Susumu Okikawa

Development of a coated wire bonding technology

A coated wire bonding technique is proposed where unnecessary coating film is removed by arc heat synchronously with ball formation after which the wire is bonded with the chip (first bonding). The wire is then bonded on a lead while the coating film is removed by ultrasonic energy and excess heat (second handling). Highly heat-resistant polyurethane is the optimum material for satisfying the requirements of no carbonization by heat during ball formation and no heat deterioration. The gas-blowing method for ball formation is effective in preventing the capillary from becoming contaminated or clogged. Short-time discharge is effective in reducing the amount of melted coating film immediately above a ball. An average voltage resistance of insulation of 850 V and a thickness of about 1.0 mu m are sufficient for the coating film to obtain satisfactory bondability and good reliability. This insulation coating film (applied to Au wire surfaces) prevents short circuits, even if wires touch. This technology facilitates the manufacture of multipin ASIC (application-specific IC) devices. >

Ball Formation in Aluminum Ball Bonding

The effects of various factors, i.e., shield gas, current density, wire polarity, and wire material, which can influence oxidation of ball surfaces were investigated. Good quality aluminum balls were found to be formed if optimum electric and shield gas conditions were employed; for example, current density geq 2.5 GA/m2wire polarity: cathode; shield gas: Ar+H 2 . Good quality aluminum balls of 1.7 to 2.5 times the wire diameter could be formed by varying discharge time and/or current density. It was found that they could be obtained independently of the wire material. The relation between morphologies of aluminum balls and the degree Of oxidation was also investigated in order to examine the mechanism for obtaining a good quality bail. A close correlation was found between ball morphologies, i.e., eccentricity, sphericity, and constriction, and the degree of oxidation. The morphologies were significantly improved as the oxide film thickness was reduced.