Cardas uses a quad-eutectic formulation which has lead, tin, copper, and silver. The point to this is that you would like the solder to contain at least a small amount of whatever metal you are soldering to, so that on a molecular level you get a very good bond. When used with copper wire, silver wire, silver-plated wire, and silver-rhodium plated connectors, I feel it is the best. If you want to save some money, though, plain old 63/37 is an excellent choice (avoid the more common 60/40; it is not eutectic).
