Corrosion study at Cu-Al interface in microelectronics packaging

The interfacial shear (IS) force of copper ball onto aluminium-based bond pad in microelectronics packaging depends on the formation and growth of Cu-Al intermetallic. This paper reports the study on the behaviour of the IS force of the Cu-Al bonds that were subjected to pressure cooker test up to 576 h. Initially, the IS force increases with test readout point until 192 h, due to Cu-Al intermetallic growth that has strengthened the bonding. However, IS force decreases significantly from 157.4 gf at 192 It to 97.6 gf at final test readout point. 576 h. The number of shear-induced cratering shows similar reduction trend after 288 h. Result of scanning electron microscopy (SEM) on bonding morphology shows the evidence of crack at the aluminium bond periphery or outer Cu-Al interface and the cracking trend continues at higher time point. Surface analysis of ball-peeled bond pad using X-ray photoelectron spectroscopy (XPS) indicated that the cracks were due to stress corrosion cracking at aluminium that has been stimulated by copper. The concentration of CuO at the surface bonded area was found to be increased at higher readout point and reached 100% at 576 h. These results indicated that the Cu-Al bond had been weakened by stress corrosion cracking at outer bond interface and reduced the IS force. (C) 2002 Elsevier Science B.V. All rights reserved.