protection of electronic equipment from electro static discharge

7/29/2019 Protection of Electronic Equipment From Electro Static Discharge

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IMPROVING OPERATIONAL RELIABILITY OF EVERY

ELECTRONIC EQUIPMENT/DEVICES FOR MISSION CRITICAL

APPLICATIONS BY

PROTECTION OF ELECTRONIC

EQUIPMENT FROM ELECTROSTATIC DISCHARGE (ESD)

Kishori Sharan Mathur

Research Scholar, Shri JJT University,

Jhunjhunu 333001, Rajasthan, India

[email protected]

1.

INTRODUCTION

Electrostatic damage of Electronic equipment and components by operating

personnel is one of the most significant problems, which directly affects the reliability,

and operational maintainability of electronic equipment.

With advances in manufacturing technologies of semi-conductor devices e.g. LSI,

VLSI, ULSI technology of ICs resulting in higher circuit densities with higher unit

performance but quite often-higher static susceptibility. Such devices, which can beeasily damaged by static discharge, are called Static Sensitive Devices (SSD). Increase in

package density and reduction in dimensions (for example the channel thickness in MOS

devices has reduced from 0.15 micron to 0.02 microns) makes almost every type of semi-conductor devices extremely susceptible to high voltage regardless of its manufacturingtechnology.

Table 1: Shows the voltage ranges at which these devices, manufactured by varioustechnologies get damaged.


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TABLE 1

REPORTED SUCEPTIBILITY OF VARIOUS DEVICES

EXPOSED TO ELECTROSTATIC DISCHARGE

SL NO. DEVICE TYPE RANGE OF ESD SUSCEPTIBILITY (volts)

1. MOS FET 100 V- 200 V

2. Ga As FET 100 V- 300 V3. EPROM 100 V

4. JFET 140 V- 7000 V

5. SAW (Surface Acoustic Wave) 150 V- 500 V

6. Op-Amp 190 V- 250 V7. CMOS 250 V- 3000 V

8. Schottky diodes 300 V- 2500 V

9. Film Resistors (Thick, Thin) 300 V- 3000 V

10. Bipolar Transistors 380 V- 7000 V11. ECL 500 V- 1500 V12. SCR 680 V- 1000 V

13. Schottky TTL 1000 V- 2500 V

Table also shows that if the voltages in working environment can be maintained to lessthan 100Volts then damage to Static Sensitive Devices (SSDs) should not exist due to

Electro Static Discharge (ESD).

2. HUMAN BODY AS A CHARGE CARRIERDuring normal body movements like walking across a carpet, we generate static

charge on our body by the sole of the shoes, which come into direct frictional contact ofthe floor e.g. a carpet. The amount of tribo electric charging depends on the molecular

structure of the shoes, and the carpet. Every time a person walks, some charges will be

generated on shoe and an opposing charge will be generated on each footprint on the

Carpet. Also the charge on shoe sole will tend to become progressively greater with eachstep.

3. ELECTROSTATIC DISCHARGE PHENOMENON:When a charged human body comes in contact with an electronic component or

electronic equipment it discharges through an available discharge path. This discharge is

called Electrostatic discharge.Even if a charged person doesnt touch the SSD, the electro static field generated willinduce static charges on the device which can also result in damage to the SSD.


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This direct injection of charge in an SSD device can damage the molecular structure of

the device resulting in permanent failure or partial damage to the device.


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4. ELECTROSTATIC DISCHARGE EFFECTS IN ELECTRONICEQUIPMENT

The ESD phenomenon can result in failure of electronic equipment in field and can be a

prime reason during handling or repair of electronic equipment. Human beings can holdcharges up to 20 KVbut unfortunately, people cant feel fields or discharges less than afew thousands volts, so they are not at all aware of the damage they are doing to the

electronic equipment unknowingly.

5. COMBATING ELECTRO STATIC DISCHARGE:Installations of Electronic equipment even for well-designed product, the majority

of actual ESD damage will often be done during installation of Electronic equipment.Unfortunately there is a tendency for operating personnel to assume that ESD failures are

at very rare occasions. But this is not always true as per one of the survey 28% SSDs aredamaged by operating personnel. It is quite common for a component (typically an IC) to

be damaged, but not destroyed. A component with such a latent failure can then fail

under the stress of continues operation at the site. There is also a tendency for personnel

to assume that ESD IS NOT PRESENT IF THEY CANT FEEL IT. This is not alwaystrue.

Normally, people cant feel ESD levels below 2 to 3 KV, but 3 KV is certainlysufficient to damage many items. Whats more, there need not to be a spark to destroycomponents. The electrostatic field it self can cause dielectric break down, even if the arc

never occurs. Because of the effect of latent failure, one cant wait for problems to occur

before implementing ESD protecting measures. Static protection must be continuouslymaintained.

6. ANTISTATIC MEASURES:

In fact static charges, which are responsible for damaging the components, are

generated by quite casual movements like walking, raising a hand or a foot, getting up

from a chair etc. Such movements are not avoidable. Hence, our anti-static measuresmust take care of such charge generating mechanisms. So, to protect the devices, we must

ensure that the electrostatic charges do not build up. We must also ensure that whatever

small charges do build up despite the precautions is dissipated safely.

It must be clearly understood, that the static-sensitive devices remain static-

sensitive throughout their working life: they are not free of danger when mounted in a


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PCB or a module. Therefore, all assemblies containing SSDs are to be treated as if they

are static sensitive themselves.

The various methods to combat ESD are described below:

7. STATIC SAFE FACILITIES

7.1. THE PROTECTED AREA.

7.1.1. Materials:

The use of materials, which are major static charge generators (Table-2), must be

prohibited.

TABLE 2

MAJOR CHARGE GENERATORS

OBJECT OR PROGRESS MATERIAL OR ACTIVITY

Work surfaces - Waxes, painted or varnished surfaces.

- Common vinyl or plastics.

Floors - Sealed Concrete.

- Waxes/finished wood

- Common vinyl tile or sheeting.

Clothes - Synthetic garments.

- Non-conductive shoes.- Virgin cotton (Below 30% RH).

Chairs - Finished wood.

- Vinyl.- Fiberglass.

Packing, Handling & stores - Common plastic-bag, wraps, envelop.

- Common bubble pack, foam.

- Common plastic trays, plastic tote boxes.

Assembly, cleaning, Test & - Spray cleaners.Repair Area. - Common plastic solder suckers.

- Soldering irons with ungrounded tips.

7.1.2. Flooring:


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For maximum protection, the floor of the work area should be conductive.

Conductive floor tiles and floor mats are commercially available. Conductive footwearoffers further protection.

7.1.3. Access:

Access to a work area where SSDs are stored or handled must be restricted topersonnel not trained in SSD protective procedures.

7.1.4. Signs/Symbols:

Warning signs can be displayed such as ESD PROTECTED AREA-

AUTHORISED PERSONNEL ONLY. One of the prominent symbols used for instantlyidentifying Static Sensitive Device (SSD) is shown in Figure 5.

Figure 5

Environment:

7.1.5.1. Humidity:

The relative humidity should be controlled between 40 and 60 percent. Where this

cannot be achieved, the use of ionized air is required (Figure6)

Figure 6


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7.1.5.2. Ionized Air:

Ionized air dissipates electrostatic charges. There are two types of ionized air

generators: nuclear activated and corona discharge. (Some corona discharge unit can

generate excessive amounts of ozone. The maximum level allowed by OSHA standards is0.1 ppm for 9 hrs exposures) (Figure 6)

8. THE WORK STATION:

The conductive work-surface (Table-top) and the floor mats are grounded

through a resistor having a value between 0.5 mega ohms and 100 mega ohms to limit the

discharge current. (This ensures operator safety. Limiting the discharge current also

protects a charged device) Figure 7 and 8.

.

Figure 7


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Figure 8

9. WORKER PROVISIONS:

9.1. Conductive Wrist Strap:

A conductive wrist-strap in both the hands must be provided to ensure that the

worker is at the ground potential. A one-mega ohm resistor is required between the strapand the Antistatic Earth to limit the discharge current

9.2. Heel Grounders:

Conductive footwear (or a conductive strap on conventional foot wear) offers

additional protection.

9.3. Conductive Garments:

Some fabrics can generate static electricity. Practically, workers should wearover-garments that dissipate charges. Few general fabrics, which are presently available.

a) Cotton.b) Polyester.c) Cotton Polyester Blendsd) Cotton Polyester Blend with additional stainless steel threads.


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9.4. STORAGE AND TRANSPORT:

9.4.1. Cabinets:

Cabinet should be conductive (e.g. steel) and should be grounded through a one-

mega ohm resistor.

9.4.2. Device-carrying containers:

Containers used to store or transport SSDs must be made out of conductivematerial. Treated cardboard and plastic are acceptable. Containers holding SSDs shouldbe stored only on conductive, grounded surfaces (Figure 11& 12).

9.4.3. Trolleys:

Trolleys should be conductive. A conductive strap (or brush) fixed to the trolley,which is in constant contact with the conductive floor (even when the trolley is moving),

provides additional protection.

9.4.4. Packing Material:

The packing material described in section (Packaging options only should be

allowed in the work area). See figure 13 & 14

9.4.5. Equipment:

All electrical equipment should be grounded to Electrical Earth. (It may be noted

that the Electrical Earth and the Antistatic Earth are not the same point). Plastic surfaces

such as CRT covers and terminals, should be treated with a topical antistat at regularintervals.


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Figure 9(I)

Figure 9 (II)


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Figure 10

Figure 11


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Figure 12

Figure 13


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Figure 14

10. STATIC SAFE HANDLING

The following general guidelines should be followed:

10.1. Operations, which require human, handling, should be minimized.

10.2 While maintaining static sensitive equipment in a place where ground straps cannot

be used, the maintenance personnel should ground themselves prior to removing

sensitive items from protective packaging.

10.3 Tools and test equipment used in protected areas should be properly grounded. If

possible, hand tools should not have insulation on the handles. (If plastic handledtools must be used, they should be treated with a topical antistat).

10.4 Ensure that all containers, tools, test equipment and fixtures used in protected areasare grounded before (and during) use; either directly or by contact with a grounded

surface. Grounding of electrical equipment should be via a grounded plug, not

through the conductive surface of the grounded workstation.

10.5 Work instructions (method-sheets), test equipment and fixtures used in protected

area should not be covered in common plastic sheeting or containers.

10.6 Workers clothing should never make contact with the device, and workers shouldavoid friction-producing activities in the vicinity of the workstation.

10.7 Workers should avoid touching leads of the devices (or contacts, in case of thesurface-mounted devices).

10.8 When devices are removed from their protective containers, they should be keptonly in the following manner:

a) Pin-side down, on a conductive surface; orb) In a conductive container, with pins in contact with the conductive surface.

10.9 Keep dielectric materials to a minimum such as floors, chairs, stools, personnel

clothing and shoes should be limited to poor charge generator such as cotton andleather (Not polyester and rubber).


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10.10 Periodic continuity and resistivity measurement should be performed. The testshould include.

a) Worker ground straps.b) Work surfaces.c)

Floor mats.d) Other ground connections.

10.11 Caution should be observed in using solvents (such as acetones and alcohol orother cleaning agents) for cleaning ESD-protective materials. The use of such

solvents can reduce the effectiveness of some protective materials, especially those

employing detergent type antistat.

10.12 When moving sensitive devices from one work station to another, the operatorsshould first ground themselves before moving or placing any devices on a grounded

work station. This will safely discharge any charge, which is developed during

transportation.

11 PACKAGING OPTIONS:11.1 Product to be shipped must be packed in one of the following:

a) Conductive foam.b) Conductive Tubes.c) Antistatic Carriers.d) Formed Antistatic Plastic.e) Fluted Cardboard.Figure 15 & 16 shows packing options and antistatic bench to be used by

logistics.

11.2 All radial leaded components should be packaged with a layer of conductive foam,

thus creating a faraday cage.

11.3 Dual-in-line package should be packed in conductive tubes. Antistatic stoppers to be

placed at each end to cushion the product. Pins will also be inserted in each end of

the tube to prevent sliding movement within the tube.

11.4 Those Dual-in-line packages, which are not possible to pack in a conductive tube, to

be packed in conductive foam.

11.5 SSD devices should have an ESD warning, stamp or label, affixed to each box or

tube.

11.6 All leadless chip carriers be packed in conductive tubes, with and ESD warning

stamp or label on each tube.


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11.7 Certain products, due to their uniqueness or the fact that optimum packaging is stillunder development, may still be packaged in non-conductive or non-antistatic

containers. These will be sealed in conductive bags (used as first level).

11.8 All PCBs should be packed in conductive bags.

11.9 All modules be packed in conductive bags, or wrapped in antistatic layer.

11.10 In general if there is any doubt regarding the exact type of packaging to be used, aconductive bag should be used.

12. UTILIZATION OF ANTISTATIC TEST EQUIPMENTS WILL FURTHER

IMPROVE REQUIRED PRECAUTIONS (Figure 9).

Figure 15


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Figure 16

CONCLUSION

Practically in todays world all aircrafts, space vehicles and missiles have ICsand other semiconductor devices (SSDs) as the electronic system. With every day

advancing technology of electronics now the emphasis is not only on small size and

weight of airborne equipment components but also reliability of electronic equipment is

of prime concern. To improve the reliability of an electronic system one of the majorareas of protection is ESD protection of electronic equipment/component. This ESD

protection can certainly improve the reliability of an electronic system. This will also

result in high maintainability of the electronic equipment, which in turn fulfills the

operational requirement and mission critical requirement of electronic equipment. Notonly will this but it may also result in savings in foreign exchange for those countries

who are not able to manufacture SSDs at their home land since spares (SSDscomponents) requirement will be comparatively less due to implementing ESD protection

for SSDs.


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protection of electronic equipment from electro static discharge

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