Stress effect on bond pad and fab of gold and copper wires / Nik Muhammad Fareez Wan Shah

This project is conducted using the explicit dynamics analysis, one of the analysis system under finite element method (FEM) to predict the stress behavior on free-air ball (FAB) of Au/Cu ball and Al bond pad during the impact stage in wirebonding process. Sometimes, there is a phenomena known as structural defect occurring on each of the FAB and the Al bond pad during the process at which each of these defects are caused by the factor of material behavior of the wire used in the process. From the founding of previous researchers regarding to the failure in wirebonding process, there are five types of major defects. Two out of five of the major defects that usually occurred are ball crack failure and the cratering or mechanical damage on bond pad and underlying material. As an effort to overcome these defects, the explicit time integration scheme study is employed to investigate the effect of two different material properties which were the gold and the copper wires. The results for these two materials are compared. The FEM regarding to the stress distribution on ball shows that the copper material has greater strength to compensate with the stress compared to the gold material as it has greater modulus of elasticity (Young's modulus) than the gold. The greater the modulus, the stiffer the material. As the copper ball is stiffer than the gold ball, the tendency of the copper ball to be cracked by the capillary is lower than the gold ball. But on the other part of the FEM regarding to the stress distribution on the Al bond pad, it shows that the copper has higher possibility to cause the cratering or crack on the Al pad and the underlay silicon die structure rather than the gold ball. Through this investigation, it is believed can help the manufacturers of semiconductor to choose which material is better to be used in wirebonding process, either the gold or the copper.