Solder

Solder, any of several metallic alloys that melt at comparatively low temperatures and are used for the patching or joining of metals. Solders are commonly classified as soft and hard solders, depending upon their melting points and strengths. Soft solders are alloys of lead and tin, sometimes with the addition of bismuth; hard solders are alloys of silver, copper, and zinc (silver solder) or of copper and zinc (brazing spelter).

In joining two pieces of metal with solder, the joining surfaces are first cleaned mechanically and then coated with a flux, usually of rosin or borax. This cleans them chemically and assists the solder in making a bond. The surfaces are then heated, either with a hot metal tool called a soldering iron or with an alcohol or gas blowtorch. When the surfaces are heated to the melting point of the solder, the solder is applied and runs freely, solidifying as the surfaces cool. In the form of soldering known as sweating, the pieces to be joined are first coated individually with solder and then clamped together and heated to form the finished joint. Soldering is not significantly different from brazing and welding, except that soldering metals and alloys used for joining have less physical strength and lower boiling points.