Strengthening mechanism of nano-Al₂O₃ particles reinforced Sn3.5Ag0.5Cu lead-free solder

To improve the properties of the eutectic Sn3.5Ag0.5Cu lead-free solder, various amounts of mixed nano-Al₂O₃ particles were added. The microstructure, thermal analysis, density, thermal expansion coefficient (CTE), and mechanical behavior were studied. The results of differential scanning calorimetry (DSC) indicate that the melting point of the composite solder doped with nano-Al₂O₃ particles is slightly higher that of the Sn3.5Ag0.5Cu lead-free solder and has a eutectic peak. The Sn3.5Ag0.5Cu composite solders exhibited lower density values and thermal expansion coefficient (CTE) values than did the unreinforced solder matrix. Compared to solder without the addition of nano-Al₂O₃ particles, the formation of dendritic β-Sn grains, the Ag₃Sn phase average size, and the spacing of lamellae decreased significantly in the composite solder matrix. The mechanical properties also improved with increasing weight percentages of nano-Al₂O₃ particles. However, the ductility of the Sn3.5Ag0.5Cu composite solder decreased. For the addition of 1 wt% nano-Al₂O₃ particles, microporosity was observed both at and along the grain boundary regions, coupled with the presence of second-phase particles (i.e. nano-Al₂O₃ and Ag₃Sn).