Inner lead gang-bonded devices with stacked Ni-Au bumps

Tape automated bonding (TAB) is a technique which is characterized by a small lead pitch, a small size, and a good high frequency behavior. To bond the inner leads of the TAB foil on the straight wall bumps on the integrated circuit (IC), thermocompression gang bonding is usually applied. If simple thin silicon nitride passivation layers are used, cracks are often observed in the sputtered TiW barrier layer (beneath the bump) and in the passivation layer of the IC. 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 behavior shows that both stress and strain at the bond pad-bump interface exceed the critical values for cracking. Plastic deformation at the bondpad is avoided if a two layer Ni-Au bump structure is used. While the plastic deformation required at the bond interface is kept constant, nickel layers with a thickness of at least 10 μm are required to avoid even the smallest cracks. If the gold layer thickness is at least 15 μm, the resulting bond strength is comparable with that of standard gold bumps. 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