Brittle cracks induced in AlSi wire by the ultrasonic bonding tool

It is postulated that the low wire bond pull strengths are the result of fatigue at the die heel due to the interaction of bending of the wire during looping and of high-frequency, low-amplitude ultrasonic vibration during first and second bonding. An electronic camera showed that considerable 60 kHz vibration is present at the die heel during first and second bonding. This is the same frequency as the vibration of the ultrasonic tool and is well above the resonant frequency (10-15 kHz) of the wires. This vibration exercises the first bond area and can create the possibility of embrittlement, leading to hidden heel cracks. It is suggested that the 60 kHz energy transmitted from the ultrasonics bond to the die heel makes a major contribution. It produces the requisite vertical wire flexure to account for most of the brittle fracture and for the formation of the hinge always found in the SEM (scanning electron microscope) photographs. It is concluded that die heel damage is induced by ultrasonic bonding to a varying degree in every wire, and that any crack on the bottom of a first bond heel is a potential reliability problem.<>