Laser Cutting Plastic Solder Paste Stencils

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Laser Cutting Plastic Solder Paste Stencils

Post by parkview » Mon Mar 22, 2021 9:34 pm

I have now made up 40+ PCB's using home made plastic stencils. I laser cut 0.1mm thick Mylar using my 60W laser cutter. When I say cut, I actually mean raster etch, as this makes for a much cleaner hole cut. The 750x750mm plastic sheet was purchased from a local Art supply shop for maybe $12, so it makes for very economical solder paste stencils.

My process for these is:
1. export the PCB front paste layer as a SVG file.
2. import the SVG file into Lightburn software
3. shrink most 0805 and larger pads inwards by 0.1 to 0.2mm, depending on pad size.
3. shrink some QFN and 0.65 0.5mm spaced inwards pads by 0.05mm
4. set the Lightburn shape layer to 'Fill'
5. set Lightburn layer to 127mm cutting speed and 27% power on a (questionable) 60W laser tube
6. laser etch/cut Mylar sheet, but leave a 20mm boarder around stencil - to help with taping down and paste placement etc
7. sand the plastic stencil on both sides with 1200 grit sandpaper - gets rid of most rounded up hole edges
8. clean with a cheap cleaning alcohol
9. label stencil and include a PCB version number

Here are a few images of the process:

Select the pads with mouse. Click on LH Lightburn icon, set the required distance to shrink inwards:
Stencil2.jpg (37.63 KiB) Viewed 1751 times
Sanding stencil:
Stencil5.jpg (30.73 KiB) Viewed 1751 times
Finished stencil:
Stencil3.jpg (38.56 KiB) Viewed 1751 times
Note the difference in hole shapes on the CP2102 pads. This is caused by the laser scanning back and forth across the sheet, so vertical pads are etched a bit more than horizontal pads. It would be great if Lightburn had a more detailed shrink GUI, but in reality, solder paste is usually pretty good and surface tension draws it up onto the closest pad.

Sometimes finely spaced pads 0.65 or 0.5mm QFN pads might get a bit too much solder paste, but it doesn't take too long to clean up with a soldering iron. You can see a few more close up microscope photos of pre-sanded 0201 laser etched holes here: viewtopic.php?f=9&t=213#p963

Note: here I am using Lightburn version 0.9.21

If you really get into cutting plastic stencils from KiCAD, you might want to automate the exporting of the Front Paste layer: viewtopic.php?f=12&t=2469

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