Why Does Solder Smoke Always Find Your Face?

For some of us the smell of rosin soldering flux vaporizing from the tip of an iron as a project takes shape is as evocative as the scent of a rose on a summer’s day. We’ve certainly breathed enough of it over the years, as it invariably goes from the piece of work directly into the face of the person doing the soldering. This is something that has evidently troubled [AlphaPhoenix], who has gone to extravagant lengths to research the problem using planar laser illumination and a home-made (and possibly hazardous) smoke generator.

He starts with a variety of hypotheses with everything from a human-heat-driven air vortex to the Coandă effect, but draws a blank with each one as he models them using cardboard cut-outs and boxes as well as himself. Finally he has the light bulb moment and discovers that the key to the mystery lies in his arms coming across the bench to hold both iron and solder. They close off an area of lower-pressure dead space which draws the air current containing the smoke towards it, and straight into his face.  It’s something that can be combated with a small fan or perhaps a fume extractor, as despite some video trickery we have yet to master soldering iron telekinesis.

39 thoughts on “Why Does Solder Smoke Always Find Your Face?

  1. Here’s me thinking it was the soldering iron is in your hands attached to you body and head which is right over the work because you are soldering some P1$$ ant component to the board. Obviously this answer is better because it has a green laser and a guy in shades in the dark in an improbable soldering position as well as improbable soldering shirt !!
    ;-P

          1. Ghost Grease was a slang term for the old heat sink compound. It was (very) white.
            Its “whiteness” may have been the reason for the name, but I felt that its ability to go through walls and appear on places you hadn’t contacted was more apropos.

  2. Very cool. A Magnificent laboratory experiment to analyze a truly nagging annoyance. Knowing the causal conditions has given me a few thoughts about simple ways to keep the solder fumes out of my snout.

  3. In still conditions the warmth of your body will create a convective updraft, and that will dominate, particulates/gases will be drawn into it. In other situations where the thing making smoke is a lot hotter than your body, then it is drawing air from all around, but because you are blocking some portion of it, the pressure is infinitesimally lower on that side, so the emitted smoke and gases will go toward the very slightly lower pressure. Also with slight drafts or breezes that you have your back to, thinking you’re sitting “upwind” a vortex will form rotating away from you at your head, but drawing toward you lower down, also having the effect that smoke goes in your face. At greater wind speeds these vortexes are not as stable or large and shed downwind.

    1. When I worked in the Fluid Mechanics lab as an undergrad we had fun with a smoke machine and a laser seeing the convection rolling upward off human beings. It is very significant even at room temperature.

  4. I have loved that smell for over thirty years. Didn’t know it was meant to be bad for us till I was already too old. But I don’t want my kids breathing it so I will get a fume extractor for them.

      1. Actually, did anyone else notice that in the last year, canned pasta sauces have gone super bland, and maybe a bit more watery. Have to stick in a bunch of stuff to bring them near to decent. I mean before you’d tweak it, but these days it seems like you’re just starting from a can of plain sauce.

  5. I never realized how dangerous the fumes from soldering were until I had to hand-solder 250 boards. I wasn’t aware enough of the dangers from soldering prototype quantities. When I scaled up production, it hurt me.

    It can seriously injure you. I didn’t suffer any permanent debilitation, but the acute impact of breathing in too much flux fumes for me was a relatively serious acute bronchitis.

    After that, I always made sure I had a vent sucking the fumes. I made it from one of those Pixar lamps, so both the light and the suction were right where I needed to work.

    1. I worked in a board-stuffing shop for almost 2 years and found that rosin-core solder caused no issues for me. The no-rinse flux, however, caused major issues after just a couple of minutes.

  6. I learned from an old bench tech to gently blow a slow breeze at the connection you’re soldering for two reasons:
    . Keep the visual path unobscured by the fumes;
    . Keep the VOCs away from your face.

    1. Same here, I’m asthmatic (since childhood). While a gentle fan is enough I often didn’t have one and I’ve always known sucking down any smoke was no good for you so I simply would lightly blow out a steady stream while soldering. It works.

    2. Yep. I’m working on a soldering fan with arms, finally, but I’ve always used a small clamp-on desk fan aimed toward my face from above. Barely notice the fan but it does a good job keeping clean air at my face.

    3. In addition to using a fume extractor, I actually trained myself to breath out while soldering. So, I’d turn my head slightly to breath in to get ready to solder, then exhale and blow across the soldering area while soldering. This tends to blow the smoke towards the fume extractor, or generally way from me if I don’t have one.

  7. Same reason smoke from a camp fire finds your face even if wind is blowing the other way. Smoke is just being a-hole. You can’t explain a-holes with science. Just like magnets.

    1. I can be with a group of 20 people yet somehow I’m the only one who ends up with a face full of smoke every time.

      Of course, I’m also the only person who can usually reach into the fire without getting easily burnt.

  8. Everything I’ve ever heard of says it started with this dude named Mr Murphy who consistently had the absolute worst luck and fortune in all of existence… immortalized by a close friend who, in observance of Mr Murphy’s apparent inability to rid himself of his never-ending string of seemingly entirely-random mishaps, was inspired to draw up a few supposed ‘natural laws’ in his friend’s honor, which of course immediately “went viral” in their (far more analog) day, long before that term even existed.

    1. So that’s the story of Murphy’s law, we can say that Poe’s law is a corollary of it, taken in the form, if there’s a wrong way to do, or interpret something, it will happen. Ergo, the impossibility in modern times of telling someone being serious from someone being sarcastic, and it’s from that that yet another famous law derives…

      Some years ago, a Sea Captain by the name of Cole found himself yearning for one of the ports of his youth, a scrubby fishing port it was back in the day down east. So he rolls in around lunchtime, and is quite put out to find the town of his youth gentrified and touristy, no longer the working port he remembered. However, his stomach rumbles, and he enters what used to be a roughneck tavern, now a Ristorante del cuisine or some nonsense, and orders up a plate of Haddock and Fries… and it is the worst fish and chips he’s ever had in his life, the batter has gone leathery, all crispness gone, the fish rubberised, the fries full of hard grey spots, and soggy with rancid grease. The one and only waiter, styling himself the Maitre D’ of course, sashays over and enquires if everything is to Sir’s liking… The Captain puts on his thickest brogue and says with a straight face, “Sure it’s almost pure perfection, the only t’ing to improve upon it, would maybe be some t’inly sliced cabbage, as raw as a new laid egg.” The Maitre D’ rushes off to attend to this request, and thus Coles Law was born….

  9. Way back when working for Philips they gave us health and safety solder training and this is the professional way to deal with this: ‘laminar flow’… place the solder extractor nozzle horizontally behind the PCB and it will draw the fumes across the desk surface and away from you.

    1. I’ve tried that, but the wisp of smoke still rises up to my face first before it turns around and goes into the nozzle.

      It even goes out of its way to loop up before going down to meet the vacuum hose.

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