Cold Soldering (2)

"Coldheat Classic Soldering Iron" Summary and Verdict

Soldering on...

In practice, sometimes the solder would melt immediately because a good contact had been made first time across the tips. Frequently, though, it was necessary to jiggle the Coldheat bit around to make the short-circuit, and prod and poke the solder onto the tip using trial and error before the solder melted properly. For the novice, the temptation will be to press the tip down harder onto the joint, in order to make a better circuit and start the heating process. This condition may damage the fragile tip, which Coldheat warns users to avoid.

The main difference in operation is that an ordinary iron, which is usually temperature controlled, is used to directly heat all components (e.g. the copper pad, and the component’s pinout/ leadout) simultaneously, and then apply solder directly to the hot joint. This results in a uniform and consistent quality of solder joint. A traditional iron can be pushed as hard as you like onto the workpiece.

In a noticeable proportion of the test joints, it was found to be a frustrating task to coax the heating cycle into commencing. Sometimes a joint could be started straight away but in a number of occasions it took some trial and error to start the joint without exerting any pressure onto the components. One joint took over a minute to make, though others were made within a few seconds. (A traditional p.c.b. joint can be soldered the traditional way by hand in well under one second.)

An l.e.d. on top of the iron indicates when current is flowing, to warn of high temperatures. The separate white l.e.d. ‘worklight’ underneath is useful to illuminate the workpiece but most electronics constructors will probably think it’s in the wrong place, for the simple reason that you can't see underneath the tip anyway: they would probably prefer the light located above the tip, shining down onto the solder tip where all the action takes place, rather than lighting up an area below it.

For the electronics hobbyist, one particularly disconcerting side-effect of the Coldheat way of doing things is that localised arcing and sparking may be produced sporadically when the heating circuit opens or closes. There is of course no such effect when using an ordinary soldering iron: sparks anywhere near a semiconductor are an anathema to the electronics enthusiast. High reverse voltages can be produced in inductive circuits as well.

This is why a Coldheat Soldering Iron should only be used with great care (if at all) on any circuit containing semiconductors (diodes, transistors, integrated circuits). The voltage across the 'split tip' can power up the circuit being soldered, or forward/ reverse bias a semiconductor junction, possibly destroying it. Here, an ultra high power l.e.d. is easily illuminated by the tip.

There is an additional warning about using the Coldheat iron with sensitive electronic components. A voltage exists across the two electrodes that is sufficient, for example, to illuminate a very powerful 3.6V white l.e.d placed across the tips (see photo). If the Coldheat tip were placed across (say) the base and emitter pins of a transistor, or across a diode and forward-biasing it into conduction, this could damage the component permanently.

Since almost all electronic circuits are full of semiconductors, often operating at very low voltages, the Coldheat iron could only be used with caution in such applications, though it is claimed to be suited to larger p.c.b.s, presumably where there is less chance of the iron's electrodes shunting or powering the components. Again, the distributors warn users about this aspect, but this runs contrary to claims that the iron is suitable for 'electronic projects'.

Care was taken by the writer to avoid the risk of damaging semiconductors, though this only made construction more fiddly than necessary.

Our Test Results

The overall results of the soldering exercise were very disappointing. Coldheat claims that the “…efficient hot-on-contact method allows for clean, tidy soldering that leaves a part that is smooth, shiny and streak-free!” but the test results on the p.c.b. were of extremely mixed quality. In particular it proved impossible to obtain any consistency in the size and quality of the joints.

The quality of the soldered joints was extremely inconsistent, due to the lack of temperature control and other variables that affect the finished joint. It would be necessary to rework these joints to bring them up to standard.

Problems with the finished joints included lack of control, insufficient or excess solder, gray/ crystalline joints and — worst — p.c.b. copper track damage.

Looking closely at the completed p.c.b., the inconsistencies in soldering quality became obvious, with perhaps 20% of the finished joints being of a reasonably good standard – that is, shiny, uniform, non-crystalline looking and proportionately sized.

The remainder of the soldered joints had a number of characteristics that left them far from ideal. It was difficult to feed solder onto the joint at a consistent rate because the heating effect was variable. Often, excessive solder was applied, which it is thought is due to the need to ‘urge’ the joint on to maintain heat before the iron switched off. Even if this did not affect the integrity of the joint, it could lead to faults when in close proximity to other solder joints, requiring skilful desoldering to cure the problem. Examples included the transistors and the radial electrolytics. Some joints were ball-bearing-like in appearance caused by excess solder being fed onto the joint.

The dull crystalline appearance of many joints points to inadequate heating, caused by the heat not sinking throughout the joint sufficiently to melt the solder thoroughly, before the iron was removed. Although a grey crystalline joint will often form a sufficiently working joint for non-critical applications, the circuit might have an intermittent fault because of this. A good proportion of the joints would have benefitted from re-working.

There is however no escaping the most fundamental principle of soldering: in order to make a good quality solder joint, all parts should be heated to the melting point of the solder so that it can flow properly. In the case of "cold-soldering" a p.c.b. (such as our Velleman kit), then only one half of the joint can effectively be heated by the iron, e.g. a cropped resistor lead that you use to short out the electrodes to generate heat.

Solder can then melt onto the hot leadout but it is then forced to flow onto the other component (the p.c.b. copper pad) which is completely cold. Inadequate through-heating of components is the main cause of dry (gray) solder joints. What little heat there is, sinks away through the workpiece and the solder never flows properly. A crystalline, dull and brittle joint is formed.

The only practical way of heating the entire p.c.b. joint (e.g. a resistor lead and the p.c.b. copper pad) with a Coldheat iron is to press the component’s lead down with the iron onto the copper pad to form an union that will heat through properly; however the Coldheat tip, unlike a traditional iron, is not rugged enough for this type of treatment. The alternative of allowing extra heating time for the molten solder to heat the rest of the joint is likely to overheat the p.c.b. and damage it, or overheat the components themselves.

There was worse news to come with our test circuit board. It was disconcerting to see afterwards that three p.c.b. copper pads were damaged by excess heat, causing the copper track to lift away from the laminate altogether. Strangely, this was the case with both of the radial lead electrolytic capacitors: the pads lifted completely, breaking the tracks and leaving the capacitors free to wobble on the board, held in place by blobs of solder. A skilled and experienced electronics constructor could repair the damage by soldering jumper wires on the board, but the novice would face the disappointment of having ruined the board altogether.

Unfortunately we failed to finish and test our sample board successfully, due to the damage caused to three copper pads.

More recently an in-store video presentation (by JML) of this item showed a Coldheat iron being used to repair wiring on a car radio. The iron was pressed onto the copper pad which had been previously tinned & soldered. The iron then melted the existing solder and a tinned wire was re-soldered to the board. In this type of electrical repair application, when simple reflow soldering is desired, the Coldheat iron would seem to be quite suited.

One distributor’s web site (www.conrad.com) shows a Coldheat iron (Part no.: 588112 - PT) being used to repair a surface mount computer card carrying an SM chip and hardly any discrete components, but the writer considers that a Coldheat iron would not be the sensible choice in that application – nor would it probably be possible to repair a PC card in any case. The Coldheat technique is probably best suited to simple electrical spot repairs such as repairing a broken wire.

Summary

The main attractions of the Coldheat iron, according to its manufacturer are:

1. Cordless battery powered, use in almost any location,
2. Instant on-off action,
3. Rapid cooling down and warning LED means improved safety.

The instant on-off action is useful to enable to cold iron to be pocketed or put away immediately where this is needed (e.g. field service use). Also, the fact that it inherently cannot be left lying around in a hot condition can only help with safety, especially if youngsters are around. Judging by the practical trials it is true to say that the iron cools down quickly – though the reviewer measured 15-20 seconds cool-down before the tip was touch-cold again.

It would appear, though, that as far as electronics hobbyist/ constructors are concerned, the cordless convenience and safety virtues of the Coldheat iron are more than outweighed by the lack of soldering consistency or temperature control that would have helped the novice DIY’er to solder the electronic kit successfully to begin with.

Strangely, the manufacturers are eager to highlight the perceived perils and evils of traditional soldering irons, and they claim that “good solderers know it’s all fun and flux until someone solders themselves… the chances of obtaining painful burns are much more likely with a regular soldering iron.”

However, in the writer’s experience, the chances of receiving accidental burns from a hot iron are extremely remote anyway, as electronics hobbyists will usually park the hot iron carefully in a bench holder made for the job. A novice would have to be extremely unlucky, inept or very careless to receive any injury from an ordinary soldering iron, which is generally less dangerous to its user than, say, a cigarette lighter.

For the average electronics constructor, accidental burns are a non-issue. They seldom if ever occur. Typical EPE Magazine readers will probably dismiss any safety-related claims of the Coldheat iron as an over-statement of the hazards associated with any type of hot tool.

The lack of any temperature control and the need to ‘coax’ the heating action into operation with trial and error means that the soldered joints were of a completely inconsistent quality. Worst, of course, was the damage to the adhesive of the copper foil resulting in three joints lifting off the p.c.b. laminate.

Unfortunately, the net result was that the writer’s review p.c.b. resembled something made by a first-time electronics amateur.

Verdict

The unorthodox “Athalite” bit is a clever use of compounds but is very fragile and likely to be easily damaged by an inexperienced constructor or novice. Unlike well established, rugged and traditional irons, spare parts do not appear to be easy to source. The maker’s claims for lower running costs when compared with e.g. butane irons may well pall in comparison with the likely costs of a new “Athalite” tip – if you can get one easily.

The Coldheat iron requires an non-conventional soldering technique. It is best used for occasional simple DIY electrical or mechanical work, such as soldering a thin wire onto a small bulb, soldering two passive components together or fixing a small broken wire; occasional spot-repairs in difficult places, where you were unable (or couldn’t be bothered) to plug in an ordinary iron and wait for it to heat up; quick, spot-joint repairs where you might want the iron to be cold enough to put away quickly afterwards; isolated electronic repairs where you were satisfied that no surrounding components would be damaged by the voltage present across the tip.

The manufacture’s web site claims boldy that “ColdHeat Exhibits Soldering Superiority”. This was not in evidence after our own practical tests. The lack of temperature control is a concern. Also, the very low impedance voltage that exists across the tip generally rules out its use in electronic circuitry except in cases such as a very simple isolated spot-repair. It is certainly not worth risking damaging e.g. a motherboard or p.c.b. with this type of iron, except in the very simplest 'spot repair' jobs once users were satisfied they would not be endangering other components in the same circuit.

With just a little practice with an ordinary electric iron (and helped by our Basic Soldering Guide!), it is perfectly possible for any novice or hobbyist to produce good quality consistent results on a p.c.b. that are superior – and faster – than the Coldheat principle. An ordinary iron lets you heat the entire joint to the correct temperature, and you can press down on wires with no risk of breaking the tip. It is also likely to cope with a far wider variety of wire gauges and tasks, and is easier to understand.

For approximately £15 (cheaper or equivalent to the Coldheat iron retail price in the UK), there are much cheaper alternatives available for the electronics hobbyist, including the excellent Antex electric irons complete with a full range of tips.

Unfortunately, I can’t recommend the Coldheat Soldering Iron to the average electronics constructor unless they are extremely accident-prone, lacking confidence or have a real need to go cordless: they should save money and buy an ordinary electric iron instead. If cordless use is an issue, then cash spent on a gas-filled iron is likely to be money better spent over the longer term.

Users stand a much better chance of producing a higher quality and more consistent solder joint with an ordinary iron using accepted techniques, and there is plenty of expertise around to help ensure that their time spent soldering will be a more rewarding one.

| Top | Back to Introduction | Back to Basic Soldering Guide |

All images and text © A R Winstanley March 2006

Alan Winstanley's EPE Basic Soldering Guide is referenced by trainees in the US Marine Corps, US Coastguard, the avionics industry and countless industrial, commercial and educational users around the world. You can E-mail the writer at alan@epemag.demon.co.uk