The effect of tin grain structure on whisker growth

Abstract

Tin whisker growth has become a major concern in the electronics industry as banning Lead application. In order to further understanding whisker growth with varied grain structures, three different grain structures (columnar, semi-columnar, and horizontal) and three mixed type deposits were prepared by changing the deposition conditions. The grain structure of deposited layer and intermetallics (IMC) were examined by focused ion beam (FIB) and chemical etching. After 4000 h of testing in 55 °C with 85% RH, whisker growth on the columnar sample was easily observed, and caused by the wedge type IMC. The semi-columnar tin grains formed small amounts of long straight whiskers accompanied with hillocks, and also seeing uneven IMC. The horizontal type tin grains formed only hillocks, accordingly finding the IMC more uniform than the others. Mixed grain structures were prepared, and consisted of different structures on the top and bottom layers. The top layer dominates the forms of whiskers or hillocks, owing to the grain boundary guiding the diffusion of tin atoms, and the bottom layer affects the density of whiskers or hillocks due to local stress building up from the formation of intermetallic phase.