Water-Soluble Flux for Pb-Alloy Josephson Device Packaging

A new water-soluble flux using a chelate system was developed for indium(In)-based low-melting-temperature solder alloys. Rosin fluxes, which are now widely used for reflow soldering processes, are not effective below 150°C. Therefore they can not be applied to Pb-alloy Josephson device chip interconnections. It was found that 5-sulfosalicylic acid compounds were very effective for In/33 percent Bi/16 percent Sn and In/48 percent Sn alloys. The new flux was active at temperatures as low as 70°C. A water-soluble flux system of dimethylamine and tartaric acid was also examined. This flux was active below 100°C, but it attacked the Pb/In/An alloy superconducting transmission lines; furthermore, it caused growth of Pb and In whiskers on the solder bumps.