Overview of the soldering iron

Soldering irons rely on the fact that electrically resistant (but still conductive) materials rise in temperature when electrical current is passed through them.

Although it requires an external electrical input, the actual work of soldering with a soldering iron is powered by the user’s hands, which places the gadget in the broad category of hand tools rather than in the category of power tools.

The handle of the iron is thermally and electrically insulated, but the exposed resistant-conductive metal heating element (usually made of copper or nickel) in the tip reaches temperatures high enough to convert metal solder wire into a molten state when subjected to a current. The temperatures are not sufficient, however, to melt the workpieces themselves.

Current is typically supplied to the heating element either via an electrical cable plugged into a mains wall socket, or via battery cables. However, other variants exist: some soldering irons store a combustible gas in a chamber which, upon combustion, produces the heat required for soldering through a catalytic heater instead of a flame. In the past, more primitive models consisted merely of a handle upon which a large copper bit was mounted (heat was provided by an external flame); these models are to all intents and purposes obsolete today.

When an electrically conductive join is required between two separate workpieces, the soldering iron, when used carefully, melts a small amount of wire made up of a fusible metal alloy called solder, which contains a cleanser (flux) in its core to clean any rust that might impair the join and improve the solder’s wetting qualities (i.e., it’s ability to maintain contact with the surface to which it is being applied in a molten state). The tip of the solder wire is positioned between the workpieces, whereupon the tip of the iron is applied to the wire until a small quantity melts off to become the new electrically conductive join.

Some soldering irons incorporate an LED indicator light in the handle to alert the user as to whether the device is on or off.

History of the soldering iron

It may seem like a modern phenomenon but historical records show that soldering with soldering irons was practiced in ancient Egypt as much as five millennia ago: goldsmiths used it to join gold in jewellery. The discovery of tin 4000 years ago increased the practise of soldering as it made a much more effective soldering metal than the materials previously used. Although precise dates are difficult to ascertain, it seems likely that soldering spread from Egypt to the Mediterranean region first, and was subsequently widely adopted in the Roman Empire, whose plumbing engineers, impressively, soldered huge 400km lengths of lead water pipe together. It arrived later in Switzerland and Hungary.

The first electrically powered soldering iron seems to have been manufactured in 1898 by the American Electrical Heater Company, although the German Ernst Sachs claimed that he was the first to invent such a device in 1921. In 1960, the Weller Manufacturing Company in the US patented the “Magnastat” soldering iron which used a magnetic component to regulate the temperature at the iron’s tip.

Soft soldering is now widely practised in the electronics industry and in the field of production engineering.

Technical aspects of the soldering iron

Simple low-power soldering irons generally come with a power rating of between 15 and 35 watts. In addition to the power rating, the temperature of the heating element in these devices is determined by several factors, including the temperature of the workpieces it is applied to, the effects of environmental cooling, and thermal equilibrium between the iron and the workpiece (their temperature typically falls when they are applied to a relatively large object such as a chassis and may be insufficient to melt solder wire as a result).

For soldering on such big objects, more sophisticated soldering irons are required. These “temperature controlled irons” include a temperature sensor and a regulating component to maintain the temperature of the tip at a steady level. They automatically draw more power when the object they are applying solder to is a large one. Although some are free-standing, some harness the control circuitry and temperature regulating capacity of a separate “soldering station” in order to function properly. Some of these stations include a visual display of the temperature at the iron’s tip.

Temperature control is achieved through a variety of means. Some controlled irons depend on a variable power control resembling a light dimmer switch, which allows the user to manually increase the iron’s temperature, if it starts to fall through thermal equilibrium. These irons do not, however, automatically measure or regulate heat output.

Others use an integral thermostat which is usually located in the tip of the iron. This automatically controls the power supply to the element, switching it on and off as required.

A number of soldering irons achieve temperature control by means of magnetic soldering tips which lose their magnetism at a particular temperature called the “Curie point”. While the tip remains magnetic, a switch delivering power to the element is held closed, permitting current to enter. When it overshoots the set temperature at the Curie point, the contacts in the switch are automatically opened, interrupting the flow of current to the heating element.

Where the soldering iron is used in manufacturing

While high-volume production processes in manufacturing require other soldering methods. The soldering iron is frequently used in low-level production work in electronic assembly, for conducting repairs, and completing installations. Other, less common, applications include plastic welding (which melts plastic workpieces together at a join rather than using separate solder material) and decorating wood with burned in designs (pyrography).

Things you do with a soldering iron

Soldering irons are ideal for soldering together transistor leads, wires, or pads on printed circuit boards (PCBs).

What makes the soldering iron unique?

Unlike soldering operations which require exceptionally high temperatures (plumbers, for example, use high-temperature blow torches and protective heat mats to provide the heat needed for soldering copper pipework together), the soldering iron is a complete instrument in its own right, requiring only an electrical power source for heating and the application of solder wire to the area where an electrical join is needed between workpieces.