Best alloy for general purpose copper-copper brazing in the Sil-Fos® family. For copper-to-copper joints the phosphorus in the Sil-Fos® product serves as the fluxing agent and no seperate flux is necessary. For brass application however, flux is recommended. For use where close fit-ups cannot be maintained Sil-Fos® 15 works well to "bridge" gaps. Highest joint ductility of the entire Sil-Fos® family to best withstand the stresses inherent in refrigeration applications. Slow Flow.