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

Comments

[Davad]

When I spray paint I have to do multiple coats to get even coverage.
Has anyone tried multiple coats of the UV paint?
Dry one coat and then apply another before exposure.

Can this stuff be thinned before application?

Last question, is UV paint available in a spray can?
IIRC a guy I new who worked in a board house many years ago said they sprayed the UV paint on then oven baked it. Incidentally he quit his job after opening the oven door with a lit cigarette in his mouth. Need I say more.

David.

[sleemanj]

I'd like to add here that readers should look at the comments on page 1 before attempting to "dry" the board.  I didn't, and wondered why I had a lot of trouble. 

Drying the board is not necessary at all, infact, it's almost impossible, and if you cook the board to dry it then you're making it very hard on yourself.  Perhaps sdelaney had a different mixture, or spread it really really really really thin, but I couldn't get good results with his instructions to dry the ink first.

The ebay sellers are actually on the money, if it sounds a little odd, put a dollop of ink on the board, put a transparent, THIN, plastic film over the top and spread it around so that it's nicely covered.  The plastic film you use needs to be as clear and thin as possible.  You don't need to be in a dark room, it's not that sensitive, but don't go doing it in direct sunlight.  You can also use a toothpick to "paint" the board first, then put on your transparent film, this might help you get a more even coverage.

Leave the film in place and align your artwork on top of the film, toner side DOWN, so you have a sandwich like this: board, then ink, then transparent film, then tonor, then whatever you printed the tonor on. 

Now put a piece of glass on top to press it together, and expose to the sunlight, or make yourself a UV exposure box, my UV box (made from an old gutted scanner bed from a multifunction printer, UV leds and a couple hours of soldering) sufficiently hardens the ink in 55 seconds or so, experiment with scrap pieces of PCB to get the timing right, you want the exposed areas hard enough to withstand wiping or light scrubbing, but the covered areas to be soft enough that they are removed.

Once hardened enough, just peel off the plastic film (it doesn't really stick to plastic), and wash the board in turps (or acetone, or ....) gently at first in case it's still a little soft, the pads will wipe clean.  Once you've got the pads cleaned, expose it for a few more minutes to fully cure the ink. 

This ink is VERY tough once exposed, VERY tough, it does not dissolve, but you can scrape it off pads with the tip of a craft knife if you need to touch it up.

[cpeniche]

Hi guys, I tired too the UV curable paint to do my boards. To be honest, for me It worked just once and never worked again. What I did was to use the dry film solder mask. It's waaaaay easier to use it, no mess and very reliable. Check the instructable for dry film solder mask.

[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

[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.

[FtForger]

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

[AtomRat]

I went and bought a rather large amount of the UV cure green mask ( glossy finish though ), and have had so many issues with application using anything but the brush you suggest, or using a rubber sqeegie to apply an even coat. If you don't, it just makes streaks and problems everywhere I know!

I feel there must still be a better solution to using this paste to get a finer finished product. Clear coat makes it look nice yes, but I still have the issue of streaks and uneven amounts over tracks.

Im going to attempt to sandwich the paste under some clear transparency that has an even pressure on it, hopefully when it dries, it will not stick to the plastic and I can remove it, leaving an even layer about .2mm thick or so of dry UV paste.

[sdelaney]

I've been using a heat gun/hair dryer or oven to bake it on before exposure, Which is what seems to remove the lustre somewhat, but makes it easier to position and setup the mask.

[AtomRat]

I'll have to try with the hair dryer now, but thanks for that tip indeed. As mentioned below as well, I have had much better results with even spreading using a piece of plastic or card like a squeegie. It even worked better than the rubber one somehow.. I think its all to do with experience / practice, I'll try more and see how I go

[vampyredh]

You can use a screen printing emulsion spreader but what we usually use for our screens is a cheap phone card or gift card they have a nice smooth edge and can do rather smal sreading jobs.

[vampyredh]

An old printers trick for your transfer. Once you laser print your transfer take some cheap dollar store clear enamel spray paint and lightly mist your transfer. All the toner absorbs the spray and darkens and solidifies. We use this trick on our velum before making screens for screen printing on t-shirts.

[sdelaney]

Nice, i'll try that out and report back

[fduraibi]

Good job, i have played with the UV curable paint before and tried different ways but couldn't get a good finish.
I will try your way with the foam brush and might even do multiple layers before the UV exposure.

[jman 31]

Nice work! That is very good to know and seems like a pretty simple process.