Dynamic Observation of Interfacial IMC Evolution and Fracture Mechanism of Sn2.5Ag0.7Cu0.1RE/Cu Lead-Free Solder Joints during Isothermal Aging

Dynamic observation of the microstructure evolution of Sn2.5Ag0.7Cu0.1RE/Cu solder joints and the relationship between the interfacial intermetallic compound (IMC) and the mechanical properties of the solder joints were investigated during isothermal aging. The results showed that the original single scallop-type Cu₆Sn₅ IMC gradually evolved into a planar double-layer IMC consisting of Cu₆Sn₅ and Cu₃Sn IMCs with isothermal aging. In particular, the Cu₃Sn IMC grew towards the Cu substrate and the solder seam sides; growth toward the Cu substrate side was dominant during the isothermal aging process. The growth of Cu₃Sn IMC depended on the accumulated time at a certain temperature, where the growth rate of Cu₃Sn was higher than that of Cu₆Sn₅. Additionally, the growth of the interfacial IMC was mainly controlled by bulk diffusion mechanism, where the activation energies of Cu₆Sn₅ and Cu₃Sn were 74.7 and 86.6 kJ/mol, respectively. The growth rate of Cu₃Sn was slightly faster than that of Cu₆Sn₅ during isothermal aging. With increasing isothermal aging time, the shear strength of the solder joints decreased and showed a linear relationship with the thickness of Cu₃Sn. The fracture mechanism of the solder joints changed from ductile fracture to brittle fracture, and the fracture pathway transferred from the solder seam to the interfacial IMC layer.