Effects of nano-Al2O3 additions on microstructure development and hardness of Sn3.5Ag0.5Cu solder

This work investigates the effects of nano-Al2O3 on the microstructure and microhardness of the Sn3.5Ag0.5Cu composite solder alloy. In comparison with solder without the addition of nano-Al2O3 particles, the formation of primary β-Sn phase, the Ag3Sn phase average size, and the spacing lamellae decreased significantly in the composite solder matrix. In addition, the eutectic areas of the composite solder were wider than that of the Sn3.5Ag0.5Cu solder. This is attributed to the adsorption of nano-Al2O3 particles with high surface free energy on the grain surface during solidification. The wettability property was improved by 0.25–0.5 wt.% addition of nano-Al2O3 particles into the Sn3.5Ag0.5Cu solder. However, when the nano-Al2O3 particles concentration up to over 1.0 wt.% decreased the beneficial influence. Microhardness improved with the addition of nano-Al2O3 particles. This improved mechanical property was due to the composite microstructure, which is close to the theoretical prediction from dispersion strengthening theory.