Key Factors in Cu Wire Bonding Reliability: Remnant Aluminum and Cu/Al IMC Thickness

The characterization of inter-metallic coverage and intermetallic phase's (IMPHs) growth in gold ball bonds on aluminum bond pads are quite well understood and there is relatively a very less literature available regarding the morphology and growth of the IMPHs and remnant aluminum requirement in copper ball bonds on aluminum bond pad metallization. Development and optimization of robust copper wire bonding processes are very popular nowadays and it requires an assessment of remnant aluminum, intermetallic coverage and copper-aluminum intermetallic growth after isothermal aging under the condition of with and without epoxy molding stages. The assessment of the remnant aluminum and inter-metallic compound formation between copper-aluminum is very important in microelectronic packaging units. Because the reliability (interfacial shear force, open failure etc.) of copper ball onto the aluminum based bond pad in microelectronics packaging depends on the remnant aluminum and Cu-Al intermetallic's formation. This study reports a minimum requirement of remnant aluminum needed in the copper wire bonding on aluminum pad that were subjected to annealing for long intervals of time from 0 to 300 hrs using different purity wires (3N and 5N). This study gives information about how much Aluminum pad thickness is needed for any purity of copper wire bonding and how long the interfacial shear force will increase/decrease with the baking time and also gives information about the intermetallic's formation in between Cu-Al. The Cu-Al inter-metallic growth strengthens the bonding, the remnant aluminum is consumed and interfacial shear force is increased in the beginning and saturated latter.