Die-attach on Copper by Pressureless Silver Sintering in Formic Acid

Sintered-silver die-attach has emerged as an excellent lead-free and reliable solution for high-temperature packaging. However, the most widely reported process involves pressure-assisted silver sintering on direct-bond-copper (DBC) substrates that are surface-finished with silver or gold plating, which adds to the cost of implementing this die-attach technology. One way to lower the cost is to die-attach on uncoated copper by pressure-less silver sintering. However, a challenge for this process is to prevent copper oxidation without significantly slowing the kinetics of silver sintering found in air. In this study, we explored the process of die-attach on copper by pressure-less silver sintering in a mixture atmosphere of formic-acid gas and air. The effects of oxygen fraction in the sintering atmosphere on the performance of an IGBT device bonded on an uncoated DBC substrate were examined. Strong, oxygen-free bond-line with die-shear strength over 30 MPa and excellent thermal and electrical performance of the packaged IGBT were obtained. The results in this study show the feasibility of a low-cost solution for implementing the silver sintering technology in manufacturing power semiconductor devices and/or modules.