Characterisation of intermetallic growth in copper and gold ball bonds on aluminium metallization

While the characterisation of intermetallic coverage and intermetallic phase (IP) growth in gold ball bonding on aluminium is quite well understood, there is relatively little literature concerning the morphology and growth of IP´s between Cu balls bonded on aluminium pad metallisation. The difference between Cu-Al IP growth compared with the well known Au-Al IP´s has been studied mainly of larger wire diameter (35-50μm) in the early 1980´s. Cu wire ball bonding has been established for many years mainly for high power devices at wire diameters ≥ 38μm and fine wire for discrete device applications. However, there is now interest in fine pitch Cu wire ball bonding at smaller wire diameters of 25μm and smaller for high pin count applications, driven mainly by cost reduction. Development and optimisation of robust copper wire bonding processes for such applications requires an assessment of intermetallic coverage and Cu-Al intermetallic growth after isothermal aging. This work describes the problems associated with coverage determination, some characteristics of Cu-Al and Au-Al intermetallic compounds and characterises the difference in the IP growth between Au-Al and Cu-Al. The relative merits of gold and copper ballbonding are also briefly discussed.