TAB inner lead gang bonding on Ni-Au bumps

The Tape Automated Bonding (TAB) technique is characterized by a small lead pitch, a small size and a good high frequency behaviour. Usually thermocompression gang bonding on straight wall bumps is applied for the inner lead bonding of the integrated circuit to the flying leads of the TAB-foil. As a consequence of this process, cracks are often observed in the sputtered TiW barrier layer (beneath the bump) and in the passivation layer of the integrated circuit. A theoretical model has been used to describe the deformation of the bump-lead structure. Comparison of this model with experimental results of the cracking behaviour, shows that both stress and strain at the bondpad-bump interface exceed the critical values for cracking. Plastic deformation at the bondpad is almost avoided if a two layer Ni-Au bump structure is used. While the plastic deformation required at the bond interface is kept constant, Ni layers with a thickness of at least 10 μm are required to avoid even the smallest cracks. The resulting bond strength is comparable with that of standard Au bumps if the Au layer thickness is at least 15 μm. Then deformation of the leads is restricted within acceptable limits, and the long term reliability is not affected. Accelerated testing has been performed by high temperature storage, pressure cooker and air to air temperature shock testing