Oxidation of Au4Al in gold ballbonds

Two different 4N (99.99% purity) gold wires were ballbonded on 1 μm thick Al–1 wt.% Si–0.5 wt.% Cu bondpad metallisation and subjected to high temperature storage (HTS) at 175 °C in air. Each wire type showed ball lift failures, Type A after 500 h and Type B after 1500 h, which in both cases was a result of Au4Al oxidation. With wire Type A the dominant compound underneath the ball was Au8Al3. A thin layer of Au4Al (≈ 1 μm thick) was observed between the Au8Al3 and the gold ball. Ball lift failures occurred in the Au4Al layer, which appeared to disintegrate due to oxidation and the resulting by products of oxidation were deposited on the underlying and unoxidised Au8Al3. With wire Type B, a double layer Au4Al was dominant after long term ageing and Au8Al3 was confined to the ball periphery. Consequently, because of the much greater volume of Au4Al, compound oxidation resulted in the formation of a large amount of a completely new microstructure consisting of gold precipitates embedded in a dark oxide matrix. The Au8Al3 compound remained unoxidised. It is speculated that internal stress and contamination may accelerate the oxidation reaction.