Lead-free solders based on the Sn-8Zn-3Bi ternary alloy with additions of In, Nd or La

The Sn-8Zn-3Bi alloy is a new lead-free solder designed for possible replacement of Sn-Pb solders for its low melting temperature and low cost, but wettability and oxidation resistance of the alloy are unsatisfactory. This paper demonstrates the effect of the fourth additive (In, La, Nd) on properties of Sn-8Zn-3Bi solders including wetting behavior, oxidation resistance, melting behavior and mechanical properties. Wetting balance method was used to measure wetting force and wetting time on Cu substrate of the Sn-8Zn-3Bi-X solders. The results show that In, Nd and La are the effective additive elements on increasing the wetting force and shortening the wetting time, when In addition was 0.2∼1.0wt%, and the rare earth element (La or Nd) addition was 0.03∼0.2wt%. The oxidation behavior was examined by a thermo-gravimetric analyzer (TGA) for solder powders. In or Nd addition obviously decreased the weight of the oxides formed at the surface of the solder. Auger electron spectra (AES) analysis was applied to determine the identity of the oxides. The results indicate that besides ZnO, La₂O₃ or Nd₂O₃ formed at the surface of the rare earth contained solders. But there was no other oxide except ZnO existed at the surface of In contained Sn-8Zn-3Bi solders. Thermal analysis for the solders was performed by differential scanning calorimetry (DSC). According to DSC profiles for the Sn-8Zn-3Bi containing rare earth elements (La or Nd), there was no obvious change on reaction temperature. But for the In contained solders, the reaction temperatures was slightly decreased. By doping rare earth element (La or Nd), the ultimate tensile strength (UTS) of the solder increases to 97.8 MPa, and the elongation of the alloys is close to 30%. By doping In, the ultimate tensile strength (UTS) of the solder increases to 90 MPa, and the elongation of the alloys had no obviously change.