Tin-whisker formation

Now that Europe s RoHS legislation restricts Pb to a level of less than or equal to 1,000 parts per miUion by weight in electrical and electronic assemblies, there is much more focus on tin whiskers. Traditionally, many electrical components had tin-lead surface finishes, as did printed wiring boards (HASL). In cases where pure tin finishes were used on components, small additions of lead were added. The resulting lead-doped tin lattice imparted some compliance to the structure and rednced the risk of tin whisker formation. For the vast majority of electrical and electronic prodncts, that is no longer an option for many classes of components. 

J. Liang, X. Li, Z. Xu, and D. Shangguan, Indentation Induced Tin Whisker Formation on Tin Plated Component Leads, Proceedings of ASME InterPack 05, July 17-22, 2005, San Francisco, CA... 

There are several postulated mechanisms for tin whisker growth discussed in detail in Chapter 21, including recrystallization due to strain aging. The main driving force for tin-whisker formation is believed to be stress. The source of the stress can be either internal (i.e., created within the... 

One might conclude that pure tin is the leading high-volume candidate to replace Sn-Pb as a Pb-free component finish as it is abundant, cost-effective, and fairly easy to plate. However, the key concern in utilizing pure Sn as a Pb-free component finish is tin-whisker growth, as mentioned previously. However, there are efforts to mitigate and hopefully eliminate tin-whisker formation. The subject of tin-whiskers is discussed in more detail in Chapter 21. 

The author wishes to thank Dr. Yun Zhang of Cookson Electronics for useful discussions on tin whisker formation. 

Xu, C., et al. Understanding Whisker Formation—Driving Force for the Tin Whisker Formation, Proceedings of APEX 2002 Technical Conference Long Beach, CA, Jan. 2002 506-2/2-4. 

Schetty, R. Minimization of Tin Whisker Formation for Lead-Free Electronics Finishing, Proceedings of IPC Works Technical Conference, Miami, FL, Sept. 2002 5-02-3/3-5. 

Other possible lead finishes such as pure Sn, Ag, Ag Bi, Ag Pd, etc. are available. However, it is important to address such issues as tin whisker formation and silver migration before deciding on a given lead finish composition. Potential reliability-related aspects of finishes must be understood. The mechanisms and kinetics of potential tin-related reliability issues are discussed in detail in Chaps. 21 and 22. 

The Structure and Kinetics of Tin-Whisker Formation and Growth on High Tin Content Finishes... 

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