Influence of Added Phosphorus and Gallium in Lead-free Bismuth-Tin Alloys on Wetting and Intermetallic Compounds

The aim of this work is to evaluate a wettability improvement and microstructure changes by addition of gallium and trace elements of phosphorus to novel low-temperature lead-free Bi-Sn solder alloys. Four different alloys Bi59Sn40Gal, Bi57Sn40Ga3, Bi60Sn40 and the eutectic alloy Bi58Sn42 were chosen. Furthermore, all these solders were investigated with an added small amount of phosphorus as well. For the wettability comparison, the wetting balance test in combination with three different fluxes was used. Moreover, these alloys were soldered to a copper plated test board and aged in a climatic chamber at the temperature of 80 degrees C for 24 days. Subsequently, metallographic cross-sections were made and analyzed by scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDX). The results of the wettability analysis showed the dominance of the chosen flux while soldering. However, it is still possible to draw the conclusion that phosphorus as an additive in Bi-Sn-Ga alloys supports the wetting, which is a crucial property of the solders. On the other hand, by the addition of gallium to the Bi60Sn40, the wetting force decreased. Regarding the microstructure, two different intermetallic compounds were identified. Namely, Cu6Sn5 at the interface between Cu board and alloys Bi60Sn40P and the eutectic one Bi58Sn42. The second detected BIC was CuGa2 between Cu and solder alloys with one and three weight percent of added gallium.