Optimization of copper wire bonding on Al-Cu metallization

This paper reports the successful implementation of copper wire ball bonding for selected TO-220 devices on a high volume commercial scale. Since August 1992, copper wire bonding has been used in production at National Semiconductor Corp. The development of copper wire ball bonding involves a three-prong approach: optimum pad metal composition, modifications to the wire bonder and optimization of the assembly parameters. the critical material parameter is bond pad hardness. This needs to be above a critical threshold value to avoid silicon cratering. The metal composition best suited for the wire bonding process is sputtered Al-Cu(2%). Typical production yields of 99.8% at lead bond are obtained with 1.5 mil (37.5 μm) copper wires, with ball shear and wire pull averaging 100±20 gms and 15±2 gms, respectively. Five issues related to copper wire bonding of TO-22 power IC packages are discussed: 1) typical bonding failure modes; 2) the relation between bond pad composition and hardness; 3) the influence of the metal deposition systems; 4) the optimization of bonding conditions; 5) the reliability of the copper wire bonded devices