I’ve decided I need to get a stencil made for a board. I’m not going to use it a lot just enough to justify making a Mylar stencil. Custom Surface Mount PCB Stencil
I want to use the PCB mounting holes for alignment. Basically, I need holes cut in the stencil at the same places as the holes in the PCB makes life a lot simpler despite the fact that the company’s instructions say to make sure you haven’t done it.
Is there any way to make a footprint that will both set up the drilling for the PCB and also produce the correct output for the stencil cutter?
Making such a footprint in KiCad is easy.
If you edit the pad properties of a pad in the Footprint Editor, you can set the Pad Type to: NPTH, Mechanical (Not Plated Through Hole), and make it a round hole of your prefered diameter.
It looks like a NPTH pad has the F.Mask and B.Mask layers enabled. To make holes in the solder stencil, you also have to enable the F.Paste and B.Paste layers.
But do check your Gerbers. If this does not work (which seems unlikely) you can also add an “SMT Aperture” pad type to your footprint, and then only use the Paste layers for that pad.
In a more general sense, I also have doubt whether it’s a good idea to do this for several reasons.
- It will hinder movement of your squeegee.
- It gets easily contaminated with paste.
- Mylar is floppy, It will not stay put very well when you move your squeegee.
- What sort of pins do you plan to put though this in the first place?
If you have a decent SMT stencil printer, it has adjustment knobs and you do not need this method in the first place.
If you do not have a SMT Stencil printer then laying the PCB flat on a table, putting some PCB pieces around it (both for making a bigger flat surface and for preventing the PCB from moving is a reasonable method. Standard method is then to fix the stencil itself with a piece of tape along one long edge too, and then hold it under tension while pasting so it does not flop around. You can pull quite hard on a piece of tape (sideways), but you can not do that when the mylar is just held with some pins. In this case you will pull the mylar from the pins.
But it’s your project, so do as you wish.
I’d be curious to know if it works for you though.
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Thanks.
I’ll only be making 5-20 boards and there are only a limited number of SMDs on the board most of it will be THT and i’ll locate them roughly together so I’ve got a straight path across them.
I’m quite clumsy which is why I want the location as simple as possible. For the pins, I’ve got some 4cm rods of the correct diameter. I’ll clamp one PCB to a bit of wood and then hand-drill some holes in the wood through the holes on the PCB.
Obviously, this isn’t suitable for anything other than amateur work and it is doubtful buying a frame or a full metal stencil is worthwhile as while production is cheap they weigh about 1kg and it adds a lot to shipping.
Oops,
I did not mean “framed stencils”, but a “stencil printer”. A gadget with a hinge and some clamps to put the stencil in and position it relative to a PCB. I changed that in my previous post. These things are both very simple and surprising expensive. If I wanted one, I’d make it myself. If it’s just for a handful of PCB’s, then I’d use a flat table some scrap PCB pieces and some tape.
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There is some useful info on this technique in this thread.
As I use this technique a fair bit, i made an alignment jig consisting of a thick slab of Delrin with 2.5mm holes drilled at 10mm centres on a CNC. I then arrange the mounting/alignment holes on a10mm grid on my pcb. If this is a one off, you could easily use a spare board to align and drill a couple of holes on a wooden board or similar. Drill the holes with a drill press - they have to be perpendicular! I’ve found that two holes are fine for a small board. I use 2.5mm stainless steel dowels - these are cheap and have chamfered ends. It is best to make the stencil holes a little larger to make it easier to lift the stencil off cleanly - mine are specified as 2.6mm
I use metal stencils - they are fairly cheap from JLCPCB -and you can specify a smaller size so you don’t have a big postage bill for a large stencil sheet with a tiny design in the middle. I tend to order them a few mm larger than the board. The metal stencils will obviously be more rigid than Mylar and you will need to think carefully about avoiding thin strips of Mylar between features.
I’ve not had too many problems with squeegee access - I try and stick the alignment holes in a place that allows a clean sweep - and they are often dual purpose as I use them as mounting holes too.
EDIT
Ive just checked the prices quickly and I paid £5 to £6 each for the last few stencils I ordered from JLCPCB and I see the Mylar isn’t any cheaper. Admittedly, these are not big boards - only 50mm x 50mm. If you are ordering the PCBs anyhow, the stencil can be included in the postage. You just need to select the option of making it a custom size so it fits in the box with the boards.You need to specig=fy them ‘unframed’.
I also sometimes put the alignment holes on a break off rail and panellise the boards. This makes squeegee access easier if the boards are small and it makes it easier to lift the stencil up cleanly. Ive attached a couple of images to show you what I mean.
Alignment_Hole_2.6mm.kicad_mod (891 Bytes)
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Just done some test
5* just board(165.1 x 114.3) the cheapest mail. £19.46
boards plus top stencil 166 x115 £28.54
boards plus top & bottom stencil each 166 x115 £37.62
boards plus top& bottom on single stencil 166x230. £35.86
Hobbytronic Mylar A4 with top and bottom 2 stencils on £9.60.
With 5 boards £29.06.
So with just a top or bottom, you can save £0.52 via JLCPCB.
If you know someone with a laser cutter you can buy 125 micron A4 mylar *5 for £3.39 on ebay and get a quicker turnaround.
'm wondering whether a vinyl cutter will do it. Vinyl definitely less suitable than mylar Cricut Joy | Hobbycraft there are some high street Repro companies that will do vinyl cuts.
Generally I try and keep the smd components on one side in my designs. If there are any on the back I hand solder them so only ever need a single side stencil - so the costings seem to be similar.
I have limited experience with Mylar but I believe you need to be careful with the laser settings to get a clean, non-blobby edge. Like most plastics, there are nasty fumes to contend with too. I’d be interested to know how the Mylar works out if you go down that route.
Someone lent me a Cricut a few years back - it looked like it was capable of the accuracy needed but I could not get it working due to software issues. The cuts tended to leave hanging chads - and we all know that doesn’t end well…
Short answer: Mike is getting quite decent results down to a QFP pitch of 0.65mm (@09:10) Below that it’s becoming more difficult.
mikeselectricstuff Cutting SMD stencils with Silhouette Cameo vinyl cutter.
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I sent the order in Wednesday night, exchanged emails Thursday, dispatched on Friday and here today. So quick. The only board I had to test on only has a single smt on it so it is simple. I specified the hole as mentioned earlier but made a slight mistake as my pins are 3.5 mm I shrank the hole to 3.5mm not the paste. The hole was therefore cut with a radius 0.25mm oversize so there is some wobble. I also had the board edge on the DXF so the stencils would be cut down to that. The combination of the hole and edge made alignment easy and I need just a bit of tape to correct for the 0.25 mm error.
Stencil
Taped into place.
Paste.
After air gun.
All fairly simple. The stencil was easily cleaned easily afterwards. There is a slight bump around the holes, advice was to scrape that off with a craft knife. Not sure it needed doing.
One weirdness was when I noticed the holes were too big I went off to find out why and ended up looking at the contents of the DXF in a text editor. No mention of circles, arcs or anything other than lines. In SVG the circular holes are defined as circles but in these DXF files it is a series of straight lines. If you magnify the DXF in a view you can begin to see it. This will at some point have an effect on type fit.
DXF does have a CIRCLE primitive so I’m not sure why it is not used. I’ve downloaded the source and will have a look. In a previous life, I wrote file importers for Timeworks Publisher DTP and have enough experience with DXF to hate it.
ACE3NOKB-F_Paste.dxf (30.3 KB)
Please don’t feel offended, but I think your avatar is upside down.
I took the liberty to have some fun with it and this is what I came up with:
Did I get the eye color right?
If you do feel offended, just send me a message and I’ll delete it immediately.
I like the “Zilog Inside” logo 
For the 0.25mm difference for your locating pins…
If you can remove the locating pins from your jig, you can put them in a drill and grind the tips slightly conical. If the tips are conical you can change the diameter by adjusting the height, and the conical tips may also help with putting the stencil over the pins.
But don’t make them too conical, then you may pull the stencil off the pins.
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Makes me look like I still have hair the eyes are the right colour though
That was just a test with what I had to hand. If I end up using it again ill just put some tape around the pins. I only mentioned it as I don’t think I would have needed tape if I’d gotten it right.
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Years ago I made some stencils with mylar and a cameo cutter that I had around for cutting custom labels. It was slow and clunky but did kinda get the job done. At the time my workflow included my little neoden pnp and a eagle plugin that generated pnp files. It was easy enough to visually align the stencil to the board sitting in a little registration nest of old boards, squeegee paste with a paint scraper, plop passives on with the neoden, drop chips on with tweezers, and reflow on the 20-dollar tilt-n-drain griddle from walmart. I made a lot of boards that way, and still do the odd small run now and then.
That seems like such a long time ago now. After I found ten-dollar stainless stencils from jlcpcb I never made a mylar one again.
I also go way back to the days of etching my own boards, and can’t believe that some folks still do that. Or even routing a board. Sheesh, it is so cheap these days to get protos and stencils in a couple of weeks.
John: Nice idea on the cnc baseplate. Also, beautiful boards!
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