Role of impact ultrasound on bond strength and Al pad splash in Cu wire bonding

Cu wire is replacing Au wire in the microelectronic industry due to its lower cost. However, during Cu ball bonding one of the main challenges is the increased stress that can damage the pad and underpad layers. Past work showed that using ultrasound super-imposed together with impact force (pre-ultrasound) results not only in a softer bonded ball, but also in a flatter ball/pad interface. In this study, Cu ball bonding processes are optimized with five levels of pre-ultrasound. The wire material is 99.99% pure Cu wire, 25.4μm in diameter. It is shown that by using pre-ultrasound of 37.5% bonds with adequately high shear strength (120MPa) are achieved and the amount of splash is reduced by 31%. Using pre-ultrasound allows for lower bonding ultrasound levels that result in less stress on the pad and underpad materials.