Strengthening effects of ZrO₂ nanoparticles on the microstructure and microhardness of Sn-3.5Ag lead-free solder

A ZrO2 nanoparticle strengthened lead-free Sn-3.5Ag-ZrO2 solder was prepared by mechanically stirring ZrO2 nanoparticles into the molten melt of eutectic Sn-3.5Ag alloy. The influence of ZrO2 nanoparticles on the eutectic solidification process, in particular, the formation of Ag3Sn intermetallic compounds (IMCs) and the associated microstructure that forms and microhardness of Sn3.5Ag solder, was systematically investigated. The addition of ZrO2 nanoparticles significantly refined the size of Ag3Sn IMCs due to the strong adsorption effect of the ZrO2 nanoparticles. The refined Ag3Sn IMCs increase the Vicker's microhardness of the prepared Sn-3.5Ag-ZrO2 solder, which corresponds well with the prediction of the classic theory of dispersion strengthening.