Investigation of the Reliability of Copper Ball Bonds to Aluminum Electrodes

The reliability of Cu/Al bonds was compared with that of Au/Al bonds to establish reliable copper ball bonding for IC's. Diffusion-controlled intermetallic compound phases of CuAl and. CuAl
2
were identified in. the interfaces of the Cu/Al bonds after isothermal aging by micro-X-ray diffraction. In addition, diffusion-controlled compound phases of Au
4
Al, Au
5
Al
2
, Au
2
Al, AuAl, and. AuAl
2
were identified in the. interfaces of Au/Al bonds. The activation energies for the formations of the compounds are 1.26 eV for Cu/Al and 1.0 eV for Au/AI bonds, respectively. The ball bonding strengths after aging of both systems are lowered as the compound layer thickncss is increased. The critical thickness of the compound layer corresponding to the degradation of bonds is about .2 µm for CU/Al and 7 µm for Au/Al bonds, respectively. The estimated degradation times for Cu/Al and Au/Al bonds at 150°C are 4 x 10
⁴
h and 1 x 10
⁴
h, respectively. Cu/Al bonds are little affected by resin, while Au/Al bonds are strongly influenced by resin. It was suggested that Cu/Al bonds possess higher reliability than Au/Al bonds. The reliability tests of resin-molded IC's using copper ball bonding have given results which are at least as satisfactory as those from conventional gold wire