Effect of Bi Addition on Melting Point and Melting Range of Sn-6.5Zn Solder Alloy

FIG. 3 illustrates the heating melting curves of Sn-6.5Zn-xBi solder with varying Bi content. It is evident from the figure that a single absorption peak is present in the heating curves of different brazing alloys, indicating stable thermal properties and no noticeable segregation. The corresponding peak position gradually shifts to lower temperatures with increasing Bi content, suggesting that Bi effectively reduces the melting point of the filler metal alloy. Notably, a minimum peak temperature of 198.7℃ is observed when the Bi content reaches 5.0 wt%. This considerable decrease in melting point is primarily attributed to the inherently low melting point of pure Bi.

Analysis of FIG. 1 reveals that the addition of trace amounts of Bi disrupts the 'island' primary Sn phase structure, leading to the dispersion of the acicular Zn-rich phase and the solid solution Bi phase. This microstructural evolution may contribute to the observed changes in the melting point. Furthermore, Figure 3 demonstrates that the melting range of the solder alloy (defined as the difference between the solidus and liquidus temperatures) gradually increases with increasing Bi content. Specifically, the melting range expands from 12.5℃ to 19.4℃ when Bi is initially absent. This widening melting range can significantly impact the solidification behavior of the filler metal alloy during practical applications, potentially causing a transition from layer to paste solidification and affecting the welding reliability.

In summary, the addition of trace amounts of Bi in Sn-6.5Zn filler metal effectively reduces the melting point and enhances the fluidity of the alloy. However, excessive Bi content should be avoided as it leads to an increased melting range, which can negatively impact solderability. Considering the balance between melting point and melting range characteristics, the Sn-6.5Zn-3.0Bi brazing alloy demonstrates the most promising potential for practical applications.