Detailed investigation of ultrasonic Al–Cu wire-bonds: I. Intermetallic formation in the as-bonded state

Scanning and transmission electron microscopy were used to study intermetallic formation and the interface morphology in copper wire-bonds. The ends of copper wires were melted in air and in a protective environment to form wire-balls. The protective environment enabled formation of symmetrical and relatively defect-free copper balls, together with a smaller heat affected zone (in comparison with wires melted in air). Detailed morphological and compositional characterization of the Al–Cu as-bonded interface was conducted using scanning and transmission electron microscopy, on specimens prepared by focused ion beam milling. Discontinuous and non-uniform intermetallics were found in regions where high localized stress was introduced during the wire-bonding process. The main intermetallic phase was found to be Al₂Cu.