Effects of Bi segregation on the tensile properties of Cu/Cu ₃Sn(1 0 0) interface

The relaxed-type tensile processes of the clean and Bi segregated Cu/Cu₃Sn(1 0 0) interfaces were investigated by the ab initio calculations based on the density functional theory. Fracture occurs at the Cu/Cu₃Sn interface which has the lowest adhesion energy and tensile strength compared to the Cu/Cu and Cu₃Sn/Cu₃Sn interlayers. The theoretical tensile strength of the clean interface is 8.04 GPa, while that of 10% Bi segregated interface drops to 4.72 GPa. According to the atomic and electronic analyzes during tensile test, failure originates from the break of interfacial bonding and the extension of charge depletion region through the interface, when the supercell stretching exceeds the critical tensile strain of 12% for clean interface and 9.5% for Bi segregated interface. Large Bi atom not only introduces interfacial distortion but also reduces the hybridization of surrounding Cu and Sn atoms, which cause the weakening of interfacial mechanical strength of SnBi solder joints.