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

A Cu-based paste containing Cu formate and Cu particles was prepared for the compression-assisted 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 ^oC 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 sinter-bonding 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.