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 1P's between Cu halls bonded on aluminium pad metallisation.The difference between Cu-AI IP growth compared with the well known Au-AI IP's has been studied mainly of larger wire diameter (35-50pm) in the early 1980's. Cu wire ball bonding has been established for many years mainly for high power devices at wire diameters >= 38pm and fine wire for discrete device applications. However, there is now interest in fine pitch Cu wire ball bonding at smaller wire diameters of 25pm 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-AI intermetallic growth after isothermal aging. This paper describes the problems associated with coverage determination, some characteristics of Cu-AI and Au-AI intermetallic compounds and characterises the difference in the IP growth between Au-AI and Cu-AI. The relative merits of gold and copper ballhonding are also briefly discussed.

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Gold ballbonding is an enabling technology in electronics packaging that accounts for around 90% of the world demand for packaged IC's. Thinner IC packaging demands lower wire loops and the stacked dies used for example in mobile telecommunications applications require a range of loop heights from very high to very low. In addition, multi-tier wirebonding also demands ultra low loop profile for its application. The lowest achievable loop heights are physically constrained by the wire dimensions and the plastic deformation behaviour of the wire. Ultra low loops result in extensive plastic deformation in the heat affected zone (HAZ). This paper examines the correlation between the wire properties (break load, modulus, hardness, grain size. HAZ length) and looping 'performance. The springback height of the lifted ball due to tension in the wire from bonding, and how it affects the looping behaviour is also discussed.

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