Silver on your copper or brass part is a solderability protection. The silver diffuses into the solder and you solder to the base metal, either the copper or brass in your case. Soldering to copper is well known. The issues are soldering to brass and silver diffusion in the solder in either base metal.
SOLDERING TO BRASS: Zinc from the brass diffuses into the tin causing solderability problems. So, you need a barrier plate between the brass and the tin. Use a flash of copper or nickel-plating between the brass and tin. Nickel has a longer shelf life, but copper is easier to control so we suggest you use copper.
SILVER DIFFUSION: Silver creates an even weaker intermetallic compound (IMC) with tin than does gold. We�re not sure of the critical Ag concentration. We have heard the 4 percent gold number that is bandied about for silver. Remember the 4 percent assumes a uniform distribution, but typically there is a much larger concentration near the interface. We know that 2 percent silver is safe.
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