SMT007 Magazine

SMT-Feb2015

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26 SMT Magazine • February 2015 Since white tin has a density of 7.31g/cm3 and grey tin has a density of 5.77g/cm3, the tin pest transformation requires a significant volume expansion. This volume expansion will typical- ly destroy any solder joint composed of white tin. Figure 8 shows a photo of tin pest forming on a tin tensile specimen. The author has pub- lished a review paper [17] on tin pest that discuss- es much of what is known about the transfor- mation, occurrence, and mitigation of tin pest. If tin pest is so hard to produce, why worry about it? The main concern should be that with trillions of solder joints in automobile, military equipment, remote electronics, etc. exposed to cold environments, some tin pest could form and cause a critical failure. The fix is relatively easy—using about 0.5% antimony or bismuth in the solder. Conclusion Tin whiskers should be taken seriously for long life, mission critical products due to the extremely high risk priority number that they present in these applications. In these appli- cations, tin avoidance or multiple tin whisker mitigation techniques should be employed. A tin whisker mitigation strategy should also be employed for consumer products with lives of less than five years. However, it appears that tin whiskers are not common and the mitigation techniques discussed in this paper should be ad- equate for these applications. The most common and effective tin whis- ker mitigation technique is to ensure that the tin coating on component leads has low com- pressive stresses. This can be accomplished by using a satin bright type of tin plating. The ad- dition of a nickel layer between the copper lead substrate metal has also been shown to have a significant effect in tin whisker mitigation. The addition of 2–4% bismuth in tin is a strong fur- ther mitigation. Parylene polymer coating has been shown to be effective in containing tin whisker penetration. The combination of all of these mitigation techniques may not eliminate tin whiskers, but surely they would reduce their numbers in a striking way. Tin pest has generated almost no concern in the community that is responsible for mission- critical products. This lack of concern seems surprising. Tin pest is admittedly much rarer than tin whiskers, but with no tin pest mitiga- tion, tin pest failures in the 40-year service life of some mission critical products would seem inevitable. The tragedy of this situation is that the fix is so easy—simply adding 0.5% bismuth or antimony to the solder. SMT references 1. NASA Tin Whisker homepage. 2. NASA Tin Whisker homepage, Lyudmyla Pashchenko. 3. Woodrow, Thomas A., "Tracer Diffusion in Whisker Prone Metal Platings," SMTAI, 2006, Chicago, IL. 4. Xu, C., Zhang,Y., Fan, C. and Abys, J., "Understanding Whiskers Phenomenon: Driv- ing Force for Whisker Formation," Proceedings of the IPC SMEMA Council APREX, ppS06-2-1- So6-2-6, Jan 2002. 5. Zhang,Y., Xu, C., Fan, C., Vysotskaya, A., and Abys, J., "Understanding Whisker Phe- nomenon-Part I: Growth Rates," Proceedings from AESF Sur/Fin Conference, June 2001. 6. Xu, C., Fan, C., Vysotskaya, A., Abys, J., Zhang,Y., Hopkins, L., and Stevie, F., "Under- standing Whisker Phenomenon-Part I: Com- petitive Mechanisms," Proceedings from AESF Sur/Fin Conference, June 2001. 7. Zhang,Y., Xu, C., Fan, C., Abys, J., and Vysotskaya, A., "Understanding Whisker Phe- nomenon: Whisker Index and Tin Copper, Tin Nickel Interface," Proceedings from IPC SMEMA APEX Conference., ppS06-1-1-S06-1-10, Jan 2002. 8. Oberndorff, P., Dittes, M., etal, "Whisker Formation on Matte Sn Influence of High Hu- midity," Electronics Components and Technol- rISK aNd MITIGaTION FOr TIN WHISKerS aNd TIN PeST continues Feature Figure 8: plumbridge's photo of tin pest forming on a tensile specimen stored at 18°c for 1.5 years.

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