Influence of Substrate Surface Roughness on Tin Whisker Growth

Implementation of Pb-free electronics has resulted in use of pure Sn surface finishes which are known to pose reliability issues due to the spontaneous growth of Sn whiskers. In this study, atomic force microscopy (AFM) has been used to investigate the effect of surface roughness on Sn whisker growth. The varying (brass) substrate roughness conditions have been created by an unpolished, an electrochemically-polished, and a mechanically-polished surface. Subsequent Sn deposition using magnetron sputtering produced a thin (~6000 Aring) Sn film on the brass. Results show that the smoother brass substrates produced significantly more Sn whiskers than rougher surfaces. Both the quantity of whiskers is greater and the average whisker length is longer as the brass smoothness increases. This is contrary to conventional wisdom which presumed that rougher surfaces offered more film stress and enhanced whisker growth.