Parameters Optimization of Copper Wire Bonding on Thin Small Outline Package

With significant rising of Au price, there is growing demand to implement Cu wire bonding as an alternative method for Au wire bonding. However, there are several challenges to overcome because Cu is easily oxidized and its property is harder than Au. The oxidation deteriorates bondability and bond reliability, while the greater wire hardness leads to the more severe wire bonding parameters (higher force, higher power, higher temperature), which results in physical damage of the bond pad and underlying layers. This paper reveals wire bonding parameters optimization in order to implement Cu wire bonding in high volume production environment with good bondability and bond reliability. The experiments were focused on Cu free air ball (FAB) formation and ball bonding process. The parameters of interest were Electronic Flame-Off (EFO) gap, flow rate of mixed gas 95% N2/5% H2 during FAB formation and force profile. The response to be characterized were FAB surface, FAB shape, FAB diameter, bond integrity testing (BIT) including Al splash and Al remaining after bonding. The EFO gap 20-35 mils, the mixed gas flow rate 0.5-0.7 L/min and the force profile with ramp-down force and low ultrasonic generator (USG) power were determined to be an optimum parameter concept for the selected product. This parameter set was applied to bonding process of Cu wire 0.9 mils diameter on AlCu0.5% metallization pad 0.65 um thickness, assembled in thin small outline package (TSOP). Bond reliability after environment stress tests was characterized through BIT, Al splash and Al remaining on pad. Electrical performance of Cu wire product was also characterized and compared to the result of Au wire product.