chemical swab test for zinc chloride/salts (soldering)? - Page 1
Forum section: electronics / general

author: coppercone2
author_rank: Super Contributor
timestamp: October 09, 2020, 01:11:02 am
content: Is there a chemical test you can do to verify that flux is gone if you are using corrosive zinc-flux (or other fluxes) in a soldering job, i.e. on a electromechanical component, cable, etcLike, after you solder something, and clean it, some thing to verify that its actually clean, to ensure electronics in the same place do not get exposed to corrosive stuff.You can use a powerful soldering gun with the heating element replaced by cheap bare copper to solder with such flux without worrying about the tip (thank you mr. carleson)the idea is to make it a reasonable cleaning job, not turn it into the andromeda strain because you chose an aggressive flux

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2020, 01:22:02 am
content: Well, zinc specifically, there are indicators. Offhand it looks like they're mostly UV fluorescent, though I'm not clear whether they only work inside cellular life or what.But that's not really the important part, the important part is chloride and other anions. Start with DI water, wash, and measure conductivity. Easy enough, fast. Does require clean water and the TDS measurement thingy.Tim

author: wraper
author_rank: Unknown
timestamp: October 09, 2020, 01:39:20 am
content: It's not possible to completely clean zinc chloride. Chlorine ions will embed into everything, including PCB substrate and sit there forever. Also it's simply impossible to get flux out of wires. It will wick in between of copper strands and under insulation and corrode wire over time.Quote from: T3sl4co1l on October 09, 2020, 01:22:02 amTry zinc chloride on PCB and it will never pass SIR test no matter how much you will wash it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2020, 02:31:43 am
content: Quote from: wraper on October 09, 2020, 01:39:20 amYeah, stranded wires are just screwed. Even solid core, if it seeps under the insulation.Or even if the surface just stinks:I swear I once did some bus bars with acid flux, and it ate through the solder, from the inside.Specifically, I think it was 14AWG solid, that had been torched for some reason or another (fast de-enameling? annealing? brazing? I don't remember). So it got a soak in HCl to remove the scale. Which leaves a microscopically porous surface, with chloride soaked into it. It turns green within hours if you merely rinse and dry it.Between that, and being heavier than my iron (at the time) could do, I soldered joints using the torch and acid flux.Some weeks later, it seemed there were a few spots of green, erupting through the tinned surface. Either it had picked up moisture through a pinhole and expanded, or it corroded all by itself no air needed. Nasty stuff.QuoteI suppose it might help to add a chelator, or acid or solvent or something to seep into those microscopic crevices and get it out. Noteworthy that zinc chloride decomposes on heating, leaving insoluble but porous oxychloride and hydroxide residues. There's not really any way to improve fluid flow into those crevices. Heat, diffusion, ultrasonic energy, and time.Probably, it takes heroic effort to clean all these up (hours, days, years of soaking?). Obviously, absolutely no reason to use such a reactive flux commercially. The problem iseminently easierto deal with on the front end. Use tin or gold plated pads/leads, store in dry climate, don't use old oxidized components. Solder with low humidity (or controlled atmosphere, even).Tim

author: coppercone2
author_rank: Super Contributor
timestamp: October 09, 2020, 02:34:47 am
content: i made a TDS machine, but it requires 1 liter to work, I guess you can put a few drops in it since its very sensitive (for measuring home water)I was hoping that the ultrasonic takes care of itGuess I should not attempt it if the surface is imbuned with the flux decomposition productsI was hoping for some kind of color test swab, perhaps also useful to verify if a PCB was soldered with the correct flux (imports)