Effect of ultrasonic vibration on the interfacial IMC three-dimensional morphology and mechanical properties of Sn2.5Ag0.7Cu0.1RE0.05Ni/Cu halogen free solder joints

Sn2.5Ag0.7Cu0.1RE0.05Ni/Cu halogen free solder joints were fabricated by an ultrasonic vibration (USV)-assisted soldering process. The effects of the USV power on the three-dimensional (3-D) morphology of the intermetallic compounds (IMC) and mechanical properties of the halogen free solder joints were characterized systematically. Results showed that the root-mean-square roughness (R_rms) and the mean spacing of adjacent peaks of the profile (λ_ave) of the interfacial IMC were linearly correlated with shear strength of the halogen free solder joints. It was more comprehensive to adopt the λ_ave and R_rms evaluation parameters to characterize the relationship between the interfacial IMC 3-D morphology and shear strength of the halogen free solder joints. With increasing USV power, the eutectic microstructure of the solder seam was refined, and the proportion of eutectic microstructure in the solder seam and microhardness increased. When the USV power increased to 130 W, the R_rms and average thickness of the interfacial IMC decreased (decreases of 37.6% and 56.4%, respectively) and the corresponding λ_ave and shear strength of the solder joint increased (increases of 68.7% and 45.8%, respectively). With increasing USV power, the fracture mechanism of the halogen free solder joint changed from brittle fracture to the ductile–brittle mixed fracture, and the fracture pathway transferred from the interfacial transition zone consisting of the solder seam and the interfacial IMC layer to the solder seam.