Whisker formation on pure Sn coating of leadframe under temperature cycling condition

Temperature cycling tests were executed to assess the reliability of pure Sn coating. After 500 and 1000 thermal cycles, Sn whiskers were found to grow initially from the thermal fatigue cracks on the surface with a length in several microns. The mismatch of the coefficient of expansion (CTE) between Sn and SnO2 produced a very large field of alternative stress in the interface of them. As a result it sheared the SnO2 and created many cracks on the surface of Sn coating. At the same time, the formation of intermetallic compound (IMC) at the interface between Sn coating and Cu substrate generated a compressive stress to some Sn grains in the coating. This compressive stress was a driving force which promoted Sn atoms out of the surface from the cracks to form Sn whiskers. At the initial stage, the growth rate of Sn whisker was slow because of the retard of oxide. By contrast, when Sn whisker broke the oxide, the growth rate of it was faster.