Driving force for the formation of Sn whiskers: compressive stress-pathways for its generation and remedies for its elimination and minimization

Compressive stress is widely accepted as the driving force for tin whisker formation. There are several pathways for compressive stress buildup in Sn coatings, which include the following: residual stress generated during plating; stress formation due to interfacial reactions between tin and copper substrate; mechanical stress; and thermal mechanical stress due to coefficient of thermal expansion (CTE) mismatch between the tin layer and substrate during thermal cycling. In order to prevent or reduce whisker growth in tin deposits, compressive stress has to be eliminated or minimized. This paper discusses the pathways for compressive stress formation and various remedies for its elimination and minimization. Particularly, a novel approach for dealing with thermal mechanical stress due to CTE mismatch is discussed.