Bismuth-based Solder Alloys: Influence of Added Phosphorus and Gallium on Wettability and Spreading

Due to the restrictions on lead solder alloy, there is an ongoing search for alternative lead-free alloys with the best possible solder properties similar to Pb-based solder alloys. Therefore, phosphorus and gallium are, for example, added to solder alloys, which could lead to an improvement in their solder properties (wettability, mechanical, lower melting point). These additives are usually used for the improvement of SAC alloys. This work focused on lead-free low-temperature solders based on tin and bismuth. The effect of added 1 wt.% gallium and traces of phosphorus on wettability and spreading was studied. These properties were observed on FR4 boards with three different surface materials: copper, copper with hot air solder leveling surface finish (HASL) and copper with electroless nickel-immersion gold surface finish (ENIG). Examined alloys were Bi58Sn42, Bi58Sn42P, Bi59Sn40Ga1 and Bi59Sn40Ga1P. The results showed that although the addition of Ga and P exhibited no significant improvement or even decrease in the wetting and spreading ability of the solder alloy on copper and ENIG surfaces, the wetting behavior of the doped alloys was better on HASL surface compared to the eutectic solder alloy.