Effects of Bi & Ni additives on the microstructures and wetting properties of Sn-Zn-Cu lead-free alloy

The microstructures and melting behaviors of (Sn-9Zn)-2Cu (SZC) lead-free solder with 3%Bi and various amount of Ni additions were studied in the experiment. The wetting properties and the interfacial reaction of these alloys soldering on a Cu substrate were also examined. The results indicated that the addition of 3% Bi could decrease the melting point and Ni would refine the microstructures and the rod-shape Cu₅Zn₈ phase changed into square-shape (Cu, Ni)₅Zn₈ phase. By using mildly active rosin (RMA) flux, the wetting angle of Sn-9Zn was 89° while that of SZC-3Bi-1Ni was 42°. The improvement on wettability was due to the addition of the interface tension depressant Bi element and the formation of the (Cu, Ni)₅Zn₈ compound in the solder which refrained the oxidation of Zn atoms at the surface of the liquid solder. The lower surface tension lead to the better wettability. In addition, the interfacial phase of the solder/Cu joint was typically planar Cu₅Zn₈, which decreased from 4 μm for SZC to 3 μm in the SZC-3Bi-1Ni alloy.