1、GEIA STANDARD Standard for Mitigating the Effects of Tin Whiskers in Aerospace and High Performance Electronic Systems GEIA-STD-0005-2 June 2006 GOVERNMENT ELECTRONICS AND INFORMATION TECHNOLOGY ASSOCIATION A Sector of the Electronic Industries Allianc Copyright Government Electronics 2002/95/EC Res
2、triction of Hazardous Substances (RoHS) and 2002/96/EC Waste Electrical and Electronic Equipment (WEEE) that restrict or eliminate the use of various substances in a variety of products that are produced after July 2006. One of the key materials restricted is lead (Pb), which is widely used in elect
3、ronic solder and electronic piece part terminations, and printed wiring boards. While these regulations may appear to only affect products for sale in the EU, due to the reduced market share of the Aerospace and High Performance Industry in electronics, many of the lower tier suppliers are changing
4、their products because their primary market is consumer electronics. Additionally, several U. S. states have enacted similar “green” laws, and many Asian electronics manufacturers have recently announced completely “green” product lines. The restriction of Pb use has generated a transition by many p
5、iece part and board suppliers fi-om tin-lead (Sn-Pb) surface finishes to pure tin or other Pb-free finishes. Lead-free tin finishes can be susceptible to the spontaneous growth of crystal structures known as “tin whiskers” which can cause electrical failures, ranging fi-om parametric deviations to c
6、atastrophic short circuits, and may interfere with sensitive optical surfaces or the movement of Micro-ElectroMechanical Systems (MEMS). Though studied and reported for decades, the mechanism behind their growth is not well understood, and tin whiskers remain a potential reliability hazard. Furtherm
7、ore, the growing number of piece parts with pure tin finishes means there are more opportunities for whiskers to grow and to produce failures. iv Copyright Government Electronics examples include circuit cards and wire harnesses. This may include soldered assemblies. Bright tin is a tin finish with
8、higher internal stresses and smaller grain size of 0.5 pm to 0.8 pm and carbon content of 0.2% to 1 .O% Critical item or function, if defective, will result in the systems inability to retain operational capability, meet primary objective, or affect safety. Customer refers to an entity or organizati
9、on that (a) integrates a piece part, soldered assembly, unit, or system into a higher level system, (b) operates the higher level system, or (c) certifies the system for use. For example, this may include end item users, integrators, regulatory agencies, operators, original equipment manufacturers (
10、OEMs), and subcontractors. EDS is Energy Dispersive (X-ray) Spectroscopy, a method for material composition analysis. High performance System or Product requires continued high performance or performance on-demand, or equipment downtime cannot be tolerated, or end-use environment may be uncommonly h
11、arsh and the equipment must function when required, such as life support or other critical systems. Lead-Free is defined as less than O. 1 % by weight of lead in accordance with Waste Electrical and Electronic Equipment (WEEE) guidelines. Matte Tin is a tin finish with lower internal stresses and la
12、rger grain sizes typically of 1 pm or greater and carbon content less than 0.050% Pb-free Tin is defined to be pure tin or any tin alloy with 3% Pb content) have some risk of whiskers, some finishes have greater risk than others. Selection of a matte tin, preferably with a nickel underplate, or a Pb
13、-free tin alloy with nickel underplate are generally considered lower risk finishes than bright tin or tin finishes over copper. Impact of underplate thicknesses and other lower risk finishes is under review in the literature. Groups such as international Electronics Manufacturing Initiative (iNEMI)
14、 may have additional guidelines on finish selection. There are also an increasing number of suppliers who provide Pb-free tin finishes that are warranted against whiskers. Use of these finishes is considered mitigating if the application environment and product life length are compatible with the wa
15、rranty. If this mitigation method is applied, references or documentation supporting selected finish performance shall be provided. For the purposes of this standard, heat treatments are considered to be in the category of lower risk finishes. Heat treatments could include reflow, fusing, or anneali
16、ng at the piece part or board levels. More information on the impact of these processes in provided in Annex C- Sectim CL1.4 and Secth C.2.2.9. 3.3.3.3 Tin Finish Replacement As stated in the definition, if all Pb-free tin finishes on the device have been replaced through replating or solder-dipping
17、 then the device is no longer considered to be tin-finished. However, if only some tin-finished surfaces have been reworked, then the actions are considered to be mitigations. For example, if tin-finished leads are solder dipped in Sn-Pb, but the dipping does not reach the piece part body, the dippi
18、ng is only a mitigation and not a 10 Copyright Government Electronics however, it is a mitigating measure. For most coatings, tin whiskers have been shown to eventually grow through thin coatings. Although there is believed to be low risk of a whisker penetrating the coating of an adjacent surface,
19、whiskers could still short to other uncoated surfaces in the area. For some piece part types, it is difficult to entirely encapsulate the individual interconnects so there remains a direct path for shorting due to whisker growth. Other drawbacks include bubbles between tightly spaced leads and conne
20、ctor mating pin overspray contamination. Coating materials and processes should also be carefully reviewed to ensure their compatibility with the hardware design and application. For example, excessively thick coatings andor mismatches of coefficients of thermal expansion may reduce life expectancy
21、of solder joints or crack piece parts. It is recommended that an assessment of the conformal coating process be performed to assure that coverage and thickness are adequate and consistent. It is also recommended that the conformal coat process be regularly evaluated and that the evaluation results b
22、e available for customer review. 3.3.4 Methods for Analysis and Evaluation of Tests and Mitigations for Tin Whisker Risk and Mitigation Effectiveness The customer and supplier should evaluate their products, applications, and environments and evaluate how test data and mitigation strategy applies to
23、 those conditions. A number of different analyses might be appropriate for this requirement. Determination of the appropriate analysis should be made by the supplier and customer. 3.3.4.1 Analysis of Application Tolerance Analyses of application tolerance might include descriptions of spacing distan
24、ces versus distribution of whisker sizes and whisker density, examination of barrier locations or conformal coat coverage, or Failure Modes and Effects Analyses (FMEA) of the impact of shorts, plasma events, and possible micromechanical dysfunction. Tolerance analyses might determine that tin whiske
25、rs have no impact or that there is some level of impact. If there is possible impact, the supplier and customer should evaluate those risks and impacts and determine if they are acceptable. 11 Copyright Government Electronics & Information Technology Association Provided by IHS under license with GE
26、IA Not for ResaleNo reproduction or networking permitted without license from IHS-,-,-GEIA-STD-0005-2 In some cases, the analysis may involve a discussion of the overall reliability model or unit design. For example, it may be that the application has adequate margin, a redundancy, or a maintenance
27、schedule to allow for a probability of whiskers without impacting mission performance. It may also be the case that field data on particular tin-finished piece parts in similar environments exists, and can be used to support a low risk of whisker impact. 3.3.4.2 Analyses of Whisker Propensity Tests
28、Although the mechanisms for tin whisker growth are still unknown, there are several test methods being used by suppliers in the industry. Industry standard tests do provide a common method and comparable data. Many piece part suppliers are involved with developing tin whisker test methods and finish
29、 qualification methods. Some examples include JESD22-A12 1 “Test Method for Measuring Whisker Growth on Tin and Tin Alloy Surface Finishes” and JESD201 “Environmental Acceptance Requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface Finishes.” Tests typically include aged samples b
30、eing put through thermal cycling, humidity, high temperature, and ambient testing. Acceptable test results alone should be considered adequate only for Level 2A hardware. If qualification or other test data is included, the analysis should include a discussion of why the test was representative and
31、how the results were generalized to the application environment and product life length. Note that, until the fundamental mechanisms of tin whisker growth are understood and acceleration factors established, customers with environmental exposures longer than the test length should be cautious about
32、extrapolating the test results too extensively. This is particularly true for systems with greater than 20 or 30 years of storage and mission life, harsh temperature cycle environments, or harsh humidity environments. Even tests that show whisker growth rate slowing should be applied with caution as
33、 periods of dormancy have been observed on whiskers. 3.3.4.3 Analyses of Field Data Suppliers field data of historic reliability on hardware using tin piece parts may provide insight into the risk of tin whiskers. Historic failure databases of these tin piece parts in these applications might includ
34、e some tin whisker failures, if failures were possible, but their cause may not have been traced to a whisker. However, care must be taken in extrapolating field data results. Different plating processes, from different suppliers, may have different propensities to whiskering. A growing number of pi
35、ece parts with Pb-free tin finishes means there are more opportunities for whiskers to grow and to produce failures. The similarity of the field data to future use must be addressed in any analysis taking this approach. 3.3.4.4 Other Analysis Issues Analyses should also discuss any applied mitigatio
36、n strategies. If the design has a mitigating feature, criteria for appropriate spacing and barriers should be agreed upon. If lower risk finishes have been selected, the finishes should be described and any available data or references supporting improved performance presented. If some Pb-free tin f
37、inishes have 12 Copyright Government Electronics & Information Technology Association Provided by IHS under license with GEIA Not for ResaleNo reproduction or networking permitted without license from IHS-,-,-GEIA-STD-0005-2 been replaced, the analysis should include a discussion of any remaining Pb
38、-free tin finishes and evidence supporting that the aftermarket processing did not damage the device. Conformal coat mitigation should include a description of the coating type, data and references supporting the effectiveness of the coating at limiting whisker growth, and any measures taken to qual
39、ify and monitor the coating process. Analyses might also include use of a risk algorithm. Although there is no industry consensus on a specific algorithm to be called out in this standard, more information regarding these evaluations is provided in Annex 15, Section 15.3. Unlike many failure mechani
40、sms, tin whiskers grow equally well, if not better, under storage conditions as compared to application environments. If the application is likely to have a long period of storage, it is recommended that customers require suppliers to address risk fi-om this period or include it in any life-time cal
41、culations. 13 Copyright Government Electronics & Information Technology Association Provided by IHS under license with GEIA Not for ResaleNo reproduction or networking permitted without license from IHS-,-,-GEIA-STD-0005-2 14 Copyright Government Electronics & Information Technology Association Prov
42、ided by IHS under license with GEIA Not for ResaleNo reproduction or networking permitted without license from IHS-,-,-GEIA-STD-0005-2 Annex A - Guidance on Control Levels, Risk Assessment, and Mitigation Evaluation A.l Introduction The determination of the suitability of the use of Pb-free tin must
43、 be performed on an application-by-application basis. Unfortunately, the current state of our understanding of the tin whisker phenomenon does not permit the quantification of the probability of failure due to tin whiskers for any particular application, even under extremely well controlled circumst
44、ances. Nonetheless, customers and supplier must choose tin control levels, mitigation strategies and weigh those decisions against the risk of tin whiskers. Potential applications of high reliability electronic systems vary fi-om “single thread“ systems where no failures can be accepted for very lon
45、g periods of time, often exceeding 20 years, to systems where it is necessary to be single failure tolerant and it is necessary to mitigate the effects of single failures utilizing multiple techniques including redundancy and field support actions. A guiding principal should be that it is not possib
46、le to have a single channel electronic system that will never fail. Thus, for high reliability electronic systems, multiple provisions and techniques are required to achieve application specific reliability and availability. These principles apply to both the risk fi-om tin whiskers as well as other
47、 failure mechanisms that have long been considered in risk and reliability assessments. A.2 Level Determination There are three basic levels: no controls on tin finishes, some controls on tin finishes, and prohibition of tin finishes. Level 2, some controls on tin finishes, has three sub-levels. The
48、 differences between the three sub-levels may seem subtle, but the differences were carefully established to allow flexibility between different program types. Full requirements are provided in the normative sections of this standard, but it may be helpful to review the following summary of requirem
49、ents when selecting a programs level. 15 Copyright Government Electronics & Information Technology Association Provided by IHS under license with GEIA Not for ResaleNo reproduction or networking permitted without license from IHS-,-,-GEIA-STD-0005-2 Level 1 Level 2A Level 2B Level 2C Level 3 Summar Documentation of Tin Use Supplier: General information on finishes used. Supplier: General information on finishes used. Customer: List of any applicati
