Experimental Research of Copper Wire Ball Bonding

The bonding procedure of copper wire ball bonding on an aluminum-metallized silicon substrate was investigated in this paper. The four crucial parameters: ultrasonic power, impact force, frequency duration and bonding temperature were optimized. It was found that the process window was quite narrow, thus the bonding process was less stable. In conjunction with the results of the process optimization, the effect of all the parameters on the bonding quality and mechanisms were investigated. The SEM analysis results of FAB (Free Air Ball) showed the solidification proceeded from the ball end towards the wire end while the orientation of the cell-type fine substructures was irregular. This was attributed to a rapid solidification of Cu during the electric sparking which was protected by mixed reducing gas. The result of SEM analysis of copper bonds showed the FAB underwent a large plastic deformation during the bonding procedure. The present work indicated that slip was the major mechanism involved in the overall deformation of polycrystalline copper, although in very limited bonds the twinning could also be found. The research results also approved that the plastic deformation benefited the bonding procedure. The results of annealing test showed that, the shear force of the ball bonds didn’t degenerate after 1500 hours in 200°, while it had some extension of increasing. This was attributed to the interface diffusion of the bonds. It was believed that some extension of interface diffusion was good for bonding quality.