For soldering of metal pipe, typically copper or brass, see sweating.

Soldering is a method of joining metal parts using a filler metal of low melting point (solder) below 450 C (800 F). Heat is applied to the metal parts, and the soft filler metal is pressed against the joint, melts, and is drawn into the joint by capillary action and around the materials to be joined by so-called 'wetting action'.

Soldering is a common and very reliable way to mount electronic components on a circuit board, and is also sometimes used to connect electric wires to devices like switches and relays. The resulting joint has good electrical conductivity, is corrosion resistant and mechanically robust.

Electronic soldering is usually done with a soldering iron of 15-40W, using soldering tin with inlaid flux. Before soldering, let the soldering iron heat up fully. If needed, apply some solder to the tip to aid in heat transfer. The parts to be soldered should be clean and free from corrosion, if needed they an be cleaned with some steel wool, fine sandpaper or similar beforehand.

To solder a joint, apply the tip of the soldering iron so that it is in direct contact with the parts that are to be joined. Apply solder to the joint, it will then melt and flow into the joint when the parts have become hot enough. Remove the soldering iron and let the joint cool off. It is important that the parts are kept still while the joint cools, or the joint will become weak.

Common mistakes:

  • Cold joints - the solder was applied directly to the soldering iron, or one of the parts wasn't heated enough. The solder will bead up instead of sticking to the surface.
  • Fractured joints - the joint was moves while it was cooling. The solder will be a dull gray instead of shiny.
  • Excessive solder - too much solder was applied. Solder may have run or even contacted a neighboring joint, creating a short.

In some cases the joint can be fixed simply by reheating it and leaving it to cool, but often it's better to remove the tin (using a desoldering wick or pump) and then solder the joint again.