Phase transformation of Sn-Pb-Bi solder for photovoltaic ribbon: A real-time synchrotron X-ray scattering study

The phase transformation of Sn-Pb-Bi solder for photovoltaic ribbon during soldering was studied using real-time synchrotron x-ray scattering. At room temperature, Sn and Pb crystal phases in the solder existed separately. By heating to 92°C, a new PbBi alloy crystal phase was formed, which grew further up to 160°C. The Sn crystal phase first started to melt at 160°C, and was mostly melted at 165°C. In contrast, the Pb and PbBi crystal phases started to melt at 165°C, and were mostly melted at 170°C. The useful result was obtained, that the solder's melting temperature decreased from 183°C to 170°C by addition of a small amount of Bi atoms to the eutectic Sn62-Pb38 (wt%) solder. Our study first revealed the detailed in-situ phase transformation of Sn-Pb-Bi solder during heating to the eutectic temperature. Considering the results of peel strength and hardness, adding 1 wt% of Bi atoms to the Sn62-Pb38 (wt%) solder produced an appropriate composition.