Factors affecting reliability of gold and copper in ball bonding

Copper and gold ball bonds were bonded on 1.2 μm thick Al-0.5%Cu-1%Si. One set of devices was aged at 175°C up to 1000 hours. A second set of devices was pre-conditioned for 168 hours at 85°C and 85% relative humidity followed by ageing at 175°C up to 1000 hours. No cratering was found with either wire in as-bonded devices but devices bonded with copper developed cratering after ageing. Neck and stitch pull strengths of copper decreased with ageing and moisture pre-conditioning slightly accelerated loss of strength. Shear strength of copper wires increased during ageing above the levels observed with gold.