Development of a System for Soldering of 2G HTS Tapes, Including Primary Exploratory Solder Joint Analysis at Various Process Parameters

Reliable low resistance joints are required for the practical application of second generation high temperature superconducting (2G HTS) REBCO coated conductors. This study presents a purpose built soldering platform that permits prescribed adjustment of soldering temperature, dwell time, and nominal applied normal force. Face-to-face lap joints with a 20mm overlap were fabricated using a low melting point Sn-Bi-Ag solder and evaluated by four-point resistance measurements at 77K and qualitative scanning acoustic microscopy (SAM). The measured terminal resistances of the regularly soldered specimens corresponded to apparent specific joint resistances ranging from 43.0 to 60.4nΩcm2, whereas a specimen containing a deliberately introduced interfacial discontinuity exhibited 117.8nΩcm2. Although the time averaged apparent specific joint resistance values showed only small differences among the regularly soldered specimens, these differences were comparable to the temporal measurement scatter. With the limited number of specimens tested at each processing condition, the results are therefore regarded as a preliminary assessment of the investigated soldering conditions. Two nominally identical specimens produced using the reference parameters exhibited comparable electrical results and similar SAM patterns, providing an initial indication, rather than statistical confirmation, of process repeatability. The deliberately defective specimen showed both a higher measured terminal resistance and a large acoustic contrast region. However, smaller edge related acoustic features showed no clear correspondence with the electrical results. An independent current decay measurement on a separate HTS loop with two soldered joints fabricated using the reference process conditions yielded an apparent specific joint resistance of 42.73nΩcm2, supporting the consistency of the four-point measurements. The findings demonstrate the feasibility of the platform for fabricating electrically continuous face-to-face REBCO lap joints under the investigated conditions. Additional replicated specimen supported by rigorous measurement uncertainty evaluation, critical current measurements, mechanical strength testing and cross-sectional characterization are required before an optimum process window or joint reliability can be established.