soldering irons and spam
You were warned.
About a year and a half ago I got a piece of spam pitching the Coldheat soldering iron, which is now available everywhere from Radio Shack to drugstores. I didn't see a price for the Coldheat right away and thought from their description that they might be a really elegant poor-man's version of a Metcal rf-inductive-heated soldering iron. Battery-powered and portable no less. The Metcal patents have been expiring, so hey, it's remotely possible if rather unlikely. For the first time in my life, I was somewhat tempted by a spam offer. I ended up writing most of this back then, an example of how long I can hold down a hairball.
I checked them out a bit exhaustively. They're not even close. They pretend they've got secret magic technology, but it's um, PATENTED. So we can look up the patent on the US Patent Office server, and see how they did it. Gee, that rhetorical trick of pretending that a patent is like a trade secret, only EVEN BETTER AND MORE SECRET, just doesn't work that well in this day and age, does it?
The patent number is #6646228. Easy enough to look up on http://www.uspto.gov; I'd give a link but they expire.
They use a resistively-heated graphite tip that's split in two. They pretend they've got some advanced materials technology in the tip, but the patent spills the beans: "For example, battery electrodes containing graphite, such as battery electrodes obtained from Eveready.RTM. Super Heavy Duty Lantern Battery Model No. 1209, manufactured by Eveready Battery Company, Inc., Cleveland, Ohio, have provided acceptable results." You have to make contact with something conductive for the tip to heat up. It's just resistive soldering. You use graphite basically because it won't melt.
I have my doubts that there isn't prior art on exactly this kind of thing: it seems straight out of the old Alfred P. Morgan "Things a Boy Can Do With Electricity" books from the thirties.
But here's the rub: the perfect soldering iron would maintain a perfectly constant temperature and have high thermal conductivity, so that it instantly heated your solder joint just hot enough to melt solder and make a clean solder joint. Could the Coldheat not only be a perfect soldering iron, but run on batteries and cost under $40, too?
Cheap-but-usable low-power soldering irons (10-35W, $5-$15 new) approximate the ideal by running without temperature control but with plenty of thermal mass. So they have a steady-state heat that's enough to make a clean joint every so many seconds, probably less time than it takes to set up the next one, and they normally won't overheat your solder joints by too much. These can be a problem when you're soldering something with a lot of its own thermal mass, because parts of it can overheat before you have the whole thing heated hot enough to solder, which can take, well, forever. They can also be a problem when you're soldering anything that's temperature-sensitive.
High-quality soldering irons have both temperature control and lots of power output, so they can put out enough heat to heat your joint quickly while a thermostat of some kind prevents them from overheating it. Some soldering irons in-between will have a control for their heat output but no thermostat. Temperature and heat output are two different things, of course: a lighter can make a plenty hot flame but you can't easily roast a turkey with it. Lots of hobbyist books when I was growing up had plans for adding such power controls (usually a variation on a lamp dimmer circuit) to a cheap soldering iron. The ARRL handbook probably still does. You can make do with something like this, but it really just confuses the issue. If you're not getting change back from a twenty, you want a soldering iron with a thermostat. A nice model that's widely available new is the Weller WTPCT.
At this point, you should be able to figure out what's wrong with the Coldheat makers' claims:
- Heats and cools almost instantaneously for comfort and safety.
- Reaches 500ºF in less than 1 second
- Tip cools to touch-safe temperature within 1-5 seconds
- Uses 4 replaceable AA batteries (not included)
- Over 700 joints per battery pack
There's neither any temperature control (because it just keeps heating to 800F), nor any thermal mass (because it cools off quickly), nor can you physically sustain high-power output from four AA batteries. Nor will the graphite tip "tin" (coat with solder) like a regular soldering iron tip for good thermal conductivity. And graphite is fragile: the Coldheat instruction manual warns about putting pressure on the tip and advises keeping the tip covered while the iron is not in use.
Admittedly, as a solution to "how do I make a portable soldering iron that runs off disposable batteries?", it's absolutely brilliant, and if you are already skilled at soldering and you already have good tools, it might be worth checking out. You'd only be out $20-$40 with shipping and all, and the Coldheat does in fact appear to do what they say. If you need a soldering iron that will run on 4 AA batteries, I don't think there's any other option. Unfortunately, even the heavily edited QVC demonstrations on their website and this unintentionally hilarious on-line review (complete with out-of-focus "macro" shots of cold solder joints presented as an accomplishment or something: http://www.moddershq.net/reviews.asp?reviewid=32&pagenumber=1) both show that the Coldheat doesn't solder anything very well in unskilled hands. It's not a matter of whether it does what it says, but whether what they claim is what you want.
Now, if you really do want the world's greatest high-tech must-have soldering iron, get on ebay and look for an older Metcal base station. These are RF inductively heated, instant on, constant temperature, high output soldering joy. They don't cool off instantly, and they won't fit in your pocket, either, but they do look sharp. They'd better, for what they cost new. Used you should be able to find one of the older base stations complete with two or three tips for around $80-$150. They have a whole line of tips for everything from fine-pitched surface-mount "rework" (i.e. repair or prototyping of modern electronics) to the sort of jobs where you'd normally use a large soldering gun, like soldering big heavy connectors. Here's an edited excerpt from a document on hand tools I shoot off to my clients when they get all impressed with my laboratory and start asking for tool recommendations (let me know if you want a copy of the whole thing):
Models to look for are the "STSS", "PS2", and "MX" series. The currently produced model is the MX500. You can get the older ones like the PS2E-01 and similar models for significantly less than the MX500 on ebay, and Metcal has an affordable flat fee for repairing them if they're DOA. Call and ask if the risk is significant to you: this could certainly have changed since I last talked to them.
The one to avoid on the used market is the SP-200; this is a new low-cost model that uses tips and accessories that are incompatible with the standard line. They make some sense if you are buying new (new, they're half the price of a new MX500), but the ebay prices are (still) way too high.
Tips are expensive (typically about $20 new) but you don't have to replace them often, especially not with light use. Used industrial surplus tips are readily available on ebay and at swap meets. My experience with used chisel tips is that they're usually pitted badly enough to be well on their way to being unusable, just not quite there yet. Other tip styles I've bought used have been OK.
I have no real affiliation with the Metcal company though one of the inventors of their technology is a colleague of past colleagues; I'm just very happy with my MX-500.
The bottom line for hobbyists: Get a Metcal if you can possibly justify the expense, but any temperature-controlled soldering iron will give you a lot more joy than anything you can find under about $30. The Weller WTPCT is a classic at around $135 new, and there are roughly comparable-to-the-Weller import models out there for under $50 if you look around.