Processing performance and microstructure of Sn-Zn based solders modified by Bi and mixed rare earth elements

The hypo-eutectic Sn-Zn based solder alloys were developed by adding Bi and mixed rare earth (MRE) elements in this study. The melting characteristics of the modified Sn-Zn-Bi-MRE solders were studied by differential scanning calorimetry (DSC), and the wettability of the solders on Cu substrate was evaluated by the spreading test. The microstructural features of the solders and the intermetallic compound (IMC) formation at the solder/Cu interface were characterized by SEM-EDS. Results show that the Sn-5Zn-5Bi solder has the best wettability among the Sn-Zn-Bi series solders. The addition of Bi element decreases the melting temperature evidently, while MRE has almost no influence on the melting temperature but can improve the wettability and refine the microstructure of the solders. During soldering, Cu5Zn8 IMC phase is formed at the solder/Cu interface, addition of MRE can make the IMC layer much thinner.