# Electric soldering iron

> Joining wires and parts with molten tin

- Date: 1880
- Era: Electric age
- Domain: [Tools & Making](https://lastcodex.org/en/domains/tools/)
- Rebuild stage: IV (Science & industry)
- Difficulty: 2/5
- URL: https://lastcodex.org/en/inventions/soldering-iron/

An electric soldering iron is a tool heated by an electric current, usually a copper bit warmed by a resistance wire, used to melt solder, an alloy of tin, onto a joint so that wires and components hold together and conduct. Irons heated in a fire were already used to join the lead strips of stained-glass windows in the twelfth century; in 1880 Charles Ball applied for a patent on an iron heated by an electric current.

![Electric soldering iron · Joining wires and parts with molten tin](https://lastcodex.org/_astro/plate.BdjpqFu7_Z1ov9Ua.webp)

## Legend

1. Tinned tip
2. Copper bit
3. Heating element wound on mica
4. Steel barrel
5. Wooden handle
6. Cord

## What it is

Soldering joins metal parts with a filler metal, the solder, that melts at a much lower temperature than the parts themselves. The parts are heated but stay solid; the molten solder flows between them, wets their surfaces and sets into a joint that holds and conducts electricity. In electronics it fixes the leads of components to the copper of a circuit board, and joins wires.

The traditional solder of electronics was tin with lead: the alloy of 63 parts tin to 37 parts lead melts at 183 °C. Lead-free solders of tin with a little silver and copper melt between about 217 and 220 °C. Solder for electronics is sold as a thin wire with a core of flux, usually rosin, a resin from pine trees. The electric soldering iron supplies the heat: a bit of copper, heated from behind by a coil of resistance wire, with a pointed tip that is brought to the joint.

## How it works

The heating element is a resistance wire wound round the rear of the bit, insulated from it by mica or ceramic. As current passes, the wire turns electrical energy into heat, as any [resistor](/en/inventions/resistor/) does. Copper is chosen for the bit because it carries heat very well, so the tip stays hot while it gives heat away to the joint. The tip is coated with a thin layer of solder, called tinning. A clean, tinned tip passes heat into the joint through a small pool of molten solder, which a dry or dirty tip cannot do.

![A joint on a board cut away to show its plated hole: a tinned iron tip pressed against both the copper pad and the component lead, solder wire fed in from the opposite side, and molten solder starting to flow around the lead](https://lastcodex.org/_astro/v3.4_RN2MAS_Z1l4tMt.webp)
_The tip heats pad and lead together; the solder is fed into the joint, which melts it._

Metals in air are covered with a film of oxide, and molten solder will not stick to oxide. The flux melts first, dissolves that film and covers the joint while it is hot. The solder can then wet the clean metal: it spreads over the surface instead of rolling into a ball, and bonds to it as it cools. A good joint therefore needs the parts themselves to be hot enough to melt the solder. Solder melted on the tip and carried to a cold joint sits on the surface without bonding, a fault known as a dry joint.

An iron that runs too hot burns the flux before it can work and gives off more fume. Many irons therefore carry a thermostat that holds the tip at a chosen temperature, hot enough for the solder in use and no hotter.

## How to do it

This is how to make a soldered joint with a bought electric soldering iron that carries a recognised safety mark and has its own stand. Use rosin-cored solder wire, lead-free if you can, and practise on two wires or on a scrap circuit board. Work at a table covered with a heat-resistant mat, in a well-ventilated room, with a small fan or an extractor drawing the fume away from your face.

Let the iron heat up in its stand. Wipe the tip on a damp sponge or brass wool and touch the end of the solder wire to it: the tip should become bright and silvery. Clean the parts to be joined. Twist the two wires together, or push the lead of a component through its hole in the board. Hold the tip so it touches both parts at once, the copper pad and the lead, for a moment. Feed the solder wire into the joint on the side opposite the tip, not onto the tip. When enough solder has flowed round, take the solder wire away first, then the iron. Do not move the parts until the solder has set.

![A finished joint on a board cut away to show its plated hole: a component lead passing through the hole, a copper pad around it, and a smooth concave cone of solder joining pad and lead](https://lastcodex.org/_astro/v2.BLkrbCN-_1TlPas.webp)
_A sound joint: the solder has wetted pad and lead and set as a smooth concave cone._

A good joint is smooth and shaped like a small cone with hollow sides, the solder flowing onto both pad and lead. Lead-free solder looks duller than tin-lead. A ball of solder sitting on the pad, or a grey, grainy surface, means the joint was too cold or moved while setting: heat it again with a little fresh solder. Cut off the spare length of lead.

The tip is hot enough to burn skin at once and to set fire to paper and cloth: always put the iron back in its stand, never on the bench, keep the cord away from the tip, and unplug it when you stop. The fume of rosin flux is a common cause of occupational asthma, so keep your head out of the rising fume and draw it away. Tin-lead solder contains lead: wash your hands after soldering and do not eat or drink at the bench. Wear safety glasses, since clipped leads and hot flux can fly. Never solder on a circuit that is switched on, and never open or repair mains-powered equipment.

## History and evolution

![A long, slender iron rod ending in a thick round head that tapers to a small point, lying on a worn wooden block beside a cast stick of tin](https://lastcodex.org/_astro/v1.wf9anYto_Z1JQEpy.webp)
_A soldering iron as Theophilus describes it, with a stick of tin solder._

Soldering is much older than electricity. In the early twelfth century a monk who wrote under the name Theophilus described how glaziers assembled stained-glass windows. The pieces of glass were held in strips of lead, and the joints were made with a "soldering iron", long and slender with a thick round head ending in a small point, filed, tinned and heated in the fire. The solder was rods of tin cast with a fifth part of lead, and wax served as the flux. Tinsmiths long worked in the same way. The workshop of the Dominy family on Long Island, whose tools are kept at the Winterthur Museum, had irons of copper and iron made between 1800 and 1812. Two irons were used in turn, one heating in the fire while the other was at work, with resin or borax as flux.

Electric heating removed the fire. On 13 December 1880 Charles E. Ball of Philadelphia applied for a patent on a soldering iron whose tip formed part of an electric circuit. Two copper rods, insulated from each other in the handle, carried current from a battery or a dynamo to a platinum tip, which the current made hot enough to melt the hard solder of jewellers, a job until then done with a blowpipe. On 5 November 1889 Willard Miner of Plainfield, New Jersey, applied for a patent on an iron heated by a separate coil: a solid copper head with a coil of German silver wire wound round a narrowed part of it, without touching it, inside an insulating shell. In 1893 Samuel Jenkins of Boston patented an iron with a copper core and a heating wire woven round it, for supplies up to 220 volts, on behalf of the American Electric Heating Company. Such irons spread with the use of electricity in workshops, and radio, then all of electronics, made the small iron with rosin-cored solder a basic tool.

Health concerns then reshaped the materials. The European Union barred lead from most new electrical and electronic equipment from 1 July 2006, and lead-free solders, which melt at higher temperatures, became the rule for equipment sold there.

## Key facts

- Iron heated by electric current: Charles E. Ball, patent applied for in 1880
- Copper bit with a heating coil: Willard Miner, patent applied for in 1889
- Fire-heated iron for stained glass: Described by Theophilus, early 12th century
- Melting point of 63/37 tin-lead solder: 183 °C
- Lead banned in new electronics in the EU: From 1 July 2006

## Requires

- [Copper Smelting](https://lastcodex.org/en/inventions/copper-smelting/): Copper won from ore with charcoal and a blast of air (c. 5,000 BCE)
- [Tin Smelting](https://lastcodex.org/en/inventions/tin-smelting/): Heavy black ore washed, then reduced with charcoal (c. 3,000 BCE)
- [Lead Smelting](https://lastcodex.org/en/inventions/lead-smelting/): Lead from galena, and the silver hidden in it (c. 4,200 BCE)
- [Resistor and rheostat](https://lastcodex.org/en/inventions/resistor/): Opposing a current by a known amount (1840)
- [Insulated electric cable](https://lastcodex.org/en/inventions/insulated-electric-cable/): Carrying current inside a sheath that does not conduct (1795)

## Leads to

- [Wired circuit board](https://lastcodex.org/en/inventions/wired-circuit-board/): Parts screwed to a board and joined by wires (c. 1905)

## Sources

- [US 421,185, Electric soldering-iron (W. M. Miner, filed 5 November 1889, granted 11 February 1890)](https://patents.google.com/patent/US421185), Google Patents
- [US 491,311, Electrically-heated soldering-iron (S. B. Jenkins, granted 7 February 1893)](https://patents.google.com/patent/US491311), Google Patents
- [Theophilus, De diversis artibus, book II, chapter 27, ed. and trans. Robert Hendrie (1847)](https://archive.org/details/theophiliquietru00theouoft), Internet Archive
- [Stained glass: an introduction](https://www.vam.ac.uk/articles/stained-glass-an-introduction), Victoria and Albert Museum
- [Soldering irons, Dominy workshops, 1800–1812](https://dominycollections.winterthur.org/tools-of-the-trade/clockmaking-shop/soldering-iron/), Winterthur Museum, Dominy Collections
- [Database for solder properties with emphasis on new lead-free solders, release 4.0](https://www.msed.nist.gov/solder/NIST_LeadfreeSolder_v4.pdf), National Institute of Standards and Technology
- [SAC305 lead-free solder alloy](https://aimsolder.com/products/solder-alloys/sac305), AIM Solder
- [Solderers: occupational asthma](https://www.hse.gov.uk/asthma/solderers.htm), Health and Safety Executive (UK)
- [Directive 2002/95/EC on the restriction of the use of certain hazardous substances in electrical and electronic equipment](https://eur-lex.europa.eu/eli/dir/2002/95/oj), European Union, EUR-Lex
- [US 236,972, Soldering-iron (C. E. Ball, Philadelphia, filed 13 December 1880, granted 25 January 1881)](https://patents.google.com/patent/US236972), Google Patents
- [English Mechanic, 25 March 1881: Ball's electrically heated soldering iron for hard solders (clipping)](https://edisondigital.rutgers.edu/document/SM057030a), Thomas A. Edison Papers, Rutgers University
- [Mining and Scientific Press, 21 May 1881: Ball's electric soldering iron (clipping)](https://edisondigital.rutgers.edu/document/SM056132a), Thomas A. Edison Papers, Rutgers University

Fact-checked on 2026-10-19.

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