Thermodynamics of the Sn-In-Ag solder system

The lead-free solder system Sn-In-Ag was studied both experimentally and by thermodynamic modeling. Thermodynamic descriptions for the phases in Ag-In and In-Sn binary systems were optimized taking into account the available thermodynamic and phase equilibria data. They were combined with the previously assessed Ag-Sn binary system to get a complete thermodynamic description of the Sn-In-Ag system. The thermodynamic functions obtained are used to model the solidifying behavior of the alloys with diagrams of the relative amount of phases (microstructures) as a function of temperature and the effect of diffusional segregation on the solidification path. The solidification of alloys of different compositions and its relevance to the microstructure and mechanical properties of the solidified solder are discussed. In the experimental research, alloys of different compositions were heat treated and analyzed by differential scanning calorimetry (DSC), optical microscopy, and scanning electron microscopy/electron probe microanalysis techniques. Ternary alloys were annealed at 250 degrees C to gain information about the location of the phase boundaries at this rather typical soldering temperature. Diffusion couple experiments were performed to investigate the interfacial reaction between Ag and binary SnIn and to find out the diffusion paths taken by the system. The melting/solidification behavior of the alloys in the Sn corner of the ternary diagram were investigated by DSC. It was discovered that a strong segregation takes place during the cooling of some In-containing solder alloys as the In component is rejected from the solid with an excess of In remaining in the liquid. At high enough In concentrations (>16%), this results in partial melting of the solder at temperatures in excess of 113 degrees C.