Polyurethane Protecting PCBs


What you see above is a home-made PCB. No, this isn’t an example of a terrible toner transfer job, but rather evidence of the ravages of time. This board is seven years old, and the corrosion and broken traces show it. Luckily, [George] already has seven years of environmental data for a cheap DIY soldermask.

Seven years ago, [George] took a piece of copper clad board, masked half of it off, and sprayed it with fast drying polyurethane. After drying, he put it on a shelf in his garage. The results were fairly surprising – the uncovered portion is covered in verdigris, while the coated half is still shiny and new.

[George] took this a bit further and experimented with other spray can coverings. He found Testors spray enable worked just like the polyurethane, burning off when the heat of a soldering iron was applied, and also passed for a professional PCB.

29 thoughts on “Polyurethane Protecting PCBs

  1. UV cure soldermask ink/paint is readily available on eBay etc in both syringes and larger pots (a little goes a very long way). Spread it on THIN, cover will cellophane (the specific type of plastic is important, some will bond with it, which is bad, I like cellophane plastic bags, work well) without any air bubbles (air inhibits curing), apply artwork (pads = black), and expose to UV for a few minutes. Wipe clean the unexposed pads, and give it a few more minutes of UV to fully cure.

    Once you have the hang of it (and don’t put it on too thick, needs a thin coating, a thick coat does not work well because the underside doesn’t cure) then provided you can align your soldermask artwork well enough the results are good, forms a very strong bond, is very tough, and heat resistant like normal soldermask (since, it is normal soldermask).

    Here’s one I prepared earlier:

  2. Back when I was in school and still was able to etch my own PCB’s and/or get one-off PCB’s from my local parts shop (this was at least 15 years ago, when environmental regulations around here weren’t as strict as they are now), I used something called Flux SK 10 to protect my boards and help the solder flow like there’s no tomorrow.
    If the board needed a thicker protection layer and/or protection against humidity, we were using Plastik 70 Red to finish the job.
    Both are still available and are made by Kontakt Chemie, as far as I know.

  3. Shock horror. Pub lacquer designed to coat the underside of pcb boards and stop corrosion actually coats the underside of pcb boards and stops corrosion??!

    Whatever next? (This stuff is like £5 a can in maplin, spray lacquer from a DIY shop and spray varnish work just as well…

    The trick isn’t knowing that a coat of varnish stops weathering, the trick is knowing that what you’ve made just now will still be useful in ten years and actually remembering to put this stuff on!!

    1. Actually this may be a shock to some people. While I’ve lacquered my boards since I first started making them I haven’t read a single PCB making article which has talked about lacquering here on HaD

  4. When I made my own boards, I used a chemical tin plating solution. After etching, simply soak the boards in the solution for a while, covering the copper with a thin layer of tin. Looks nice, prevents corrosion, and easy to solder.

    1. Both an actual product, and I think a typo: They probably meant “Testors spray enamel.”

      It’s in the model section of any hobby-ish store, and lots of large department stores, at least stateside. It’s not the cheapest product out there, by volume, but it’s easy to get.

  5. I usually just brush the board with a thin layer of pure rosin flux (violin rosin dissolved in alcohol), it will protect the traces very well with the added bonus of having them pre-fluxed when you decide to solder the board at a later date.

  6. Also evidence that the acid was NOT neutralized properly. if you guys think a water rinse is enough, well you need to read up again about etching boards. if you properly etch and process the board it significantly reduces this problem.

    Just use a standard enamel and call it done. has the exact same effect and is far more serviceable in the future.

  7. If you’re using a solvent-based coating, there’s a technique intaglio artists call the ‘sugar lift’ that might be useful: you literally mix sugar with ink and paint it onto the board as a resist. For a PCB, you’d put it on the solder pads that you want to remain bare after the soldermask is applied.

    Once the sugar/ink has dried, you spray on your solvent based film and let that dry, then wash the board in water. The water will dissolve the sugar, leaving a void between the solvent-based film and the copper. A bit of scrubbing will peel away the loose film, leaving a nice, clean hole in the soldermask.

    For water-based films, you can do the same thing with Crisco.

    The main problem is that applying the sugar/crisco resist layer takes forever.

  8. While “burning off” sounds convenient, it’s a guaranteed way of contaminating your solder joint, unless you burn-off first, clean, then solder. How bad this is in practice, YMMV.

  9. A few years back we did something similar (with either spray varnish or spray lacquer from Duh Home Depot) with some small boards containing an opto-interrupter and an LM319 and it ruined (at least) the analog on each and every board that we made. We had to go back and sand the coating off of all of them, after which they all worked fine. Parts were all 0.1″ DIPs, so the physical isolation between signals was about as big as one can get.

    Fixing all of those ruined boards was a huge PITA. I’ve never been in any hurry to try this again — got some Liquid Tin instead. Didn’t try polyurethane though.

    Setting boards in the garage is only one measure of performance. An end-product that actually functions is kinda nice.

  10. Back in the old days my father used three methods to coat the boards:
    – rosin (the violin type) thinned with alcohol and “painted” on the board with a brush
    – nail polish, regular type
    – tinning

    I’m sure some of you will complain about unknown effects and corrosion and whiskers and whatever but the boards went on for many years (15+). The applications were 12V power supplies, timer for staircase lights, RF antenna amplifiers and other stuff, mostly analog.

    What NOT to use is acetic acid-based [bathroom] silicone. It has a pungent acetone smell and it will corrode joints within a month or so.
    Hot glue does a decent job but it’s not airtight since it shrinks after cooling. It’s relatively easy to remove if a rework is needed though.

    There are some rubber coatings available in spray cans that should also do the job just fine. Some of them are meant to cover car parts to protect them against light abrasion so they probably don’t attack copper nor solder.

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