Always braze 5-15% all have proven good. I use 5% on txvs and the like due to the lower melting temp. Also get the guide made by the mfr of the braze rod it will greatly help when brazing disimiliar metals like stainless to copper. I dont care what anyone says if you are not using the right stuff on stainless your joint is crap, (learned the hard way) 45% silver on stainless with silver flux is the proper combo here. I have seen mfrs like liebert use silver solder and their systems and they work fine but I still braze it it holds up better to vibration and needs less prep or flux than soldering which produces less contaminates.