In-situ fusion process and Cu–Sn compounds evolution mechanism of nano-intermetallic compound mixed solder joints

Abstract

This study investigated the fusion and phase-transformation processes of nano-intermetallic compound (nano-IMC) mixed solder pastes. In particular, the interfacial reaction with the Cu substrate, the kinetics and mechanism of Cu6Sn5 growth inside the solder joint, and the shear strength of the solder joint were investigated. In-situ SEM and XRD results showed that the SAC305 and Sn–10Cu powders fused and the liquid solder ball completely melted at 227 °C. A higher initial Cu concentration in the mixed solder joints resulted in a lower Cu concentration gradient that inhibited the diffusion of Cu atoms. The growth activation energy of the IMC layer of a SAC305 + 20 wt% Sn–10Cu solder joint was 104.81 kJ/mol, which was higher than that of the SAC305 IMC layer (75.75 kJ/mol). During aging, Cu6Sn5 rods within the mixed solder joint formed gullies on the outer facets, and the Cu6Sn5 rods continued to grow along the [0001] crystal direction. The number of Sn grains and the proportion of high-angle grain boundaries in the mixed solder joints were higher after reflow. After aging at 150 °C for 1000 h, the shear strength of the SAC305 + 20 wt% Sn–10Cu solder joints was 34.25 MPa, slightly higher than that of SAC305 (33.12 MPa).