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 Au 4 Al 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.

A Review of Eutectic Au-Ge Solder Joints

“…A variety of materials systems combined with Au-Ge have been evaluated in the pertinent literature, including Al, [34] Ti/Ti-W, [17] W, [20,32] and glass. [17] Lang et al showed that it was possible to bond to an Al metalization creating strong joints, > 50 MPa, and with stable electrical performance results.…”

“…[17] Lang et al showed that it was possible to bond to an Al metalization creating strong joints, > 50 MPa, and with stable electrical performance results. [34] Long-term evaluation or other stress tests of that system was not 222 performed, and one might suspect that the Al-Au-Ge 223 system may form the well-known problematic Al-Au 224 IMCs such as Al 2 Au (purple plague) and Al 2 Au 5 (white 225 plague). [55] 226 IV.…”

“…[11,12,[15][16][17][18]27,28,31,35] Thin film techniques by 231 deposition and patterning have also been evaluated, [37] 232 along with solder balls [8,9] and pastes. [34] Bonding has 233 been performed in a vacuum, inert atmospheres, reduc-234 ing atmosphere, flux, and air. 235 Recent process parameters that have been used in 236 literature are compiled in Table II…”

“…[15,26,32,33] Au-Ge joints 67 have been evaluated as materials for die-attach purposes 68 in numerous devices and configurations and various 69 other uses in recent years. Applications include silicon 70 carbide (SiC) power devices, [10,19,28,[34][35][36] SiC 71 diodes, [10,[18][19][20]26,29,32,33,35] SiC dummy dies, [15,30,31] Si 72 dummy dies, [8,9,11,12,17,22] microwave circuit, [27] MEMS 73 device, [37] for wafer bonding, [37] creating nanowires, [38] 74 forming ohmic contacts, [6,39] and hermetic seals [40,41] to 75 mention some. Ceramic substrates (circuit boards) have 76 most commonly been the substrate of choice in the 77 evaluated systems.…”

“…Ceramic substrates (circuit boards) have 76 most commonly been the substrate of choice in the 77 evaluated systems. In particular, substrates of silicon 78 nitride (Si 3 N 4 ) [12,15,17,20,30,31,[34][35][36] or alumina 79 (Al 2 O 3 ) [10][11][12]18,27,28] have been used. Other ceramic 80 substrates include AlN [15,19] and BeO.…”