Laser Cut Solder Paste Stencils

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Andy Smith's picture
Andy Smith

PCBs are increasingly cheaper and easier to manufacture these days - from milling it yourself, chemical etching or using one of the numerous super cheap pooled manufacturing services (my favourite being DirtyPCBs). Designing your own PCBs means you can easily take advantage of cheaper and smaller surface mount components, but assembling them can be a long and tedious processes. For small-run batches of boards that don't warrant paying for a professionally made steel solder stencil, I generally use a syringe with a blunt tip to manually apply solder paste to each pad on the board. Not only does this take a long time, it leads to sore fingers and a lack of control of the amount of paste on each pad. Having inconsistant amounts of paste usually results in problems during reflow like tombstoning components and bridged IC pins that take a bit of time cleaning up afterwards. With a solder paste stencil, you simply lay it over your board and use a credit card to apply a nice even layer of paste on each pad in one go. Easy! 

I'll explain the process of how to export your board design from Eagle, clean it up in Adobe Illustrator, then cut it out with APS-Ethos and our CadCam FB500 laser cutter. You'll need some Mylar stencil sheet - I bought some 125 micron A4 sheets from eBay for around 71p each. 

First of all, you need to change the layer settings in Eagle to reveal just the surface mount solder pads. To do this, hit the layer button on the left toolbar, deselect all the layers, and reselect the 'tCream' layer. Then hit the 'Change' button to set the fill style to empty (the first icon). I found that having any other fill style than empty (e.g. solid or hatched) would end up leaving multiple paths in the resulting file that would cause issues later on.
You should now be left with something like this..
Now export your file as a PDF. Go to File > Print, and choose 'Print to File (PDF)' as the printer. Uncheck the 'Caption' option, and make sure that the scale factor is set to 1.
Open the resulting file up in Illustrator, then select one of the pads, right click it and select 'Release clipping mask'.
You'll probably find there's an invisible rectangle around your pads, and you'll need to delete this before continuing. I also changed the outline colour at this point to make it a bit easier to see.
You can now save this file as an Illustrator CS4 file (for compatibility with the laser cutting software), and open it up in Ethos. Select all the pads, right click and select Properties. Go to the Raster tab, and set a 0.1mm scan line resolution, 1mm overscan, and medium power contrast.
With the pads still selected, set the effect to 'raster'. See our laser cutter tutorial video for how to change cutting effects.
You're now ready to cut. In the material manager window, choose the Mylar stencil preset I created. For reference, this is set to raster engrave at 152mm/sec at 40% power. You might want to make a copy and tweak this if you're using a thinner or thicker stencil sheet (I'm using 125 micron).
Done! I was surprised how well the TQFP-32 package came out and was expecting the material between the pins to just melt away, but it's actually very sturdy. I've yet to see how well it can deal with finer pitched QFN or BGA packages, but I don't hold much hope.