Soldering is a process of joining two metal pieces by means of a low temperature fusible alloy called solder applied in molten state. Solders are alloys of low melting point metals like lead, tin, cadmium and zinc. Of these tin-lead alloys are most common and are called soft-solders. A combination of 62% lead and 38% tin produces the lowest melting point and is called 60–40 solder. This corresponds to the eutectic composition of Pb–Sn series and has a fixed m.p. of 183°C. Increasing tin content produces better wetting and flow qualities. Hard solders are also available and have higher melting points.
Before applying solder, the surfaces to be joined are cleaned and a flux like ammonium chloride is used. Then the solder is melted and spread upon one surface, while the other surface is applied to it under pressure. When the solder solidifies, the two pieces get joined. The process of soldering does not call for any joint preparation. A common example of soldering can be seen in joining electrical wires of P.C.B. circuits.
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