thumb|Electric soldering iron
thumb|A gas-fired soldering ironA soldering iron is a hand tool used in soldering. It supplies heat to melt solder so that it can flow into the joint between two workpieces.
A soldering iron is composed of a heated metal tip (the bit) and an insulated handle. Heating is often achieved electrically, by passing an electric current (supplied through an electrical cord or battery cables) through a resistive heating element. Cordless irons can be heated by combustion of gas stored in a small tank, often using a catalytic heater rather than a flame. Simple irons, less commonly used today than in the past, were simply a large copper bit on a handle, heated in a flame.
Solder melts at approximately . Soldering irons are designed to reach a temperature range of .
Soldering irons are most often used for installation, repairs, and limited production work in electronics assembly. High-volume production lines use other soldering methods. Large irons may be used for soldering joints in sheet metal objects. Less common uses include pyrography (burning designs into wood) and plastic welding (as an alternative to ultrasonic welding).
History
thumb|Historical soldering irons (front) and torches (back)
Before the development of electric soldering irons, the typical soldering iron consisted of a copper block, with an appropriately shaped point, supported on an iron rod and held in a wood handle. Immediately before use, the iron was heated over a fire or in a charcoal brazier, and it had to be reheated whenever it became too cool for use. Soldering irons were primarily used by tinsmiths and coppersmiths to work with thin sheet metal.
A large copper block was required in order to have sufficient thermal capacity to provide
useful heat after removal from the fire, and copper is expensive. This led to the development of
soldering irons that had a small copper tip attached to an inexpensive cast-iron block. Some irons even had removable and replaceable copper tips.
The first electric soldering iron had a very lightweight platinum tip heated by electric current flowing through the tip itself. By 1889, electric soldering irons were being developed with a resistance wire wrapped around the back end of the copper head and enclosed in a protective shell. Alternatively, the heating element could be enclosed in a relatively light-weight hollow copper head.
In 1894, the American Electrical Heater Company began manufacturing electrical soldering irons on a large scale in Detroit. They started producing them and shortly after American Electrical Heater Company released their line of soldering irons. In 1905, Scientific American Magazine, published a tutorial on making a soldering iron that
clearly explains how early irons were made.
In 1921, a German company founded by Ernst Sachs developed an electrical soldering iron similar to American Electrical Heater Company iron. in 1926, William Alferink applied for a patent for the first soldering station.
Actual "Form factor" of soldering irons
thumb|Electric soldering iron for electronic work
In 1946, Carl E. Weller applied for a patent for his soldering gun that could heat instantaneously and began production of the "Speedy Iron" in Pennsylvania. It was manufactured through the Weller Manufacturing Company, and this product was the first instantaneous thermal soldering gun. Few years later, they released to the market a soldering iron on with self-adjusting temperature. In 1951, the company WEN Products began manufacturing its own instantaneous soldering iron. After a three years trial Weller won for patent infringement. This iron could control the temperature by using a temperature-sensitive magnetic tip. The "Magnastat" became a best seller and it was included it in the W-TCP soldering station in 1967. In fact, within the patent, as a complementary description, it defines what today has become the "de facto", the redundancy is worth it, "form factor" of the vast majority of current Japanese and Chinese irons: Hakko, Baku, etc... The now-expired patent, which even Weller has stopped using on some models, described an outer tube holding the coated copper tip, clamped with a nut to the handle.
Types
thumb|Soldering iron in use
Simple iron
For electrical and electronics work, a low-power iron, a power rating between 15 and 35 watts, is used. Higher ratings are available, but do not run at higher temperature; instead there is more heat available for making soldered connections to things with large thermal capacity, for example, a metal chassis. In some models, the firmware for the control circuitry is free software that can be modified by the end-user.
Another approach is to use magnetized soldering tips which lose their magnetic properties at a specific temperature, the Curie point. As long as the tip is magnetic, it closes a switch to supply power to the heating element. When it exceeds the design temperature it opens the contacts, cooling until the temperature drops enough to restore magnetisation. More complex Curie-point irons circulate a high-frequency AC current through the tip, using magnetic physics to direct heating only where the surface of the tip drops below the Curie point.
Soldering station
thumb|left|Temperature-controlled soldering station
A soldering station has a temperature control and consists of an electrical power supply, control circuitry with provision for user adjustment of temperature and display, and a soldering iron or soldering head with a tip temperature sensor. The station will normally have a stand for the hot iron when not in use, and a wet sponge for cleaning. It is most commonly used for soldering electronic components. Other functions may be combined; for example a rework station, mainly for surface-mount components may have a hot air gun, vacuum pickup tool, and a soldering head; a desoldering station will have a desoldering head with vacuum pump for desoldering through-hole components, and a soldering iron head.
Soldering tweezers
thumb|Soldering tweezers in use
thumb|Soldering iron stand
For soldering and desoldering small surface-mount components with two terminals, such as some links, resistors, capacitors, and diodes, soldering tweezers can be used; they can be either free-standing or controlled from a soldering station. The tweezers have two heated tips mounted on arms whose separation can be manually varied by squeezing gently against spring force, like simple tweezers; the tips are applied to the two ends of the component. The main purpose of the soldering tweezers is to melt solder in the correct place; components are usually moved by simple tweezers or vacuum pickup.
Hot knife
A hot knife is a form of soldering iron equipped with a double-edged blade that is situated on a heating element. These tools can reach temperatures of up to 1,000 degrees Fahrenheit (538 degrees Celsius) allowing for cuts of fabric and foam materials without worry of fraying or beading. Hot knives can be utilized in automotive, marine, and carpeting applications, as well as other industrial and personal uses.
Stands
A soldering iron stand keeps the iron away from flammable materials, and often also comes with a cellulose 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 sometimes displayed.
Tips
alt=Image of four typical soldering iron tips, namely conical, chisel, bevel and spoon.|thumb|Some common soldering iron tips (also known as soldering bits). Note that there are different tip style naming conventions from manufacturer to manufacturer. Some very typical names are listed here.
thumb|Used plated tip with remains of solder flux
Most soldering irons for electronics have interchangeable tips, also known as bits, that vary in size and shape for different types of work. Common tip shapes include: bevel, chisel, and conical. Tip selection depends upon the type of work and access to the joint; soldering of 0.5mm pitch surface-mount ICs, for example, is quite different from soldering a through-hole connection to a large area. A concave tip well is said to help prevent bridging of closely spaced leads; different shapes are recommended to correct bridging that has occurred. Due to patent restrictions not all manufacturers offer concave tips everywhere; in particular there are restrictions in the USA. A nickel-chrome outer plating may be used further back from the very tip, as solder does not stick well to this material: this avoids solder wetting parts of the tip where it would be unwanted. other models' tips may float at arbitrary voltages unless an additional grounding wire is used.
High temperature
When operating, the iron can reach temperatures in the range of 200-480°C.