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24 SMT Magazine • February 2015 test coupons did not represent the coating chal- lenges of the contoured surfaces of typical elec- tronic packages. One aspect seldom discussed in coating stud- ies is that the whiskers must penetrate two coat- ing layers in order to cause havoc. Many studies show whisker penetration through the coating on the surface that the whisker is growing out of. However, it is not common for researchers to discuss whisker penetration of a coating that protects another surface. One might expect the whisker to bend as it tries to penetrate the sec- ond coating surface. Considering this point and the promising results of some coatings, such as parylene, it would appear that the judicious ap- plication of the best coatings would significant- ly aid in tin whisker mitigation. Tin Whisker risk assessment Failure modes and effects analysis (FMEA) is a good strategy to use to manage tin whisker risk. The central part of FMEA is the risk prior- ity number (RPN). For tin whiskers, the RPN is equal to the product of three metrics: (1) the probability of tin whiskers (P); (2) the severity, if a tin whisker exists (S); and (3) how hard it is to detect a tin whisker (D). In equation form: RPN = P*S*D As a first example, consider a consumer product like a mobile phone with a life of five years. With mitigation, on a scale of 1–10, P might be 2. For S, we might rate at 3, as a failure in the device would not cause bodily harm to anyone. Detection (D) is a problem because the tin whiskers that form later cannot be detected during manufacturing; hence, we would have to rate D as a 10. So, the RPN is: 2*3*10 = 60, which is not too high. Therefore, a tin whisker mitigation strategy would likely be successful for a consumer product. It should be pointed out that determining the RPNs would almost certainly require supporting data, brainstorm- ing sessions, and a buy-in from the entire prod- uct team. Now consider a mission critical product, such as some types of military equipment. If we assume that the electronics have a service life of 40 years and that a failure could cause bodily harm or death, we could likely end up with a consensus that RPN = 10*10*10 = 1000, the highest RPN possible. This situation would demand that special tactics be used to address the tin whisker risk. Woody [15] presented strategies developed by PERM (Pb-Free Electronics Risk Management). PERM is a consortium of companies that devel- op and manufacture mission critical electron- ics for the military and aerospace industries. In her presentation, Woody shared PERM's "Three Bears" approach to managing tin whisker risk, as shown in Figure 7. Level 1 shows no control, while Level 3 shows tin avoidance. CALCE has developed a tin whisker risk as- sessment calculation software tool. [16] This tool would appear to be a vital aid for those with mission critical applications with the risk of tin whiskers. Tin Pest As stated earlier, it is easy to process tin plat- ing so that tin whiskers will result, and it is not difficult to minimize tin whisker occurrence with appropriate mitigation. Tin pest, on the other hand, is very difficult to produce, even in the lab. There have been numerous cases where researchers could not produce a virgin tin pest, even under ideal conditions. Tin pest is an allotropic transformation of white beta phase tin to grey alpha phase tin that can occur at temperatures below13°C. rISK aNd MITIGaTION FOr TIN WHISKerS aNd TIN PeST continues Feature Figure 7: perM's approach to control tin whisker risk.