Solder

Solder is a metal alloy that has a low melting point and is used to join two pieces of metal. It can be used to make a physical joint as in plumbing, or an electrical one as in electronics.

Solder is usually 60/40 tin and lead, though 63/37 tin and lead is also offered because it melts at a slightly lower temperature, and refreezes more suddenly.

Lead free solder is available, and required by law on consumer electronics in the European Union. It sometimes avoids the use of lead, which is hazardous, by using other hazardous metals like bismuth, antimony, and cadmium. Alloys of copper, tin, and silver are also available. It melts at a higher temperature than regular solder, leading to a higher incidence of cold solder joints and more heat stress on components and equipment.

Some solder has flux in it, rosin or acid. Only rosin should be used on electronics. The function of flux is to clean and deoxidize the metals to be joined. Fumes from flux being consumed are toxic.

Some solder has silver in it. Some people think it makes a better electrical conductor.

Cold solder joints are bad. They look dull, clumpy, and can cause electronics to fail. For this reason military and medical electronics use solder with lead in it.

Soldering irons are used to melt solder. They come in various wattages and some have adjustable temperature settings. The tips can be interchangeable, and can be cleaned with a damp sponge or a ball of wire. They come in pen or gun styles.

In French "solder une dette" means to pay a debt off entirely. Go figure.