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February 2015 • SMT Magazine 53 of the tin layer, the larger grain size means few- er locations for tin whiskers to form compared to bright tin with finer (less than 1 micrometer) surface grains for the same surface area. How- ever, matte tin should not be interpreted as be- ing whisker-free. With regard to high tempera- ture annealing, this process provides relief from whisker formation by promoting a uniform and enlarged interfacial intermetallic layer between the tin film and the substrate and reducing stress in the tin film. However, research indi- cates that the stress state continues to evolve, delaying, but not eliminating, whisker growth. With regard to a nickel underlayer, the porosity of the nickel layer may still allow for an evolv- ing stress state favorable to whisker formation. Further, neither annealing nor a nickel under- layer prevents external stress sources from pro- moting whisker formation. As a result, equip- ment manufacturers should not consider part manufacturer mitigations to be sufficiently ef- fective, particularly in electronic systems where failure can result in significant losses. However, advancements in tin plating and treatment of substrates prior to plating continue to show promise as further reducing whisker growth. To assist equipment manufacturers of high performance systems with mitigating the poten- tial risks associated with tin whisker formation, the GEIA-STD-0005-2:2012, "Standard for Miti- gating the Effects of Tin Whiskers in Aerospace and High Performance Electronic Systems," has been developed. For mitigation strategies, this document identifies the use of hard non- conductive barriers, encapsulation, and confor- mal coating. The document also identifies the separation of tin-finished surfaces as a strategy as well as the use of tin-lead solder-covered tin surfaces with solder occurring in the reflow and wave soldering process. For the reasons previ- ously discussed, GEIA-STD-0005-2:2012 does not consider part-manufacturer-applied tin whisker mitigation methods to be sufficiently effective to stand alone. However, the docu- ment does indicate that tin-finished parts that have been subjected to part-manufacturer-ap- plied tin whisker growth mitigation methods are preferred over tin-finished parts with non- defined tin whisker growth mitigation meth- ods. In addition to mitigation strategies, the document identifies program control levels for handling tin-finished materials in electronic products. The control levels are created to al- low organizations to consider the consequences of failure and the use of appropriate tin whis- ker mitigation strategies, with the highest level, Level 3, calling for the complete prohibition of lead-free tin. While a complete ban on the use of tin, par- ticularly for safety-critical applications, would be preferred, the cost and potential risks of eliminating tin-finished parts may be prohibi- tive. Further, manufacturer mitigation meth- ods, such as encapsulation, conformal coating, and fault tolerant circuit analysis, may be suf- ficient for safe operation. However, any mitiga- tion strategy should be assessed for its effective- ness prior to implementation and should be au- dited to ensure that the proscribed mitigation method meets the defined requirements. For in- stance, the coverage of tin-finished surfaces pro- vided by current conformal coating processes may be insufficient to prevent whisker-induced Figure 2: poor coverage of acrylic conformal coating (bright areas on lead wire represent little to no coating). (Courtesy of CAlCe/university of Maryland.) Feature TIN WHISKerS reMaIN a CONCerN continues