The Reliability and the Effect of NCA Trapping in Thermo-Compression Flip-Chip Solder Joints Fabricated Using Sn-Ag Solder Capped 40 µm Pitch Cu Pillar Bumps and Low Temperature Curable Non-Conductive Adhesive (NCA)

The effects of nonconductive adhesive (NCA) trapping on the reliability of low-temperature (150°C) thermo-compression (TC)-bonded flip-chip joints were investigated in this study. Both rough and smooth Cu pads were employed to investigate the effects of surface roughness on NCA trapping, with Sn-Ag solder-capped Cu pillar bumps bonded onto the Cu pads via low-temperature TC bonding. The NCA trapping in the rough Cu pad sample was much greater than that in the smooth Cu pad sample after TC bonding. In addition, the NCA trapping increased with decreasing bonding pressure. The electrical resistance for both the rough and smooth Cu pad samples increased after preconditioning (moisture sensitive level 3) and thermal cycling (-55°C/125°C) reliability tests. The high electrical resistance of the rough Cu pad sample was due to the crack propagation caused by the expansion of the trapped NCA. The reliability of the flip chip joint increased with increasing bonding pressure increased and decreasing surface roughness.