The role of ultrasonics in formation of bonding for solid state solder interconnections

For the purpose of process R&D on inter-die vertical connection for 3D integration, the role of ultrasonic vibration in forming a low-temperature bonding joint needs to be studied in-depth. Rather than focusing on the ultrasonic-induced interfacial reactions by molten solder, this study used low power ultrasonics to bond solder with flat Ni and microcone-structured Ni substrates respectively under solid state temperature, and interpreted the mechanisms of ultrasonics functioning on the formation of interfacial adhesion. Results of joint shear tests and interfacial observations showed that ultrasonics worked in totally different ways on two types of Ni surfaces, the transverse vibration could either promote microscopic contact between two sides, or deteriorate the joint strength if lasted too long, depending on the type of Ni used. The effect of frictional heating brought by ultrasonics on bonding formation was less prominent.