Strengthening mechanism of electroplated Cu layer on sintered Cu joint

This study fabricated two joint types, rolled Cu/sintered Cu/rolled Cu and electroplated Cu/sintered Cu/electroplated Cu, referred to as the rolled and electroplated Cu joints, respectively. The microstructures, interconnection mechanisms, and shear performance of the joints were investigated, and effects of the substrate crystallographic and microstructural characteristics on these properties were clarified. The grain size of electroplated Cu substrate (i.e., electroplated Cu layer) was considerably smaller than that of rolled Cu substrate. The grain boundaries (GBs) in the electroplated Cu substrate were wider, providing more GB diffusion pathways for Cu atoms. During sintering, the wider GBs, higher GB density, and higher initial dislocation density of the electroplated Cu substrate affected the recrystallization of the sintered Cu layer. These features facilitated the high diffusion of Cu atoms along the GBs and dislocations from the electroplated Cu substrate into the sintered Cu layer, supplying abundant Cu atoms for recrystallization nucleation and refining sintered Cu grains. Meanwhile, in the rolled Cu joint, the sintered Cu layer tended to eliminate pores primarily via grain coalescence because of limited atom supply. Further, the rolled Cu substrate retained large grains and exhibited weak recrystallization and high dislocation density, with the subgrains not promptly eliminated. After sintering, the electroplated Cu substrate exhibited strong recrystallization, extensive subgrain elimination, and a low dislocation density. The shear strength of the electroplated Cu joint was markedly higher. These results indicate that electroplated Cu provides a simple and practical route to improve the reliability of sintered Cu joints in power devices.