Tin is widely used as a coating in the electronics industry because it provides excellent solderability, ductility, electrical conductivity, and corrosion resistance. Unfortunately, tin whiskers often grow spontaneously from pure tin electrodeposits and short-circuit finely pitched electrical components. Adding a few percent lead inhibits whisker growth, but environmental concerns have resulted in a demand for lead-free (Pb-free) surface finishes, and an attendant demand for whisker-mitigation strategies.
Two conditions in the tin coating are necessary, but not sufficient, requirements for whisker growth. First, a compressive stress must exist in the film. Second, energy relaxation mechanisms such as grain growth and recrystallization must be impeded, e.g., by grain-boundary pinning. We are investigating a number of strategies to prevent tin whisker formation, including the elimination of columnar grain shape through pulse deposition techniques and suppressing the intermetallic reaction of tin with the substrate. Our focused ion beam (FIB) milling has revealed this internal microstructure. We have also developed models of stress accumulation and relaxation, enabling interpretation of x-ray and cantilever beam stress measurements and a coherent description of whisker growth.