The role of Sn grain orientation in Cu depletion of Sn-based solders

Abstract

The miniaturization of solder bumps in flip-chip packages results in large variability in their microstructure making the prediction of joint lifetime challenging. This is of particular importance in Sn-rich solders due to the anisotropic diffusion behavior of Sn and the presence of fast diffusion pathways, such as grain boundaries and twins that vary from joint to joint. We developed a multi-physics phase-field model to predict the electromigration-enhanced evolution of intermetallic compounds in Cu/Sn system with different grain structures. We present simulations of solder joints with polycrystalline Sn with grains with different orientations to observe the effect of Sn anisotropy and grain boundaries on electromigration failure. The simulations predict Cu depletion in the cathode and the formation and accumulation of intermetallic compounds along Sn grain boundaries dependent on the Sn grain orientation. Additionally, Cu6Sn5 clusters are formed in the Sn matrix when high diffusion rates at the interfaces are considered.