Approaches of Modeling and Simulation of Stresses in S Finishes

This chapter discusses the constitutive model of Sn grain as the foundation of finite element method (FEM) modeling. It introduces the strain energy density (SED) as the failure criterion. The chapter presents a common technique in Sn finish FEM modeling and grain rotation. It provides two FEM examples to explore the impact of the grain orientation on whisker growth in Sn finishes. The first example focuses on the elastic behavior of three immediately adjacent grains without considering the effects of grains in longer distances, while the second example considered elastoplastic behavior of a much larger group of grains. In order to describe the geometric characteristics of the textural Sn finish on a Cu leadframe, a Voronoi diagram method is used to generate the grain pattern. The simplified plasticity model is useful if the SED criterion is adapted for Sn whisker growth.