Rosin-core solder combines a metal alloy with a pitch-like organic compound; together, the substances melt easily and adhere well to copper and other metals, creating a strong bond that conducts electricity. In particular, rosin-core solder is used extensively in building electronic circuits, connecting wires and components together cleanly and permanently.
Rosin-core solder consists of a metal tube that surrounds a thin core of rosin. Traditionally, the metal is an alloy consisting of 60 percent tin and 40 percent lead; in recent years, efforts to eliminate lead from consumer products have resulted in solders that use bismuth and other metals. Solder is available as a “wire” wrapped on a spool; this form makes it is easy to use for both hobbyists and professional technicians. It is also sold in bar form for high-volume soldering operations.
When you apply heat to solder, the metal and rosin melt together. The solder and rosin flow onto an electrical connection -- typically, copper wires twisted together. The rosin helps the solder flow onto the metal and bond with it. The solder cools after a few seconds, forming a solid, conductive joint. Because some of the rosin evaporates, soldering in a well-ventilated room is highly recommended. Always, but especially when you are finished handling traditional tin-lead solder, wash your hands thoroughly to remove traces of lead.