A ZrO 2 nanoparticle strengthened lead-free Sn-3.5Ag-ZrO 2 solder was prepared by mechanically stirring ZrO 2 nanoparticles into the molten melt of eutectic Sn-3.5Ag alloy. The influence of ZrO 2 nanoparticles on the eutectic solidification process, in particular, the formation of Ag 3 Sn intermetallic compounds (IMCs) and the associated microstructure that forms and microhardness of Sn-3.5Ag solder, was systematically investigated. The addition of ZrO 2 nanoparticles significantly refined the size of Ag 3 Sn IMCs due to the strong adsorption effect of the ZrO 2 nanoparticles. The refined Ag 3 Sn IMCs increase the Vicker's microhardness of the prepared Sn-3.5Ag-ZrO 2 solder, which corresponds well with the prediction of the classic theory of dispersion strengthening.

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A series of Sn-Ag solders were prepared by arc melting and their phase evolution was investigated as a function of cooling rates. It was found that bulk Ag 3 Sn intermetallic compounds (IMCs) separated out only in the slowly cooled Sn-4.0Ag solder. This would be explained by the strong kinetic undercooling, arising from the rapid cooling conditions, which leads to the actual eutectic point shifts in the direction of higher Ag concentration. Thus, the eutectic and hypereutectic alloys experience a metastable hypoeutectic solidification route instead. All formed fractions of bulk Ag 3 Sn IMCs in solders, measured by quantitative microstructural analysis and thermal analysis, are larger than those predicted by the equilibrium phase diagram. The reasoning for this could be attributed to fine Ag 3 Sn phases, which cling to the primary Ag 3 Sn crystal during the eutectic reaction for their matching crystalline orientation relationship. Furthermore, the fraction of bulk Ag 3 Sn IMCs increases gradually with increasing the cooling rate in the slowly cooled Sn-4.0Ag alloy, which fits with the prediction of eutectic solidification theory: the increase of cooling rate would decrease the surface energy of fine Ag 3 Sn particles and primary Ag 3 Sn crystal, and make fine Ag 3 Sn particles cling to primary Ag 3 Sn crystal easily to form bulk Ag 3 Sn IMCs.

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