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