Effect of Polarity on Heat Transfer in the Ball Formation Process

Ball formation is the first step in wire bonding of semiconductor chips to their lead frames. An electric discharge melts the wire and surface tension causes the melt to roll up into a ball; the ball is subsequently pressed onto the proper bond pad on the chip to make a ball bond. In the ball bonding process, the electrical discharge characteristics, the associated heat transport, and the wire material determine the nature of the ball formed. The discharge shows sensitivity to the polarity of the wire. Experiments with upscaled aluminum and copper wires in air at reduced pressure show that the heat transfer to the wire associated with the discharge is significantly greater when the wire is the anode than when the wire is the cathode. These experiments included temperature measurements in real time made by fine thermocouples implanted in upscaled wires. In aluminum the anode wire receives twice as much energy from the arc as the cathode wire. In copper the anode wire receives three times as much energy from the arc as the cathode. Our conclusions are also reinforced by metallographic examination of sectioned balls.