Investigation of ultrasonic copper wire wedge bonding on Au/Ni plated Cu substrates at ambient temperature

Copper wire is attracting more and more attention in wire bonding technology due to its advantages in comparison with gold or aluminum wire. This paper presents an achievement of ultrasonic wedge bonding with 25 μm copper wire on Au/Ni plated Cu substrate at ambient temperature. A detailed investigation from the aspects of process optimization, bonding mechanism, interdiffusion, ultrasonic effects on microstructure and microhardness of the bonding materials were performed. The results show that it is possible to produce strong copper wire wedge bonds at room temperature, and the thinning of the Au layer was found directly below the center of the bonding tool with the bonding power increasing. Interdiffusion between copper wire and Au metallization during the wedge bonding at ambient temperature was assumed negligible. The wedge bonding was achieved by wear action induced by ultrasonic vibration. The ultrasonic power did contribute to enhancing deformation of the copper wire due to ultrasonic softening effect which was then followed by the strain hardening of the copper wedge bond, and the dynamic recovery or recrystallization of the copper wire caused by ultrasonic vibration during wedge bonding was also found.