A soldering iron is a device for applying heat to melt solder for attaching two metal parts.
A soldering iron is composed of a heated metal tip and an insulated handle. Heating is often achieved electrically, by passing a current, supplied through an electrical cord or a battery, through a heating element. Another heating method includes combustion of a suitable gas, which can either be delivered through a tank mounted on the iron (flameless), or through an external flame.
Some heat up and cool down in a few seconds, but others take minutes.
For electrical work, wires are usually soldered to printed circuit boards, other wires, or small terminals. A low-power iron (15-30 Watts) is suitable for this work. In earlier days wires were frequently soldered to large chassis made of heavy metal, but this high-power requirement is now rare. Higher power is used for non-electrical metal-work.
Small battery-operated or gas soldering irons are useful when there isn’t a convenient source of electricity.
Some soldering irons have interchangeable tips for different types of work. Pyramid tips with a triangular flat face are useful for soldering sheet metal. Fine round or chisel tips are typically used for electronics work.
A soldering iron stand keeps the iron away from flammable materials, and often also comes with a sponge and flux pot for cleaning the tip. Some soldering irons for continuous and professional use come as part of a soldering station, which allows the exact temperature of the tip to be adjusted, kept constant, and displayed.
For best results, particularly in electrical work where good electrical contact is required, the iron should be used to heat the work piece, and solder applied to it: this helps to prevent "cold joints", where hot solder is applied by the iron to a relatively cold target, shrouding it in solder to look like a good joint, but without wetting it properly, and without forming a good connection.
Some electrical solder contains flux cores (the purpose of the flux is to clean the oxides off the metals to permit a good joint). If the solder is applied to the iron first then the flux is rapidly burnt off (the wispy white smoke you get from the tip of the iron) and cannot serve its purpose on the joint. In heavier applications, including plumbing, flux is normally applied completely separately.
A variety of means are used to control temperature.
The simplest of these is a variable power control, much like a light dimmer, which together with the loss of heat from the iron to the environment roughly sets the temperature.
More complex is some form of thermostat, which is often inside the tip itself, which switches power on and off to the elements.
Another approach to use magnetized soldering tips which lose their magnetic properties at a certain temperature (the curie point). As long as the tip is magnetic, it clings to the heating element. At the design temperature, it loses contact, cooling down.
Other complex irons similar to those produced under the Metcal brand, circulate a high frequency AC current through the tip, and using magnetic physics to direct heating only where the surface of the tip drops below the curie point.
The most advanced soldering irons are digital and control temperature via a microprocessor in the station that monitors a thermocouple inside the tip, increasing or decreasing the power on the tip based on the selected temperature. Brands like JBC and a few models from ERSA and Weller use this heating system.
Many soldering stations come with a sponge which is dampened, and used to wipe the hot iron's tip clean. Wet denim performs the same job.
A small amount of fresh solder is usually then applied to the clean tip in a process called tinning. A metal edge is sometimes used carefully to remove very severe oxidisation, though this risks damaging plating.
The working surface of the tip should be kept tinned (coated with wet solder) to minimise oxidation. Oxidation blocks heat transfer, corrodes the tip and contaminates the joint.
Bare copper tips that are repeatedly resurfaced, as typically found on old high power irons, can last decades in use, and the shaft of the copper tip is prone to corrosion over such long times. It is therefore best practice to remove and wire-brush the bit to remove shaft corrosion about once a decade, to maintain effective thermal contact with the iron.
With the very few irons that use a steel tip, this needs carrying out more frequently, as tip shaft corrosion (rust) is liable to break the iron.
Ready made tips for old irons are not always available. When needed, replacements are made from any suitable piece of copper, or if necessary a copper alloy.
Soldering irons can be used for many tasks that require a controlled source of heat. This will often damage the iron, or make it difficult to use for its proper purpose.