Author Topic: Mylar Stencil thickness (Read 7134 times)

Pack34 · « **on:** January 21, 2016, 04:24:47 pm »

I've used some mylar stencils in the past with some mixed results. The main issue that I think I found with them was that the material was too malleable. This seemed to cause the paste to push up a bit around the pads. This caused everything to smear a bit. Because of this, I try to avoid them for anything that has a real fine pitch.

I was looking at some stenciling options and I noticed that PCBStencils.com has a couple different options when it comes to the material type.

https://www.pcbstencils.com/index.php?option=com_k2&view=item&layout=item&id=2&Itemid=2

They have two different thickness options for the standard stencil, but I'm intrigued by their "Stabil-Flex M3-4000" stencil. It looks like a completely different material. I'm guessing it's more rigid?

Does anyone here have experience with it? I'm tempted because I typicaly use iTead's framed stencil offering since it's cheap ($70 for a framed laser cut stencil). But the lead time tends to be a killer. I think I typically get them in two weeks. With a mylar stencil made locally I can get them in a day or two. A lot of the time I typically use these for one-off prototypes before I send them off to production.

Mainly, I'm looking at the quickest and economical way to do quick prototyping.

theatrus · « **Reply #1 on:** January 22, 2016, 04:07:20 am »

That looks like a Kapton (polyamide) like material. Easy to laser cut.

I've used Kapton stencils from OSHStencil for fine pitch QFN without issue, and now have some sheets I can run in my laser cutter (since I usually iterate on apertures a bit)

Pack34 · « **Reply #2 on:** January 22, 2016, 04:37:15 am »

Quote from: theatrus on January 22, 2016, 04:07:20 am

That looks like a Kapton (polyamide) like material. Easy to laser cut.

I've used Kapton stencils from OSHStencil for fine pitch QFN without issue, and now have some sheets I can run in my laser cutter (since I usually iterate on apertures a bit)

I've been able to get the orange material from OSHStencil to work, but only after switching to a less rigid applicator. The included credit-card-like applicator just didn't work well for me. I was probably pressing too hard.

calexanian · « **Reply #3 on:** January 22, 2016, 06:48:20 am »

I use both the Polymide and stainless ones from StencilsUnlimited. Both work fine, but i do like the rigidity of the framed ones they make. Frame refers to perforations a little way in from the edges you bend up. Makes the paste come out sharper, but in all honesty after you flow it, no real difference is made.

uncle_bob · « **Reply #4 on:** January 27, 2016, 08:15:46 pm »

Hi

You have (at least) three variables involved:

1) Your solder paste
2) Your applicator
3) Your stencil (material, thickness, hole size ...)
4) The pitch of your parts (solder pad size)

Yes, there are a few others, but these are the big ones. Change any one of these and you will impact your outcome. In general, the tighter things get, the more careful you have to be. Your stencil likely will get thinner as things get tighter. Your solder paste may be different for a fine pitch part (smaller particle size) than for something massive (like a SO-8 package).

Best approach - set up a frame for printing and control the tension on the stencil (regardless of the material). Index your applicator against the frame (constant pressure etc). Come up with a hold down for the board you are working on. (might be two sided tape).

There is no need to get crazy expensive with this. You can do it with junk you have lying around. Just plan on a bit of experimentation / trial and error. Once you get a formula worked out, things will repeat pretty well.

Bob

jdraughn · « **Reply #5 on:** January 28, 2016, 09:05:14 am »

I haven't done a whole lot with stencils, probably 4 or 5 different boards total, but I have made a few mistakes and also discovered what works best so far in my case.

Mistake - Make sure your solder paste is room temperature. I store mine in the fridge and it was pretty stiff and didn't spread very well at all. I didn't realize just how much trouble it causes until I cleaned up the stencil and board and did it again with the same paste at room temperature and it went on smooth and even.

Make absolutely sure that you are supporting all sides of the board you are spreading the paste on with a jig or other PCB's of the same height. Otherwise the stencil will bow up and cause some paste to be forced under the stencil.

As far as using a not very ridged applicator, in my case I actually use a razor blade. for the rigidity. You can tell I have only done tiny boards since I can use a razor blade, so maybe some of these rules change if your doing a much larger board.

I have done pretty small SMD parts though - 0203 passives and a QFN 100ish microcontroller. I have had success with all my boards so far.

rich · « **Reply #6 on:** January 28, 2016, 11:39:12 am »

Razor blade is an interesting idea from very small boards. Are you using stainless or mylar stencils ?

jdraughn · « **Reply #7 on:** January 28, 2016, 12:09:17 pm »

Mylar from OshStencils

Quote from: rich on January 28, 2016, 11:39:12 am

Razor blade is an interesting idea from very small boards. Are you using stainless or mylar stencils ?

kony · « **Reply #8 on:** January 28, 2016, 12:11:51 pm »

Word of advice: don't bother with plastic stencils, especially if not held in frame. 20$ for proper lasercut stainless stencil is very reasonable, for example here: http://www.elecrow.com/laser-cut-pcb-stencil-p-450.html. If you want to save on shipping, use otpion without the frame - it is still stiff more than enough for prototyping.

janekm · « **Reply #9 on:** January 28, 2016, 12:15:16 pm »

Quote from: jdraughn on January 28, 2016, 09:05:14 am

Mistake - Make sure your solder paste is room temperature. I store mine in the fridge and it was pretty stiff and didn't spread very well at all. I didn't realize just how much trouble it causes until I cleaned up the stencil and board and did it again with the same paste at room temperature and it went on smooth and even.

Absolutely... it's actually standard practice to mix up the paste thoroughly before application, mainly to warm it up a little so it flows into the apertures well. Of course that doesn't work with the paste in syringes so I don't recommend that (you'd be better off buying one of the cheapo tubs from China (like Mechanic brand) and occasionally adding a little isopropanol to freshen it up if it gets too dry). Mixing up the paste before application also lets you judge whether it has gotten too dry, of course.

And yeah I agree with others on getting laser-cut stencils, especially if you get a decent chemically / electro-polished one, much nicer.

jdraughn · « **Reply #10 on:** January 28, 2016, 06:19:09 pm »

Quote from: kony on January 28, 2016, 12:11:51 pm

Word of advice: don't bother with plastic stencils, especially if not held in frame. 20$ for proper lasercut stainless stencil is very reasonable, for example here: http://www.elecrow.com/laser-cut-pcb-stencil-p-450.html. If you want to save on shipping, use otpion without the frame - it is still stiff more than enough for prototyping.

My first experience with Elecrow was pretty horrible because of how long it took for me to get my boards back. I placed my order and didn't get any response back for quite a few days. Then the Chinese new year happened so I had to wait another week, but finally my design went to the fab, it took a very long time to get back from the fab, and then it took like 4 or 5 weeks for me to receive my order. All told it took like 2 and a half months to receive my boards.

zapta · « **Reply #11 on:** January 28, 2016, 07:38:29 pm »

OshStencils over two stencil thicknesses, 3mil and 5mil. I used the 5mil because it prints more paste.

The trick not to smear too much is to have only one slow pass. If the first pass didn't go well, don't do a second one, clean the board and redo.

From my experience, metal stencils such as the ones from electrow are more accurate and better for small pads (0402, QFN, etc). It also helps to have fine paste, e.g. T5 rather than T3.

calexanian · « **Reply #12 on:** January 28, 2016, 08:30:50 pm »

proper hinging of the stencil is critical to prevent smearing. I use masking tape. also as said above, using another piece of PCB of the same thickness to border the subject board is essential. a flat surface, masking tape, and scrap PCB cuts actually can make an effective little setup. I do it here for sample batches and flow the boards in a convection toaster oven. Very handy.

zapta · « **Reply #13 on:** January 28, 2016, 08:44:30 pm »

Apropos brackets, if you have access to 3D printer, you can print brackets with customized thickness and size.

http://www.thingiverse.com/thing:530949

Kasper · « **Reply #14 on:** February 19, 2019, 05:02:55 pm »

I recently got access to a 45W CO2 laser so now I want to make my own stencils. It can't cut metal but mylar should work. I've had good results with 3 mil mylar stencils made by polulu.

I usually make small boards < 3 sq in. I was searching for 3 mil mylar but this 4 mil mylar is what came up on amazon.

Does this look like the right stuff?

Does anyone recommend anything else?

https://www.amazon.ca/mil-Blank-Mylar-10-5-sheet/dp/B00RDATMOU/ref=mp_s_a_1_fkmr0_1?keywords=3+mil+mylar+sheets&qid=1550594283&s=gateway&sr=8-1-fkmr0

Bassman59 · « **Reply #15 on:** February 19, 2019, 05:23:25 pm »

Quote from: zapta on January 28, 2016, 07:38:29 pm

From my experience, metal stencils such as the ones from electrow are more accurate and better for small pads (0402, QFN, etc). It also helps to have fine paste, e.g. T5 rather than T3.

The metal stencils from OSH Stencil work well.

texaspyro · « **Reply #16 on:** February 20, 2019, 04:07:19 am »

I cut prototype stencils out of red "vellum" from Hobby Lobby or Michaels using a 1 watt 405 nm laser diode mounted on a small CNC mill. The laser kerf is less than 10 microns. The vellum is a plasticized translucent paper, not true sheep skin vellum. I've never worn out a stencil, but I don't usually make more than 10 boards.

I use assist air from a small air pump blowing down a 3D printed tube with a nozzle on the end. The laser shines down the tube and exits the nozzle. The assist air produces a very clean cut and eliminates any invisible threads that can keep the cutouts from releasing from the body of the stencil.

Kasper · « **Reply #17 on:** February 20, 2019, 08:02:05 pm »

Quote from: texaspyro on February 20, 2019, 04:07:19 am

I cut prototype stencils out of red "vellum" from Hobby Lobby or Michaels using a 1 watt 405 nm laser diode mounted on a small CNC mill. The laser kerf is less than 10 microns. The vellum is a plasticized translucent paper, not true sheep skin vellum. I've never worn out a stencil, but I don't usually make more than 10 boards.

I use assist air from a small air pump blowing down a 3D printed tube with a nozzle on the end. The laser shines down the tube and exits the nozzle. The assist air produces a very clean cut and eliminates any invisible threads that can keep the cutouts from releasing from the body of the stencil.

That's pretty impressive. Whats the smallest pitch components you've soldered with it?

texaspyro · « **Reply #18 on:** February 20, 2019, 08:24:29 pm »

I've done 0402, but always try to use 0805 if possible. I've also done tiny wafer scale solder bump chips.

BTW, mylar tends to be a very poor material for stencils. When cut by a laser the edges of the cuts tend to swell. Some lasers are better than others. I can also cut Kapton, but the edges of the cuts tend to char. The vellum works very well and a 12x12" sheet costs around a dollar.

Kasper · « **Reply #19 on:** February 21, 2019, 02:43:27 am »

Ok, thanks, I'll try to find some vellum

texaspyro · « **Reply #20 on:** February 21, 2019, 02:57:15 am »

Quote from: Kasper on February 21, 2019, 02:43:27 am

Ok, thanks, I'll try to find some vellum

What you want is fake vellum... vellum paper. Not the real vellum made of animal skin. Also you want it to be a color that absorbs your laser light. For blue lasers red seems to work very well. I have also had luck with a gold-ish yellow.

texaspyro · « **Reply #21 on:** February 21, 2019, 03:45:05 am »

BTW, the vellum that I have been using is around 0.003 inches thick.

jmelson · « **Reply #22 on:** February 21, 2019, 09:07:42 pm »

Quote from: texaspyro on February 21, 2019, 03:45:05 am

BTW, the vellum that I have been using is around 0.003 inches thick.

Yup, and after some experimentation a decade ago, I also settled on .003" stencils. I make them with PCB etching technology our of brass shim stock material.
I get 100 foot rolls of 12" wide shim stock, it is a bit of a special order item, but most machinist supply outfits stock it in 6" width.

Jon


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Author Topic: Mylar Stencil thickness (Read 7134 times)

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