Brittle cracks induced in AlSi wire by the ultrasonic bonding tool vibration

Brittle first bond heel breaks in AlSi wire have been observed on 68 lead cerquad flat packs at two assembly houses. These low pull strengths of 0.1-2.5 g (annealed) all have the same appearance. Scanning electron microscope photographs of the fractured surfaces after pull testing all show a grainy brittle structure above and below a center ductile line. These brittle heel cracks start at the bottom of the heel and can be observed before wire pull. Considerable 60 kHz vibration is consistently present at the first bond heel during the first and second bonding either on the chip or frame. 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 bottom heel cracks especially with wire with elongations of <1%. It is concluded that die heel damage is induced to a varying degree in every wire, and that any crack on the bottom of a first bond heel is a reliability problem.<>