There’s an old saying about something being a “drop in the ocean.” That’s how I felt faced with the prospect of replacing a 12 V heated bed on my printer with a new 24 V one. The old bed had a nice connector assembled from the factory, although I had replaced the cable long ago due to heating issues with that particular printer. The new bed, however, just had bare copper pads.
I’m no soldering novice: I made my first solder joint sometime in the early 1970s. So I felt up to the challenge, but I also knew I wouldn’t be able to use my usual Edsyn iron for a job like this. Since the heated bed is essentially a giant heatsink for these pads, I knew it would require the big guns. I dug out my old — and I mean super old — Weller 140 W soldering gun. Surely, that would do the trick, right?
Well, the Weller…
Obviously, it didn’t, or you wouldn’t be reading this right now. It could be it just needed a new tip — the thing is seriously old, but it just wouldn’t get the pads hot enough for solder to truly flow. I did finally get the wires to stick, but the solder joints were so bad I could not imagine they would hold up to constant flexing.
I thought about trying a hot air gun, but I decided I’d try something even different: flame. You can get butane soldering irons and torches from a variety of places at many different price points. Being cheap, I picked up a Schneider-branded iron from Harbor Freight. The handle has three attachments that nest. One collar just shoots flame like a torch. A little tube that fits the collar keeps the flame away and you get hot air out the end. That tube can also take a soldering iron tip.
We think the promotional picture on the Harbor Freight website (see adjacent) might be a bit optimistic. We don’t recommend this iron for doing surface mount work on a PCB. However, I needed a lot of heat and, as the video below discusses, the thing puts out almost too much flame. You can mod it like [marshkid1] does in the video to make it put out less, but that wasn’t my problem.
Turns out, it isn’t as easy to get butane as it used to be. Besides that, you really want butane made for this sort of tool so it doesn’t clog. I settled on just using lighter refill butane and took my chances. So far, so good.
There was only one problem. Despite the vigorous flame, the solder just wouldn’t really melt enough to flow. The tinning on the wire would melt and a little of the top surface of the pads, but with the entire surface area of the PCB resistance to sink heat, it was impossible to get a good joint.
What to Do?
I contemplated heating the whole board up in an oven or with a hot plate. I even thought about soldering under an IR lamp. In the end, I used a two-pronged approach. I removed the soldering tip from the iron and let it jet hot gas over the connections. Then I used the Weller and I did finally get reasonably good solder flow. The hot air alone, however, was not sufficient.
The joints aren’t the best-looking ones I’ve ever done, but it seems to be holding even under use. Of course, I soaked everything with rosin, tinned everything, and made sure everything was clean. There was just too much heat capacity.
I may yet try again, even though it is working well enough for now. Maybe I need one of those huge automotive soldering irons. Or maybe I should use the SMD heat gun to preheat the joints. I’m sure I’ll get some things to try in the comments.