Propagation mechanism and metallurgical characterization of first bond brittle heel cracks in AlSi wire

Bottom die heel cracks in 1.25-mil Al-1%Si wires were found to cause opens and shorts, which were attributed to brittle-looking fatigue die heel breaks. This failure mechanism has found to be directly caused by the 60-kHz ultrasonic energy vibrating the wire behind the tool during the first bond. In order to increase the frequency of these brittle cracks, hard AlSi wire was deliberately produced for this work. The frequency of the bottom cracks increased from one in a thousand to eight out of ten wires, when normal wire (19-g tensile breaking force) was replaced with hard wire (35-g tensile breaking force). The wire bonding sequence was partitioned to determine the onset of the crack and its propagation. Results show that the ultrasonic vibration during first bond produces an incipient crack that further propagates during the remainder of the wire bond cycle. Full metallographic characterization of the wire, which included optical and scanning electron microscopy, was performed in order to investigate the matrix/particle involvement in the failed specimens. The possible fracture formation and propagation characteristics are described, and the specific contribution of the AlSi alloy system is discussed