Effects of lead on tin whisker elimination efforts toward lead-free and whisker-free electrodeposition of tin

Spontaneous tin whisker growth is one of the direct consequences of complete removal of Pb from tin electrodeposits. To pursue a greater understanding of the whisker growth mechanism, the effects of Pb on the properties of matte tin electrodeposition at different alloy compositions were investigated with respect to cathodic polarization, crystallographic and microstructural characteristics of the deposits, and the formation of Cu-Sn intermetallic compounds (IMC) under ambient conditions at the interface between the Sn or SnPb films and the Cu substrate. Furthermore, internal stress of pure Sn and 60Sn-40Pb deposits was measured as a function of storage time using the bent strip method. The observations strongly suggest that Pb prevents tin whisker growth due to the formation of an even Cu-Sn IMC. In turn, upon the IMC growth the SnPb deposit exhibits lessening of compressive stress. It is evident that the presence of Pb enhances the lattice diffusion of Cu and increases the ratio of Cu-Sn IMC formed through lattice diffusion and grain boundary diffusion. Other contributing factors such as predominant tin [220] textures and the equiaxed grain structure may also play supporting roles in the whisker retarding effect of Pb.