SMT007 Magazine

SMT-Feb2015

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82 SMT Magazine • February 2015 electroplate IO finishes to mitigate the risk of whisker growth. Annealing the matte finish is also reported to further reduce the risk by re- ducing the stress forces that produce whiskers [7] . Tin is a popular surface finish for PCBs used in automotive electronics. During the days of eutectic tin-lead solder, it proved to be a very solderable finish, without the issues of co-planarity associated with tin-lead HASL. However, with the advent of lead-free tech- nology, soldering the back- side of a once reflowed board could mean soldering to Cu 6 Sn5 if the PC fab shop was in a hurry with the immer- sion tin process and did not allow a sufficient layer thick- ness of tin to form over the copper substrate. The forma- tion of Cu6Sn5 underneath a tin finish also causes stress on the tin. Cu6Sn5 has 58% more molar volume than the base metals used to form the compound [9] . Producers of the tin chem- istry also have developed matte tin finishes that lower the chances of whisker formation in any areas not covered with solder. The remelting/annealing associated with SMT and through hole processing also re- moves stress from the tin surface finish. Conformal coatings have also been shown to inhibit the effect of tin whisker formation [4] . Parylene conformal coating may not eliminate the formation of whiskers, but one study shows that it prevents whiskers from breaking through the parylene coating, eliminating shorts or other issues with whisker formation. Urethane conformal coatings have also shown to be effec- tive in eliminating the effect of whisker growth. Dipping assemblies in conformal coating also eliminate shadow effects associated with spray application of conformal coating. Even if a whisker forms, and breaks through the confor- mal coating, it will not create a short circuit if it contacts an adjacent component lead covered with the coating. In summary, tin whiskers can cause cataclys- mic failures in long life, high-reliability circuits. When developing an approved material list for lead-free assemblies, OEMs who need to protect their brand names should consider specifying SAC alloy solder, matte tin plated compo- nents, matte tin surface finish and the use of parylene or urethane conformal coatings to reduce or eliminate the risks associated with tin whiskers. SMT references 1. NASA tin whisker homepage. 2. Leidecker, et al. "Electri- cal Failure of an Accelerator Pedal Piston Sensor Caused by a Tin Whisker…." 5 th Inter- national Tin Whisker Sympo- sium, September 2011. 3. P. Daddona, "Reactor Shutdown: Dominion Learns Big Lesson from a Tiny Tin Whisker," The Day (New Lon- don, CT), July 4, 2005. 4. T. Woodrow and E. Led- bury, "Evaluation of Conformal Coatings as a Tin Whisker Mitiga- tion Strategy," April, 2005. 5-. M. Osterman, "Mitigation Strategies for Tin Whiskers," August, 2002. 6. Yaowu Shi and Hu Hao, "Tin Whiskers with Special Morphology." 7. Y. Zhang, C. Xu, J. Abys, A. Vysotskaya, "Understanding Whisker Phenomenon, Whis- ker Index and Tin/Copper, Tin/Nickel Inter- face," IPC APEX EXPO 2002. 8. C.Hillman, G. Kittlesen, R. Schueller, "A New (Better) Approach to Tin Whisker Mitiga- tion." Mitch Holtzer is global director of customer technical service (CTS) for Alpha. He may be reached at mholtzer@alent.com. His column, across the board, will be appearing monthly in SMT Magazine. MINIMIzING THe rISK OF TIN WHISKer FOrMaTION IN Lead-Free aSSeMbLIeS continues aCross the Board parylene conformal coating may not eliminate the formation of whiskers, but one study shows that it prevents whiskers from breaking through the parylene coating, eliminating shorts or other issues with whisker formation. urethane conformal coatings have also shown to be effective in eliminating the effect of whisker growth. " "

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