Interfacial reaction and mechanical properties of Sn-Bi Cu core solder joints

Sn-Bi solder, considered to be a lead-free solder that can be used in the packaging of heat-sensitive devices, has been attracting increasing attention from researchers due to its excellent mechanical properties. The microstructure evolution, interfacial reaction, shear properties, and fracture behavior of Sn-Bi Cu core solder joints during reflow and thermal aging processes are investigated in this study. The results show that the Cu core can significantly reduce the thickness of intermetallic compound (IMC) after reflow, and no Cu3Sn is present in the IMC at the interface. As thermal aging proceeds, the morphology of Cu6Sn5 evolves from irregular island-type after reflow to layer, with the thickness increasing and showing a linear relationship with the square root of thermal aging time. The solder joints possess the highest shear strength after reflow, and the shear strength of the solder joints decreases with the progress of the thermal aging. The fracture behavior of the solder joints mainly occurs inside the solder and at the upper interface layer.