Pd coated Cu wire bond on XoAA material in LQFP package

Abstract

Pd coated wire is increasely being used as a substitute for bare Cu wire. Being a noble metal, Pd coated wire has high resistance to oxidation enabling longer shelf life. Its chemical properties also exhibit better second bond-ability on micro PPF lead frame enabling simple bond process translating to high throughput and yield. It has higher stiffness which is able to minimize the wire sweep especially for LQFP, as well as thermo-mechnical robustness. However, there are a few challenges to be overcomed before the bonding process can be released. Basically, Pd will diffuse non-uniformly into FAB after EFO sparking. The formation of Pd-Cu alloys will increase FAB hardness resulting in higher risk of oxide crack issue. Technically, due to its physical properties, Pd coated wire will produce higher bonding impact on the bond pad in order to achieve stable and reliable 1 st bond process. Conversely, this approach is not feasible for XoAA material. A new 1 st bonding process has to be developed that can produce a stable bond yet able to meet all buy off requirements. This paper will show the study of Pd coated wire interaction with pad metallization of NiP/Pd/ Au on XoAA material. In the 1 st bond process technology development, the effect of capillary on pad structure are examined. The traditional capillary design appears to be detrimental to XoAA material. A special capillary design was introduced that incorporated a different bond mechanism with the consideration of wire properties and pad structure. An extensive Design of Experiment (DOE) is carried out to define a robust process window. New analysis method by using optical profiling was also introduced for quick and reliable assessment for pad deformation. Bond interface was also validated. Transmission Electron Microscopy (TEM) with EDX line scan analysis showed the presence of Cu-Pd at the bond interface. This study also established two criterias to control oxide crack issue. This process technology is proven and able to meet automotive requirement. In short, the requirements to achieve stable bondability and reliability has been developed in this study.