Joining of CC Tapes With Lead-Free Solders

Overlap joints of the SuperPower 2G HTS tape SCS12050 with surrounding copper stabilizer were prepared, in controlled conditions, using a lead-free solder and two types of fluxes in air atmosphere. A clamping device was developed, in order to control the evolution of temperature during the process. The reflow time (during which the solder melting is expected) and the pressure exerted on the joint area were utilized as parameters in the optimization of the soldering procedure. The melting point of the used Sn96.5Ag3Cu0.5 alloy was 217 degrees C; soldering temperatures slightly exceeded the latter temperature. Quality of the joint was evaluated by measurement of the voltage-current curve, from which the joint resistance, critical current, and n-value were determined. Resilience against temperature cycling was tested on one of the samples. Electrical characterization is complemented by structural investigation with scanning electron microscopy, which was focused on the microstructure of the soldered joint layer. Surprisingly, the best joints were obtained at the lowest pressures used in these experiments. With regard to the reflow times and used fluxes, a slight reduction in the joint resistance was observed at longer duration of melting conditions and using rosin flux.