Phase transformation of lead-free Sn₄₃Bi₅₇Ag_x (wt%) solders for photovoltaic ribbon: A real-time synchrotron x-ray scattering

The phase transformation of lead-free Sn₄₃Bi₅₇Ag_x (wt%) solders for photovoltaic ribbon during annealing was studied using real-time synchrotron X-ray scattering. At room temperature, the crystalline phases of (tin) Sn and of bismuth (Bi) in the solder existed separately. The silver (Ag) atoms in the solder formed an alloy phase (Ag₃Sn) after reacting with the Sn atoms but not with the Bi atoms. By increasing the temperature, the crystalline phase of Sn disappeared first, followed by the crystalline phase of Bi, despite the addition of Ag atoms. We revealed the existence of an amorphous solid phase below the melting temperature (139°C) of eutectic Sn₄₃Bi₅₇ (wt%) solder. The addition of Ag atoms decreased the amorphous solid phase temperature of the Sn₄₃Bi₅₇Ag_x (wt%) solders.