Effects of Bi Doping on Zn-Rich Phase Evolution and Physical and Chemical Properties of Sn-6.5Zn Lead-Free Solder Alloy

In the domain of electronic packaging, the application of Sn-Zn-based solders is recognized for its low melting point and satisfactory wettability. However, the reliability of solder alloy is compromised by the presence of the Zn-rich phase. This study focuses on the effect of Bi doping on the microstructure evolution of Zn-rich phase, thermal properties, wettability, oxidation resistance, and corrosion resistance of Sn-6.5Zn solder alloy. Compared to Sn-6.5Zn solder alloy without Bi, the introduction of Bi led to the trend where the size of the Zn-rich phase initially decreased and then increased, with the minimum size reaching 2.4 µm at the Bi concentration of 3.0 wt.%. Beyond the Bi addition of 3.0 wt.%, the small white dot-like Bi particles precipitating from the β-Sn matrix were observed. These particulate Bi phases tended to cluster around the Zn-rich phase, eventually forming lamellar structures. The incorporation of Bi served to lower the eutectic temperature, yet it widened the melting range. This phenomenon is attributed to the inherent low melting point of Bi; its presence extended the eutectic reaction temperature range and broadened the melting region. At the Bi content of 3.0 wt.%, the alloy demonstrated superior wettability and corrosion resistance, with corrosion products being small spherical ZnO, which is mainly attributed to the presence of finer Zn-rich phase in the alloy. When the Bi content was limited to 1.0 wt.%, the alloy showed the preferable oxidation resistance, possibly due to the Bi being dissolved in the Sn matrix after addition, leading to smaller Zn-rich phase size and finer β-Sn phase. These results indicate that the addition of an appropriate amount of Bi has a certain improvement on the physical and chemical properties of Sn-6.5Zn solder alloy, further enhancing the theoretical research of Sn-Zn-based solders, which may have an important impact on their electronic applications.