Superconducting and Mechanical Properties of Low-Temperature Solders for Joints

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, Bi
₄₉
Pb
₁₈
In
₂₁
Sn
₁₂
(Cerrolow 136), Pb
₂₀
Sn
₃₄
Bi
₄₆
, Pb
₂₀
Sn
₆₀
Bi
₂₀
, In
₅₂
Sn
₄₈
, Pb
₃₈
Sn
₆₂
, and Pb
₅₇
Bi
₃₆
Sb
₇
, and the mechanical properties of three of the six solders (Pb
₃₈
Sn
₆₂
, Cerrolow 136, and Pb
₂₀
Sn
₃₄
Bi
₄₆
). We have found that all the solders except Pb
₃₈
Sn
₆₂
and In
₅₂
Sn
₄₈
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 Pb
₃₈
Sn
₆₂
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.