A solid-state bonding technique of large copper wires for high power devices operating at high temperature

Abstract

Solid-state processes of bonding 1 mm copper (Cu) wires on Cu substrates and silicon (Si) chips, respectively, were developed. To overcome Cu oxidation issue, the bonding surface on the wire was plated a silver (Ag) layer. An annealing step followed to make the Ag layer much easier to deform and conform to the Cu or Si bonding surfaces. The bonding process was performed at 300 °C with 6.89 MPa. Wire-bond cross sections were studied using optical and electron microscopy, respectively. The images obtained exhibit nearly perfect Ag–Cu bonding interface. For wire-bonds made on Cu substrate, in-plane (shear) pull test measured a breaking of force 20.7–23.7 kg, comparable to the 22.5 kg breaking force of the Cu wire itself. Breaking forces on vertical (peel) pull test are about one-half of in-plane pull test results. For wire-bonds made on Si chip, breaking forces are about 80 % of those made on Cu substrate. Fracture modes were evaluated in details. 90 % of the wire-bonds broke with three modes mixed together: near Cu–Ag plating interface, inside Ag layer, and Ag–Cu bonding interface. Thus, the bonding interface is as strong as other regions of the wire-bond. This solid-state wire bonding process is expected to have valuable applications in high power and high temperature devices and modules where the wire-bonds have to stand alone without protection due to lack of high temperature molding compound and reinforcing materials.