Having got a few hundred PCBs ordered from 3pcb.com, it was time to think about how we were going to solder these up when they arrived! Immediately, the obvious solution was to create some kind of solder mask.
We've tried making thin metal solder masks in the past with mixed success (the drinks-can etching approach didn't really work, whereas etching thin sheets of modeller's hobby brass using the same techniques as etching a PCB turned out quite nicely).
This time, we thought we'd try laser-cut mylar.
Creating a solder paste stencil from an ExpressPCB file is relatively easy - although it does involve a few steps, so pay attention: the first thing is to makes sure you have CutePDF installed. This great little program creates a "virtual printer" which allows you to create a PDF of just about any document that supports printing.
So in ExpressPCB, print the pads and silkscreen layer
Wait for the PDF to be generated, then open this in Inkscape. The first thing to notice is that the entire object is grouped - selected everything and click "ungroup" (ctrl + shift + G) until there are no more groups in the selection. Now individually select the things we don't need - like the board outline, silkscreen shapes and text etc.
When you have only the pads remaining, select everything and remove the fill, while setting the stroke to 0.1mm. This should now show you the shape of your stencil-to-be.
With all of the pads still selected, choose Object -> Transform from the menu and set the scale to around 90%. This is because when cutting the mylar, we're allowing for the material to shrink back a little around the "burnt" edges. It's also creating a slightly-too-small-for-the-pad opening, so that each pad doesn't get fully flooded with solder paste.
It's important when re-scaling to apply the scale function to each object individually, not to the selected group as a whole (otherwise some of the pads will become out of line)
There are two ways to go forward from here. At first, we saved the document out as a DXF file, with polyline options. This makes the file compatible with most laser cutters (or even routers, if you're crazy enough to want to route this stencil from very fine metal - although you may want to reduce the pads even further, to allow for the thickness of the router cutting bit). This means we were (vector) cutting the lines from the mylar with our laser cutter.
It didn't really work - as the laser tried to outline each of the pads, the heat build-up caused the bits of plastic between the pads to simply disintegrate.
Instead, we printed our design to CutePDF, opened in Inkscrape, then exported as a bitmap at 600dpi. The result was a raster image that we could engrave, rather than cut. This gave a much nicer finish to the cut edges.
But even then, the finish was slightly off. It looked like the mylar was warping as the holes were being burned into it. So one last try, this time with a small piece of mylar taped to a piece of scrap mdf
This time the stencil looked just fine with very little sign of warping. When it came out of the laser, the holes lined up perfectly with the pads on the PCB.
Next we created a jig to hold our PCB, and a frame for the stencil/mylar to be mounted onto. Some locating holes ensure that the two line up properly during use.
Getting the stencil and the pcb to line up with each other as well as the the alignment holes was a bit fiddly, but we got there in the end!
For some reason, our image contained a set of pads which are not actually on the manufactured PCB - so we're either using an old design, or a super-new one, including changes we've made since having these boards made up! Any paste deposited through these two holes on the right can simply be wiped off
The mylar film was fixed to the frame using good old fashioned double-sided tape, and a small amount of solder paste applied in a line across one edge. Using a bit of an old plastic from some food packaging, we swiped the solder paste over the face of the stencil, pushing it onto the PCB underneath
Whereas before it could take three or four minutes to apply solder paste to a board - with a pin or fine-tipped implement - it now takes just seconds. The end result isn't perfect - yet - but apparently, even with the best made, jet-cut, steel stencils, learning to apply the paste properly is something of a fine art.
Although the paste is a bit blobby around the hall sensor connector, the rest of the pads look just fine. It's much more precise than our current application method (which is to just lay a slug-trail of solder paste across all the pads anyway) so in the morning we'll place some components and give them the hot-air treatment, to see how the excess is handled.
Mylar film stencils are not as hardwearing as metal ones, but are generally good for about a hundred passes (maybe more). So, until we're churning out our PCBs by the hundred or thousands, this seems like an ideal way of creating stencils for applying solder paste to our prototype boards!