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. All the failures were attributed to brittle looking fatigue die heel breaks. This failure mechanism was traced and reproduced through experimentation and found to be directly caused by the 60-kHz ultrasonic energy vibrating the wire behind the tool during the first bond. Bonds with these cracks exhibit low pull strengths (<1.0 g for annealed wires and <2.0 g on unannealed wires). In order to increase the frequency of these brittle cracks, hard AlSi wire was deliberately produced for this study. 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. The magnitude of the cracking is described at five points in the bonding cycle. 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 as well as scanning electron microscopy, was performed in order to investigate the matrix/particle involvement in the failed specimens