Cold Soldering - better than a traditional iron?

"Coldheat Classic Soldering Iron"

Full review and practical test

Coldheat Soldering Iron (JML UK packaging)Traditionally, the principle of soldering involves heating components that are to be soldered together, in order to raise their temperatures to that of the melting point of solder. A small quantity of solder alloy is then melted over the hot joint, the source of the heat is removed and the solder is allowed to cool and solidify.

Modern electric soldering irons are often temperature controlled using an electronic thermostat, though the more basic fixed temperature irons may be “thermally balanced” which means that their mechanical design helps to regulate the temperature of the tip.

Recently a device called a “cold soldering iron” arrived on the market that is radically different from any traditional soldering iron. It is a battery-powered cordless iron produced exclusively by Hyperion Industries Inc. under their Coldheat trademark ( It is an unorthodox device with a number of intriguing features. I decided to check them out in detail, and put an iron through its paces.

In the UK the Coldheat Classic iron that is the subject of this review is marketed by JML ( and at the time of writing is on sale in a number of retail outlets, including Maplin (£14.99) and Homebase DIY (£19.99). JML has said (February 2006) that supplies are quite limited at the moment, but they can be bought online from JML Direct at £19.99 here.

Coldheat Iron from the outside

Coldheat Iron

The Coldheat iron is roughly the size of a large pen-style handheld multimeter. It is purposeful looking, fits together well and is finished to a reasonably high standard. The business-end of the iron is of course its tip: Coldheat is tight-lipped about the composition of the exclusive soldering bit, and only alludes to a patented composite material dubbed “Athalite”.

The iron features an on-off slide switch, a white l.e.d. to light the work area (more on this later) and a bi-colour l.e.d. on top to indicate that the iron is heating. The iron is supplied with a sturdy plastic case, and a translucent clip on cover protects the tip when the tool is not in use. The see-through cover also warns that the iron is powered up, to save the batteries.

The most prominent manufacturer’s claim is that the tip heats up instantly and cools in seconds: they state the patented tip rises to 800 degrees Fahrenheit (420 degrees C) in just over a second, returning to room temperature in less than three. The main benefit of “cold soldering” then, apart from its cordless convenience, seems to be the instant on-off action that is unique to this device: when not actually soldering a joint, it is claimed the tip temperature is at a safe low ambient level.

The packaging declares that the Cold Soldering Iron is appropriate for most electrical soldering with wires up to 0.5 – 1.0mm diameter, small jewellery repair and electronic projects (my italics). For best results, they advise to use 18-20 gauge/ 0.8 – 1.0mm diameter solder. The manufacturers also now market a more powerful “Pro” version using 5 AA cells that is “perfect for computers, jewellery, electrical and more”. This version does not seem to be available in the UK but may be available via the web.

This review describes the results of using a Classic Coldheat Soldering Iron to solder together a small electronic project, and it starts by taking a close look at the iron itself.

The Coldheat "Cold Soldering Iron" "Athalite" Tip

The patented "Athalite" soldering tip of the Coldheat Soldering Iron is actually two electrodes, separated by the plastic outer moulding.
The contacts within the main unit that connect to the split tip. Push-fitting the "Athalite" soldering tip into the main unit.
The split tip "sandwich" construction of the Coldheat Soldering Iron, located in the main unit. The tip fits either way round (bevel facing up or down). The white l.e.d. is a worklight. A slide switch and l.e.d. are on top.

A closer examination of the tip shows that it consists of two electrodes formed from a graphite-like material, retained by an injection moulding. It exhibits an electrical resistance and is a form of conductive high temperature compound. The tip is extremely lightweight, weighing only 2 grammes compared with say a 10 gramme bit of a typical traditional soldering iron. There is therefore very little thermal mass available in the Coldheat tip. So how can it possibly heat up to melt the solder?

The answer is that the tip’s electrodes have to be bridged by applying them across an electrically-conductive part (such as a component lead). This causes both electrodes to short together, and a substantial current then flows. The tip temperature rises which is sufficient to melt narrow-gauge solder.

A measurement with an ammeter showed a current flowing through the tip of up to 4.5 amps from a fresh set of 4 “AA” Duracells. It is claimed that 700 - 1,000 joints could be made with fresh batteries.

The removable tip push-fits into the iron either way round and has a precise fit to it: the photos on the packaging show the angled tip facing down, which encourages users to press the iron down onto the workpiece, but many constructors would traditionally want the tip the other way, facing upwards, to form a working “anvil” surface where you can see to dab solder onto the joint.

The “Athalite” tip appears to be very lightweight and delicate, and only time will tell whether it can withstand the rigours of working in the hands of DIY’ers, handymen or unskilled solderers. Some users have complained in forums about the cost of replacement tips, and in the UK at least, the importer has failed to provide an answer regarding the cost of spares. Damaged tips can be filed to shape using a nail file, says the maker.

At this point it is worth mentioning the warning label stuck inside the plastic carrying case:

THIS TIP IS FRAGILE. PLEASE DO NOT PRESS HARD. The red light on the tool will indicate whether the soldering tip is creating heat upon contact with your joint. If the red light does not turn on, please reposition the tip so that both electrodes make contact with your electrically-conductive work or joint. Do not press hard on the tip, as this will not solve the problem and may in fact damage it. This tool is intended for light-duty soldering projects. For best results, use solder of 18AWG [American Wire Gauge] (0.040” diameter) to 20AWG (0.032” diameter)…”

Coldheat Soldering Iron — Internal View

Looking at the internal construction of the iron itself, the plastic case has a battery cover held on with captive screws. The manufacturer’s web site alludes to the iron containing some clever and complex electronics, and it was second nature for the writer to “pop the hood” and examine the internals of the Coldheat iron, to see what all the fuss is about.

The blue plastic top cover can be removed with a screw and after undoing five more self-tappers the body unclipped neatly. The electronics – as far as they go – consists of a tiny surface-mount chip on a thumbnail-sized board. It is unclear whether it uses a custom chip because the markings were erased during production, but it may be a custom programmed microcontroller. Some interwiring connects the switch, batteries and l.e.d.s.

The small printed circuit carries a surface mount chip (possibly a microcontroller), markings have been scrubbed out by the manufacturers.

Some standard interwiring hooks up the battery, tip, slideswitch and l.e.d.s.

Coldheat Soldering Iron on test

It was decided to test the iron by building a basic electronic kit with the Classic Coldheat Cold Soldering Iron. The Velleman kit #MK148 chosen was a simple l.e.d. flasher based on a transistor astable multivibrator circuit. It represents a small electronic circuit that an electronics novice would hopefully feel confident enough to try to assemble, and it has a number of discrete parts but no integrated circuits.

The kit is of a high quality: it has an excellent quality printed circuit board (approx 50mm x 30mm) with full silk-screen printing and solder resist. It has roughly 50 solder joints. The writer would expect a simple kit like this to be built by an experienced constructor within 20 minutes, with an expectation that it will work first time. A novice would need a little longer, though there would probably be no component orientation or identification problems once the "continental European style" instructions were understood.

The Velleman kit used for the test consists of a high quality but simple glass fibre printed circuit board, silk screen printed on the top side to aid with component placement, and coated with solder resist (the green coating). Instructions are very clear and all information necessary to help with pinouts and identification are included in the kit's instructions.

In the event, it turns out to be critical to understand how the iron works, to stand a chance of making a solder joint successfully.

As mentioned earlier, the Coldheat iron requires that the two electrodes be shorted together to create the high current flow and heat that melts the solder. It was found that there was a certain knack to getting this right as the process was a bit temperamental (pun intended).

All the kit’s components were gradually soldered into place on the printed circuit board, using as much care as possible, and with considerably more skill and experience in electronic soldering than a novice or handyman/ DIY’er would possess.

The components included ordinary ¼ Watt carbon resistors, two radial lead electrolytic capacitors, five transistors, two light-emitting diodes, a preset potentiometer and two switches. A variety of discrete parts were therefore soldered onto the p.c.b that possessed a variety of tag or pin sizes. The Velleman p.c.b. track sizes are relatively rugged, and the solder resist helps to avoid excess solder being applied.

Please now continue to the next page - Summary and Verdict.

| Continued - Summary and Verdict |

All images and text ©A R Winstanley March 2006