Effect of Ag grain size on high temperature joint formation in Ag-In system

After many experiments in developing fluxless Ag-In joints, we realize that the success of producing a joint relates to microstructure of the Ag layer. Ag with small grains results in rapid growth of solid Ag2In intermetallic compounds through grain boundary diffusion. Thus, a joint is not obtained because of lack of molten phase, (L). To coarsen Ag grains, an annealing step is added to the Ag-plated Cu substrate. This step makes Ag grains 200 times coarser compared to the as-plated Ag. The coarsened microstructure slows down the Ag2In growth. Consequently, the (L) phase stays at molten state with sufficient time to react with the Ag layer on Si chip to produce a joint. Nearly perfect joints are produced on Ag-plated Cu substrates. The resulting joints consist of pure Ag, Ag-rich solid solution, Ag2In, and Ag3In. The melting temperature exceeds 650°C. Using the present process, high temperature joints of high thermal conductivity are made between Si chips and Cu substrates at low bonding temperature (200°C). We foresee the Ag-In system as an important system to explore for various fluxless bonding applications. This system provides the possibilities of producing joints of wide composition choices and wide melting temperature range. Present study provides preliminary but useful information on how the microstructure of Ag affects the bonding results.