Transient liquid phase bonding using Cu foam and Cu–Sn paste for high-temperature applications

This study presents a new structure for transient liquid phase (TLP) bonding using Cu foam and Cu–Sn paste. Cu foam and Cu–Sn paste with large reaction surface areas were used to reduce the long bonding time, which is a disadvantage of the conventional TLP bonding process. After applying the Cu–Sn paste at an optimized ratio to the top and bottom of the Cu foam, pressure was applied to fill the pores inside the Cu foam with the paste, and a bonding process was performed at 260 ºC under a pressure of 3 kgf. Bonded joints composed of only the Cu skeleton and Cu–Sn intermetallic compounds (IMCs) were confirmed within a bonding time of 5 min. In addition, the proportion of the Cu3Sn phase in the joint increased with the bonding time. After a bonding time of 40 min, the joint was mostly composed of a Cu skeleton and Cu3Sn, with a measured shear strength of approximately 39.2 MPa. After the joint analysis of the as-bonded samples, a high-temperature storage test (HTST) was conducted to evaluate their high-temperature long-term reliability. As a result of HTST, because most of the joint was converted to Cu3Sn after the bonding process, the shear strength did not decrease despite heat treatment at a high temperature of 240 ºC, and a similar microstructure of the joint was maintained.