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solder mask - UV solder mask procedure

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r00n:
Hi. Lovely Day for PCBs.

I am trying to do my first solder mask  ---> I have had about 20 failed attempts and I am down to my last bit in the tube. It is green of course. All the information I have on masking obtained from you-tube videos so I thought if I documented my recipe you may be able to determine where I am failing.

Here goes:

1. photocopy the mask in duplicate onto 2 pieces of what we used to call Overhead transparencies, the plastic sheets that lecturers used.  I tape the two sheets together being careful to line up the black masks to block out more UV.

2. sand copper with a bit of really light grade wet and dry paper. wash the copper with isopropanol.

3. put enough UV solder mask down on the copper to cover the board and place another transparent sheet over the top. Use a credit card to squidge out the UV solder mask with an even covering over the copper.

4. place my duplicated photocopy mask over the top plastic lining up with the solder pads.

5. place under UV light for about 4 minutes.  (I completed a test exposing the UV solder mask to UV for varying times incrementing in 30 secs.  For 7 minutes the porridge was too hot, and 2 minutes it was too cold)

6. remove the plastic masks and sheet and wash the pads with a small amount of alcohol and a paint brush being gentle.

should be ready to solder then...................................but no.

The error occurs during removal of the mask and plastic sheet. The mask sticks to the OHP transparency and not the copper. Not all - but most.  For the UV mask left on the copper, the pads are still moist enough to clean off.
Any tips you have would be well received. Or perhaps there is a better method altogether?

thm_w:
https://learn.adafruit.com/how-to-solder-mask-pcbs/applying-resin

I don't think the transparency print needs to be duplicated?

What UV light is used?
Photos?

coppercone2:
printing on transparancies is a little dodgy. I put it this way, I put a piece of kapton tape on a PCB during exposure to keep the transparency on. The area under the kapton tape was nearly perfect but the traces were a bit like swiss cheese for the same exposure time. Set with maximum darkness on the printer.

But I did do a long etch, that is.. no polygon on top. So it was in the solution for a lot longer then necessary. If I put polygon I could have drastically reduced etch time and there would be no problem on the ground plane.

my result was acceptable, but I can see it being improved.

i use presensitized boards though.

I heard people using some kind of translucent tracing paper with better results then transparencies. BTW transparencies are not so transparent to UV anyway. For instance a clear acrylic sheet, put under the UV light, you can see strong attenuation. SO paper is not that much worse, apparently. You do use glass to press it down on the pCB?

r00n:
thm_w - thanks for the link. im not certain on the light that i am using but it is strong enough to cure the resin.  After looking and reading the link I tried a shorter exposure time of 30 seconds and a thinner spread of resin.  The resin cured and the mask didnt - which is exactly what is required, however it was stuck to the transparent film and not the copper.


--- Quote from: coppercone2 on November 28, 2023, 01:36:15 am ---I heard people using some kind of translucent tracing paper with better results then transparencies. BTW transparencies are not so transparent to UV anyway. For instance a clear acrylic sheet, put under the UV light, you can see strong attenuation. SO paper is not that much worse, apparently. You do use glass to press it down on the pCB?

--- End quote ---

Coppercone2, I'm certain that the transparencies are the fault.  If you have any more info on that tracing paper that would be useful info. not using glass but will give that a try

edit - maybe i could just paint it on with a brush and leave the solder pads :)

shapirus:

--- Quote from: thm_w on November 28, 2023, 12:54:13 am ---I don't think the transparency print needs to be duplicated?

--- End quote ---
I guess it depends on the printer, but in my case (for etching with photoresist, not for solder mask, but the reasoning is I think the same) I absolutely had to print it twice, normal + mirror, then, after drying the ink (I'm using an ink jet epson L805) put the two together, ink sides facing each other, align, fix them together using sticky tape, and it's good to go.

With a single print it was a consistent failure because of very low contrast: black areas were half-opaque with multiple small transparent dots without any ink in them, so it was impossible to expose uncovered areas enough and not expose the covered ones at the same time. However, with two prints combined it became a consistent success (the biggest problem now becoming the reliable application of the photoresist film onto the board), and with a very decent resolution, too: 10/10 is not a problem at all, and, judging by how well the copper text with component reference markings serving as poor man's silk screen is etched, even thinner traces and clearances should be possible.

Can also print twice on the same surface for the same effect (after thoroughly drying the ink which I do in a ventilated oven at 50 degrees C), but, even though the printer does a very good job at consistently aligning the image (or the sheet?) to print in exactly the same place, second printing spoils thin white (transparent) lines, which get even thinner, because the already printed areas can't soak enough of the new ink and it spreads a little outside of where it's supposed to stay. So I abandoned it and kept using two transparencies, at least where traces are thin and dense.

Firm pressing (with some glass) of the transparency against the board is vital, too.

I wonder what can go wrong using the same technique for solder mask. I've never tried it, and am now tempted to try it too.

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