Wetting and spreading mechanism of CuSn pre-alloyed powder-cored composite silver solder

In this paper, the wetting and spreading process of 30% − 70% CuSn pre-alloyed powder composite silver solder on T2 copper matrix was studied, and the wetting and spreading mechanism of CuSn pre-alloyed powder composite silver solder on copper matrix was discussed. The results show that the initial contact angle decreases from 119° for 30% CuSn to 94° for 70% CuSn during the wetting and spreading process with the copper substrate. The final contact angle decreased from 15° for 30% CuSn to 7° for 70% CuSn. When the content of CuSn alloy powder in the powder core is 60%, the wetting area of the solder on the copper plate reaches 530.04 mm², an increase of about 66% compared to when no CuSn alloy powder was added. The initial and final contact angles decreased with the increase of CuSn pre-alloy powder content due to the precursor wetting effect of the low melting point CuSn pre-alloy powder, which melted on the outer skin of BAg30CuZnSn brazing material during the brazing process, forming a thin layer of molten CuSn liquid alloy and reducing the solid-liquid interfacial tension. Subsequently, the molten BAg30CuZnSn foil strip spreads over the pre-melted CuSn liquid thin layer and reacts with it by diffusion of solute atoms, eventually forming a liquid composite braze. The addition of the low melting point CuSn pre-alloyed powder improves the wetting performance of the composite silver brazing material on copper significantly. The reaction wetting interface of the composite silver braze with the copper substrate is uniform and dense with the addition of 40% CuSn pre-alloyed powder, where the white Ag-rich phases are interconnected and the Sn elements are mainly distributed in the Ag-rich phase and the surrounding tin bronze phase.