Low temperature bonding to 304 stainless steel for high temperature electronic packages

In this study, silicon (Si) chips were bonded to 304 stainless steel (SS) substrates using silver-indium (Ag-In) binary system without any use of flux. 304SS substrates were also bonded to 304SS substrates to develop low temperature fluxless processes to bond and seal two 304SS parts together. In the bonding design, Ag and In were deposited separately in layered structure. Various processes and solutions were experimented to plate Ag on 304SS. We have not found the process that could plate Ag directly on 304SS without an intermediate layer. So far, the most successful intermediate layer is nickel (Ni). Thus, Ni was plated on 304 SS, followed by Ag. The resulting 304SS substrates were annealed to increase Ag grain size if grain growth is needed for successful bonding. Nearly perfect joints were produced on Si to 304SS bonding and 304SS to 304SS bonding. The resulting joints are composed of Ag, Ag-rich solid solution (Ag), Ag3In, and Ag2In. The joints were fabricated at only 190°C of bonding temperature. The melting temperature of the joints exceeds 650°C. This new bonding process should be valuable for packaging electron devices that need high operating temperature. It is also useful for bonding 340SS parts together at low temperature.