Effect of process parameters on pad damage during Au and Cu ball bonding processes

Abstract

Cu wire bonding is one of the hottest trends in electronic packaging due to the cost and the electrical and thermal 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 ball bonding process. This high stress is not desirable as it leads to pad damage or cratering in the silicon under the pad. Another issue is pad splash where the pad material is squeezed outside the bonded area, which in severe cases can cause Al pad thinning and depletion. This paper compares the pad stress during wire bonding for equivalent Cu and Au process using state of art piezoresistive microsensors integrated at the bonding pad. 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) on the microsensor test pad while measuring the pad stress signals in real time. The measured pad stress in ultrasonic direction did not show any significant difference between the Au and Cu ball bonding process. 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, the Cu process requires higher BF and USG. Such higher settings 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 pre-bleed USG (USG level applied during impact portion of the bonding) is investigated. One of the most important findings from the DOE is the reduction of pad damage when higher pre-bleed USG levels are used.