Soldering iron

How it works

The tip is heated until it can melt solder and bring the joint to a temperature at which the solder wets the metal.

Hobart’s 1919 treatise explains the older form: a mass of copper heated in a fire pot or furnace stores heat and imparts it to the work as it is used. In the electrically heated tool there is no need for a large heat-storing mass, because heat is applied continuously and passes to the bit as fast as soldering draws it off. The heater is a coil or loop of resistance wire wound on a non-conducting core of asbestos or clay inside the enlarged shank.

Weller’s instant-heat gun, patented in 1946, replaces the stored heat with a step-down transformer. The patent describes a secondary winding giving 200 amperes at one-quarter volt, which heats a copper tip loop quickly for intermittent use.

Anatomy

Tip (bit): Copper in the traditional form.
Shank and heater: In Hobart’s description of the electric copper, the shank is enlarged to contain the resistance coil or loop.
Handle: Wood in the traditional form; the early electric forms in Hobart’s book enlarge the handle or shank to hold the heater.
Cord and plug: Hobart notes that the flexible cord can break inside the handle, a common fault.
Stand: Hobart says a support should be provided so that a hot tool can be laid clear of the bench.

Variants

Fire-heated soldering copper: A forged copper bit on an iron shank with a wooden handle, heated in a fire pot, furnace or by a Bunsen burner (Hobart).
Blow-lamp-heated iron: The Science Museum Group holds a blow-lamp-heated soldering iron marked Kibraz Geant, made in Paris.
Self-heating iron: The Science Museum Group holds a Merx axe-type self-heating soldering iron in a tin box with briquettes and flammers, made by Merx Ltd of Sevenoaks, 1940–1970, which the record says was used for jointing overhead lines.
Electric resistance iron: Hobart describes the common form of the electric soldering tool, with a coil heater in the shank.
Instant-heat soldering gun: Weller’s pistol-grip transformer tool, patented by Carl E. Weller in 1946 on an application of 1941.
Temperature-controlled iron and station: Ersa introduced the TC 70 in 1971 as the first soldering iron with mechanical temperature control built into the handle, and the TE 50 station in 1974 as its first electronically temperature-controlled iron. See the soldering-station entry.

Specifications

Instant-heat gun secondary · A at V: Weller’s 1946 patent describes a secondary winding of 200 amperes at one-quarter volt, from a 115 volt primary.
Filter lens shade for soldering · shade number: OSHA 1910.252 lists shade 2 for soldering in its filter lens guide.

Made to

IPC J-STD-001
IPC J-STD-004
ISO 9453
IEC 60335-2-45
IEC 61340-5-1
OSHA 29-CFR-1910-1025

Safety and care

Safety

Keep exposure to lead from solder under control; OSHA names solder as one source of lead exposure, and its lead standard sets a permissible exposure limit of 50 micrograms per cubic meter as an eight-hour average (OSHA).
Rosin (colophony) solder flux fume is a known cause of occupational asthma; the HSE advises controls such as good general ventilation, temperature-controlled irons and local exhaust ventilation (HSE INDG249).
The HSE notes that fume levels can triple between 250 and 400 °C iron temperature (HSE INDG249).

Care

Do not use an electric soldering copper as a hammer or throw it on the bench, since the coil can be thrown out of its groove; Hobart says to handle the tool as carefully as glass.
Do not dip an electric iron in acid or let acid fumes reach the heater; Hobart states that the fumes attack the heater and joint of the shell, and that the tool may be in bad shape within two or three months of such treatment.
Do not leave an electric iron on its stand at full current for long periods; Hobart says that it is apt to overheat when not in use, with possible damage to the insulation or heating unit.

Tips

Tin a copper bit and keep it tinned
Hobart describes several ways to tin a copper, including rubbing it on a brick with resin and solder, with sal ammoniac added as a quicker variant. He advises that a copper that has been removed from the fire and shows thin or defective tinning can be restored with a rub or two on the brick.1
A copper that shows colors is too hot
Hobart writes that for a good seam the copper must be clean, well tinned and hot, but not too hot. If the film of solder on the copper is hot enough to show colors it is oxidizing fast and will soon burn off.1
Match an electric iron to its supply voltage
Hobart says electric soldering tools must be bought for the current to be used. A tool wound for 100 volts would burn out almost instantly on a 500 volt circuit unless a resistance such as a lamp was placed in series.1
Heat a soldering gun only while the trigger is held
Weller’s 1946 patent describes the gun as electrically heated soldering apparatus for intermittent use. A spring normally holds the trigger out so that the switch is open and the current cut off, and the iron is heated by tightening the finger grip on the trigger.1

History

Earliest documented

c. 2600–2500 BCE (soldering of gold; the tool is not documented)

Ur, Mesopotamia

The 2016 Metalla volume on the Royal Tombs of Ur dates the large tombs to the Early Dynastic III period (about 2600–2500 BCE) and describes two soldering techniques in Bronze Age goldwork. One is a metallic hard solder, an alloy of gold with silver and copper placed in tiny particles at the joint and melted by heating. The other is a reaction solder, made by heating a copper salt powder with an organic glue. The volume says analysis is needed to distinguish the two with certainty. This is evidence for soldering, not for a soldering iron1

Soldering is far older than the soldering iron. The Ur material shows gold jewelry of about 2600 to 2500 BCE joined with an alloy solder, and the Metalla authors discuss both a metallic solder and a copper reaction solder as possible methods. None of the sources read documents the tool used for soft soldering in antiquity.

The traditional iron is a heat store. Hobart’s 1919 treatise describes a soldering copper as a mass of copper heated in a fire pot or furnace, and explains that the mass is needed so that enough heat is stored to be imparted to the work. Workers had to tin the copper, keep it from overheating and burning off its tin, and return to the fire for reheating. Hobart counts the time spent on the furnace, on dressing and tinning the copper and on waiting for reheating as a cost in the shop. The Science Museum Group’s collection includes irons heated by blow lamp and by a self-heating briquette.

Electric heating made the mass unnecessary. Hobart writes that the electric bit is cut down in size because heat is supplied continuously, and that the heater is a resistance wire wound on an asbestos or clay core in the shank. He records resistance from workers used to a furnace copper: the electric tool melted solder more slowly on first contact and could not be crowded for short intervals, but once the workers learned its characteristics, he says, most preferred it. The date of the first electric iron is contested. Ersa calls Ernst Sachs’s 1921 H1 the first, and a collector’s history cites a 1894 start and an 1898 advertisement for an earlier American iron (see disputed).

The gun and the station followed. Weller’s 1941 application, granted in 1946, replaced the heated mass with a transformer that passes some 200 amperes at a quarter volt through a copper loop, a design the patent describes for intermittent use. Johnson reports that Weller won an infringement suit over the gun in 1954 and that the company was sold to Cooper Industries in 1970. Ersa’s TC 70 of 1971 and TE 50 of 1974 brought temperature control to the iron itself.

The tip and the solder changed with electronics. The HSE reports that fume levels from rosin flux can triple between 250 and 400 degrees Celsius, which is one reason temperature-controlled irons appear among its recommended controls.

Timeline

c. 2600–2500 BCE
Gold jewelry from the Royal Tombs of Ur is joined with metallic hard solder or reaction solder.1
1894
The American Electrical Heater Company of Detroit begins making electric soldering irons, according to Steve Johnson’s collector’s history; the same page cites an 1898 advertisement for the company’s iron.1
1919
The second edition of James F. Hobart’s treatise describes the electric soldering tool, with a resistance coil in an enlarged shank, as a form that has come largely into use in late years.1
1921
Ernst Sachs files a patent application for the Ersa H1 on 8 July and on 18 November registers his “First Special Factory for Electric Soldering Irons” in Berlin, according to Ersa, which calls the H1 the first electric soldering iron.1
1925
Ersa adds lighter irons for the electrical industry to its 200-watt irons.1
1937
Supreme Instruments of Greenwood, Mississippi, produces a pistol-grip soldering iron designed for radio repair, according to Johnson’s collector’s history.1
1941
Carl E. Weller applies on 14 July for a patent on an electrically heated soldering apparatus with a transformer, a quarter-volt secondary and a copper tip.1
1946
US Patent 2,405,866 is issued to Weller on 13 August. Johnson’s page says Weller reportedly made some 275 guns by hand in his basement before forming a company after the war.1 2
c. 1946
An electric soldering iron of about this date is recorded in the Science Museum Group collection.1
1949
Ersa attends the first export trade fair in Hanover and its Ersa 30 soldering iron begins its success.1
1971
Ersa launches the TC 70, the first soldering iron with a mechanical temperature control built into the handle; in 1974 it follows with the TE 50 electronically controlled iron.1

Where the sources disagree

Which was the first electric soldering iron?

Ernst Sachs’s Ersa H1, for which he filed a patent application on 8 July 1921, was the first electric soldering iron.1
Ersa
The American Electrical Heater Company of Detroit began making electric soldering irons in 1894; the collector has an 1898 advertisement in The Electrical Engineer for its iron, 23 years before Sachs, and a General Electric iron with patent dates earlier than 1921.1
Steve Johnson’s collector’s history
The electric soldering tool had come largely into use in late years by the 1919 edition; the chapter does not date the first iron.1
James F. Hobart (1919)

Gaps in the record (8)

Removed for sourcing: the Ersa soldering primer (2012) was cited from a distributor’s copy at sos.sk, and no copy on ersa.com, kurtzersa.com or archive.org could be located or fetched. Every claim that rested on it is therefore gone: the three phases of soldering and the lowest-temperature advice, copper and coated tip description, tip cleaning and tinning care, the filing warning, the hand-washing and iron-holder safety items, the 200 to 450 °C and 2 to 5 second specifications, the tips on hot-tip cleaning, soldering time and joint appearance and wetting angle, the 2006 RoHS timeline event, and the sentence on joining of gold in ancient Egypt.
The earliest soldering iron form (the heated copper bit) has no sourced date; the Ur evidence concerns the process of soldering gold.
The 1894 and 1898 American Electrical Heater Company dates come from a private collector’s site; the advertisement and any patents have not been located.
Hobart’s chapter on electric tools may have been added in the 1919 edition; the 1912 first edition has not been read.
The Ersa H1 patent has not been located; Ersa’s statement that it was the first is a company claim.
The Smithsonian wooden prototype of Weller’s gun, mentioned on Johnson’s page, has not been checked against a museum record.
Weller’s 1946 date for the start of production and the 1954 lawsuit are taken from Johnson’s page and a trade publisher’s news item and are not confirmed in court records.
No standard for soldering iron construction has been read; the standards slugs are guesses.