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Solders are metal alloys (often of tin and lead), usually with low melting points, that are melted and used to join metallic surfaces, especially in the fields of electronics and plumbing.

In electronics, tin/lead solders are normally 60/40 by weight in order to produce a near-eutectic mixture (lowest melting point - below 190°C). The eutectic ratio of 63/37 corresponds closely to a Sn3Pb intermetallic compound. According to the European Union Waste Electrical and Electronic Equipment (WEEE) and Reduction of Hazardous Substances (ROHS) directives, lead has to be eliminated from electronic systems by July 1 2006, leading to much interest in lead-free solders. These contain tin, copper, silver and other metals in varying amounts.

In plumbing, a higher proportion of lead was used. This had the advantage of making the alloy freeze more slowly, so that it could be wiped over the joint to ensure watertightness. With the replacement of lead water pipes by copper, the lead in plumbing solder was replaced by copper, and the proportion of tin increased.

Hard solder, as used for brazing, is generally a copper/zinc or copper/silver alloy, and melts at higher temperatures.

Solder is commonly mixed with, or used with flux, which is a reducing agent designed to help remove impurities (specifically oxidised metals) from the points of contact to improve the electrical connection. For convenience, solder is often manufactured as a hollow tube and filled with flux. Most cold solder is soft enough to be rolled and packaged as a coiled making for a convenient and compact solder/flux package.

See also: soldering iron