Forming Mechanism of Cu@Ag Composite Preforms by Electromagnetic Compaction for Power Electronics

Unlike conventional solder preforms, Cu@Ag composite solder joint can be directly achieved at low temperatures and pressure by Cu@Ag composite preforms prepared through the electromagnetic compaction (EMC) technology. The high compactness of the preforms is mainly attributed to the extreme force applied to the Cu@Ag powders in a rapid time during the forming process. In this work. the rapid forming mechanism of Cu@Ag preforms was elaborated. The Cu@Ag solder preforms were prepared at different compacting voltages. By observing the microstructure and element distribution, it was found that the compactness of the Cu@Ag composite preforms improved as the voltage increased. Diffusion interconnection phenomenon occurred between different Cu@Ag particles, as well as at the Cu core/Ag shell interfaces. The finite element simulation of the compacting process revealed that the Cu@Ag particles were plastically deformed during the forming process, while the stresses and equivalent strains were concentrated in the bonding area of the particles. Moreover, molecular dynamics simulations were also used to investigate the interfacial diffusions at the particle/particle and core/shell interfaces. It was noted that the increasing voltage enhanced the diffusion degree and the compactness of the Cu@Ag solder preforms. Additionally, the dislocation evolutions and numbers were also analyzed to deeply understand the deformation mechanism during the EMC forming process.