Characterization of Bi–Ag–X Solder for High Temperature SiC Die Attach

Higher operating temperature (200 °C) SiC power devices simplify the thermal management system for power electronics modules in hybrid and electric vehicle applications. However, common lead-free solders have a melting point of only 217 °C-229 °C, limiting their use in this application. In this paper, a Bi-Ag-X solder paste with a solidus point of approximately 265 °C was characterized on direct bond copper (DBC) and CuMo reactively brazed alumina substrates. The Cu and CuMo surfaces were plated with Ni:B and Au. During high-temperature storage (200 °C) of test vehicles on DBC, there was an initial decrease in die shear strength followed by relatively constant die shear strength with further aging (through 5000 h). The initial decrease in shear strength was determined to be the result of NiBi3 intermetallic formation. The formation of the intermetallic layer was limited by the thickness of the initial Ni plated on Cu or CuMo. Similar results were observed for CuMo samples thermal cycled (-55 °C to +195 °C) up to 2000 cycles.