Though silver solder is designed mainly for bonding its namesake, it’s useful for just about any metal. The stuff ships in thin sheets which are designed to be cut into small pieces (pallions, for the picky), so you can pre-place exactly as much as you need in precisely the right location. The process is similar to brazing, but is less likely to damage fine or thin metals. A careful user can fuse two 0.025 in. copper wires end-to-end. The end result is also much cleaner than lead- or tin-soldered joints, and with proper technique, stronger as well.
You’ll need an acetylene or butane torch to work with this stuff, and you’ll need one of four grades depending on the metal you’re fusing. Easy solder has a low 1145° F melting point, and the Hard stuff, 1365° F. In a nutshell, metals with low melting points (e.g. silver) are best served with Easy solder, high melting points (steel) by Hard, and the middle of the range (brass) gets Medium, but it should be noted that silver solder does not work with aluminum.
Art supply houses carry silver solder sheets for around $5 a piece, which is enough to last quite a while. You can also find it online from Contenti or other jewelry supply houses.
Silver Solder [Contenti]