Microstructural evolution of ultrasonically bonded high purity Al wire during extended range thermal cycling

This paper concerns the reliability of ultrasonically bonded high purity thick aluminium wires at elevated temperature. To date, the evolution of the microstructure of wire bonds during thermomechanical exposure and its influence on reliability have not been fully characterised and understood, particularly as they pertain to thermal cycling regimes which exceed 125 °C. Shear testing, indentation hardness and fine-scale microstructural data are reported here which show that the rate of wear-out can be influenced not only by the thermal cycling range (ΔT), but more importantly by the maximum temperature and duration to which bonds are exposed. There is evidence that significant annealing occurs during thermal cycling regimes with high Tmax values, which results in the removal of some of the damage accumulated and a reduction in the rate of crack propagation. The rate of bond degradation is also found to be faster for 99.99% (4 N) than 99.999% (5 N) pure Al wires. Analysis of the two wire compositions after thermal cycling suggests that this difference could be attributable to a difference in their creep resistance. In conclusion, our findings suggest that high purity Al wire bonds may be suitable for operation at temperatures which exceed 125 °C.