An investigation on secondary EFO copper wire - from a nanoscale perspective view

Abstract

The aim of present research is to investigate the characteristic of secondary EFO (electronic flame-off) Pd-coated copper wire. The term of “secondary” is twice EFO performed by a bonding apparatus, K&S 1488 wire bonder. As a result, secondary EFO Cu wire demonstrates a longer heat affected zone (HAZ) and a softer free air ball (FAB) which results in a decrease in the squeeze of aluminum bond pad. It is also observed that the surface roughness is a major factor to reduce the aluminum pad squeezing. The interfacial coefficient of frictional force at a nanoscale surface level was measured by Atomic Force Microscopy (AFM). The coefficient of frictional force needs to measure both the normal force (corresponds to loading force-distance curve) and a lateral force (corresponds to friction force) on a sliding contact. In addition, mechanical property of FAB surface on Pd-coated copper wire was determined by nanoindenter. A well-defined contact area is measured to study the frictional force and friction stress. Thermal aging effect has been conducted to reduce the strength of Cu wire and increase the reliability. The study of roughness parameters corresponds to evaluate the friction and the interfacial strengths. Local variation in nano tribology is also measured. Nanotribology is crucial in describing manipulations of molecules behavior. The measured surface topography (3D profiles) are then applied to determinate the potential energy in molecular dynamic (MD) method to study the atomic scale frictional interactions. A series of experimental works and MD predictions are conducted to investigate the interfacial behavior along the Cu FAB and Al Pad.