Unusual Observations in the Wear-Out of High-Purity Aluminum Wire Bonds Under Extended Range Passive Thermal Cycling

This paper reports on the reliability of ultrasonically wedge-bonded 99.99% (4N) and 99.999% (5N) pure aluminum wires under different passive thermal cycling ranges, namely, -40°C to 190°C, -60°C to 170°C, -35°C to 145 °C, and -55°C to 125°C. The rate of bond strength degradation during cycling was found to be more rapid in the wire bonds subjected to lower peak temperatures (T_jmax) and lower temperature ranges (ΔT) for both wire types. This observed effect of ΔT cannot be described by the commonly accepted empirical relationships based on damage accumulation, such as the Coffin-Manson law. In addition, the 4N wire bonds were found to degrade more rapidly than the 5N bonds under the cycling ranges investigated. Microstructural characterization and nanoindentation of the bond interfaces indicated differences in microstructural restoration in wires subjected to the different cycling ranges. These differences have been attributed to annealing phenomena occurring in the wires during the high-temperature phase of cycling, which are believed to remove some of the damage accumulated during the low-temperature phase. A model is proposed for the prediction of wire bond wear-out rate, which incorporates both damage accumulation and damage removal mechanisms. We conclude that the rate of annealing during cycling varies exponentially with temperature; the annealing effects which occur can reduce damage accumulation and therefore influence wire bond reliability.