Oxidation of Au4Al in un-moulded gold ballbonds after high temperature storage (HTS) in air at 175 °C

BGA devices with 10,000 Å thick Al–1%Si–0.5%Cu pad metallisation were ballbonded with an experimental 4 N gold wire and subjected to HTS (high temperature storage) in air at 175 °C for up to 2000 h. As far as 1000 h pull strengths were normal with failure occurring in the neck but at 1500 and 2000 h all balls lifted away the Si chip. Cross-sections of devices aged to 1000 h showed a relatively thin upper layer of Au4Al adjacent to the ball and beneath it a thicker layer believed to be another form of Au4Al that forms by phase transformation of Au8Al3. However, in ballbonds subjected to 1500 and 2000 h HTS, cross-sections show that both layers of Au4Al transformed into new microstructures, the upper Au4Al layer into an oxide scale and the lower Au4Al compound into a two-phase microstructure of gold precipitates in a dark oxide matrix. Ball lifts occurred by separation between the scale on the underside of the ball and the two-phase microstructure adhered to the chip. The in situ phase transformation of the lower Au4Al compound appears consistent with internal oxidation. The Au8Al3 compound present at the ballbond periphery under the inner chamber did not appear to appear affected by HTS. It was concluded that the root cause of ball lifts very low adhesion between the new microstructures derived from the upper and lower layers of Au4Al.