Influence of temperature gradient bonding on the micromorphology and shear performance of Sn-based solder joints: Experiments and first principles calculations

In electronic packaging technology, solid-liquid interdiffusion bonding processes under temperature gradients allow for rapid preparation of intermetallic compounds (IMCs). In this study, the differences in micro-morphology and shear performance of Cu/SAC305/Cu and Cu(hot end)/SAC305/Ni(cold end) solder joints were investigated in detail under this process. And ultrasonic vibration was introduced in the process to further investigate the difference between the two kinds of joints under the synergistic effect of temperature gradient and ultrasound. The composition of IMCs was analyzed by XRD and EDS, which showed that the main IMCs in the Cu/SAC305/Cu joints were Cu6Sn5, whereas (Cu,Ni)6Sn5 was produced in the Cu/SAC305/Ni joints in addition to Cu6Sn5, and ultrasound did not change the composition of IMCs. The morphology of IMCs was analyzed by SEM and EBSD, and the results indicated that (Cu,Ni)6Sn5 IMCs had finer grains and more grain orientation in Cu/SAC305/Ni joints. And the ultrasonic treatment resulted in grain refinement and more orientation of IMCs in both joints. Shear tests revealed that the shear strength of the Cu/SAC305/Ni joints was higher, and the shear strength of both types of joints was enhanced by ultrasound. In addition, the study of (Cu,Ni)6Sn5 and Cu6Sn5 crystals using first-principles calculations demonstrated that the bulk modulus, shear modulus, and Young's modulus of (Cu,Ni)6Sn5 crystals were higher than those of Cu6Sn5 crystals, and that the addition of Ni improved the structural stability and mechanical properties of Cu6Sn5 crystals. This is the main reason for the better performance of Cu/SAC305/Ni joints.