there are at least two different "silver solders", one is for "hard" soldering when you heat the metal red hot with a gas flame. This solder is brass based with up to 50% silver in it. Its for jewelery model steam engines etc. Then there is silver solder for "soft" soldering for electronics, this was lead and tin based with up to 10% silver added. The reason for the silver was, that High grade electronics use lumps of ceramics as the insulator with silver contacts sputtered on and the components are soldered to these silver areas. If the solder used did not contain silver the solder would dissolve away the silver into the joint and not leave any adhering to the ceramic, so the leads would just fall of.
Frank