Critical study of thermosonic copper ball bonding

Copper wire is getting well accepted as a reliable design alternative to the gold wire in the wire bonding process. In the present study, the shear strength results of the copper ball bond show better performance than the gold bond. Additionally, microhardness results also confirm that Cu ball bonds reveal a higher hardness of 111 VHN than that of initial Cu ball (84 VHN). The increased shear strength and hardness can be rationalized based on the strain hardening process due to application of the ultrasonic energy. Microstructural study of the deformed ball confirms the presence of subdivided grains (or cells) of a few microns in size. In addition, slip bands inside the cells are also observed.