Wanted - UV cured PCB paints

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Wanted - UV cured PCB paints

Postby Dan Eakin » Fri Mar 14, 2014 9:57 am

Hello guys and gals!

I'm looking for something that i'm hoping you might be able to help me find. I'm trying to find someone who has UV cured acid resistant paints for PCB manufacturing. I'm trying to get into making PCBs with silkscreen, and I'm finding it incredibly frustrating finding a supplier for the acid resist, solder mask, and component color layer (prolly white). I find that the white component paint is also acid resistant but will that work to bath the board for etching? Looking forward to hearing suggestions!
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Re: Wanted - UV cured PCB paints

Postby Doug Coulter » Fri Mar 14, 2014 12:27 pm

Dan, it seems most people do it like this: viewtopic.php?f=34&t=33
There are other ways, of course. You can make your own negative resist with PVA and a dichromate (potassium or ammonium). PVA is a long-chain alcohol (PolyVinyl), more or less water soluble as it comes (powder or bait baggies). Mixed with a few percent dichromate, it becomes cross-linked and insoluble with UV exposure (or time, you add the dichromate just before use). Not that there isn't better stuff out there, but...they tend to sell it in 55 gal barrels only, and it doesn't have infinite shelf life, or that's been the issue when I've tried to buy it. Makes the pre-sensitized boards look pretty cheap if you're not starting a PCB business.

I've tried UV cure adhesives with high hopes, but they tend to "creep cure". EG, exposing an edge to light - the curing spreads throughout the stuff, so it can't make nice patterns.

GC used to sell the positive stuff mentioned in the link in spray cans, but people had issues getting the board pre-cleaned well enough (it's THE issue in plating too) and getting a uniform coat (didn't realize how easy it is to spin-coat things).

Plated through holes are a bear. You more or less use a completely different process, which I've witnessed but don't have the stuff for here.
a: drill all the holes - a robot is best for this.
b: dip in (weak but expensive anyway) PdCl solution
c: plate copper - the original surface copper was made on the thin side to accommodate this. This step about doubles the thickness of the Cu.
d: use holes to align artwork for etching
e: expose and develop

The PCB house I watched in action then used negative resist and plated solder to use as the etch resist, which also got into the holes nicely to protect the thinner copper there. There are other ways.

I just use APCircuits in Canada these days, they do a fine job at a reasonable price if I want pro-grade boards. And they are very fast from order to delivery.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Wanted - UV cured PCB paints

Postby Dan Eakin » Fri Mar 14, 2014 1:13 pm

i was reading your other post - i would really like to establish a silk screen friendly method (and of course do a bunch of how to videos :) ) that is 'basement' friendly and doesnt' require a specialty lab. this alternative, can it be applied through a silk screen without destroying the silks? The reason for silks is not just professionalism, but also repetitivity. I have aspirations of developing learning kits and stuff for schools, which means my income side will be very low, meaning self manufacture is really the only option. Once i get up and running smoothly though, maybe i can offer small prototyping batches for members?
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Re: Wanted - UV cured PCB paints

Postby Doug Coulter » Fri Mar 14, 2014 1:35 pm

I've tried silk screen and toner transfer. Sadly, even with the best setup, the resolution is *horrible* compared to the easy repeatability of a laser printer film and photo-lithography - by roughly factor 10 as tested here. Look at any well made new PCB. Notice how when the letters get small you can barely read them? And that track widths are 1/10th of that, yet perfectly clear? Well, those letters are silkscreened on by people who do it all day and have refined that process about to the limits. Contact printing isn't the hangup here, it's pretty fast, and you can of course, do more than one at a time...it's called "panel-izing" - you can do what amounts to a cut-paste operation in the layout software such that one exposure makes 10-20 small boards, for example, or just print out more transparencies.

There are people who do silk-screen for low resolutions, but that leaves out most of modern tech - some parts are not even available in big old through hole styles any more. It kind of depends what you want to make, or be able to make.

The methods I use don't require a speciality lab at all. I've used hamburger packing trays from walmart and regular plastic bottles for the chemistry parts. I did build a contact printing frame (like a silkscreen frame, but far easier and less specialized). I did built a custom UV exposing setup, since back then you needed UV. The presensitized boards sold now don't even require that!
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Wanted - UV cured PCB paints

Postby Dan Eakin » Fri Mar 14, 2014 2:28 pm

right, but on the far side of things, you cannot do component labeling or anything with tonor transfer methods, or the other laser printing solutions. You can get quality boards from laser printing, but not production quality boards, since there's missing steps.

I'd still like to at least experiment with silks, I have some very close friends who run their own silkscreen shop so i have the talent nearby to help me fix the any problems with the actual print itself. i've also see some videos of really nice looking boards that were silked. They don't say what their resists were in the video though, which sucks.

they did say they use the 300thread silks, which is a special order item. most silks from like hobby lobby are only 100-120 thread, and are incapble of the resolution SMD parts need. maybe thats where the silks failed for you?
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Re: Wanted - UV cured PCB paints

Postby Doug Coulter » Fri Mar 14, 2014 2:44 pm

Well, I can't have any objection to *you* doing silks. If you want to make it useful and write it up - power to ya. I tried, and for me, it just didn't deliver in any of:
Setup costs
Speed
Quality of results
Complexity
(General electric found this out the hard way, FWIW, way back when - I sure did get paid to fix a lot of defective silked boards - back when I did that kind of thing)

I regularly label components just using copper...it's easy, but I'm not "in the make large amounts of consumer hardware" so I often simply don't bother - I'm making something I designed, after all, and by golly know where all the parts are.
Most silk paints will work fine as resists against most etchants. A little simple testing will show what works there - of course, you want a paint that you can easily dissolve off once done as well - the presensitized boards are either solder right through the resist, or a single wipe with acetone, literally just one stroke of a paper towel. It's the fuzzy edges and open tracks due to slightly missed coverage (especially when you get down to .004" - .008" tracks) that kill things with silks.

When you "get to volume" I'd personally recommend just using a PCB house (or perhaps even a job-house that also builds the boards and solders them) - that business runs on pretty darn low margins, it's very hard to beat them in either price or quality, much less both. Anything over a few $K - take it to Taiwan or even Canada. You save money AND it's better quality.

I do PCB's when I need 1 or a few. Maybe I want something special it's not worth it to have made, like the very simple board I demoed in that thread, where I wanted something a little better than perf board, only needed one, and wanted it simple and reliable. I've also made a lot more complex PCBs, but once the design is good - get them made in a pro shop. They are such a small percent of the total costs it just doesn't make sense for me. It's cheaper and better to just buy an axe, rather than mine ore, refine it, forge it, make a handle...just to get an axe that will last longer than I'll live and only costs a fraction of my hourly rate to buy. I guess it's just different strokes.
After all, if the world economy collapses, as quite a few think it might - it could be a good business to be in for awhile, since the big houses will probably go out of business (but then, so will your silk process suppliers). But that's not something I sweat too much personally.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Wanted - UV cured PCB paints

Postby Dan Eakin » Fri Mar 14, 2014 3:20 pm

yeah, my electronics work for SHTF stuff doesn't require circuit board manufacture :P

its good to see your take on solutions for developing in house. I'll be adapting my own image and plan to whatever is required to make it all turn out the way i want :D
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