Gold Ball Bonding on Copper Substrates at Ambient Temperatures

Ultrasonic Au ball bonding on Cu substrates was performed at ambient temperatures to study the effect of process parameters on bond shear force and the extent of bonding. Response surfaces were developed to gain an understanding of the effect of ultrasonic power, bonding force and time on bond shear force and etched bump surfaces were examined to study the extent of bonding. Optimum Au-Cu ball bonds of about 65 μm in diameter with bond shear forces greater than 25 g (shear strengths equivalent to 4.5 g/mil²) were obtained. The present study showed that adequate level of ultrasonic vibrations, bonding force and time are required to obtain optimally bonded bumps. Although the application of higher ultrasonic power and possibly bonding force result in more bonded areas, they also result in excessive deformation of the bump, which may not be acceptable. Amongst the three bonding parameters studied, ultrasonic power was found to have the maximum influence on macroscopically observed bonded regions.