Effects of reflow on the microstructure and whisker growth propensity of Sn finish

The initiative of removing Pb from materials sets within the microelectronics industry has created many challenges for research and development. One of the issues is to replace the Pb-containing finish with a Pb-free finish for leaded packages. Pure Sn and alloys with high Sn content have been the leading choices for many manufacturers. These finishes, however, have the tendency to spontaneously grow whiskers on the surface, which is considered by some to be a potential reliability concern as the whiskers can continuously grow and may cause shorting between leads. In this study, leaded components with as-plated Sn finish and reflowed Sn finish are subjected to several Sn whisker acceleration tests including air-to-air thermal cycling and temperature/humidity storage. The results indicate that the whisker growth propensity on the two different finishes is rather different. Microstructure analysis of the Sn finish is also performed with scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). Significant microstructure differences are observed between the as-plated and the reflowed Sn finish including grain size, grain orientation, and intermetallic compound (IMC) thickness. As Sn whisker growth is mostly a stress related phenomenon, these findings of microstructure change are used to interpret the process of stress buildup and relief in both as-plated and reflowed Sn. Additionally, at room temperature Sn has a very anisotropic lattice (body centered tetragonal) and this property contributes greatly to the stress concentration in the Sn finish. The implication of this property on Sn whisker growth is also discussed.