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
