Reliability Study of Wire Bonds to Silver Plated Surfaces

The influence of temperature and environment on bond integrity was investigated for both aluminum ultrasonic and gold thermocompression wire bonds made to silver plate. Storage of aluminum-silver bonds at 85°C-8S-percent relative humidity (85/85) (no bias) resulted in progressive bond strength degradation and a shift in the primary failure mode from wire breaks to bond lifts. Degradation was attributed to massive aluminum corrosion. Bond degradation was not observed for the gold-silver system during storage at 85/85 (no bias) for 1500 h. In addition to humidity testing, gold-silver bonds were isothermally aged at 150, 250, 350, 450, and 540°C for a minimum of 10 min at each temperature to a maximum of 2000 h at 150°C, 1500 h at 250 and 350°C, 1000 h at 450°C, and 500 h at 540°C. Although rapid silver diffusion was observed in samples aged above 150°C, the system was determined to be mechanically reliable. An effective silver diffusion coefficient for the wire bond system was calculated from destructive wire pull data and was found to obey the equation