View Full Version : Solder stop
06-12-2003, 10:24 AM
I'm doing a bit of soft soldering on brass and would like a sugestion as to what to use to stop the solder from flowing all over the piece. If there is something that might be locally available, that would be super. If it's mail order, I can live with that too. I'm using a small (HF) butane torch and tin/lead.
06-12-2003, 10:44 AM
I haven't tried it myself, but somewhere I read a suggestion that if you soot up the areas where you don't want the solder, with a carburizing flame it prevents solder from adhering. Or maybe soot up entire piece and only clean areas where solder is to stick.
I think I've read other methods, but can't recall any right now.
English author George Thomas's solution was to be exceedingly precise in his placement of solder. (This was silver soldering, but no matter.) He'd cut small bits of solder, just as much as required, and place them exactly in or around the joints. Then,when he'd heat the work, the solder would flow, fill the joints, and there wasn't any excess to flow where it wasn't wanted.
Use more flux and less solder and get everything very clean. If there is enough flux and the joint is clean capillary action will suck the solder through the joint and there will be no tendency for the solder to escape the narrow fit up. Not enough flux is one of the most common mistakes other than not getting things clean enough. The solder should immediately go in the joint if you have enough flux and the right temperature. Make a practice joint with the heat high enough to instantly melt the solder when it is applied. Use the minimum amount of solder; when you cut the joint open you should see that the solder completely filled the joint.
06-12-2003, 12:14 PM
You can soot the area as stated, or I've heard that soapston works too....I've used graphite from a pencil to limit silver solder, never tried it for soft solder though....easy to apply.
I've used toothpicks or wood slivers to prevent flow into capillary tubing when soft soldering it in place.
06-12-2003, 01:24 PM
Use a pencil to outline the area to be soldered. Sometimes you don't even need the graphite to stick, just rubbing the pencil will 'dirty' the metal enough to prevent solder flow. All other advice applies, get it clean to begin with, use the flux, etc. Or spray some barbecue paint into a small container, and use a brush to paint a barrier. Depends on the job, maybe spray the paint over the whole thing, and erase away areas to take solder with the pencil eraser. Cotton swab the erased areas to clean up enough to solder.
06-12-2003, 04:12 PM
I've soldered all my life and my main problem was always how to get it to take to things like brass. Seems like even the slightest bit of corrosion will render brass forever unsolderable. RCA used some unplated brass pins in their circuit boards that drove me nuts. The bottom wire often was not soldered and the corrosion on the pin made it very difficult to correct.
Back to your question. There is a professional solution that's used by PC board manufacturers. It's called a solder mask and it is a transparent green ink. You can check it out here:
I never used it but I know that it works very well. Even on old boards it still holds up well. As for removing it, I don't know. When doing repair work I usually just scrape it off as I will be soldering that spot anyway. I'm sure there must be some solvent or steel wool or something.
[This message has been edited by Paul Alciatore (edited 06-12-2003).]
06-12-2003, 07:23 PM
Many years ago I ran a silver soldering line for precision waves guides (lots of brass and copper) with many attachments. We used Milk of Magnesia (drug store product) applied in a small line, applied with a fine tip artist brush. The silver solder would not cross the line. I don't remember if we used the same for soft solder. WTH give it a try. No big loss as you can always use the remainder for constipation.
06-12-2003, 09:09 PM
Maybe try white-out, mistake eraser for typists. I also agree about keeping the problem under control with sparing use of solder.
06-16-2003, 03:04 PM
Thanks for all the advice. My technique can use a tune up, and the white-out and MofMag are easilly gotten. I also note that Micro-Mark carries something called TIX Anti-Flux.
My problems will be solved, now that I can get back to the little job.