Soldering of non-wettable Al electrode using Au-based solder

Abstract

In manufacturing three-dimensional SiC power modules, the Al electrode of SiC power devices should be soldered to the substrate. However, the Al electrode is difficult to be bonded by a solder due to the naturally formed aluminum oxide on it. In this paper, we describe an effective approach for soldering the non-wettable Al electrode by fabricating a Au-stud bump in the Al electrode together with a Au-20 wt% Sn or Au-12 wt% Ge solder. The soldering initiated at the Au bump and spreaded on the Al electrode. The soldering featured as reactive wetting, realized by the reaction of liquid Au in the Au-base solder and the Al electrode. The activation energy of the Au-20 wt% Sn soldering the Al electrode was Q = 159 kJ/mol. A continuous Au4Al layer formed at the Au-20 wt% Sn bond interface. The shear strength exceeded 60 MPa, ∼1 order magnitude higher than the required shear strength. For the bond with Au-12 wt% Ge solder on the Al electrode with a Au bump, the liquid Au-12 wt% Ge solder reacted with the solid Al electrode and formed a Au-Ge-Al solid solution after solidification. The shear strength of the Au-12 wt% Ge solder on the Al electrode with a Au bump was beyond 50 MPa. Little electrical characteristics of the SiC-SBD changed after the Al electrode was bonded to a circuit substrate using this technology.