A Study on the Bonding Process of Cu Bump/Sn/Cu Bump Bonding Structure for 3D Packaging Applications

This study reports the kinetics of the bonding process of a Cu bump/ thick Sn bonding layer/Cu bump bonding structure, popularly adopted for three-dimensional (3D) packaging, and the mechanical properties of the joints. Characterizing the bonding morphologies of the joints using scanning electron microscopy disclosed that a drastic intermetallic phase transformation occurred around the melting temperature of Sn in a manner similar to the wetting process of SnPb solder on Cu. We also delved into the mechanical behavior of the joints using lap-shear testing to illuminate that a unique feature of the joints is a lack of ductility. In addition, the samples exhibited a sensitive dependence of the joint strength on the bonding conditions (temperature and pressure).