Stencil for solder paste for printed circuit board
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I used a Silhouette vinyl cutter to make a stencil to apply solder paste for printed circuit boards.
[I don't recommend this approach, rather I suggest you order from OSH Stencils, but if you want to try, here's what worked for me.]
I used mylar (overhead transparency stock from Office Max, $0.40 per sheet).
At 6.5 mils (measured with Mitsumi digital caliper), this was thick, actually much thicker than Kapton stencil from OSHStencils, and maybe too thick. Suggest obtaining thinner transparency stock than I got. If stencil is thicker than needed, more solder paste is applied than is needed, and solder could short between adjacent pads.
I:
-started with Gerber files (one can also export cream/paste layer from PCB CAD software, as SVG)
-opened cream/paste layer in Gerbv (FOSS) and printed to PDF
-opened PDF in Inkscape, set strokes to black
In Inkscape 0.93's "Path" menu, using the Combine, Stroke to Path, and
Union operations, in that order, ended up completing my workflow from
paste/cream layer in Eagle CAD (free electrical CAD software) to messy
PDF (in which some shapes were, as in the original, filled with a
back-and-forth vector pattern) to simple continuous outlines for
laser-cutstencils.
One can do something similar in Illustrator based on its "expand," which converts a stroke with finite width to a filled shape. This is like "Stroke to Path" in Inkscape. That should facilitate the same result in Illustrator.
Union operations, in that order, ended up completing my workflow from
paste/cream layer in Eagle CAD (free electrical CAD software) to messy
PDF (in which some shapes were, as in the original, filled with a
back-and-forth vector pattern) to simple continuous outlines for
laser-cutstencils.
One can do something similar in Illustrator based on its "expand," which converts a stroke with finite width to a filled shape. This is like "Stroke to Path" in Inkscape. That should facilitate the same result in Illustrator.
-thickened strokes to make the vector "fill" look solid black
-exported as PNG
-re-imported to Inkscape
-traced bitmap (to get a red vector outline around each pad)
-added a board outline (probably should have printed this layer from Gerbv along with the other layer)
-printed on Cameo using the above method (mylar was stuck to the the Silhouette 12x12 tacky cutting mat).
This same file, with both black raster and red vector outlines, probably would haveworkedin our Trotec laser engraver/cutter, either for cutout ablation (via raster engraving; see previous links) or outline cutting. If the latter, suggesting putting something under the mylar sheet for the chads to adhere to -- maybe a second mylar sheet.
What didn't work on laser cutter:
When I went to laser cut the stencil file I created yesterday, the laser cutter engraved away my entire work piece, even the parts that looked white. And when I used a white fill, it left the pads but still obliterated the (white looking but apparently slightly gray) background.
On close inspection I saw that in the exported & re-imported PNG bitmap, each black object (pad) faded into gray and transcended its red vector tracings.
Presumably it took infinitely many pixels to fade all the way to pure white, so that Trotec job control was viewing the entire background as some shade of gray and equating that to black.
I did select "remove background" when I traced, but that didn't remove the original bitmap.
Today I did manage to remove the bitmap using this extension, which puts the trace on a different layer:
On close inspection I saw that in the exported & re-imported PNG bitmap, each black object (pad) faded into gray and transcended its red vector tracings.
Presumably it took infinitely many pixels to fade all the way to pure white, so that Trotec job control was viewing the entire background as some shade of gray and equating that to black.
I did select "remove background" when I traced, but that didn't remove the original bitmap.
Today I did manage to remove the bitmap using this extension, which puts the trace on a different layer:
http://wiki.evilmadscientist.com/Post_process_trace_bitmap
That's one of the extensions in "Eggbot extensions." I installed it on the rightmost (easternmost) of the electronics area PCs on loan from Robbie.
I dragged the trace off the bitmap and then deleted the bitmap.
I ended up with a pure vector file. I used a lime green fill and blue outline, avoiding red and black so that I don't have to adhere to any conventions. I can define a custom material in Job Control that either engraves the lime green fill or cuts on the blue outline (and skips the other) at my option, depending on which works best.
By the way, the first mylar stencil (from vinyl cutter) worked pretty well, even though the lines actually came out angled, skewed from the intended horizontal and vertical. Solder mask repels solder, and stray solder's surface tension will cause it to "snap" to pads.
Due to the skew, our vinyl cutter may not be accurate enough to produce SMTstencils for very small parts such as 0603 and smaller.