Role of process parameters on bondability and pad damage indicators in copper ball bonding

Cu wire bonding is one of the hottest trends in electronic packaging due to the cost and performance advantages of Cu wire over Au wire. However, there are many challenges to Cu wire bonding, one of which is the increased stress transmitted to the bond pad during bonding. This high stress is not desirable as it leads to pad damage or cratering in the Si under the pad. Another issue is pad splash in which the pad material is squeezed outside the bonded area, which in severe cases can cause Al pad thinning and depletion. To study the root cause of the increased stress, ball bonding is performed with Au and Cu wires using the same levels of ultrasound (USG), bonding force (BF), and impact force (IF). The bonding is performed on a bonding test pad with integrated piezoresistive microsensors and the in situ pad stress is measured in real time. The ultrasonic pad stress did not show any significant difference between the Au and the Cu ball bonding processes. This indicates that the cause of increased stress cannot be attributed to material properties such as hardness alone, and that the differences in bondability and bonding parameters required for the Cu process might be more influential. To achieve optimal bonding results in terms of shear force per unit area, the Cu process requires higher BF and USG settings, which are the main causes of pad damage. To understand the effect of bonding parameters IF, BF, and USG on pad stress, a detailed DOE is conducted with Cu wire. In addition to conventional bonding parameters, the effect of a non-zero USG level applied during the impact portion of the bonding (pre-bleed USG) is investigated. One of the findings is the reduction of pad damage when higher pre-bleed USG levels are used.