Professional Home Brew PCB: Creating a Solder Mask Using UV Curable Paint

I noticed that on ebay you can purchase UV curable paint for creating solder masks on your home etched PCBs , but the attached instructions that it comes with are woeful. On the auction description and the bit of paper that eventually came with it all it said was.

-  Apply paint on your PCB
-  Spread it on the PCB with covering the transparent mask film
-  Expose it under UV light ( or sun ) for a few minutes
- Wash away the extra paint by gasoline or oil base solvent

Which sounds a bit sparse and inaccurate for my liking, so after some investigation and experimentation i've come up with this guide.

Step 1: Requirements

To complete this you will need:

- UV Curable PCB ink  - Ebay
- Foam brush or Roller (You will use less paint with the foam brush) - Dollar shop
- Inket or Laser transperency sheets - Office supplies store
- Inkjet or Laser printer
- Old toothbrush
- Mineral Turpintine - Hardware store
- The Sun
- A sheet of glass

Step 2: Applying the Paint

You'll need to find yourself a spot out of direct sunlight, indoors under a weak fluro bulb  etc should be fine.

First of all squeeze out a little of the paint along the top edge of your PCB and scrape it down using a foam brush, like you get from a craft or dollar store and try to get an even coverage.

You dont need much to get coverage, applying too much increases the drying time significantly. work the paint back and forward across the board until you get a light even coat over the board all your tracks should have a green (or whatever colour you are using) tinge to them.

Step 3: Applying the Mask

Now set the board aside somewhere dark to dry for a several hours. I put mine in a sealed cardboard box to make sure it was not exposed.

You will now need to print the solder mask that will cover the board and protect the pads you wish to solder from being hardened in the sun. If you are using Fritzing to design your PCB then when you export it it will create a solder mask PDF you can print as well.

If you use a laser printer to print your mask then make sure you use the highest quality possible so it has the most toner on it to create an effective block for the UV light from the sun.

Cut your mask down to size so it is the same size as your PCB.

Once your PCB is dry it's time position your mask over your board and then use a sheet of glass to push the mask down flush on the board. This will avoid the chance of shadows on the board protected things you did not intend.

Place it all in direct sunlight for about 30 minutes.

Now the UV light should have hardened all but the areas protected by the mask, take it inside and remove the mask and lightly scrub the pads with turps using a toothbrush.

The non exposed areas should wash away. Rinse the board in water to remove any excess turps. It should now be ready to solder.

Step 4:

Your pcb should now have a nice green solder mask protecting your circuit

Possible enhancements

I found i still get some brush strokes using the foam brush as well, it might be necessary to apply a few coats to get an even covering. I'm not sure how this will affect the UV exposure though.

The ink i purchased has a matt finish , which is fine from a functional standpoint, but to improve the look of the board it might be worth a coat of clear polyurethane after soldering is complete.

There are many different colours available on ebay, i might try blue,red and yellow also

Comments

[ahernandez5]

Just watched a video of a guy pressing it with a piece of glass with plastic film and let the plastic film fuse to the solder mask, and it came out flawless:

[rpotts2]

I just saw a video on YouTube where a guy put a dollop on the board, put a THIN plastic film over that, then squeegeed with a credit card. after it was all spread then he put the printed mask on for UV exposure.

when all was said and done there were NO strokes visible and the solder mask looked extremely well done.

[MatthieuR]

I might have seen the same video. In the end the mask looks incredibly smooth and shiny.

[FtForger]

has anyone tried using this same ink to mask off the traces for etching?

[mouseas]

There is a similar material designed for etching. The main difference is that this mask material is not designed to come off, whereas etch resist is designed to come off with a removal chemical.

[summat]

One thing I've just tried - roughly spread the paint around using squeegee etc, then place the board in a plastic bag, and feed through a COLD laminator (we all have one of those!) The results were a pretty good, thin, uniform layer (and a dollop of excess paint squeezed out of the bag and all over the laminator - hint, use bags that accomodate a bit of excess paint!

[Schwuuuuup]

Hi,

I am testing this eBay solder mask as well and did not yet have any good result.

But my latest finding is:

YES you can put transparent printer foils directly on the wet paint. If it is really cured you can remove it and have a glossy surface.
BUT the Toner has to be UP - e.g. NIT touching the wet paint.

Unlike exposing photo resist, where you want the black toner as near as possible to the resist to avoid shadows, this does not work with my (blue) solder mask: It dissolves the toner!

You mention storing the painted pcb for a while. My solder mask never hardens. In fact my cellar gets more and more covered in solder mask because it never get's dry and makes everything it touches blue and sticky.
(My girlfriend is mad at my, because I stepped into a tiny drop of solder mask and walked around in the house leaving blue spots everywhere :-/ )

[madisoz]

I have the same blue as Schwuuuuup has. It really doesn´t dry. I keep the bottle in a box with some stains in the box that are still sticky after 3 months. So storing away for 2 hours or even 2 months doesn´t make much sense. That applies also to heat drying.
The solder mask paint/paste is actually UV curable resin. These resins have problem when cured in open air because oxygen acts as inhibitor to curing process.

That means the ebay sellers instructions are correct - covering paint with film avoids the paint being exposed to oxygen

So far my best results come this way
- make sure the pcb is absolutely clean and dry
- drop nessecary amount of paint on the centre of the board
- cover the paint with plastic film. My best results come with film cut from thin transparent glossy document sleeves
- squeegee the paint through the film with some plastic card from centre of board to the edges. This helps greatly to avoid the forming bubbles
- place and align laser printed solder mask film over the film covering the paint
- put a sheet of glass on top of solder mask for weight
- expose under UV light. I use 36W UV nail lamp bought from ebay. Time depends of paint thickness and quality and exposure distance. Usually 20-60min
- remove the film and wash the pcb with denatured alcohol - works better than gasoline.
- I am not quite sure but it seems that peeling off film from cured paint immediately is not a good idea - let it be for a while - the curing process might go on for a while without UV light

[Angelo S]

Hallo madisoz

I try your method for some days, (I use black paint) and the few times the paint drys it remains on the film, not on the PCB. What am I doing wrong???

[madisoz]

Hi Angelo S,

Only thing that comes to my mind is that you have strong case of UV underexposure. From my experience I can say that green paint cures a lot better/faster than green. This calls for black paint to be even worse than blue. The paint is only cured under the film being insufficiently cured on PCB side

[Angelo S]

Thank you for your reply, madisoz!!!

I am thinking the same, so I left it under UV for a hole night... more than 10 hours!, I also tried to decrease the quantity of the paint, but the outcome was disappointing.

I would like to ask you, what kind of UV lamps do you use? Mine are transparent sterilization lamps, a true pain for your eyes if you look for more than 30 sec. I have 3 for an A4 area, and they take about 1h 20 min of exposure before I can remove the photoresist with sodium hydroxide....

I am thinking to replace with four blacklight lamps, but first I would like your advice.

[madisoz]

Hi Angelo S,

the UV soldermask paint is designed for 365nm wavelength UV. So is the UV curable nail paint.
That´s why I use lamp like this
hxxp://www.ebay.co.uk/itm/36-Watt-UV-Gel-Nail-Art-Lamp-Dryer-Manicure-Light-Shellac-Gelish-Timer-36W-/271229033014?pt=UK_Health_Beauty_Nails_Manicure_Pedicure_CA&hash=item3f26829a36

works pretty good with solder mask, photoresist etc.
Sterilization lamps are too shortwave - about 250nm. Blacklight is closer to a needed wavelength.
Nail drying lamps like one linked above do not cause any significant eye stress

[Schwuuuuup]

Hey Madisoz,
is there a special trick for aligning the laser print?
The paint is very dark and so I have difficulties to see the pads (TQFP48).
when I have a very thin layer of paint, I squeeze almost all of the paint off the traces

[madisoz]

Hey Schwuuuuup,

here is blue coated pcb (actually 3 of them on 100x160mm board). Sorry for photo quality. In real life the board looks a lot better. Uneven coating is not so visible as on photo. Soldermask has nice glossy surface

[madisoz]

Hi Schwuuuuup,

I just use a bit larger pcb than actually needed, so I can place alignment marks on four corners outside pcb edge. I make 4 films - copper front, copper back, solder mask front and solder mask back. All films have alignment marks. So the etched pcb has also these alignment marks. When squeegeeing I try to not cover the alignment marks or squeeze them visible. Also I do not actually use laser transparencies - they deform too much (especially for pcb densities like TQFP needs) in printer oven - I use 90g tracing paper instead.

[AtomRat]

I've attempted this myself as well and I too have a single tub of this paint, but in green again. Whenever I have tried to place the transparency mask directly on top of the wet mask and then cure, there's always at least a %40 chance that the mask peeled up in sections where it cured to the plastic, rather than the pcb board. Maybe my exposure time has been too little, but I usually would leave it under for almost 40mins - 1hour just to make sure, yet it still sticks a fair bit.

I too have been told off by g.f for streaks of green on carpet... coffee table.. sink nasty stuff. Keep a moist towel / rag close by always hehe

[sdelaney]

I'll have to try that , i never tried applying the transperency with the paint still tacky, instead i dried it with a heat gun or in the oven (which i forgot to put in the instructable). I'll try it out and revise the instructable.

[PeterMetzger]

I recently purchased the eBay variety solder mask in white and blue. After a few application tests, and reading the comments below, I'm wondering if the best way to get smooth, streak-free coverage isn't just to use a small foam roller. As sdelaney said, a foam roller would probably absorb more paint initially. However, as most of you seemed to concur, this stuff basically never dries out. So, I figure a cheap roller from Menards or Home Depot should do the trick. It may soak up some extra paint to start, but if you plan to do lots of painting, it should work for a long long time.

I was also intrigued by the note about not exposing the pant to air during curing. That would make sense now that I've tested the paint a couple times. Parts of my test boards with paint not covered by transparency seemed like they never cured, even when left under UV for 12+ hours.

[mikeanthoney]

Thanks for this instructable!!! It took me a few tries before I got it right. Now it works every time. I was having some issues because I was using laser printer transparencies as the first layer, which stuck to the ink when it cured, not such a good idea in retrospect. Instead, I cut up some clear glossy report covers and they worked great. I used a fluorescent black light (like you would see at a dance club) for 15 minutes and the plastic peeled right off. A light scrub with acetone to clean the pads and another 15 minutes under the black light and they were perfect.

I used the black jars of UV curable solder mask with all of the Chinese writing on them that you can buy on eBay. Green color. This finish is nice and glossy. My only issue now is getting an even layer. Otherwise they're very nice.

[mechagen]

Is your last name is Delaney? If so then oh the irony.