An investigation of microstructure and mechanical properties of novel Sn3.5Ag0.5Cu–XTiO2 composite solders as functions of alloy composition and cooling rate

In the present study, the influence of both TiO2 nanoparticle addition and cooling rate on the melting temperature, microstructure, and mechanical behaviour of Sn3.5Ag0.5Cu (SAC) solder alloys was studied. The composite solders were prepared by mechanical mixing of TiO2 nanoparticles with SAC solder. With the addition of TiO2 nanoparticles into the eutectic SAC alloy, a novel SAC composite solder was successfully prepared. The melting temperature for the SAC composite solders was found to be only 1.56 °C higher than that of the SAC solder, indicating that the novel SAC composite solder is fit for existing soldering process. The cooling rate and TiO2 nanoparticle addition affected the solidification of the microstructure dramatically. Notably, SEM observation of the microstructure of the SAC composite solders under the rapid-cooled condition revealed fine dot-like nano-Ag3Sn IMC in the solder matrix. The ultimate tensile strength (UTS), 0.2% yield strength (0.2YS), and microhardness of the SAC composite solder increased with the increase of TiO2 nanoparticle content by 0.25–1.0 wt.% and the cooling rate, which could be attributed to the dispersion strengthening mechanisms. However, the ductility of the composite solders was found to decrease because of microporosity at the Ag3Sn network grain boundary.