Homebrew PCBs

Soldermask UV cured ink is available on eBay in various colours either 
in 10cc syringes or 100g pots.

In theory, you coat the board, cover with plastic, expose to artwork 
(pads black) for a couple minutes, and clean off the pads, then 
re-expose without artwork for 30-40 minutes.

In practice it can be very frustrating, firstly it has to be exposed 
wet, you can't dry it, the type of plastic you cover it with is very 
important (bonds to most when cured, you need something it doesn't bond 
to), you can't have any air bubbles between the plastic and the ink or 
it won't work, getting a smooth finish is tough because it's very 
viscous, but you also have to get it very thinly coated because too 
thick and it won't work well, then trying to align this whole mess over 
your artwork!  For small boards, a couple cm on a side, it's not too 
bad, but once you're at something like 5x5 then it's a real pain.

Anyway I've been perservering on and off for a few months trying to get 
a workable method for using it, and finally, I think I have come up with 
something that is actually repeatable.  Still needs a bit of practice 
but so far this is working.

Supplies:

   First you must find the "special plastic".  I am using some 
self-sealing cellophane gift style bags, I suspect that they are 
actually OPP/BOPP ([Biaxially] Oriented Polypropylene), I bought mine 
from a local cheap-chinese-junk shop, they look the same as these
      http://www.craftumi.com.au/detail.asp?id=128785#prettyPhoto
   you know the type of thing I'm sure, perfectly clear and flat, can be 
sharply creased, very thin.  I just pull them apart along the sides and 
cut into two sheets.

   Now the artwork, I'm just using laser printed tracing paper, single 
sheet.

   Some self adhesive transparent contact book covering film.

   A glue stick (white/transparent), and some tape.


Process:

   Cover your artwork with the contact book covering.

   Spread some glue stick on top of the book covering.

   Now stick a piece of your special plastic on top smoothly.

   Trim one end so it's all the same length there.

   Stick your PCB (etched, cleaned) down to your desk (or rather, some 
cardboard/paper because it'll get ink on it for sure) lightly so it 
won't move about, or if you have some extra length on the PCB you can 
hold it there.

   Take the prepared artwork and align it over the PCB, tape it down at 
the trimmed end so that it now forms a hinge, check you can hinge it up 
and down and that it will be aligned when down.

   Lift the artwork, and place a bead of ink along the inside of the 
hinge, now using something stiff as a squeege gently smooth the artwork 
down slowly pushing the bead ahead of the "interface" between artwork 
and pcb so that there is no air trapped, ideally you want a little to 
ooze out of all the edges.  Once it's down use the squeegee to smooth it 
out more if necessary and ensure there is an even (as you can be 
bothered) coverage.

  Carefully remove the board from your bench and expose to UV for a few 
minutes (clamping/vacuum framing it probably won't be necessary but if 
you do, be gentle or you'll push all the ink out).

   Peel off the artwork, wash the board gently with a solvent. Acetone 
works very fast (literally wipes away the pads), but a bit too agressive 
for the partially cured ink and can mess it up easily, you might have 
better luck with isopropyl, or turpentine.

   Expose again for a long exposure.

   NB: If you have more than 1 of the same board to do, you can clean 
off the ink from the special plastic, or peel it off totally if it's too 
dirty and stick a new piece to your artwork (the glue stick just holds 
it in place lightly, easy to peel off).

I have not heard of this soldermask and am puzzled. You expose it the 
first time with the artwork and then expose it again for a lot longer 
time without the artwork. I am curious as to how this works. It seems 
like it would all just harden. Can you post some links to this that 
may explain it better?

You can find the stuff on EBay, UV Curable Solder Mask, but you won't find any more information than that you see here. The Chinese shops sell it but don't have any details on the stuff.

When you expose board with mask the ink harden. But if mask isn't full 
black (the laser print isn't full black) then there is practice to not 
expose too long time as unwanted area may be exposed too. So you just 
keep the swet spot when difference exposed/unexposed is big enought to 
wash the unwanted ink off. Then you expose developed board again to 
fully harden the ink.

The first exposure is long enough that the areas you want to stay on are 
somewhat hard, take it out, remove the artwork, wash off the uncured 
pads, then re-expose (without artwork since you have cleaned off the 
areas you were masking anyway) to fully cure.

Paul Alciatore wrote:

>  
>
> I have not heard of this soldermask and am puzzled. You expose it the
> first time with the artwork and then expose it again for a lot longer
> time without the artwork. I am curious as to how this works. It seems
> like it would all just harden. Can you post some links to this that
> may explain it better?
>
>

twgray2007@... wrote:
>  
> Walmart in the wrapping paper section.  If you are using plastic 
> transparencies you can dispense with the celophane layer and place it 
> directly on
>
> 

The stuff bonds to most plastic including printable transparencies in my 
experience, maybe you struck it lucky with the type you tried :)

Oh, I missed the step where you wash off the uncured areas. Now it 
makes sense.

Thanks!