Laser cut solder paste stencil
VonSzarvas
Posts: 3,446
As mentioned in another thread: https://forums.parallax.com/discussion/comment/1563198/#Comment_1563198
Here are the details of the paper stencils that can be cut using a diode laser cutter. Ideal for applying solder-paste to a PCB!
Most important detail is to use thick paper- for example: 250g weight standard color laser copier paper.
The photo shows two stencils. One was for a complete board layout; the other sheet was cut with lots of smaller RGB LED footprints that were cut out individually and used to replace parts on a damaged disco light!
I thought I'd need one stencil per LED, but in the end I think I replaced 12 or so LEDs with only 2 or 3 stencils- and probably that many because I had them, and it was quicker than cleaning! So the point being, the paper stencils are robust and work well for multiple uses. (Or they are cheap enough to use a clean one every time, if that suits!)
Settings / Specs
For cutting the stencil I'm using a basic 20W diode laser machine.
In LightBurn the settings need some experimentation of course- the stencils in that photo were cut on 250g paper with: 350, 50, -0.08 (Speed, Power, Kerf).
160, 200 and 250g standard color copy paper covers the ranges I use in terms of stencil thickness.
Observations
The stencils work consistently well and are easy to align on the PCB without fuss. I've not tried with super-tiny ICs; that would be a challenge to dial in the settings, if even possible with the accuracy of this machine. In those cases I tend to order something metal and save the time :-) Certainly 0402 or larger works great, without much "dialing in" effort.!
I've not tried cranking up the power to find out at what point the paper burns. Hmm, now I think about it, how did I miss that opportunity ??! 
- Anyway, keep safe as always; having any machine located such that a fire won't ruin your day is always a great idea!!
What other materials could work?
If anyone has similar tips or ideas, please share! I've tried laser-cut kapton sheets, and also etching thin steel in the past, but in the end paper works just as well for low density boards, and the disposable nature of them sure saves a lot of cleaning and chemicals!
Comments
What I've found that works best (so far) is Mylar (polyester) drafting film. We tested a wide range of synthetic and art papers before coming back to the Mylar, which was originally purchased as plotter film in the 1980s or early 1990s. I couldn't get as good results from kapton, annoyingly. We didn't try anything too thick like the paper you're describing.
Ours is a 80 watt CO2 laser and needs the power turned way down, almost as low as it can go. That operating point actually varies day by day with temperature, so needs to be found experimentally, its about 6 or 7 percent. Lower powered lasers would be better for this kind of thing.
After cutting, we give the mylar a very light sand using very fine sandpaper, to remove any beads that sit up above the plane. We then blow out any dags out using compressed air, then its good to go. I think the Mylar could be good for several passes, and cleaning with isopropyl is fine.
Lightburn is able to read a pdf, making it useful for any software that can print a paste mask, or even (if you deleted the tracks) just a top layer. Once imported from pdf, you can grow or shrink the pads, which is super useful for fine tuning the amount of paste delivered. This can be done on all pads, or just selected pads. For smaller pitch like the p2's 0.5mm pitch you end up with single pass lines - drawing a rectangle causes a blowout when it stops and reverses direction.
@Tubular
About the Mylar sheet, are you able to share a link to your regular supplier product page, or some specs? I always struggle to find a supplier locally- maybe seeing some detail will help me refine search terms.
I suspect paper is much cheaper though, and can also be re-used several times. Scrapping it clean with a painters scraper between passes is effective to remove paste from the holes, before going again.
The thicker color copier paper is very smooth and almost gloss-like compared to ubiquitous 80g A4 copier paper, so I think you need to see the thicker stuff to appreciate how well it can work. Will add a link below later.
I would like to try Mylar too- curious to compare results and toxicity of the laser cutting process.
The problem I see with paper is that it sucks up solvents and flux resin of the paste but not the tin balls, of course. So your paste gets diluted and dries out more and more when being applied to the paper stencil multiple times. The lead free paste is so expensive you don't want to throw residues away each time you print.
Polyester film, for example film to be used with laser printers for overhead projectors, should work. It doesn't creep and expand when heated so much like vinyl film does.
Found 7.5 mil (On the thick side for a stencil...) PET film in black on Amazon.
In old forgotten memory was a list of plastics that were safe-ish for laser cutting, and some which were definite no no's.
Pretty sure the first thing we cut for testing the new machine was some random office binder, that turned out to be one of the definite "don't do it" category plastics, and let out a really obnoxious stink - probably poisonous !!!!!
Doesn't anyone have experience with that sort of thing? Which plastics should be regarded safe for laser cutting ?
Good point, and agreed! - Paper is probably suited for one-off repairs, or small builds, etc... But not the most economic (unless you are super careful with paste dosing), for production runs.
I guess once you are building more than 10 of something, the cost (and the time it takes to order) a proper metal stencil become non-issues. And you'd probably be using a dedicated stencil holder/applicator, which from my experience speeds the process of alignment to multiple boards too.
My friend put some decent docs together,
https://github.com/CCHS-Melbourne/Laser-Cutters/wiki/Knowledge-Base
and the materials list it points to,
https://github.com/CCHS-Melbourne/Laser-Cutters/wiki/Materials-List
Safe materials include PMMA (Acrylic), PP (Polypropylene), kapton. I'd suggest PE (polyethylene) and PET (polyester) are also OK. PETG, teflon, acetal can be cut in small doses (eg stencils), but you want good fume extraction for. Polycarbonate can be cut but has yukky brown edges and needs good fume extraction.
PVC is a big no-no.
Happy to post you some of that mylar (polyester) drafting film if you want to experiment. I also bought a lot of synthetic (plastic) papers, but found the original mylar the best, but could also include those for you to try, you might get different results