Low-Temperature Bonding to 304 Stainless Steel for High-Temperature Electronic Packaging

In this paper, silicon (Si) chips were bonded to 304 stainless steel (304 SS) substrates using silver-indium (Ag-In) binary system without any use of flux. 304 SS substrates were also bonded to 304 SS substrates to develop a low-temperature fluxless process to seal two 304 SS parts together. In the bonding design, Ag and In were deposited separately in a layered structure. Various processes and solutions were experimented to plate Ag on 304 SS. So far, we have not found a process that could plate Ag directly on 304 SS without an intermediate layer. At present, the most successful intermediate layer seems to be nickel (Ni). Thus, Ni was plated on 304 SS, followed by Ag. The resulting 304 304 SS substrates were annealed to increase Ag grain size if grain growth is needed for successful bonding. Nearly joints were produced on Si to 304 SS bonding and 304 SS to 304 SS bonding. The resulting joints consist of Ag, Ag-rich solid solution (Ag), Ag₃In, and Ag₂In. The joints were fabricated at only 190°C. Yet, the resulting joints exceed 660°C in melting temperature. This new bonding technique should be valuable for packaging electronic devices that need a high operating temperature. It is also useful for bonding 340 SS parts together at low temperature.