Thermo-Compression Sinter-Bonding in Air Using Cu Formate/Cu Particles Mixed During Reduction of Cu₂O

Abstract

A Cu-based paste containing Cu formate and Cu particles was prepared for the compressionassisted sinter-bonding of Cu-finished wide-bandgap power devices onto a Cu-finished substrate at a relatively low bonding temperature of 250 oC in air. A mixture of Cu formate and Cu particles was designed to mitigate the tremendous volume shrinkage during reduction of Cu formate, which approaches approximately 90%, and could be a significant obstacle in the formation of a high-density bond-line. The mixture was spontaneously formed during the 15-min reduction of the initial Cu₂O particles by a simple wet process using formic acid. In the bonding, pure Cu generated in situ from the Cu formate at a temperature exceeding 200 °C exhibited significant sinterability, and the generated hydrogen reduced oxide layers on the Cu finishes. Furthermore, the mixed particles resulted in low volume shrinkage in the bond-line during bonding, compared to the use of Cu formate particles alone. Consequently, a robust die shear strength of 22.2 MPa was achieved by sinterbonding for even 10 min at low temperature and the compression of 10 MPa, even though Cu oxide shells were formed in the bond-line because of the long sintering in air. The simple wet process provided an efficient preparation of an effective filler system before the paste formulation for the sinter-bonding.