Focused Heat Treatment after Bonding for Cu Wire Bond Reliability Improvement

Currently, there is a great interest in applying copper material to wire bonding of fine pitch assembly packaging particularly to improve reliability through minimizing the deterioration of bonded ball shear strength after thermal aging as well as cost saving purposes. However, since copper is much harder than gold, coupled with the work hardening effect of the USG power and force during wire bonding have caused bond pad crack and pad cratering, especially on sensitive pad devices. In this study, two existing solutions and one new solution were studied to resolve the bond pad crack and pad cratering problem on sensitive pad devices. One of the existing solutions is to use a heater system which continually blows heated forming gas during and after the formation of the FAB (Free Air Ball) before bonding. The purpose is to soften the FAB before bonding. Coupled with bonding parameters optimization to ensure the forces, power and time do not initiate a crack to the underlying metallization and silicone oxides structure, it was observed that as the USG power decreases, the % of cracks decreases, which is a good trend. However, a lower USG corresponds to a weaker adhesion of the bonded ball to the die pad, which is a disadvantage. Another existing solution is to optimize the heat to the die by adjusting the pedestal temperature. In this study, the wire bonding was performed at an optimized heated temperature on the die.