Theoretical Approach Towards Elastic Anisotropy and Strain-Induced Void Formation in Cu-Sn Crystalline Phases

The Cu-Sn alloys have been used since ancient times. At present they attract much interest since the formation and growth of Cu-Sn intermetallic compounds, namely, Cu₃Sn and Cu₆Sn₅, play an important role in the kinetics of the soldering reaction in microelectronics packaging. Their formation kinetics as well as mechanical properties has shown to be crucial for the integrity of solder joints. In this work, we report elastic properties of Cu₃Sn and Cu₆Sn₅ crystalline phases using first-principles calculations based on the density functional theory. The elastic anisotropy of these phases, which is difficult to resolve from experiments, is fully discussed. Our results show that both crystalline phases have the greatest stiffness along the c direction. In particular, Cu₃Sn exhibits in-plane anisotropy, which is associated with the lattice modulation within the superstructure. We also propose a void formation mechanism based on the computed bond anisotropy of Sn-Cu and Cu-Cu in Cu₃Sn.