Effects of ultrasonic vibration on undercooling and microstructures of SAC305 alloy

One model experiment in this study was conducted to investigate the influences of ultrasonic vibration (USV) on solder joints by fabricating Cu/SAC305/Cu sandwich structure samples. The samples were first heated to 270°C and then cooled in air to 250°C, at which temperature USV with five different powers were applied to samples during their cooling procedure. Undercooling is significantly decreased as the power increase, while the soldering quality improved. The grain size is significantly refined under the ultrasonic treatment, with a reduction of almost 90% from 100μm to 10μm. And Cu₆Sn₅ length scope decreases from more than 200μm (0W) to 10~100μm as the ultrasound power increases. However, the Sn-rich phase changed from dendritic type to equiaxed cell as the power increase. The average width of β-Sn dendrite is about 5 μm, while the diameter of Sn-rich cell grows to almost 50 μm after 267W USV treatment. Besides, the Ag₃Sn phase translates from needlelike to plate form as the ultrasound power increases.