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Polyurethane for protecting one-off PCBs

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One of the pitfalls of starting out in electronics (I found) is that you have a lot of hair-brained ideas that you want to try. And sooner or later you outgrow the usefulness of the bread board and will want to start making “real” PBCs. By “real” PCBs, I mean good ‘ole copper clad boards etched at home using the toner transfer method. It’s a real mess – but I think it’s a really valuable skill for a beginner to learn. And, unlike a breadboard, you actually have something to keep and call your own!

(yes, the cost of board houses for small runs has dropped dramatically recently, but there is nothing like the instant gratification of holding a finished board in your hands in less then an hour)

The downside to a home etched board, is that it doesn’t have a solder mask layer to protect the copper traces from the elements.  Raw copper will quickly tarnish.  And if there is moisture in the air, it can corrode the traces beyond repair.  Urrr.  Well that’s no good!

Raw, uncleaned PCB that is about 7 years old

Anyways, about 7 years ago, I saw that someone said that you could protect the finished copper clad board with polyurethane once you were done soldering. So, I grabbed a scrap of PCB and scrubbed it with a scotch bright pad till the copper was shinny. I then sprayed 1/2 the board with polyurethane, and left the other half bare. I put it on a shelf in my garage, and it’s stayed there. That was about 2006-7.  I just let it sit there in an unheated garage for ~4 years. Then I took some pictures.  As you can see, it worked very well!  What a difference.

Left side is bare copper clad. Right side is protected with polyurethane. The board was aged about 4 years at this point.

I know the test could be more scientific (like measuring the thickness before and after) but I think you can see the results and make your own judgment how well it works. You can still de-solder and re-solder pads if you need be – the polyurethane just melts away under the heat of the iron.  Usually a hot iron helps with the ~700F.

You can see I’m using the “Minwax” brand. I think I paid $3 a can for it at Walmart. I prefer the satin or “semi-gloss” finish. The gloss looks great, but if you get any dust or lint in it while drying it ruins the effect. The satin seems to play the part much better. Drying time is about 30 minuets. Sometimes I speed that up with a heat gun.  So – for small run of boards that you need on a short notice, it a very good solution.

If you’re looking to do some a bit fancier, there is some translucent hobby store paint made by a company called “Testors”.  Because it’s translucent, you can do a decent job of faking a “real” solder mask.  Here you see the component side of a one sided PCB.  You can also coat the solder side.  Just one note:  this stuff takes forever to dry.  At least 48 hours.  If you touch it any sooner, you’ll leave finger prints into the paint.

Two of the paints you might be interested in are:

Testors 1601 Candy greeen

Testors 1605 Spray Candy Red

 

I recenly pulled the same PCB out of tool box.  And I didn’t try to match the lighting, but the PCB looks the same as the 4 years ago.  So, for this little test – 7 years in total – I’m really happy with how well polyurethane is holding up!

So – this isn’t a “real” solder mask, but it plays the part very, very well.  And will protect your projects for years and years.  Maybe I should call it a “poor man’s” solder mask for those making boards at home.

If you’re looking for a more professional product, MG Chemicals has a line of conformal coating that you might be interested in:  http://www.mgchemicals.com/products/protective-coatings/conformal/

But at $20 +shipping for a can it may not be worth it.

One thing you do want to do is to clean the PCB before applying the polyurethane.   But, I’ll save that for another blog post.

 

6 Responses to “Polyurethane for protecting one-off PCBs”

  1. […] 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. […]

  2. […] 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. […]

  3. […] 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. […]

  4. Phil Combs says:

    When I was etching my own PC boards years ago, I would spray clear enamel paint on the copper trace side after completing the project. It worked very well. Then I started using Tinnit, which also works well but has a shelf life of about 2 months once it’s mixed up. As I was doing boards in small quantities that method was a bit costly for me (and not eco-friendly either).

  5. Shannon says:

    What do you do about the solder pads though? It looks like they get covered as well. Will you be able to solder to them?

    • Yes. If need be, you can re-work the board. But it’s not ideal. The poly simply melts and burn off. You’ll get some contamination of the joint. So it’s best to solder suck the joint clean. But, you’d really use this type of protection for a finished board, that you’re happy with the design.

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