting carbon fibre threads. Only the initial part of the

discharge curve is shown in the figures.

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Failure mechanisms and failure thresholds of ESD sensitive electronics have been studied inDeliverable report D1.2 “Preliminary evaluation of the risk of ESD damage to sensitive devices arisingfrom ESD from garment surfaces” by J. M. Smallwood and J. Paasi . Five types of damage thresholdwere be envisaged:

♦ A peak discharge current limit♦ A peak power dissipation limit♦ A peak energy dissipation limit♦ A limit to electric field developed across MOS gates or other internal circuit parts as a result

of internal current flow♦ A limit to electric field developed across MOS gates or other internal circuit parts as a result

of differential charging of a device internal capacitance.

All these types of threshold are applicable during different ESD situations. However, to attempt to testmaterials as ESD sources against all these thresholds would be difficult, even if the threshold valuescould be determined for each case. Some simplification could be done by taking into account that thelimits of power and energy are also related to discharge current. So, it seems reasonable to place athreshold on discharge current in order to control these aspects. The damaging effect of electric fielddeveloped across MOS gates or other internal circuit parts as a result of internal current flow or devicecharging is likely to be very device specific and unpredictable. It is possible that the peak dischargecurrent threshold or charge transferred in a discharge may serve as a good guide for the assessment ofrisks for damage of voltage sensitive devices due to direct discharges.

The results of D1.2 are in accordance with the results of Holdstock [11] which found thresholds of ESDpeak current, voltage at the device pins, and charge transferred in the discharge for different levels ofdamage to a CMOS IC. Because voltage at the device pins is very device specific parameter, and thusnot applicable as parameter characterising ESD protective performance of a material, we conclude topay attention to

♦ peak ESD current from garment or garment material, and♦ charge transferred in an ESD

when assessing ESD garments with respect to their ability to minimise risks of ESD failures to devicesdue to direct ESD from charged clothing into a victim device.

3.2 Device charging and CDM damage risks

The second important ESD risk with reference to garments is associated with garment originatedcharging of a device. The device becomes, at first, charged and, after that, gets ground contact givingrise to electrostatic discharge (CDM type of ESD). A device may charge by four methods.

♦ Firstly, accidental rubbing of the package against the garment material may cause charging bytriboelectrification.

♦ Secondly, a device in an electrostatic field arising from the garment will have voltage inducedon it in response to the field. Component parts of the device (an input gate of IC, or acomponent on a PWB) may also experience induced voltage stresses.

♦ Thirdly, ions present in the air may be deposited on the device and give a non-zero voltage.♦ Fourthly, a device may become charged when it becomes in direct contact with a charged

object and the device is not grounded.

Electrostatic fields external to the garment may be dependent on;♦ Suppression of fields by coupling to the body of the person wearing the garment

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♦ Triboelectric chargeability of the garment material♦ Rate of charging of the garment materials♦ Rate of charge dissipation of the object materials♦ Ability of the garment material to shield the electrostatic field of underlying garments

An electrostatic field external to the garment leads to risks of induced charge on a device that could leadto CDM ESD damage. The risk threshold is reached when the charge induced on the component reachesthe device CDM withstand voltage level. Thus from the ESD control point of view it seems reasonableto place threshold on device charging. A device package can be charged also through rubbing on amaterial. Then risk threshold is, again, reached when the component is charged to the CDM withstandvoltage level.

Another case of importance is when an entire Printed Wiring Board (PWB) becomes charged due toclothing. The four charging mechanisms presented above for device charging are valid also for PWBcharging. With reference to garments the main mechanisms are charging by induction when a board isin the vicinity of charged garment (or charge enclosed by the garment) or by accidental rubbing of theboard by the garment fabric. A PWB conductor has much higher capacitance than a single device and,thus can store much more charge than a device. For voltage sensitive devices, a situation may arisewhere there are an inducved stress voltage over the device exceeding the gate breakdown voltage of thedevice. Also a direct discharge of a charged board, i.e. a charged board model (CBM) ESD, can happen.For a device assembled on the board, a discharge of a charged board is external to the device unlikecharged device model type of ESD. That is, the direction of discharge current in charged board model,CBM, type of ESD is mainly into the victim device resembling the characteristics of MM ESD, unlikein CDM ESD where the direction of ESD current is outwards from the device. Some of the chargestored in the PWB assembly could be in or on the victim device but most of the charge is elsewhere(outside the device) before ESD. Paasi et al. have shown that under some circumstances devicesassembled on a PWB are more sensitive to ESD failures than before the assembly [21]. With referenceto garments, any potential charging of PWBs should be minimised. Electric field external to garmentshould be low in order to minimise induction charging of the PWB. Perhaps more important is to designthe garment so that charging by accidental rubbing is minimised. Triboelectric charging propensity ofthe garment fabric with respect to PWB assembly should be low and the design of the garment shouldbe such that possibilities for accidental rubbing, for example by loose cuffs, are minimised

In summary, from the ESD garment point of view the key parameter to control, in order to minimiserisks of ESD damage due to CDM or CBM type of ESD with reference to charged clothing, is theelectrostatic (electric) field external to the garment. Because electrostatic field decays rapidly withdistance, charging by induction is a serious risk only when the garment/operator is sufficiently close tothe device. But in that case, the ability to minimise risks of CDM or CBM type of ESD is an importantfeature of a good ESD garment.

3.3 Risk from indirect discharges

Risks from indirect discharged (i.e. radiation) are related to EMI currents induced by nearby ESD, suchas a discharge from operator’s ordinary clothing to his grounded ESD-garment. Induced EMI currentscan damage ultrasensitive components, but a strong ESD source would be required. According toDeliverable report D1.3 the risk, with reference to garments, seems to be relevant only when handlingMR heads or other components with ESD withstand in the range of a few volts [13].

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4 Evaluation criteria of test methodsThe purpose of ESD garments is to protect sensitive electronics from ESD failures or alternatively tominimise risks of ESD failures to sensitive electronics. Any good test method for garments or garmentmaterials should assess that. The required protection level would vary a lot between different productionlines and plants, depending on current and anticipated, future sensitivity of the devices in production.The tendency that ESD control programs will be tailored and optimised to companies needs also makesadditional challenge for developing a good standard test method or a group of standard test methods forESD garments used in electronics industry. The same applies for any recommendations related to theuse, acceptance limits, structure, etc. related to these garments. It is clear that the present standards donot give sufficient support for all that. New documents are required.

4.1 Characteristics of good ESD garments

According to Baumgartner [14] an ideal ESD garment is a garment that possess all the desiredproperties:

♦ Low resistance (fast dissipation of generated charge)♦ High resistance (safe slow dissipation of generated charge)♦ Total suppression of electrostatic fields from charge under and on an ESD garment surface♦ An anti-static material that does not generate a charge when contact is made to any other

material.The end result of an ideal garment cannot be achieved without compromises. The first two items arecontradictions. A low resistance will cause a fast discharge that may not sufficiently limit the energytransferred, and a device may be damaged. A high resistance may not reduce the electric field strengthin a time period short enough to prevent field-induced damage (device charging by induction andconsequent CDM ESD). Electrical safety, in those tasks where it is of concern, calls for high resistivity.Enclosing the whole person for total suppression of charge under the ESD garment is not practical. Aconductive garment will still tribocharge. Although an ideal garment can never be achieved in practicebecause of competing or unrealistic demands, the properties of the ideal garment will give good insightinto the design of ESD garments for electronics industry as well as for the testing and validation of suchgarments.

A non-charging garment material is the target, but not a realistic one. Realistically, materials used inESD garments should not charge easily and, if charges are generated, they should be safely dissipated.There are three mechanisms by which static dissipation occurs and by which the charge on aheterogeneous ESD fabric will be neutralised, Fig. 7 [22]:

♦ Conduction: If the fabric is grounded, the charge on or near the conducting element of theconducting thread will be conducted to earth.

♦ Induction: The charge on the insulating base fabric induces a charge of opposite polarity ingrounded conductive threads leading to partial neutralisation of the total charge. Thephenomenon can also be understood as an increase in the capacitance caused by the groundedthreads which lower the effective potential to be observed.

♦ Corona: Partial neutralisation of charges on the base fabric may also occur by receiving the airions formed in nearby corona discharges on conductive threads, if the corona onset fieldstrength in the region is exceeded.

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The only mechanisms of the three that is directly related to the resistivity of the full material isconduction. Charge transport on composite fabrics have been studied in WP 2 “Physical mechanism infabrics” and results from experiments are presented in Deliverable report D2.3a “Charge behaviour onfabrics” [23]. The different orders of magnitudes in the resistivity of conductive threads and base fabrichave a direct influence on the charge transport behaviour of an inhomogeneous composite fabric. Thereare two different times scales in charge transport (conduction): the charge transport in conductivethreads is fast while it is slow in the base fabric. The resistivity of the conductive threads has aninfluence also on the charge decay by induction mechanism. In induction mechanism the resistivity ofbase fabric is of minor importance. The internal structure of the fabric, especially the spacing of thegrounded conductive threads, have the strongest influence on the effective potential observed on thefabric through the induction mechanism [24]. The fineness and cross sectional shape of the threads willhave a big influence on the corona onset field strength. It should be pointed out that the coronamechanisms do not require grounded threads but surface charge density should be relatively high toinitiate the corona.

Electrostatic shielding of charge enclosed by the garment, suppression of electrostatic field outside thegarment due to grounded operator body, and, in some cases, shielding of radiated EMI pulse are alsoimportant features of a good ESD garment. Some composite fabric structures would give betterperformance than others. Another question is related to electrical safety in those works where that isrelevant. Personnel safety requirements can be contradictory to ESD protection requirements.

+

+

+

+

+

+

+

++

+

+

-

--

-Conduction

Induction

Corona

Figure 7. Three charge dissipation mechanisms of ESD fabrics

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Protective performance of a garment is highly reduced or in some cases even completely lost, if thegarment is not grounded. Grounding is typically realised by a direct collar or sleeve contact to thegrounded operator. Improper use of the garment may easily disconnect the ground path. Thereforealternative grounding methods could be considered or continous ground connection monitoring systemsbe developed in order to guarantee the desired function of the protective clothing.

4.2 Evaluation criteria for fabric and full garment test methods

Any good test method for ESD protective garments should assess the garment’s ability to provide ESDprotection. If the real failure mechanisms of ESDS are related either to discharge energy or power (bothrelated to discharge current) or to internal device overvoltage (often related to charge), it is reasonableto ask the question that ‘are we measuring the right parameter?’ For example, current practice methods(standard resistive methods and other commonly used methods) too often measure only the electricalperformance of the net formed by the conductive threads, ignoring several factors which are importantwhen evaluating the protective performance of the garment. When testing the ESD protectiveperformance of a garment or a garment fabric, it is important to realise that we are not interested inspecific physical behaviour (even though that would be interesting for a measurer) but we are evaluatinghow well the garment would protect ESD sensitive electronics from failures of electrostatic origin.

Based on the output from WP1 “Assessment of the risk of damage to sensitive devices from ESD fromtextiles and garments” and conclusions from Chapters 2, 3, and 4.1 of this report, we have derivedevaluation criteria for test methods characterising the ESD protective performance of ESD garments orgarment materials. The criteria are listed below.

The following parameters have been identified as the key parameters to control in order to minimiseESD failures with reference to garments:

♦ Peak ESD current♦ Charge transfer in a discharge♦ Device charging due to electric field external to the garment.

These parameters, however, are not necessarily such which could be easily, reliable and repeatablemeasured in garment or garment material tests. Therefore we processed the problem further and definedfactors influencing to the key parameters. The factors were identified as “easily” measurableparameters.

Peak ESD current and charge transfer in a direct discharge from charged fabric is largely determined by♦ the resistivity of conductive threads♦ the grid density and grid structure♦ the amount of retained charge

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Device charging due to electrostatic field external to the garment is largely determined by♦ the chargeability of the garment fabric♦ the rate of charge dissipation of the garment/garment material, which can happen in three

mechanisms:1. Conduction, which depends on the resistivity of the threads as well as base material

and possibly also on the resistance over garment seams,2. Induction, which depends on the resistivity of the conductive threads, grid structure

and capacitance of the garment system,3. Corona mechanisms, which depends on the structure of the conductive threads.

♦ the electrostatic shielding property of the garment material, which depends on the resistivity ofthe conductive threads, base material and the grid structure

♦ voltage suppression, which depends mainly on the distance of the garment to nearbyconductive objects (usually earthed, e.g. the wearer’s body), but conductive thread resistivityand grid structure have also some influence.

Risks of direct discharges from garment into devices are minimised when all the garment panels aregrounded satisfactory well. Also charge decay through conduction and induction mechanisms dependson the grounding of the garment. In some cases there can be a proper ground path for charge directlyfrom each garment panel to operator’s body, either directly through a skin contact or through underlyinggarments if they have sufficient conductivity. Alternatively the ground could be brought directly to thegarment panel by a ground wire to a groundable garment point. In the worst case there is no conductivepath for a charge to flow from each panel directly to operator’s body, and the charge must flow throughone or more seams to find the path to the body. Therefore, the integrity of the electrical resistance of theseams is important and special attention is paid for that in the IEC 61340-5-1 and ESD STM2.1standards.

Most of the factors above are directly related to the structure and electrostatic properties of the garmentmaterial and, thus, can be studied by fabric tests. Only few factors require potentially full garment orsystem tests. The list of the factors has been used for the evaluation of existing test methods for bothESD fabrics and full garments. The results of the fabric test methods are reported in Deliverable reportD5.2 “Report on the evaluation of existing test methods for fabrics” [10]. The results of the garment testmethods are presented in Chapters 6 and 7 of this report.

Additional challenge for the evaluation of test methods for garments and garment fabrics are given bythe industrial need to have different levels of tests:

♦ Evaluation tests, which would be a series of tests that determine the properties of interest andwhich would be done in specialised laboratories under controlled conditions. Evaluation testswill be predominantly done for new products to enter the markets and/or during productdevelopment stages.

♦ Acceptance/Periodic test(s) which would be simple test or tests done in sites or laundries andwhich focus only on one or more critical properties. These tests will be done in sites for newgarments when taken in use, during auditing. These tests could be done also in laundries whenverifying the performance of used garments after washing.

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5 Selected existing methods for the testsThe first task of the work was to made a survey of existing test methods for the characterisation of theelectrostatic (ESD protective) performance of ESD garments. The survey covered international andmajor national standards worldwide as well as a few commonly used industrial and laboratory methodswithout a standard status. Some preliminary screening of the methods was done based mainly on theexperience from the European research project SMT4-CT96-2079 “The evaluation of the electrostaticsafety of personal protective clothing for use in flammable atmospheres” [6,7] and the output fromWP1. Special attention was paid to include test methods applying different kind of charging techniquesand potentially monitoring factors listed in Chapter 4.2. The selected test methods after the pre-screening for the evaluation of existing test methods in WP 5 and WP6 are listed in Section 5.1.Selected test methods for full garment tests are described in more detail in Section 5.2.

5.1 List of selected test methods

The following existing test methods for ESD fabrics have been applied in the studies of WP5, reportedin detail in Deliverable report D5.2 [10]:

♦ Resistive methods of IEC 61340-5-1 (surface resistance, point-to-point resistance)♦ Resistive methods of EN 1149-1 /-2 (surface resistance, vertical resistance)♦ Surface resistivity according to EN 100015-1♦ Induction charging test according to prEN 1149-3♦ Triboelectric charging test according to prEN 1149-3♦ Charge decay test by contact charging (VTT-method)♦ Corona charging test according to IEC 61340-5-1♦ “Capacitance loading” test of John Chubb (corona charging) using JCI155 and JCI176

measurement devices.

The following existing test methods for full ESD garments have been applied in the studies of WP6,results presented in this report:

♦ Resistive methods of IEC 61340-5-1♦ Resistive methods of ESD STM2.1♦ VTT’s method for the measurement of charge decay time of ESD-protective clothing♦ SP Method 2175 “Measurement of charge decay time of ESD-protective clothing”♦ STFI test method No. PS 07, version 01/03Rev. A “Test method to determine the body

potential and the charge transfer by wearing of electrostatically dissipative protectiveclothing” (charge transfer – garment test)

♦ Shirley method 202 “Test method for measuring static electricity generated when removinggarments from the human body”

♦ JIS L 1094:1997 “Frictionally charged electricity-amount measuring method”

Both the fabrics and full garment tests were done with similar samples supplied from WP4. It wasdecided to perform the tests with modern ESD protective materials which are known to be the mostchallenging from the electrostatic testing point of view. The selected garments and garments materialsincluded surface, core, and hybrid conductive threads in pure polyester base fabric. The test garmentsare presented in more detail in Chapter 6 and, in particular, in Deliverable report D4.2 [25].

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5.2 Description of the selected test methods for full garments

A short description of the selected test methods for full garments are given here by method by methodbasis. For more details, see the original reference documents.

5.2.1 Resistive methods of IEC 61340-5-1 and ESD STM2.1

Resistive methods of the both major standards, IEC 61340-5-1 [3] and ESD STM2.1 [4], for fullgarments measure point-to-point resistances and/or resistance to a groundable point when the garmentprovides such a groundable point. In the IEC-standard it is said that care should be taken to include,where applicable, one seam between the two measuring electrodes. The ESD STM2.1 says that point-to-point test is intended to test the electrical resistance between any two points on the garment, which mayinclude the electrical resistance across the seams of the garment.

The point-to-point resistance measurements in both standards are done for a garment under test layingon an insulating support (Fig. 8). There are minor (in practice negligible) differences in the electrodesused in the point-to-point tests. The experiments to be presented in Chapter 6 are done using theelectrodes according to ESD STM2.1 but otherwise following the procedures of IEC 61340-5-1,Appendix A.3. According to IEC 61340-5-1, Appendix A.3, the line joining the axis of the twoelectrodes shall be given different orientations relative to the axis of the garment for the threemeasurements. The measurements are typically taken from sleeve-to-sleeve, sleeve-to-hem (or leg),across back piece for 30 cm distance between the electrodes. The measurement voltage is typically100 V.

ESD STM2.1 describes a different arrangement for sleeve-to-sleeve resistance measurement. Thesleeve-to-sleeve measurement is done for garments hanging from each sleeve with electrically isolated,stainless steel clamps acting as electrodes. The purpose of the sleeve-to-sleeve method is to test theintegrity of the electrical resistance across the seams of the garment.

Figure 8 Point-to-point resistance measurement of ESD garment: Resistance from sleeve-to-sleeve, thegarment on a flat insulating support.

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5.2.2 VTT’s point-to-point method for full garments

VTT’s method for the measurement of charge decay time of ESD-protective clothing is based on thepoint-to-point or sleeve-to-sleeve resistance measurement arrangements of IEC 61340-5-1 and ESDSTM2.1. An alternate name for the method is charge decay time from point-to-point. The method iscommon in Finland.

In the method garment under test is charged using contact charging by one of the electrodes used in theresistance measurements to a voltage exceeding 1000 V. Then the sample is earthed through the otherelectrode, and the discharge time from 1000 V to 100 V is recorded. Measurements could be taken atthe same locations as the point-to-point and sleeve-to-sleeve resistance measurements. It is important toincluded measurements over a seam in the tests to measure the charge transfer across a seam. Themeasurements could be done either in a flat or hanging garment. The measurement arranged for hanginggarments is shown in Fig. 9.

Figure 9 Method used at VTT for the measurement of charge decay time of ESD-protective clothing

5.2.3 SP Method 2175

The SP Method 2175 “Measurement of decay time of ESD-protective clothing” [26] is a laboratorymethod, which has, in practice, an unofficial status of national standard in Sweden. There are alsocompanies outside Sweden who like their garments to be tested according to this method.

The SP Method 2175 is intended to verify that each panel of the garment, intended as protection forESD-sensitive components in electronics manufacturing, has a connection to the ground. The method isa system measurement. The measurement is performed with the garment worn by a person, targeting

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that also other phenomena, such as charge spread out on the complete garment, voltage suppressionappearing in real world situations, are simulated. The application of the charge is performed by transferof a charge from a capacitor, charged to 550-600 V, to any fabric panel of the test garment, Fig. 10. Ifthe garment is conductive or dissipative, the charge will spread over the complete garment and thevoltage will be suppressed by the capacitance of the garment to the test person. The test person shallstand with horizontal underarms and with the cuffs of the sleeves extending 5 to 8 cm outside the cuffsof the clothing worn underneath, Fig. 11. The test person is grounded through a wrist strap and, thus, thecharge shall be drained away from the test garment. The decay time from 500 V to 100 V of thecharging electrode is measured and reported.

Figure 10 Sketch of the SP Method 2175 “Measurement of charge decay time of ESD-protective

clothing”

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Figure 11 Measurement of charge decay time of full garment using the SP system method 2175.

5.2.4 STFI test method No. PS 07

STFI method No. PS 07 “Test method to determine the body potential and the charge transfer bywearing of electrostatically dissipative protective clothing” is a new laboratory method developed forthe characterisation of ESD protective clothing used in explosion dangerous areas [27]. Version “01/03Rev. A” was used in the current studies of ESTAT-Garments. The initial purpose of the method is todetermine the ignition risk of protective clothing, made from dissipative materials, due to possibledangerous electrostatic charging of the human person or the garment.

The test set-up and the procedure is sketched in Figs. 12-13. An operator is clothed in a specifiedundergarment (cotton) and in the protective clothing as outer garment. The outer garment is highlycharged triboelectrically by a third person rubbing with selected textile material (pure wool fabric orpure polyamide fabric ) on the back of the operator. The body potential, which is caused due to thegenerated charge on the clothing, is measured by an electrostatic voltmeter. An earthed, specified ballelectrode [28], Fig. 14, is then (as soon as possible) approached to the charged area of the garment tomeasure possible charge transfer in types of spark or brush discharges by a fast storage oscilloscope,Fig. 15. The operator is in turn earthed and unearthed. Ten parallel measurements are made both withthe person earthed and unearthed. Between measurements, the tested garments are neutralised by anioniser.

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electrostaticfield meter

recorder

metal plate

foot stoolswitcher

resistormetal plate

hand probe

test personpolyamide / wool


recorder

electrostatic fieldmeter

Figure 12 Procedures of STFI method No PS 07: Garment charging and potential measurement.

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recorder

ball electrode

oscilloscope


recorder

neutralizer

generator

Figure 13 Procedures of STFI method No PS 07: Initiation and measurement of a discharge andneutralisation of test sample.

Figure 14 The ball electrode of von Pidoll type used in the method.

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Figure 15 Measurement of discharge from charged garment.

5.2.5 Shirley method 202

The Shirley method 202 “Test method for measuring static electricity generated when removinggarments from the human body” is a laboratory method of BTTG (British Textile Technology Group)[29]. In accordance with its title, the method specifies procedures for measuring the static electricitygenerated when garments are removed from the human body. The method is applicable to all types ofgarments. For reference purposes, standard under garments are specified.

In the test procedure the test subject (person) stands with bare feet on the metal base plate and ismomentarily earthed to remove residual charge, Fig. 16. The Faraday pail shall also be momentarilyearthed. Then the test subject removes the test garment and drops it into the Faraday pail taking carethat the garment does not touch the outside of the pail. Measurements of body voltage and charge on theremoved garment are recorded. The test procedures are done ten times in total. The procedure isrepeated for each combination of test garments and reference garments. In the ESTAT-Garments teststwo different reference garments were used, the garments made of polyester and cotton.

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Figure 16 Sketch of the Shirley Method 202: 1) Electrostatic voltmeter, 2) Recording device, 3)Faraday pail with charge measuring device (3a), 4) Metal base plate, 5) Insulating support.

5.2.6 JIS L 1094:1997 “Frictionally charged electricity-amount measuringmethod”

JIS L 1094:1997 is a Japanese Industrial Standard “Testing methods for electrostatic propensity ofwoven and knitted fabrics” including several test methods [30]. Method “Frictionally chargedelectricity-amount measuring method” is originally for pieces of fabric, but it is generally applied alsofor full garments. In the ESTAT-Garments project it was applied for garments for two reasons: at first,the tribocharging method of prEN 1149-3 is about having a widely accepted status in Europe as theprincipal tribocharging test method for ESD fabrics, and, secondly, we would like to include a secondfull garment test method, in addition to the Shirley Method, in the tests.

In the “Frictionally charged electricity-amount measuring method” a whole garment (or a piece offabric) is placed on a metal plate covered with polyamide or a polyacrylic fabric and lain on a woodenplace attached with a plastic bar from the sleeves, Figs. 17 and 18. The sample is rubbed five times atthe rate of one trial per second with a PVC rubbing bar covered with a polyamide or a polyacrylic fabricby hand. Immediately after rubbing procedure the sample is thrown in a large Faraday cage and theelectrostatic charge is measured. The procedure is repeated five times for both polyamide andpolyacrylic charging fabrics. The results are here given in nanocoulombs because the tests was made tofull garments. When the test is done for a small piece of fabric, the results are given as charge per unitarea (C/m2).

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Figure 17 Example of rubbing procedure in the Frictionally charged electricity-amount measuringmethod (JIS L 1094:1997).

Figure 18 Rubbing procedure for full garments and the used Faraday cage at VTT.

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6 Test results for full garments

6.1 Test samples

Preliminary test were done for samples having stainless steel fibres (SS), surface conducting fibres (SC)and core conducting fibres (CC) in a base fabric of cotton, polyester or a mix of both. For the final teststhe Project Management Committee decided to restrict on garments which would, according to theexperience of the project group and the preliminary studies, be the most challenging one by the existingstandard test methods. The garments should also represent state-of-the-art technology and be incommon use in electronics manufacturing industry.

As a result, three different kinds of garments were selected for the final tests of WP5 and WP6. Therewere three samples of each type. All the garments had pure polyester as the base fabric, representingmaterial of both high resistivity as well as high cleanliness performance (cleanroom compatible). Thegarments different mainly by the conductive threads they have: one has surface conductive (SC) threads(Resistat F901), one hybric9 conductive (HC) threads (Megana), and one core conductive (CC) threads.The SC garment was a coat made by Fristads. HC and CC garments were overalls used in cleanrooms(cleanroom garments) made by Alsico, Fig. 19. The conductive threads in the Alsico HC and CCgarments form a grid of size 5 mm x 5mm. The Fristads SC garment has a diagonal grid of 3.3 mm x3.3 mm, Fig. 20. Cross-sectional optical images of the conductive threads used in the garments areshown in Fig. 21. The samples were washed per EN ISO 6330 before the studies. More detailedphysical analysis of the garment material is given in Deliverable report D4.2 [25].

Figure 19 Pictures of a Fristads ESD-coat, code SC, on the left and a Alsico overall, code HC, on therigh. Alsico overalls, code CC, look similar to the HC samples.

9 Fibres with a conducting core reaching the outskirts.

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CC-GAR

HC-GAR

SC-GAR

Figure 20 Pictures of the garment fabrics, in scale 1:1.

CC-GAR

HC-GAR

SC-GAR

Figure 21 Optical images of the cross-section of the conductive threads used in Alsico CC and HCgarments and in the Fristads SC garment.

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6.2 Test results

The samples were tested at VTT in the IEC 61340-5-1 low humidity conditions of 12 % RH (±3% RH)at (23 ± 2) °C, with the exception of additional measurements with the STFI method done at STFI at 25% RH (±3% RH), (23 ± 2) °C. The samples were conditioned in the conditions above at least 72 hbefore the measurements.

Note! Results show power of 10 as “E+” due to the spreadsheet software(e.g. 2x103 = 2E+03).

The results are presented both as tables and diagrams. Separate diagrams are given for the test results ofindividual samples and for the averages of the three samples of each type. More detailed result analysisand discussion is done in Chapter 7.

6.2.1 Point-to-point resistance (Resistive IEC 61340-5-1 and ESD STM2.1methods)

Results of the point-to-point resistance measurements are given in Tables 4-5 and Figures 22-24. TheIEC 61340-5-1 requirement for ESD protective garments is that the point-to-point resistance is less than1x1012 Ω. (Rp≤1x1012 Ω). In IEC 61340-5-1 there is no minimum value of resistance for the garments.However, a minimum resistance value may be required for the protection for safety. The recommendedelectric resistance range according to ESD STM2.1 is 1x105 Ω to 1x1011 Ω.

The test results show that all Fristads SC coats fulfil the IEC 61340-5-1 requirements but only one theESD STM2.1 requirements (Coat B). The other two coats gave too high resistance values. Alsico HCoveralls fulfil neither requirements, when the measurement includes a seam. When no seam wasincluded in the measurement path, the results were within the acceptable limits. Alsico CC overalls gavetoo high resistance values according to both standards.

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Table 4 Results of point-to-point resistance measurements, flat garments.

POINT-TO-POINT RESISTANCE, EN 61340-5-1

2.3 kg sensors, level surface []SampleMeasuring point

A B C average

1. From sleeve to sleeve 5,8E+11 2,1E+08 2,7E+11 3E+11

2. From sleeve to hem 4,5E+11 1,9E+08 2,4E+10 2E+11

3. Across a side seam 5,6E+10 2,9E+08 5,3E+10 4E+10

4. Horizontally, no seam 2,4E+11 1,1E+11 7,3E+10 1E+11

1-3 Fristads SCcoat

5. Vertically, no seam 1,5E+06 7,0E+05 9,6E+05 1E+06


2. From sleeve to leg 1,2E+12 7,4E+11 2,6E+12 2E+12



4-6 Alsico HCoveralls



2. From sleeve to leg 1,4E+12 4,2E+12 1,2E+12 2E+12



7-9 Alsico CCoveralls


1E+00

1E+01

1E+02

1E+03

1E+04

1E+05

1E+06

1E+07

1E+08

1E+09

1E+10

1E+11

1E+12

1E+13

1E+14

FristadsESD-coat

SC, a

FristadsESD-coat

SC, b

FristadsESD-coat

SC, c

Alsicooveralls HC,

a

Alsicooveralls HC,

b

Alsicooveralls HC,

c

Alsicooveralls CC,

a

Alsicooveralls CC,

b

Alsicooveralls CC,

c

Po

int-

to-p

oin

t re

sist

ance

, afl

at [

ohm

]

from sleeveto sleeve

horizontally,no seam

vertically,no seam

Figure 22 Point-to-point resistance, flat garments

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1E+00

1E+01

1E+02

1E+03

1E+04

1E+05

1E+06

1E+07

1E+08

1E+09

1E+10

1E+11

1E+12

1E+13

Fristads ESD-coat SC Alsico overalls HC Alsico overalls CC

Po

int-

to-p

oin

t re

sist

ance

, afl

at [

oh

m]


horizontally,no seam

vertically,no seam

Figure 23 Point-to-point resistance, flat garments – averages.

Table 5 Results of point-to-point resistance measurements, hanging material.

POINT-TO-POINT RESISTANCE, EN 61340-5-1

5 cm clamps, []SampleMeasuring point

A1 A2 B1 B2 C1 C2 average1. From sleeve tosleeve

3,2E+11 3,4E+11 1,5E+10 4,8E+10 1,2E+11 1,0E+11 2E+111-3FristadsSCcoat A

2. From sleeve tohem

1,3E+11 1,7E+11 4,5E+08 1,6E+08 3,4E+10 9,2E+10 7E+10

1. From sleeve tosleeve

8,0E+11 1,0E+12 5,3E+11 9,5E+11 4,4E+11 5,5E+11 7E+114-6 AlsicoHCoveralls


6,3E+11 6,0E+11 6,7E+11 4,6E+11 3,5E+11 7,4E+11 6E+11

1. From sleeve tosleeve

7,2E+11 1,5E+12 1,1E+12 1,8E+12 1,0E+12 1,2E+12 1E+127-9 AlsicoCCoveralls


3,1E+11 3,2E+12 8,9E+11 9,0E+11 1,0E+12 6,6E+11 1E+12

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1E+00

1E+01

1E+02

1E+03

1E+04

1E+05

1E+06

1E+07

1E+08

1E+09

1E+10

1E+11

1E+12


Po

int-

to-p

oin

t res

ista

nce,

han

gin

g [o

hm]


sleeve to hem / leg

Figure 24 Point-to-point resistance, hanging material – averages.

6.2.2 Charge decay time from point-to-point (VTT’s method for garments)

Results of VTT’s point-to-point charge decay time measurements for full garments are given in Tables6-7 and Figures 25-29. For the performance requirements VTT applies the general IEC 61340-5-1 10%requirement for charge decay: from 1000 V to 100 V in less than 2 s.

The test results show that only one garment - the Fristads SC coat B - fulfil the requirement. The chargedecay time for all the other garments was too long (>2 s). The charge decay time for Alsico HC coatswas within the acceptable limits when no seam was included in the path of the charge. When a seamwas included, the charge decay was too slow.

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Table 6 Results of charge decay time from point to point measurements, hanging material.

Sample

Measuring point Measurement

pos. neg. pos. neg. pos. neg.

1 47,7 49,6 0,1 0,1 6,9 6,2

2 34,2 48,3 3,7 3,9 13,9 12,6

3 - - 0,1 0,1 8,2 4,5averageaverage

1 18,2 18,8 0,1 0,1 2,0 1,5

2 26,8 25,0 0,1 0,1 9,4 9,9

3 - - - - 2,3 1,9averageaverage

1 >60 (297 V) >60 (285 V) >60 (578 V) >60 (540 V) >60 (299 V) >60 (288 V)

2 >60 (302 V) >60 (319 V) >60 (622 V) >60 (605 V) >60 (289 V) >60 (310 V)

3 - - - - - -averageaverage

1 >60 (312 V) >60 (322 V) >60 (322 V) >60 (291 V) >60 (322 V) >60 (330 V)

2 >60 (318 V) >60 (324 V) >60 (302 V) >60 (293 V) >60 (340 V) >60 (318 V)


1 >60 (1562 V) >60 (1542 V) >60 (1713 V) >60 (1774 V) >60 (1499 V) >60 (1554 V)

2 >60 (1551 V) >60 (1488 V) >60 (1708 V) >60 (1747 V) >60 (1580 V) >60 (1472 V)

3 - - - - - -

averageaverage

1 >60 (1645 V) >60 (1558 V) >60 (1728 V) >60 (1892 V) >60 (1566 V) >60 (1546 V)

2 >60 (1705 V) >60 (1570 V) >60 (1922 V) >60 (1833 V) >60 (1541 V) >60 (1552 V)


- --

7-9 AlsicoCC

overalls

1. From sleeveto sleeve

- - --

2. From sleeveto leg

--

4-6 AlsicoHC

overalls


- --


- -

A B C

1,4 8,7

-

22,2 0,1 4,5

-

8,9

1-3Fristads

SCcoat



18,345,0

5 cm clamps [s]

CHARGE DECAY TIME FROM POINT TO POINT

G6RD-CT-2001-00615


50(80)

0,02,04,06,08,0

10,012,014,016,018,020,022,024,026,028,030,032,034,036,038,040,042,044,046,048,050,052,054,056,058,060,0


Ch

arg

e d

ecay

tim

e fr

om

po

int

to p

oin

t, h

ang

ing

[s]


sleeve to hem / leg

Figure 25 Charge decay time from point-to-point, hanging garments – averages.

G6R

D-C

T-2

001-

0061

5

DE

LIV

ER

AB

LE

RE

PO

RT

D6.

1

51(8

0)

Tab

le 7

Res

ults

of c

harg

e de

cay

tim

e fr

om p

oint

to p

oint

mea

sure

men

ts, f

lat g

arm

ents

.

Sam

ple

Mea

suri

ng p

oint

pos.

neg.

pos.

neg.

pos.

neg.

1. F

rom

sle

eve

to s

leev

e>

60 (

644

V)

>60

(54

8 V

)0,

20,

20,

90,

6-

2. F

rom

sle

eve

to h

em>

60 (

482

V)

>60

(56

1 V

)0,

20,

20,

40,

4-

3. A

cros

s a

side

sea

m>

60 (

345

V)

>60

(29

0 V

)>

60 (

275

V)

>60

(24

0 V

)>

60 (

225

V)

>60

(23

3 V

)-

4. H

oriz

onta

lly,

no

seam

54,5

54,5

1,0

0,9

>60

(14

9 V

)>

60 (

152

V)

-

5. V

erti

call

y, n

o se

am0,

10,

10,

10,

10,

10,

10,

1

1. F

rom

sle

eve

to s

leev

e>

60 (

428

V)

>60

(41

4 V

)>

60 (

464

V)

>60

(39

2 V

)>

60 (

416

V)

>60

(39

1 V

)-

2. F

rom

sle

eve

to h

em4,

37,

20,

81,

11,

21,

22,

6

3. A

cros

s a

side

sea

m0,

60,

61,

20,

8>

60 (

145

V)

0,5

-

4. H

oriz

onta

lly,

no

seam

0,1

0,1

0,1

0,1

0,1

0,1

0,1

5. V

erti

call

y, n

o se

am0,

10,

10,

10,

10,

10,

10,

1

1. F

rom

sle

eve

to s

leev

e>

60 (

1718

V)

>60

(15

18 V

)>

60 (

1859

V)

>60

(18

34 V

)>

60 (

1711

V)

>60

(13

22 V

)-

2. F

rom

sle

eve

to le

g>

60 (

1733

V)

>60

(15

92 V

)>

60 (

1901

V)

>60

(19

05 V

)>

60 (

1743

V)

>60

(16

41 V

)-

3. A

cros

s a

side

sea

m>

60 (

1618

V)

>60

(14

65 V

)>

60 (

1869

V)

>60

(18

52 V

)>

60 (

1669

V)

>60

(14

64 V

)-

4. H

oriz

onta

lly,

no

seam

>60

(88

5 V

)>

60 (

895

V)

>60

(87

5 V

)>

60 (

832

V)

>60

(57

8 V

)>

60 (

822

V)

-

5. V

erti

call

y, n

o se

am>

60 (

530

V)

>60

(33

6 V

)>

60 (

376

V)

>60

(43

3 V

)>

60 (

360

V)

>60

(33

7 V

)-

BC

CH

AR

GE

DE

CA

Y T

IME

FR

OM

PO

INT

TO

PO

INT

2.3

kg w

eigh

t se

nsor

s, [

s]

aver

age

7-9

Als

ico

CC

over

alls

1-3

Fris

tads

SC

coa

t

A

4-6

Als

ico

HC

over

alls


52 (80)

02468

1012141618202224262830323436384042444648505254565860

FristadsESD-coat

SC, a

FristadsESD-coat

SC, b

FristadsESD-coat

SC, c

Alsicooveralls HC,

a

Alsicooveralls HC,

b

Alsicooveralls HC,

c

Alsicooveralls CC,

a

Alsicooveralls CC,

b

Alsicooveralls CC,

c

Ch

arg

e d

ecay

tim

e fr

om

po

int

to p

oin

t, a

flat

[s]

from sleeve to sleeve pos. from sleeve to sleeve neg. horizontally, no seam pos.

horizontally, no seam neg. vertically, no seam pos. vertically, no seam neg.

Figure 26 Charge decay time from point-to-point, flat garments.

0,02,04,06,08,0

10,012,014,016,018,020,022,024,026,028,030,032,034,036,038,040,042,044,046,048,050,052,054,056,058,060,0


Ch

arg

e d

ecay

tim

e fr

om

poi

nt

to p

oin

t, a

flat

[s]

from sleeve to sleeve horizontally, no seam vertically, no seam

Figure 27 Charge decay time from point-to-point, flat garments – averages.


53 (80)

Charge decay time from point to point, aflatFristads SC, coat B

-500

0

500

1000

1500

2000

0 10 20 30 40 50 60

Time (s)

Po

ten

tial

(V

)

from sleeve to sleeve

from sleeve to hem

across side seam

horizontally, no seam

vertically, no seam

Figure 28 Examples of charge decay curves from point-to-point for Fristads SC coat “B”.

Charge decay time from point to point, aflatAlsico HC, overalls B

-500

0

500

1000

1500

2000

2500

0 10 20 30 40 50 60

Time (s)

Pot

entia

l (V

)

from sleeve to sleeve

from sleeve to hem

across side seam

horizontally, no seam

vertically, no seam

Figure 29 Examples of charge decay curves from point-to-point for Alsico HC overalls “B”.


54 (80)

6.2.3 SP Method 2175 (System measurement)

Results of SP’s system measurements “Measurement of charge decay time of ESD-protectiveclothing” are given in Table 8 and Figures 30-34. The ESD-approval requirement used by SPfor complete garments is that all decay time measurements at discharges from 500 V to 100 Vshall be less than 20 s.

The results show that only one garment - the Fristads SC coat B - fulfil the requirement. Thecharge decay time for all the other garments was too long (>20 s).

Table 8 Results of charge decay time for full garmet system measurements.

CHARGE DECAY TIME FOR FULL GARMENTSP METHOD 2175Sample

Measuring point 5 cm clamps, [s] A B C average

1. From back hem to right wrist >60 1,9 30,2 -2. From back hem to left wrist >60 1,7 2,4 -1-3 Fristads SC

coat3. From front hem to left wrist >60 0,6 26,9 -1. From left leg to right wrist 19,0 >60 >60 -2. From left leg to left wrist >60 >60 >60 -4-6 Alsico HC

overalls3. From right leg to left wrist 1,0 >60 >60 -1. From left leg to right wrist >60 >60 >60 -2. From left leg to left wrist >60 >60 >60 -7-9 Alsico CC

overalls3. From right leg to left wrist >60 >60 >60 -


55 (80)

Figure 30 Charge decay time, SP Method 2175.

0,02,04,06,08,0

10,012,014,016,018,020,022,024,026,028,030,032,034,036,038,040,042,044,046,048,050,052,054,056,058,060,0


Ch

arg

e d

ecay

tim

e, S

P M

eth

od 2

175

[s]

From back hem to right wrist From back hem to left wrist From front hem to left wrist

Figure 31 Charge decay time, SP Method 2175 – averages.

0,02,04,06,08,0

10,012,014,016,018,020,022,024,026,028,030,032,034,036,038,040,042,044,046,048,050,052,054,056,058,060,0

FristadsESD-coat

SC, a

FristadsESD-coat

SC, b

FristadsESD-coat

SC, c

Alsicooveralls HC,

a

Alsicooveralls HC,

b

Alsicooveralls HC,

c

Alsicooveralls CC,

a

Alsicooveralls CC,

b

Alsicooveralls CC,

c

Ch

arg

e d

ecay

tim

e, S

P M

eth

od

217

5 [s

]

From back hem to right wrist From back hem to left wrist From front hem to left wrist


56 (80)

SP-method, from back hem to right wristFristads SC

-200

-100

0

100

200

300

400

500

600

700

800

900

0 10 20 30 40 50 60

Time (s)

Pot

enti

al (V

)

coat A

coat B

coat C

Figure 32 SP Method 2175 – Example charge decay curves for the three SC garments.

SP-method, from back hem to right wristAlsico HC

-200

-100

0

100

200

300

400

500

600

700

800

900

0 10 20 30 40 50 60

Time (s)

Po

ten

tial

(V

)

overalls A

overalls B

overalls C

Figure 33 SP Method 2175 – Example charge decay curves for the three HC garments.


57 (80)

SP-method, from back hem to right wristAlsico CC

-200

0

200

400

600

800

1000

0 10 20 30 40 50 60

Time (s)

Po

ten

tial

(V

)

overalls A

overalls B

overalls C

Figure 34 SP Method 2175 – Example charge decay curves for the three CC garments.

6.2.4 Charge transfer - garment test, STFI-Reference No. PS 07

Results of STFI’s charge transfer – garment test done at VTT are given in Tables 8-12. Testresults done at STFI are presented in Tables 13-14. The test method has been developed todetermine the ignition risk of protective clothing, made from dissipative materials, due topossible dangerous electrostatic charging of the human person or the garment. For theassessment of the electrostatic ignition risk the highest single value of the measured chargetransferred will be used. The charge transfer must be less than the ignition level of theexplosive subject with an adequate safety margin. No maximum allowable charge transfervalue for garments used in electronics manufacturing industry has been defined.

From the results we can see that no ESD could be measured from any test garment, when theperson (operator) was earthed. When the person was unearthed, maximum discharge of 780pC was measured from a Fristads SC coat. No direct discharges from the Alsico HC and CCoveralls were measured at VTT. In the tests at STFI, the SC coat gave a maximum ESD of2500 pC. The corresponding values for Alsico HC and CC overalls were 2500 pC and 500pC, respectively. The measured transferred charge values at STFI are significantly higher thanthose measured at VTT. The same applies also for the measured body and garment surfacepotentials. In some cases the potential measured at STFI was more than an order of magnitudehigher than the corresponding potential measured at VTT.


58 (80)

Table 9 Results of charge transfer, garment test measurements at VTT - person unearthed.

BODY POTENTIAL AND CHARGE TRANSFER BY WEARING OFELECTROSTATIC DISSIPATIVE PROTECTIVE CLOTHING

STFI-Reference No. PS 07

Person unearthedSample

Measurement Charge transfer[pC]

Body potential[V]

Surface potential[kV]

1 720,1 -560 -0,562 633,5 -610 -0,563 743,5 -620 -1,234 787,1 -630 -0,645 - -455 -0,49

average 721 -575 -0,7

1. Fristads SCcoat A

standard deviation 64,6 72,3 0,31 - 225 -0,122 667,1 -580 -0,493 - -270 -0,224 - -355 -0,325 - -325 -0,27

average 667 351* -0,3

2. Fristads SCcoat B

standard deviation - 296,2 0,11 - -335 -0,282 - -330 -0,343 - -240 -0,284 - -415 -0,605 651,9 -565 -0,67

average 652 -377 -0,4

3. Fristads SCcoat C

standard deviation - 122,0 0,2* Individual measuring results have different signs. Average are calculated from absolute values.


59 (80)

Table 10 Results of charge transfer, garment test measurements at VTT - person unearthed.


STFI-Reference No. PS 07Person unearthed

Sample


Body potential[V]


1 - -596 -1,722 - -810 -2,283 - -470 -1,724 - -935 -2,325 - -1000 -1,65

average - -762 -1,9

4. Alsico HCoveralls A

standard deviation - 224,6 0,31 - -490 -1,452 - -860 -1,603 - -770 -1,524 - -815 -2,295 - -715 -1,35

average - -730 -1,6

5. Alsico HCoveralls B


average - -918 -2,2

6. Alsico HCoveralls C


average - -762 -2,7

7. Alsico CCoveralls A


average - -784 -3,3

8. Alsico CCoveralls B


average - -641 -3,4

9. Alsico CCoveralls C

standard deviation - 121,8 0,4


60 (80)

Table 11 Results of charge transfer, garment test measurements at VTT - person earthed.

BODY POTENTIAL AND CHARGE TRANSFER BY WEARING OFELECTROSTATIC DISSIPATIVE PROTECTIVE CLOTHINGSTFI-

Reference No. PS 07Person earthed

Sample


Body potential[V]


1 - -55 -0,922 - -65 -0,653 - -55 -0,704 - -60 -0,705 - -70 -1,12

average - -61 -0,82



average - -62 -0,25



average - -55 -0,36



average - -55 -2,76



average - -58 -1,73



average - -52 -1,46




61 (80)

Table 12 Results of charge transfer, garment test measurements at VTT - person earthed.



Person earthedSample


Body potential[V]


1 - -55 -5,132 - -50 -4,783 - -50 -3,394 - -45 -4,005 - -50 -3,72

average - -50 -4,20



average - -49 -3,78



average - -54 -3,37



Table 13 Results of charge transfer, garment test measurements at STFI - person unearthed,averages.



Person unearthedSample

Measurement Charge transfermax [nC]

Body potential[kV]


Fristads SCCoat average 2,5 -5,2 -10,9Alsico HCOveralls average 2,5 -6,1 -8,69. Alsico CCOveralls average 0,5 -6,5 -8,0


62 (80)

Table 14 Results of charge transfer, garment test measurements at STFI - person earthed,averages.



Person earthedSample


Body potential[kV]


-Fristads SCCoat average - 0 -1,9

-Alsico HCOveralls average - 0 -2,3

-9. Alsico CCOveralls average - 0 -2,5

6.2.5 Static electricity generated when removing garment, ShirleyMethod 202

Results of the Shirley Method 202, full garment’s tribocharging tests are shown in Tables 15-17 for the SC, HC, and CC garments, respectively. The document describing the test method[29] do not comment about recommended limits for body voltage and garment charging.

For the Fristads SC coats typical body voltages were less than 100 V with cotton referencegarment and around 300 V for polyester reference material. For Alsico HC overalls typicalbody voltages were 300-400 V with cotton reference. With polyester reference they werebelow 200 V. Highest body voltages were measured with Alsico CC overalls when valuesexceeding 400 V were measured with both reference materials. Garment charging was in theorder of tens of nC for the SC and HC garments. For the CC overalls the garments werecharged to about 100 nC with both reference materials.


63 (80)

Table 15 Results of static electricity generated when removing garment.

Body voltage [V]

Charge [C]

1 110 7,0E-092 35 5,0E-093 65 6,5E-094 65 6,0E-095 60 5,0E-09

average 67 6E-09standard deviation 27,06 9E-10

1 305 5,7E-082 270 6,8E-083 245 8,9E-084 365 5,7E-085 350 5,1E-08


1 50 6,0E-092 65 6,2E-093 95 7,0E-094 -80 5,0E-095 -100 5,0E-09

average 78* 6E-09standard deviation 89,40 9E-10

1 325 5,7E-082 315 1,6E-073 320 5,8E-084 365 3,7E-085 325 4,4E-08


1 -75 5,0E-092 -95 4,0E-093 40 4,5E-094 60 5,0E-095 65 6,0E-09

average 67* 5E-09standard deviation 77,57 7E-10

1 380 7,3E-082 300 9,8E-083 350 4,2E-084 330 1,1E-075 265 9,0E-08



Cotton

Polyester


Cotton

Polyester

Polyester


Cotton

Sample

STATIC ELECTRICITY GENERATEDWHEN REMOVING GARMENT

Reference garments

MeasurementSHIRLEY METHOD 202

* Individual measuring results have different signs. Averages are calculated from absolute values of theindividual measuring results.


64 (80)


Body voltage [V]

Charge [C]

1 -360 -5,4E-082 -390 -7,1E-083 -370 -4,7E-084 -400 -6,4E-085 -215 -4,9E-08

average -347 -6E-08standard deviation 75,47 1E-08

1 -220 -2,8E-082 -235 -3,5E-083 -185 -2,4E-084 -155 -2,3E-085 -165 -2,5E-08


1 -310 -5,2E-082 -370 -8,0E-083 -320 -7,0E-084 -355 -5,1E-085 -325 -5,8E-08


1 -185 -3,4E-082 -200 -4,5E-083 -175 -4,2E-084 -170 -3,8E-085 -140 -4,3E-08


1 -265 -4,7E-082 -330 -5,4E-083 -292 -5,8E-084 -325 -6,6E-085 -295 -5,4E-08


1 -126 -2,8E-082 -145 -3,0E-083 -130 -1,7E-084 -65 -1,6E-085 -115 -2,0E-08



Cotton

Polyester


Cotton

Polyester


Cotton

Polyester

Sample


Reference garments



65 (80)


Body voltage [V]

Charge [C]

1 -345 -1,4E-072 -370 -1,4E-073 -430 -1,3E-074 -430 1,2E-075 -415 -1,6E-07


1 -335 -1,5E-072 -305 -1,3E-073 -250 -1,3E-074 -330 -1,8E-075 -320 -1,3E-07


1 -440 -1,1E-072 -395 -1,3E-073 -395 -1,2E-074 -400 -1,4E-075 -370 -1,3E-07


1 -435 -1,8E-072 -500 -1,6E-073 -410 -1,7E-074 -450 -1,9E-075 -430 -1,3E-07


1 -360 -1,3E-072 -380 -1,2E-073 -320 -1,2E-074 -335 -9,3E-085 -290 -1,1E-07


1 -410 -9,2E-082 -430 -1,3E-073 -398 -1,4E-074 -345 -1,1E-075 -335 -1,1E-07



Cotton

Polyester


Cotton

Polyester


Cotton

Polyester

Sample


Reference garments



66 (80)

6.2.6 Frictionally charged electricity, JIS L 1094

Results of the JIS L 1094, frictionally charged – electricity amount method are shown inTables 18-20 for the SC, HC, and CC garments, respectively. The JIS L 1094:1997 standard[30] does not give recommendations for allowable garment or fabric charging.

For the Fristads SC coats the maximum garment charging was 487 nC. For the Alsico HC andCC overalls the corresponding values were 149 and 316 nC, respectively (all values areabsolute values). If comparing to the tribocharging results of the Shirley 202 Method, the JISL 1094 method gave systematically higher garment charging.


67 (80)

Table 18 Results of frictionally charged electricity measurements.

Rubbing material

MeasurementCharge

[nC]1 1942 277

Polyamide 3 3234 3625 373

average 3061 3622 399

Polyacrylic 3 4184 4375 391

average 401maximum value

1 3462 352

Polyamide 3 3654 3855 379

average 3651 3632 387



1 3662 370

Polyamide 3 3564 3735 364

average 3661 4592 389




437


387

Sample

FRICTIONALLY CHARGED ELECTRICITY-AMOUNT MEASURING METHOD

JIS L 1094


487


68 (80)


Rubbing material

MeasurementCharge

[nC]

1 -1062 -110

Polyamide 3 -744 -905 -95

average -951 -442 -15

Polyacrylic 3 -184 85 37

average 24**maximum value

-742 -91

Polyamide 3 -984 -1045 -118

average -971 -342 -57

Polyacrylic 3 -504 -835 -41

average -53maximum value

1 -892 -24

Polyamide 3 -764 -885 -136

average -831 452 112



Sample


JIS L 1094


-110*

-118*

149



* Maximium values are calculated from absolute values and the sign of the value is noticed.** Averages are calculated from absolute values of the individual measuring results.


69 (80)


Rubbing material

MeasurementCharge

[nC]1 -1562 -216

Polyamide 3 -1844 -2295 -231

average -2031 -1932 -197

Polyacrylic 3 -2184 -2065 -242


1 -2502 -280

Polyamide 3 -3054 -2915 -315

average -2881 -1802 -226

Polyacrylic 3 -2204 -2535 -257


1 -2672 -300

Polyamide 3 -2474 -2905 -316

average -2841 -1602 -206

Polyacrylic 3 -1824 -2115 -221



-316*



-242*

-315*

Sample


JIS L 1094

* Maximium values are calculated from absolute values and the sign of the value is noticed.


70 (80)

7 Evaluation of the test methods for fullgarments

The selected existing test methods for full garments are evaluated here by method by methodtaking into account the evaluation criteria presented in Chapter 4.2 and the test results shownin Chapter 6.2.

7.1 Resistive methods of IEC 61340-5-1 and ESDSTM2.1

According to the results of Chapter 4.2, resistance (or resistivity) is a key factor for mostparameters important from the ESD protective performance point of view of ESD garments.But in most cases the relevant resistance, in the case of modern ESD garment material, is theresistivity of the conductive threads, not the overall resistivity of the material. The resistivityof conductive threads has a direct influence on peak ESD current, on charge transfer in anESD, on the rate of charge dissipation of the garment and garment material by conduction andinduction effects, on the electrostatic shielding property of the material and weakly also on thevoltage suppression in full garment system. The resistivity of the base fabric has a significantinfluence only on the charge dissipation by conduction. The resistance over garment seams isimportant, when each garment piece is not in a direct contact to ground, for the rate of chargedissipation as well as for the amount of retained charge as a possible source of direct ESD.

The electrodes used in the resistive methods of IEC 61340-5-1 and ESD STM2.1, however,do not measure a specific resistance of the conductive threads or the base fabric, but anoverall resistance over a large contact area. In the case of ESD garment material that includesconductive threads and more or less insulating base fabric. Even more serious is the problemthat good electrical contact between the electrode and the targeted subject (e.g. conductivethreads) is not guaranteed. For example, in the case of core conductive threads the measuringelectrode will not be in contact with the conductive fibre but with the insulating surface of theconductive threads. Electrical contact between the electrode and the conductive threads can bepoor also with surface and hybrid conductive threads when the conductive fibres are insidethe material, which can easily happen not only for used garments but also for new garments.

For homogeneous materials resistance measurements are the best way to evaluate the ESDprotective performance of the material or product. The measurement is simple to perform,reproducibility is high, and it covers most of the factors influencing the protectiveperformance of the material.

For inhomogeneous materials the situation is no more straightforward. The measuredresistance may not correspond to the correct resistance that must be controlled. The mostserious issue is that the actual conductive element of the material may not be contacted by ameasuring electrode, which would directly lead to misleading results. A product may berejected for inappropriate reasons. For garments having core conductive threads, a properelectrical contact between the conductive elements and the measuring electrodes wouldinherently be never realised. The contact may be poor also with other types of modernconductive composite threads.


71 (80)

ESD garments are sometimes used in electronics industry also in works where electricalsafety issues are important or even critical. The test methods of IEC 61340-5-1 and ESDSTM2.1 (point-to-point resistance and resistance to groundable points) are right methods toevaluate the electrical safety issues of the garments. The only comment is that, whenevaluating the electrical safety, the measuring open voltage should for ‘high resistance’garments be higher than the 100 V specified in the standards, for example 500 V, to reflect thehazardous voltage levels likely to be encountered in practice. Or alternatively a ramp of testvoltage up to a few kV could be used, like in the resistive signature method, underdevelopment at Centexbel.

As a conclusion, the resistive methods of IEC 61340-5-1 and ESD STM2.1 do not satisfactorywell characterise any parameter controlling the protective performance of modern ESDgarments. A garment may be rejected by inappropriate reasons when the point-to-point orsleeve-to-sleeve resistance measurements are used. Vice versa, on the other hand, is quiteunlike. If a garment passes the resistance tests, the risk of ESD failures to ESD sensitivedevices with reference to garments is small, supposing that the garment is correctly used andgrounded and that the garment fabric structure is correct (that is, sufficiently small grid size ofthe conductive threads). Measurement of point-to-point resistance, or resistance to agroundable point is, however, necessary when evaluating the garment from the electricalsafety point of view. For the electrical safety measurements, IEC 61340-5-1 and ESD STM2.1methods are suitable.

7.2 VTT’s point-to-point method for full garments

VTT’s charge decay time measurements from point-to-point characterises the chargedissipation capability of full garment mainly by conduction. There is a weak aspect of chargedissipation through induction mechanisms in the method, when the conductive threads of thetested garment are grounded by a measuring electrode. The method simulates the practicalsituation of garment charging by contact and charge migration to ground through seams.Basically the measurement result is mainly determined by the resistance experienced by thecharge on its way to ground. That is influenced by the resistivity of the conductive threads,integrity of the electrical resistance across seams (when there are seams on the path) andcontact resistances between the charging electrode and the conductive fibres and between thegrounding electrode and the conductive fibres. In that sense the method comes very close tothe point-to-point or sleeve-to-sleeve resistance measurements of IEC 61340-5-1 and ESDSTM2.1.

The results of the charge decay time tests are in line with the results of the resistancemeasurements of IEC 61340-5-1 and ESD STM2.1. Those garments who failed in the IEC61340-5-1 point-to-point resistance tests failed also in the point-to-point charge decay timetests. However, some garments which passed the IEC point-to-point resistance test, failed inVTT’s point-to-point charge decay time test. That is not surprising as it is well known thatIEC’s charge decay time criteria (to 10% of maximum potential value in less than 2 s) is aharder requirement than the resistance criteria of less than 1x1012 Ω.10 The charge decay timemeasurement contains an element of charge storage (effective capacitance) as well as theresistance element.

10 The relation between resistance and charge decay time has been discussed in more detail inDeliverable report D5.2 “Report on the evaluation of existing test methods for fabrics”.


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As a conclusion for the point-to-point charge decay time measurement method, it wouldcertainly work well for the characterisation of the ESD protective performance ofhomogeneous materials. With inhomogeneous, modern composite textile materials it faces thesame problems as the resistive methods of IEC 61340-5-1 and ESD STM2.1. The method iscritical with respect to the electrical contact between the electrodes and the conductive fibresof the garment. When the contact is poor, factors related to the protective performance of thegarment are not correctly characterised by the measurement.

7.3 SP Method 2175

The SP system measurement test “Measurement of decay time of ESD-protective clothing” isintended to verify that each panel of the garment has sufficient electrical connection toground. It is also targeted to take into account other phenomena, such as charge spread out onthe complete garment and voltage suppression appearing in real world situations.

For the authors it is not clear whether the voltage suppression due to charged operator insidethe garment has any practical influence for the measurement. The capacitive couplingbetween the operator and the garment increases the time constant of the charge decay, but thatmay have been taken into account when specifying the acceptance limits for ESD protectivegarments. The garment potential is measured by monitoring the potential of the chargingelectrode connected to charging capacitor and charged plate. The influence of voltagesuppression due to grounded operator inside the garment would be better measured directly onthe garment surface by a field meter or by a non-contacting electrostatic voltmeter. Chargespread out on the garment panel connected to the charging electrode, however, could beidentified from the decay curves of Figs. 32-34.

The operator inside the garment, however, has an important and very relevant function inproviding real, worst case11 ground paths for charge, corresponding to real situation in a workbench. The garments with core conductive threads failed in the tests evidently because of theinsulating layer between the conductive fibres and the operator, which makes too high barrierfor charge to overcome within an acceptable time. The problem is fundamentally the same asin the case of resistance measurements.

We can conclude that the method characterises well whether each panel of the garment hassufficient electrical ground connection in a practical, worst case situation where the chargehas to flow from one panel to another, across a seam, to find a way to operator’s body. Themethod also characterises well the charge dissipation capability of the garment in realoperation conditions when the charging is done through a contact with charged material. Itremained open how well the case corresponds to situation encountered more frequently inpractice – garment charging by rubbing (triboelectric charging). Such a study should be donein a further stage of the project.

11 Worst case ground paths in that sense that the charge is forced to flow across a seam. In practice acharged garment panel may have a direct contact to operator’s skin or a contact to operator’s bodythrough sufficiently conductive undergarment. The garment panel may also have a groundablegarment point without a necessity for a charge to flow across a seam.


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7.4 STFI test method No. PS 07

The STFI charge transfer – garment test is a new test method and it has been originallydeveloped to characterise protective clothing used in flammable atmosphere. For suchpurposes it seems to be very suitable. It focuses on the correct key parameters to control:charge transfer in a discharge, the surface potential of the garment, and the body potential.The method has also a great potential for the assessment of ESD garments used in electronicsmanufacturing industry, where the key parameters to control are peak ESD current, chargetransfer in a discharge, and device charging due to electric field external to the garment.

The method, however, is not ready and would require better specification to achieve greaterreproducibility than obtained now. The difference between the test results at VTT and STFIwas too high. That clearly indicates that all important parameters influencing the results arenot controlled in a satisfactory way. Furthermore, the probes etc. used in the method havebeen designed for the evaluation of risks in flammable atmospheres. They may not be idealfor the assessment of risks in electronics manufacturing environment. And finally, acceptancelimits with respect to the key parameters to control have to be specified for garments used inelectronics manufacturing industry.

In summary, the STFI method No. PS 07 has potential for a test method to be used whenassessing the ESD protective performance of garments used in electronics manufacturingindustry. Some improvements for the method, however, are required to improve the reliabilityof the results and to modify the method for the specific needs of electronics industry.Acceptance limits for the garments used in electronics manufacturing industry have to also bespecified.

7.5 Shirley method 202

The Shirley method 202 “Static electricity generated when removing garment” is a generallaboratory test method for any kinds of garments. It focused on a major parameter to controlin ESD protective clothing: chargeability of the tested garment by triboelectric charging. Themethod is simple and, besides a large Faraday cage, only basic electrical measurementequipment is required. Reproducibility of the results was also satisfactory good, taking intoaccount that it is a tribocharging method. Comparability between results from coat andoveralls tests, however, is not straightforward because the way of tribocharging (removingprocess of the garments) is not exactly the same for coats and overalls. Also surface areas ofcoat and overall garments are not the same.

The major weakness of the method, if looking from the point of view of ESD garmentevaluation, is that it simulates the situation strictly forbidden in ESD protective area (EPA).Simply from this argument, the method has no potential for an international standardcharacterising ESD garments used in EPA. Furthermore, it may be difficult to specifyjustified acceptance limits for ESD garments based on real failure thresholds of ESD sensitivedevices.


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7.6 JIS L 1094:1997 Frictionally charged electricity-amount measuring method

The Japanese Industrial Standard method is another tribocharging test method selected forthese full garments tests. In contrast to the Shirley method, the approach of the JIS method ishighly acceptable in electronics manufacturing industry. The method intends to evaluate thechargeability of the garment or garment material. Other factors related to the ESD protectiveperformance of garments are not considered in the method.

Reproducibility of the results was satisfactory. For the needs of electronics manufacturingindustry, different rubbing materials could be chosen. A minor criticism of the method is thatgarment charging is measured only after a delay of about 1-2 s. Furthermore, there are norecommended limits for the approval of ESD garments used in electronics manufacturingindustry. As in the case of the Shirley method, it may be difficult to specify justifiedacceptance limits for ESD garments based on real failure thresholds of ESD sensitive devices.

Although the method is in use in Japan and is basically good, measuring one criticalparameter to control in ESD protective clothing used in electronics manufacturing industry,we do not recommend the method for the consideration in the subsequent work in the project.The reason for that is the increasing implementation of the tribocharging method of prEN1149-3. The prEN 1149-3 tribocharging test covers more factors important in the evaluationof the protective performance of a garment than the JIS method. In addition to thechargeability, the prEN 1149-3 tribocharging method takes into account charge dissipationthrough conduction mechanisms and to some extent also through induction and coronamechanisms.

7.7 Discussion

When evaluating the garment test methods, we should keep in mind that full garment tests areonly a part of the study. Much important information is covered by the garment fabric test,reported in Deliverable D5.2. A general view is obtained only after the results from bothgarment fabric and full garment tests are combined. All the important factors influencing theESD protective performance of garments used in electronics manufacturing industry, andpresented in Chapter 4.2, are not taken into account in the garment tests presented above.Some of them are considered only in the fabric tests. When summarising the results fromD5.2 and this report D6.1, special attention have to be paid whether all the factors of chapter4.2 are considered satisfactory well.

ESD garments where the conductive elements are core conductive threads are the mostproblematic for the assessment of their true ESD protective performance. The conductiveelements can not be easily contacted or grounded satisfactorily. Therefore, the garmentstypically fail verification tests (that may require good electrical contact) for inappropriatereasons. The principal charge dissipation mechanism in material having core conductivethreads is typically different to the charge dissipation mechanisms of surface conductivethreads (see Chapter 4.1). The charge dissipation performance of core conductive garments ismainly based on the corona discharge mechanism, the effect of which depends mainly on the


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structure of the conductive threads not on electrical parameters of the material. In coronadischarge, the corona current is quenched when the local electric field has decayed to a certainlevel. At that level the material still has quite a lot charge. The residual charge would hardlycause a direct ESD of such level causing failures to ESD sensitive devices, because of thehigh resistance of the conductive grid. Instead, the electrostatic field external to the garmentdue to the residual charge may be strong enough to charge a nearby device by inductioncreating a risk of subsequent CDM type of ESD and potential failure of the device. Furtherwork is required for the assessment of the ESD safety of garments using core conductivethreads as the conductive garment elements. ESD sensitive devices in the vicinity of such agarment may be at the risk of ESD damage due to the residual charge easily remaining on thegarment surface, when core conductive threads are used. It may also appear that there shouldbe different test methods for garments with core conductive threads than for other types ofESD garments used in electronics manufacturing industry.

The industrial need for different level of tests – laboratory tests covering all important aspectsand simple factory tests which may not cover all aspects – has not been taken into account inevaluation of the existing methods. The evaluation has been only at the highest level by takinginto account all important aspects. The industrial need for simple methods for periodic testsusing basic instruments would require its own analysis. Such an analysis should be done inthe next project phase.


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8 Conclusions

Current standard test methods for full garments do not characterise satisfactorily well theprotective performance of modern ESD garments used in electronics manufacturing. None ofthe laboratory test methods for full garments selected for the study could assess the protectiveperformance of an ESD garment satisfactorily. Completely new methods or modifications forthe existing test methods are required.

The following parameters have been identified as the key parameters to control in order tominimise ESD failures with reference to garments:

♦ Peak ESD current♦ Charge transfer in a discharge♦ Device charging due to electric field external to the garment.

Only the STFI method PS07 addresses the charge transfer in direct ESD from worn garmentsPeak ESD current is not measured in the STFI PS07 test, but would be an easy enhancementof the method. The STFI PS07 method also measures the electric field external to thegarment. So, this test seems to be a promising basis for further development.

Typically a garment has a conductive grid that, if charged, is capable of sourcing a potentiallydamaging ESD event. The garment may rely on effective grounding of the grid to bringsurface fields to a low level. In these cases, it is vital to verify the grounding of the garment.The SP method 2175 achieves this, via a charge decay method.

Garments made from conductive core materials would probably not pass the SP2175 test asconnection to the conductive grid is poor. The question remains, would this material, ifineffectively grounded, source potentially damaging ESD or have significant external fields?If not, then there may be no practical necessity to ground the conductive parts and pass theSP2175 test. The modified STFI PS07 test performed on the material, would answer thisquestion. A simple test protocol based on this logic is given in Figure 35.

The simple resistance measurements currently used in practice would seem to be completelyinadequate for evaluation of full garments. If modified to give a resistance to groundmeasurement this type of measurement may have some value in verifying effective groundingof such materials that require it.


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Is the garment intended to begrounded in use?

Perform modifiedSP 2175

Perform modifiedSTFI PS07

(garment grounded or ungroundedaccording to design)

Yes

Figure 35. Simple test protocol based on SP2175 and STFI PS07

From the existing test methods the SP method 2175 and the STFI method No PS 07 are themost promising full garment tests for further modification. However completely new methodsare potentially required to cover all factors influencing the ESD protective performance of agarment. Finally, we should take into account that fabric tests, which have been considered inanother Workpackage, form an important part of the garment performance evaluation. Ageneral view is obtained only after the results from both garment fabric and full garment testsare combined.


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We should also take into account that the work in Workpackage 1 “Assessment of the risk ofdamage to sensitive devices from ESD from textiles and garments” and Workpackage 2“Physical mechanisms in fabrics” is not yet completed and the work in Workpackage 3“Interrelations between different components of the system” is in delay. Those studies areproducing important input for the evaluation of the test methods for full garments andgarment fabrics. A re-evaluation of the conclusions done in this report should be done afterthe final results of WP1 and WP2 and more output from WP3 are available.

An important question to be studied is whether the ESD protection level provided bygarments, where the conductive elements are core conductive threads, is sufficient for themost sensitive devices. Such garments are increasingly used especially in cleanroomproduction of electronics due to the good durability, washability and cleanroom properties ofthe core conductive threads. Such garments will systematically fail in performance tests doneusing current standard test methods, although their protective performance is not so bad ascould be expected based on the standard tests. The charge dissipation performance of the coreconductive garments is mainly based on the corona discharge mechanism, the effect of whichdepends mainly on the structure of the conductive threads not on electrical parameters of thematerial. The true ESD protective level of garments including core conductive threads,however, is not clear.

Additional challenge is given by the fact that there is actually a need for different level oftests. A new product should be assessed with respect to all important parameters influencingthe protective performance in controlled laboratory conditions. These tests can includedmeasurements requiring special equipment. On the other hand, there is a need for simpleperiodic control tests done in manufacturing sites or in laundries after washing. These tests ora test may not have to cover all important aspects and could restrict on one or a few criticalparameter. These test should be carried out using basic measuring instruments. The issue ofdifferent level of tests was not considered in the present work but should be taken intoaccount in further project work.


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[12] Standard draft prEN 1149-3: Protective clothing – Electrostatic properties – Part 3: Testmethods for measurement of charge decay, 2003.

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[22] M J Dyer. The antistatic performance of cleanroom clothing: Do tests on the fabric relate tothe performance of the garment within the cleanroom. Proc. EOS/ESD Symp. Vol. EOS-19.,1997, pp 276-286.

[23] G. Coletti, F. Guastavino, E. Torello, Charge behaviour on fabrics. Part a), ESTAT-Garments (G6RD-CT-2001-00615) Deliverable report D2.3a, 2003, unpublished.

[24] K Tappura and S Nurmi. Computational modelling of charge dissipation of fabricscontaining conductive fibers. J. Electrostatics Vol. 58, 2003, pp. 117-133.

[25] P. Lemaire, Results on physical tests, ESTAT-Garments (G6RD-CT-2001-00615)Deliverable report D4.2, 2003, 15 p., unpublished.

[26] SP-Method 2175, Measurement of decay time of ESD-protective clothing, Rev. 3, 2000.[27] Test Method STFI-Reference No. PS 07, Version: 01/03 Rev. A, Test method to determine

the body potential and the charge transfer by wearing of electrostatically dissipativeprotective clothing, 2003.

[28] U. von Pidoll, Determining the incendivity of electrostatic discharges without explosive gasmixtures, PTB Braunschweig, 2002.

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[30] Japanese Industrial Standard JIS L 1094:1997 Testing methods for electrostatic propensityof woven and knitted fabrics, 1997.

d61 evaluation of existing test methods...(hbm) esd test [1]. and the number of ultrasensitive...

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