Superconducting and Mechanical Properties of Low-Temperature Solders for Joints

Abstract

All large superconducting applications require electrical connections between superconductors or between superconductors and other metallic parts of the systems. Such connections are essential components for these applications and are usually made by melting the solder (or soft soldering). In general, the melting point, electrical resistivity, mechanical and wetting properties, and toxicity are all important criteria when choosing the best solder for a specific application. If the solder itself is superconducting under the operating conditions of the application, then there is also the possibility of extremely low resistance joints. In this work, we report on the superconducting properties of six low-melting-point solders, namely, Bi49Pb18In21Sn12 (Cerrolow 136), Pb20Sn34Bi46, Pb20Sn60Bi20, In52Sn48, Pb38Sn62, and Pb57Bi36Sb7, and the mechanical properties of three of the six solders (Pb38Sn62, Cerrolow 136, and Pb20Sn34Bi46). We have found that all the solders except Pb38Sn62 and In52Sn48 have relatively high upper critical fields (higher than 2 T) at temperatures lower than 3 K. Although the good ductility and high maximum fracture elongation of Pb38Sn62 is useful when using low-temperature superconductors, other solders may be preferred with high-temperature superconductors because their lower melting temperatures mean they are less likely to delaminate them.