Effects of trim and form on the microstructure and whisker growth propensity of Sn finish

A microstructure study was performed on electroplated Sn finish on leadframes both before and after the forming process. Two types of plastic quad flat pack (QFP) packages of different dimensions were used. On the leadframes prior to the forming process, the Sn plating had a very uniform grain size. After the trim/form process, however, the grain size was considerably larger at the deformed areas on the leads, while the non-deformed areas maintained their grain size. This difference is believed to be caused by the recrystallization and growth of the deformed Sn plating. The same post-forming packages were also subject to two types of accelerated whisker growth tests. After the tests both the density and maximum length of whiskers in the deformed areas on the leads are reduced compared to the non-deformed areas. Considering the theory that Sn whisker growth is driven by the compressive stress buildup in the finish, these results are explained by the facts that grain recrystallization and growth relieves the pre-existing stress in the finish and reduces the triple points and grain boundaries in the plating. Based on these findings, it is concluded that Sn finishes with larger grain sizes would have less whisker growth. Dimension of the leads on packages may also affect the overall whisker growth propensity due to the difference in the size of deformation zones.