Glue selection for robust wire bonding process related to non-stick on pad

Cu wire has gained a significant interest in the semiconductor industry due to its cost competitiveness as well as its electrical and thermal performance, as compared to Au wire. When Cu wire bonding is paired with the development of wafer front side metallisation, Cu wire bonding becomes more challenging especially with the presence of variations during high volume production. Yield loss due to lifted ball bond and non-stick on pad has triggered process optimisation such as glue cure profile change, reduce bond line thickness, different wire bond capillary design and wire bond process window optimisation. The effect of glue on wire bonding process is however not yet well understood. In this study, various glue types with different modulus stated in the suppliers' technical datasheet (TDS) were characterised via dynamic mechanical analysis (DMA) to standardise the test method. Different glue cure profiles were also assessed. It is observed that prolonged cure duration and/or increased in cure temperature increases the glue modulus, though cure temperature has a more significant effect. With the glue modulus obtained from DMA, different test vehicles were modelled using 3-D finite element analysis (FEA) to investigate the effect of glue modulus, bond pad locations on die surface, die size, capillary design and bond force on die z-displacement, corresponding to the die displacement when bond force is applied vertically on the die surface during wire bonding process. Based on the simulation results, glue modulus has a significant effect on die z-displacement where higher modulus glue shows lower die z-displacement regardless of bond pad locations, die size, capillary design and bond force. This is also validated in the wire bond assessment where higher modulus glue shows thinner ball bond, bigger ball diameter and higher ball shear strength with zero non-stick on pad with 30um, 50um and 65um Cu wire.