Micro structure-based stress modeling of tin whisker growth

A 3-D finite element method (FEM) model considering the elasticity anisotropy, thermal expansion anisotropy, and plasticity of beta-Sn is established. The Voronoi diagrams are used to generate the geometric patterns of grains of the Sn coating on Cu leadframes. The crystal orientations are assigned to the Sn grains in the model using the X-ray diffraction (XRD) measurement data of the samples. The model is applied to the Sn-plated package leads under thermal cycling tests. The strain energy density (SED) is calculated for each grain. It is observed that the samples with higher calculated SED are more likely to have longer Sn whiskers and higher whisker density. The FEM model, combined with the XRD measurement of the So finish, can be used as an effective indicator of the Sn whisker propensity. This may expedite the qualification process significantly.