Reliability Practices for Satellite Design and Assembly Focusing on

FMECA, Cleanliness and X-ray Inspection

İlknur Baylakoğlu#1, Barış Çal#2, Murat Harmandalı#3, Engin Köksal#4, Vedat Gün#5 1, 2, 3, 4, Reliability Laboratory Expertise Group, 5 Clean Room Expertise Group

#TÜBİTAK Uzay, Space Technologies Research Institute Ankara, Turkey

1ilknur.baylakoglu@uzay.tubitak.gov.tr 2baris.cal@uzay.tubitak.gov.tr

3murat.harmandali@uzay.tubitak.gov.tr 4engin.koksal@uzay.tubitak.gov.tr

5vedat.gun@uzay.tubitak.gov.tr

Abstract—Because most space systems are extremely complex, and need to service long enough under extreme environmental conditions, it is necessary to perform, execute, and provide a method of risk identification during the design stage. The Failure Modes Effects and Criticality Analysis, FMECA has been recognized as such an effective approach that provides the necessary visibility during the design period by identifying mission critical failure modes and provides an input for design alteration. Besides design improvement tools, the manufacturing and inspection procedures of space application products like satellite need to take care special attention. The cleanliness requirements of soldering and assembly areas should be supplied to avoid the occurrence of failure mechanisms caused by contaminants like particulates; such as dirt, sand, industrial fumes, hair cosmetics, dead human skin cells, fibers and lint from clothing, chips and burrs from machined surfaces and etc. As a sophisticated inspection tool, X-ray is particularly beneficial for the evaluation of solder joints and assemblies that involve advanced packaging technologies such as: Ball grid array (BGA), land grid array (LGA), chip size package (CSP), flip chip (FC), wafer level packaging (WLP), system in package (SIP) and quad flat-pack no leads (QFN) etc. The use of X-ray inspection is forced by the standards for space application products BGA solder joints. The defects can be detected by X-ray inspection are; shorts, voiding, delamination, missing part, open, misalignment, warpage, counterfeit components etc.

Keywords-reliability, FMECA, satellite assembly cleanliness and x-ray inspection

I. INTRODUCTION Space systems are complex and need to perform under

extreme environmental conditions for its expected life, so it is vital to use methodological risk identification in the design stage. Besides design, it is also vital the consideration of cleanliness requirements and necessary inspections to find defects in the early phase of assembly of subsystems and integration steps to avoid the reoccurrence. X-ray inspection

has become an irreplaceable tool for design evaluation, process improvement, quality assessment and rework verification. While X-ray image processing, fault detection and failure analysis tools provide the ability to quantify and fine-tune manufacturing processes, they also improve quality, yield and increase reliability.

The Failure Modes Effects and Criticality Analysis (FMECA) is an effective approach that provides the necessary visibility during the design period to identify mission critical failure modes and provides an input for design alteration. The FMECA analysis needs to be performed for each equipment, subsystem, and system of the satellite and updated by keeping an ascending coherence until the configuration of every level is finalized. The formal spread sheets are used for each potential failure within an assembly is recorded together with its resultant assembly, subsystem, and system effect including the severity and criticality level of failure modes.

The FMECA application can be performed by description, identification and analyze of failure modes, categorization of failure effects by their severity level, calculation of criticality numbers (CN), determining criticality levels and critical components, design and execution of failure detection, isolation and compensation methods, performing maintainability analysis, identifying intervention methods and modifications, providing precautions, recommendations and corrective actions to prevent or minimize such failures. FMECA provides increase in reliability by minimizing failures.

There are several standards to perform FMEA/FMECA applications for various industries. ECSS-Q-ST-30-02C, Failure modes, effects (and criticality) analysis (FMEA/FMECA) standard is applied at TÜBİTAK UZAY. Section II reviews the use of FMECA methodology in design.

Cleanliness requirements and contamination control for space application devices manufacturing process need to be followed strictly to avoid the occurrence of failure

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mechanisms caused by contaminants. Brief information will be outlined in this paper for measurement and control of cleanliness and contamination including the standards especially for space application.

Cleanliness requirements and the type of applicable test methods and their advantages and disadvantages for measuring cleanliness will be described in Section III. The importance of cleanliness of PCB assemblies for high reliability harsh environmental stress applications and cleaning methods and critical situations and validation of cleaning procedures and TÜBİTAK UZAY’s clean room capabilities will also be presented.

The use of X-ray inspection method will be described in Section IV with the related applications and type of failures detected. Solder joint defects are caused by a variety of unique thermal and mechanical conditions that occur during the assembly process and it is a must by ECSS (European Cooperation for Space Standardization) standards to inspect BGA assemblies by X-ray which is performed 100% at TÜBİTAK UZAY. At the end the effects of FMECA, cleanliness/contamination control and X-ray inspection on reliability improvement are presented at the conclusion Section V.

II. FMECA IN DESIGN

A. FMECA Methodology FMECA applications focus on identifying and examining

potential failure modes in products and processes. During these identification and examination processes, failure effects and criticality are derived and obtained by the related failure modes. FMECA studies are performed within a team work leaded by reliability engineers.

In order to describe all possible failure modes and effects in systems and subsystems effectively and as early as possible, several preemptive approaches are provided. Depending on application and test environment, both qualitative and quantitative analysis or a mix of those can be applied. The product which is going to be a subject for a FMECA application is described by the help of the following information [1], [2]:

• Definition, theory and idea about the product and its operational environment

• Definition of ground rules and assumptions which are used in design procedures

• Functional block diagrams and reliability block diagrams

• Interfaces, schematics and bill of materials (BOM) lists which contain detailed specifications and characteristics of the product and its components

• Relationships, dependencies and definitions about the product

• Operational modes and functionality • Expected performance and mission phases

The information above is necessary to begin a FMEA/FMECA application. The inputs can be summarized as below [3], [4], [5]:

• Determining a system’s limits; that is, deciding which parts to be included and which parts are not

• Specifying main system functions, missions and functional requirements

• Considering operational and environmental conditions • Collecting available information about the system’s

description • Collecting information about previous similar

examples and their design Subsequently, the FMECA application can be continued

with failure-related topics after defining the inputs and information about the product. Those can be summarized as below [5]:

• Description, identification and analyze of failure modes, effects and remarks in component or functional level

• Categorization of failure effects by their severity level • Calculation of CN, determining criticality levels and

critical components • Design and execution of failure detection, isolation

and compensation methods • Performing maintainability analysis, identifying

intervention methods and modifications • Providing precautions, recommendations and

corrective actions to prevent or minimize such failures After enumerating the inputs and failure-related topics,

FMEA/FMECA outputs are going to emerge from available information and data. These outputs can be summarized as below [6]:

• The failure modes which are encountered in different levels of the system

• The necessary preemptive actions • The executed actions • The outcomes of those actions • Information about minimizing the risks • Information about the success rate while minimizing

each failure mode In the structures of today’s contemporary electronic designs

and systems, complex devices and components can be encountered. The failures can be critical in field programmable gate arrays (FPGAs), piezo-electric crystals in oscillators, memory units, latches and logic operation integrated circuits (ICs), converters etc. The examples can be extended. As a result of these failures, the most critical failure effects can emerge. In order to compensate or minimize the failure effects, the precautions or methods below can be executed for space application products:

• Use of radiation tolerant and antifuse FPGAs • Use of transistor-transistor logic (TTL) technologies

and magnetoresistive RAMs wherever possible • Use of space qualified and flight heritage components • Performing screening-shielding wherever possible • Performing cold redundancy • Providing stable electronic circuit designs with the

reliable features; such as, high drive currents, low fan-out values, wide gain ranges, high noise immunity, etc [19].

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A FMECA application is not only the report itself, but also the related FMECA and auxiliary worksheets. For that reason, a detailed BOM list is crucial to prepare the worksheets. In the mentioned BOM lists, all components’ types, references, values and manufacturer codes have to be shown as well as their descriptions. Therefore, FIT values of mentioned components need to be obtained. The probability number (PN), severity number (SN) and CN are calculated. The PN of any component and its related failure mode determines the failure tendency of that component/failure mode during a certain time of operation. FMECA worksheet is also provided as the attachment of report. At end of these analysis a FMECA report should include the main titles such as, objectives, scope, product and design details, ground rules and assumptions including the methodology, failure detection and isolation criteria, results and recommendations, critical items, and finally failure effect summary list.

B. FMECA Standards There are several standards to perform FMEA/FMECA

applications. They are published by different organizations and corporations according to their main field of activity and they can be differed by the demands of applications as given in Table I. The reliability and the quality of the products are ensured by FMECA report as a deliverable according to the proper standard requirement.

TABLE I. PROMINENT FMEA/FMECA STANDARDS

Standard Number

Standard Name

ECSS-Q-ST-30-02C

Space Product Assurance-Failure modes, effects (and criticality) analysis (FMEA/FMECA)

SAE J1739, Jan, 2009

Potential Failure Mode and Effects Analysis in Design, Potential Failure Mode and Effects Analysis in Manufacturing and Assembly Processes

JEDEC JEP131A, May 2005

Process Failure Mode and Effects Analysis (FMEA)

IEC 60812.2006

Analysis techniques for system reliability-Procedure for failure mode and effects analysis (FMEA)

SAE AIR 4845, Jun

1993

FMECA Process in the Concurrent Engineering (CE) Environment

III. CLEANLINESS IN ASSEMBLY FOR SPACE SYSTEMS Cleanliness has a fundamental importance for the space

system performance. Cleanliness and contamination control activities include cleaning of hardware, cleaning monitoring and verification in all phases of design, assembly, launch and mission of a space system.

Cleanliness control starts with the selection of the materials by considering the outgassing requirements according to ECSS-Q-ST-70-02 standard, providing training to the personnel in assembly/test and providing conformance at the printed circuit board (PCB) products and the facilities to meet cleanliness requirements. The satellite assembly, integration and test (AIT) area is a clean environment and the selected materials greatly affect the level of contamination.

Production of space electronics done in clean rooms reduces the risk of contamination compared to the ordinary production sites.

In addition to the contaminations caused by the staff in clean room, selection of materials used in clean room and production has great effects on the level of contamination.

Materials which remain in the facility for long periods of time, or are/become permanent in the facility, may be a contamination concern.

Especially gloves are very good examples for different undesirable situations in clean rooms. Gloves are one of the sources of contamination, especially when used with solvents. Solvents will extract residues and “cross contaminate” items during the cleaning process of spacecraft hardware. Also higher amounts of residue from latex gloves (with isopropyl alcohol or acetone etc.) can be encountered. Finally, with lower amounts of residue, nitrile and poly gloves are better choices for low level of contamination [10].

The cleanliness of PCB assemblies which are tested for the validation of the contamination level needs to be below the failure causing values. The validation of cleaning methods are provided by the use of some test methods like resistivity of solvent extract (ROSE), surface insulation resistance (SIR), NaCl Salt Equivalent Ionic Contaminant Test and Ion chromatography.

A. Types of Contaminants and Their Effects 1) Particulate Contaminants [8]: Many particulate

contaminants, such as dirt, sand, industrial fumes can be excluded from clean rooms by filtering to protect the space systems. But the contaminants caused by human sources, contaminants from walls during the test, machined surfaces, bacteria, fungi, viruses, etc. are released in the integration and test area. Particles can be charged with ambient plasma or photoemission and attracted by electrically charged surfaces[12].

2) Molecular Contaminants [8]: Molecular contaminants are produced during all on ground phases of the space system and they include atmospheric gases, desorbed water, leaks in sealed units like freon, hydrazine, helium, neon and krypton, outgassing products from organic materials like monomers, plasticizers, additives and solvents, vapors from packaging materials and test facilities, vapors from substances used in clean rooms and secondary products coming from microorganisms. Some liquids like residues from cleaning agents, fluxes and some other contaminants like finger prints, salt, acid, etc.

The molecular materials can decompose because of thermal, solar radiation, electromagnetic and charged particles, atomic oxygen, impacts by micrometeoroids or debris, electrical discharges and arcing to lower molecular weight higher volatility species [12].

3) Ionic and Non-ionic Contaminants: When the cleanliness of a PCB is discussed, the absence of harmful residues or contaminants is expressed and classified into two major categories: ionic and non-ionic.

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Ionic residues are the materials that, in presence of moisture, disassociate into negatively and positively charged residues. Once this event occurs, conduction of the eventual solution increases.

Non-ionic residues are the organic residues that can remain on a PCB after production. These residues are typically polymers, oils, or greases.

The two most common failures due to ionic contamination are corrosion and dendrite growth, see Fig. 1. Both of these conditions can cause device failure. The most common sources of ionic contamination are flux, cleaning fluids (e.g. tap water), and plating chemistry residue (from the surface finish).

Figure 1. Dendrite growing between two traces on a SIR coupon [18]

The most common failure modes due to non-ionic contamination are reduced solderability, lack of connectivity and sensor malfunction. Since non-ionic contaminates are nonconductive, they can disrupt the flow of electricity through these connectors if they are present on an edge card connector or inside a socket [7].

Cleanlines Contaminants which can cause several failure mechanisms such as electrochemical migrations, electrical leakage, copper corrosion, etc. reduce reliability level of boards. Different residues can create same failure mechanism; for example, bromide and chloride can cause electromechanical migration [9].

Residues caused by fabrics and materials used in production can be reduced or extinguished by selecting right material. While performing cleaning operations, using nitril gloves instead of fabric gloves reduces the risk caused by residues [10].

B. Standards and Test Methods There are several standards related to cleanliness in

electronics directly such as IPC-TM-650, 2.3.25 “Detection and Measurement of Ionizable Surface Contaminants by Resistivity of Solvent Extract (RoSE)” and also indirectly such as clean room standards or some parts of other standards.

The choice of the cleaning method shall be determined by the following criteria:

1. The type of contaminants to be removed. 2. The physical or chemical nature of the item to be

cleaned. 3. The actual situation on ground phase. Due to the fundamental differences of these contaminants,

they require different methods of testing. The most common tests applied are a derivative of the RoSE test and Sodium Chloride Salt Equivalent Ionic Contamination (Omega Meter)

test especially in space applications. A summary of cleanliness test methods and standards is shown below and in Table II.

TABLE II. SOME CLEANLINESS STANDARDS

Standard Name Standard Number Space product assurance–

Cleanliness and Contamination Control

ECSS-Q-ST-70-01C

Space Engineering-Space Environment

ECSS-E-10-04A

Cleanliness requirements for spacecraft propulsion hardware

ECSS-E-ST-35-06C

Detection of organic contamination of surfaces by infrared spectroscopy

ECSS-Q-ST‐70‐05C

Resistance of Solvent Extract / ROSE

IPC-TM-650, 2.3.25 NASA-STD-8739.2, para.11.6

Sodium Chloride Salt Equivalent Ionic Contamination (Omega Meter)

NASA-STD-8739.2, para.11.7 ECSS-Q-ST-70-08C 11.3.3

Ion Chromatography / IC IPC-TM-650, 2.3.28 Surface Insulation Resistance /

SIR IPC-TM-650, 2.6.3.3

Electrochemical Migration IPC-TM-650, 2.6.14.1 1) RoSE Test [7]: This response is generally reported as a

number of sodium chloride (NaCl) equivalents per unit area. This test is good for quality control, but is not suited for investigations or failure analysis.

2) Sodium Chloride Salt Equivalent Ionic Contamination or Omega Meter Test: This test is used to respond amount of NaCl per unit area. It is very similar to RoSE test and limit value for space electronics is 1.55µg/cm2 [11]. Omega Meter (Ω-meter) test is the method to measure cleanliness in both ECSS-Q-ST-70-08C and NASA-STD 8739.2.

3) Ion Chromatography or IC Test [7]: An ion chromatograph test is the most common tool for precision testing and process baseling. This system can quantify and identify specific ionic species that are present on an electronic device. The most common test method is the IPC TM-650 2.3.28. IC test method can measure up to 15 ionic species in detail.

4) Surface Insulation Resistance or SIR Test [7]: It is an environmental test that measures the effect of ionic contamination.

5) Fourier-Transform Infrared (FT-IR) spectroscopy [7]: For testing non-ionic species, the most common method is FTIR spectroscopy.

This is an advanced test and is normally only carried out in some independent laboratories. Nor are there any thresholds established by IPC regarding how much of each type of residue is permitted on the PCB. However, in time some organizations have started to provide recommendations for appropriate values, primarily for chlorides, sulphates and bromides [12].

C. TÜBİTAK UZAY’s Clean Room TÜBİTAK UZAY facilities consist of four main

compartments:

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1st Compartment and 2nd Compartment: Class 10.000, Dimensions: 5.4m x 7.8m x 4m and 5.4m x 7.3m x 2.8m respectively, the assembly, AIT activities of middle and small scale satellites are fulfilled in this compartment.

3rd Compartment: Class 10.000, Dimensions: 7m x 8m x 2.8m (W x L x H), the satellite modules assembly activities are fulfilled in this compartment.

4th Compartment: Class 10.000, Dimensions: 5.4m x 3.4m x 4m (W x L x H), the main entrance of clean room and the entry section of thermal vacuum room take place in this compartment.

The employees consisting of engineers and technicians, who take part in the Class 10.000 clean room which was established to be in use of satellite studies, have the knowledge of production and execution in ECSS standards (ECSS-Q-70-08, ECSS-Q-70-08, ECSS-Q-70-18, ECSS-Q-70-28, ECSS-Q-70-38, ECSS-Q-70-26 & 70-30). The assembly and inspection technicians are certified for these standards.

Figure 2. Clean room activities

In the clean room, the equipments, which are used during production and examination activities, such as microscopes, special purposed solders and related devices, mini-vacuum room for chemical applications and curing ovens are available.

A. Clean room air conditioning and cleaning systems: In TÜBİTAK UZAY facilities, the main purpose is to provide a dust free environment rather than biological hygiene. The air which is taken from outdoors and circulated indoors is filtered by panel, bag and high efficiency particulate air (HEPA) filters, respectively. The system provides 22±3°C temperature and 55±15% humidity (applicable with the ECSS standard) for the environment by the help of heaters and humidifiers regardless of different weather conditions.

IV. X-RAY METHOD FOR INSPECTION The most useful non destructive methods for inspection of

solder joints to area array devices are X-ray techniques. Not only solder joints but also failures through packages, including encapsulation, heat sinks and metallic shielding to reveal obscured connections and identify potential quality issues can be detected by X-ray inspection non-destructively.

The necessity for the evaluation of voids in soldered joints in the space applications is a must, because the voids may affect the reliability of joints as the devices get smaller. Voids may impact reliability by weakening the solder balls and reducing functionality because the reduced cross-section will

have lower heat transfer and current carrying capabilities in BGA and LGA.

X‐ray inspection technique is in two separate groups, transmission and cross‐sectional (laminographic). Missing solder ball or bridges are detected using transmission X‐ray techniques. Inadequate reflow of the solder paste and open joints can be detected by inspecting the device from angle. Thick ceramic devices, boards with a very large number of layers and components on both side of the board, solder joints are inspected by using laminography techniques [14].

Solder joint defects are caused by a variety of unique thermal and mechanical conditions that occur during the assembly process and it is a must by ECSS standards [14] to inspect BGA assemblies by X-ray. The defects can be detected by X-ray inspection are; shorts, voiding, delamination, missing part, open, misalignment, warpage, counterfeit components, etc.

A. The Effect of Void on BGA Solder Joint Reliability Voids occur in BGA balls are one of the most critical

failures detected by X-ray. Rate of voids in solder ball acceptance criteria is given as maximum 25% of the BGA ball cross (6.25% by area) section in ECSS-Q-70-38C and IPC-A-610D [13]. Place of void is important for long term reliability of solder, Fig. 3 [15].

Figure 3. Place of voids in BGA balls

M. Yunus et al. have concluded in their study that voids which are greater than 50% of the solder joint area cause potential reliability problems with reduction in solders joint life in mechanical testing and in thermal cycle testing. Besides this they also resulted that, if the distance from the void–solder interface to the corner of the solder joint is lesser than 6–7 mil, there is a probability of crack propagation. In addition to these, the multiple small voids line up on the component side affect the solder joint life. It has also been concluded that, there is 30% reduction in characteristic life of solder joints with big voids in thermal cycling and a 44% decline in the characteristic life of solder joints with big voids in mechanical testing [16].

B. Space Standards and X-ray Inspection Strategy at TÜBİTAK UZAY A good X-ray inspection strategy provides adequate detail

information for failure analysis, and results can be the design change, process improvement methods, training of operators, etc.

X-ray inspection of electronic devices in space applications is a must in ECSS and NASA standards. So there are several standards about X-ray inspection directly and indirectly such as ECSS-Q-70-38C, EN 61191-6, IPC-A-610E and IPC-7095B. The IPC standards requires that solder ball offset should not violate minimum electrical clearance,

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soldered connection should not has any soBGA solder balls contact and wet to thecontinuous elliptical round or pillar connectio

IPC-7095B defines the acceptability oreliability Class III products as given in Tabl

TABLE III. ACCEPTABILITY OF V

Location of Void Class III

Void in ball 30 % of diameter

Void at interface of ball and substrate

20% of diameter

There are several failures detected by XTÜBİTAK UZAY such as short circuits beSDRAM and balls of BGA (Fig. 4), open circan be detected by different angles, voids in5). All area array components have been inafter assembly according to the IPC and ETÜBİTAK UZAY.

Figure 4. Short circuits and open cir

Figure 5. Voids in BGA balls

V. CONCLUSIONS Expected reliability performance of a

product can be mostly reached by applying Fcontrolling cleanliness and contaminationmaterials, providing clean areas and inspectiarea array solder joints with X-ray. FMEdesign tool but it can also provide informimprovements. Cleanliness and contaminmonitoring the facilities and the assembselection of materials according to space providing the validation of cleanliness higspace system reliability. Voids at area array the reliability of the product and its cinspection provides the avoidance of unacce

older bridging and e land forming a on.

of voids for high le III [14].

VOIDS

9% of area

4% of area

X-ray inspection at etween the legs of rcuit pattern which n BGA balls (Fig. nspected by X-ray ECSS standards at

rcuits

space application FMECA in design, n in selection of ing assemblies for

ECA is used as a mation for process nation control by bled products, by

standards and by ghly improves the components affect control by X-ray eptable product to

install to the system and mprocess and design improveme

There are some other reliUZAY and will be presented in

ACKNOWL

The authors would like to abeen supported by RASAT which is financed by DPT (STurkey) under grant contract no

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[7] Technical Tips Documents, “CoUS Navy’s National Electronic(EMPF), Jan.2002

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Void

make it possible the necessary nts. iability practices at TÜBİTAK n the future events.

LEDGMENT acknowledge that this work has Satellite Development Project, State Planning Organization of o 2007K120030.

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