DIY tenting failure possibly solved

[throbscottle]

I'm guessing that if anyone else makes home brewed PTH PCBs you'll have had the same problem. You get your board beautifully ready with all the holes tented over, chuck it in to etch, and a few holes lose their plating because your pad alignment isn't quite good enough. You curse your favourite deity, scream WHY??? a few times, and sob quietly in the woods until you feel ready to face the awful prospect of soldering component leads on both sides of the board, and/or little bits of wire into those lost holes. Again. So I've been experimenting with filling materials to try to alleviate the problem. I recommend avoiding anything based on PVA glue, if you are trying the same!

Well, whilst cleaning up a different kind of failure with the beloved blue film, the answer struck me. See if I can dissolve a bit of film and pour it into the holes (well OK squeegee it in). Then the holes will have photoresist in them, which should become contiguous with the rest of the film when it goes through the laminator, and come off with it when stripping. I tried isopropanol first and that did the trick, so no need to try other solvents.

So far as the experiment was able to go, it worked. The caveat is that I already had some PVA based filler in there, which contained oil (to soften it), which was creeping out during cleaning and making the board greasy (resulting in the above mentioned "different kind of failure"), but was hollowed out at the tops, so was still able to be filled on top of. However the dissolved film successfully stopped the oil seepage, filled some very small holes the filler had missed, and made a (very uneven because I used my finger) coat on the board. And it turned out lovely (caveat - it would have been a lot more lovely had it not been for the serial forck ups leading up to this point! Oh well...)

So the next experiment will have to be lining freshly plated holes that haven't suffered abuse and ill conceived filling...

And yes I know you can get 75000 boards for £3 or something from your favourite board house in China. I'm not interested in that, I'm interested in the process, and time spent learning from these mistakes is time well spent, IMHO.

[IconicPCB]

why would soldermask etch remove plating from within holes?
We are talking via hole tenting where the soldermask cover via holes?

[jpanhalt]

I don't quite see the question.  Is PVA "oil" real oil or just a description of its consistency?  PVA is usually in water.

As for removing residue, try Zep acrylic floor stripper (butyl cellosolve + a little ethanolamine + water) diluted to about 10% cellosolve final concentration + ultrasonic cleaning to get the tiny holes clean.  Butyl cellosolve is a versatile solvent and is found in many household cleaning agents.  You'll recognize its odor.

My initial impression, however, was why bother?  Just discard those experimental boards and use your new process.

[throbscottle]

Well, it's not a question, I just excited because of my revelation and wanted to share!

It's not soldermask tenting, it's etch resist tenting. People who use the blue film professionally use tenting to protect plated through holes during etching. As hobbyist I just can't do that reliably enough.

Forget the PVA mixed with oil stuff. I tried making a filler using a mix of white glue and cornflour, with oil mixed in to make it softer and easier to remove. Problematic as described but I left it in since it was below the surface and going to be covered up. I mentioned it for the sake of completeness.

So the next board I do will use the new method only, and we'll see how that goes.

[abeyer]

If pad alignment is so off to start with don't you still risk not developing the resist in your through holes and failing to plug them up? Maybe I'm missing something, but it seemed like tackling pad alignment issues might have been the way I'd go. (But I'm also making a bunch of assumptions based on what you've said, because you didn't really explain what your process was so it wasn't obvious why you were trying to do this at all.)

Also, is there any reason to do the roundabout dissolving solid resist vs just adding a bit of liquid resist?

[throbscottle]

I was using what I had to hand. I might invest in some liquid resist.
You read it right.
Alignment isn't so bad that it's leaving the hole uncovered at any spot, although it can be a bit marginal. I've started enlarging pads a bit to alleviate the problem, and if it's very bad, moving them slightly. It's the larger holes that tend to be affected, 1mm and over. What I find happens is the resist tends to tear. I'm putting it down to DIY methods like wet applying the resist, so it's bubble free, but water gets trapped in the holes, which I guess weakens it.

I realised that my resist mask was printing wider than my drilling template, by ~0.15% as near as I can measure it. Correcting that made things massively easier! Drilling holes accurately enough is working well now since I fixed the issues with my drilling machine, which has just drilled the replacement board for it's own prototype controller - and which prompted this thread.

I had a fairly awful time making this one, and it needs a bit of rework at the ends where it didn't get etched enough. It's shiny on the back because I resorted to taping over the top and etching the back first. Since the holes were in a poor state after that first etch, I re-plated the board, taped over the bottom (so it stayed shiny) then got into my filler experiments before etching the top. Absolutely ridiculous! I've never done it that way before and I hope to never do it again! But at least it worked.