Soldering

Soldering is a fastening technique for metals, in which the pieces to be joined are heated sufficiently that a metal alloy, with a lower melting point (defined as less than 800 degrees Fahrenheit/427 degrees Celsius, wets the work, and, as they all cool, harden together. It is not as mechanically strong than brazing or welding, but has many applications, especially in electronics when the components are heat-sensitive.

Most solders (i.e., the low-melting-point filler) are made of mixtures of lead and tin, perhaps combined with a cleaning agent called a flux. The higher the percentage of tin, the lower the melting point. Specialized alloys are used for aluminum, stainless steel, and some other metals. "Silver solder", which has a higher melting point, is actually used for brazing, not soldering.

There are different ways to apply heat to the work. In all cases, the metal must be clean and shiny for soldering to work.

For small pieces, especially manual electrical assembly, the usual tool is an electrically heated soldering iron. For joining copper pipe, the pieces are usually heated with a gas flame. The basic rule is to heat the pieces to be joined sufficiently that the solder melts onto them; melting the solder and letting it flow onto colder metal gives an unreliable cold joint.

A slight variation, tinning, is useful when joining sheets of metal or heavy wire: preheat each piece and let solder spread onto it, let them cool, put them into position, and then apply heat to metal not covered with solder, so the solder melts when the work is fully heated.

When soldering copper pipe, it is convenient to slide the copper pieces together, then heat them with a torch, touching the solder to the space between them. When the work is sufficiently hot, the solder will melt and be drawn into the opening by capillary attraction. The flow of solder, once the pieces are sufficiently hot, is quite fast, and some plumbers find it an almost magical effect to watch.

Large-scale electronic work with printed circuit boards often is done with wave soldering, where the board is preheated, then passed over a bath of melted solder. This is an industrial process requiring extreme control.