The mechanism of ultrasonic wire wedge bonding, one of the die/chip interconnection methods, was investigated based on the characteristics of the ultrasonic wire bonding joints. The Al-1%Si wire of 25μm in diameter was bonded on Au/Ni/Cu pad and the joint cross-section was analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The results indicated that it is irregular for the ultrasonic bond formation, non-welded at the centre but joining well at the periphery, especially at the heel and toe of the joint. Furthermore, the diffusion and/or reaction at the cross-section interface are not clear at C-zone, while there exists a strip layer microstructure at P-zone, and the composition is 78.96 at. pct Al and 14.88 at. pct Ni, close to the Al3Ni intermetallic compound. All these observations are tentatively ascribed to the plastic flow enhanced by ultrasonic vibration and repeated cold deformation driving interdiffusion between Al and Ni at bond interface. The mechanism of ultrasonic wire wedge bonding, one of the die / chip interconnection methods, was investigated based on the characteristics of the ultrasonic wire bonding joints. The Al-1% Si wire of 25 μm in diameter was bonded on Au / Ni / Cu pad and the joint cross-section were analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The results indicates that it is irregular for the ultrasonic bond formation, non-welded at the center but joining well at the periphery, especially at the heel and toe of the joint. Furthermore, the diffusion and / or reaction at the cross-section interface are not clear at C-zone, while there is a strip layer microstructure at P-zone, and the composition is 78.96 at. pct Al and 14.88 at. pct Ni, close to the Al3Ni intermetallic compound. All these observations are tentatively ascribed to the plastic flow enhanced by ultrasonic vibration and repeated cold deformation driving interdiffusion between Al and Ni at bond intents rface.