Impact of Ga Addition to InSn Solder on ReBCO Joint Performance

Reliable, robust, low-resistance joints are essential for long-length ReBCO conductors used in high-field superconducting magnets. Yet, common solder joints, such as those made with InSn, suffer from limited mechanical robustness and degradation. This study investigates the influence of adding gallium (Ga) as a minor alloying element (1-7 wt.%) to the eutectic InSn solder for joining commercial ReBCO (YBCO) coated-conductor tapes. Face-to-face lap joints were fabricated using In-Sn-Ga solders and characterized in terms of electrical resistance, microstructure, and mechanical properties. Adding Ga to the eutectic InSn solder decreased the melting points, which is beneficial for joining delicate YBCO tapes. The results showed the formation of a Ga-rich interfacial layer with a CuGa2composition, leading to a slight increase in the joint electrical resistance compared to binary eutectic InSn. However, mechanical testing revealed that moderate Ga contents (in the range of 1-3 wt.%) improved the joint strength by up to 46% compared to binary eutectic InSn solder. These results indicate that controlled Ga additions can mechanically reinforce the joint without severe electrical degradation, providing a practical approach towards stronger, more reliable joints for ReBCO magnet applications.