The X-carve / Shapeoko 2 seems to be pretty good at milling pcb boards when dialed in, but what about producing the stencil for solder paste application? Has anyone tried this? Typically these are laser cut from mylar or stainless steel films, but it would be convenient to mill one myself.
The problem with milling is keeping such a thin material clamped down.
You might be better off just strapping a laser to the side of the spindle.
Cheers
Ian
Hi Kyle,
I’ve cut SDM stencils from Mylar sheets (125-190 micron thickness) with a 0.1mm tip 10 degree V-bit successfully. I have a sacrificial platform on top of the wasteboard that I skimmed flat and I fixed the sheets down with double sided tape. In my first attempt using Eagle I ran a ULP to export the cream layer as SVG and loaded that into Easel - that worked fine so I’ve stuck with it. Note as Easel does’t support V-bits you will need to do some trig. to figure out what diameter bit to tell Easel you are using - or you can just take a guess.
What’s really nice exporting SVG is that it’s easy to tweak if you need to. For example when I was bringing up a new PCB I decided to only populate half the board, just make sure the MCU was working OK. I could easily remove the stencil areas I didn’t need to create a custom bring-up stencil. Similarly you can slightly inflate of reduce the size of the pads if needed.
I’ve been very happy with this process so far. I’m not so interested in creating PCBs with the X-Carve, but being able to cut stencils - which have been costing me more to purchase than 10 off of my PCBs - is really nice.
I’m doing fairly ‘large’ 0805 and 0604 components, so your mileage may vary, but I’d encourage you to give it a try!
Andy
IanWatkins: I know a laser would work better, but I’m hoping to use close to the stock machine to make the stencil.
Good to hear mylar cuts well, because I doubt stainless would. I’d say my goal is to be able to put paste down on 0.65mm pitch pins (msop-8 size), with the paste not bridging between pins. I find this the difficult part of the paste application, whereas 0805 components are a lot more forgiving.
I actually tried a rough test already similar to your method. I cut an aluminum can apart and flattened it out. I used double sided tape to adhere the entire film to a wasteboard. Since aluminum cans have a plastic coating, I sanded it off of one side, and then used chilipeppr’s autolevel function to probe the surface. It makes a big difference if the plastic is left on because the probe needs to push harder to connect the circuit.
As was pointed out, it is the milling of the thin stock that is the main problem. I had to plunge much deeper than expected to cut through the ~4mil aluminum, possibly due to the softness of the tape and the relative hardness of the aluminum. The edges were also pretty rough. None of the bars separating the 0.65 mm pitch pins survived and even some of the 0805 separating bars were broken.
I though aluminum would be pretty easy compared to cutting copper in PCBs, but removing the double sided tape bends the aluminum out of flat, which makes the actual paste application more difficult.
I’ll see if I have better luck with the mylar. Also any recommendations on a relatively low tack and thin double sided tape for this application? Maybe mylar won’t have the plastic deformation issue on delaminating?
Hi Kyle,
I used a fairly strong carpet tape - no trouble removing from the wasteboard or the stencil. I did a quick sand of the stencil as there were some rough edges. I also cleaned up a couple of parts with a scalpel. No more than 5 minutes of cleanup in total.
If small pin pitch ICs cause trouble (I need a 0.5mm soon) - then I plan to chop up an old steel stencil and reuse parts of it. Typically my PCBs have many discrete parts (0805, 0604 or similar) and 1 or 2 small pitch components (I always use the same MCU and CPLDs) so I’ll try either try a two pass approach or create some kind of Frankenstencil… . .
If that doesn’t work out then I might well join the laser club (using Mylar) - but still some mileage in the current method I think (as I haven’t attempted to refine it much yet).
Andy
Just a quick update on the stencil making.
I found that there is an error in the pcb2gcode eagle extension. The program uses the wrong offset for path generation, only 1/4 the diameter instead of 1/2, so all the paste layer holes were coming out oversized. I just increased the bit size entry to compensate for the error.
I went back to using lite duty carpet tape which is thinner than the other tape I had been using and more rigid.
I purchased some 5mil metallized PET for making stencils, with the idea that the metallized surface would allow for autolevel probing. Unfortunately, the bit I was probing with is just too sharp and half the time cuts into the plastic without registering the surface contact. Possibly using a blunt tip would produce better results.
Besides the plastic, I also cut open some aluminum cans (tall Arizona brand cans) and polished the coating off one side. These cans are a bit thicker at 5.25mils.
The X-carve can cut both the plastic and the metal, but both leave a little bit of burrs. These are slightly easier to remove with sandpaper from the aluminum one. And of course, with enough polishing, the plastic stencil becomes transparent.
At this point I’ve only tested the paste application, not whether the reflow indicates there is the correct amount of solder. In the best case, I’ve gotten separate paste pads on 0.65mm pitch pins, but putting a chip on spreads the paste a bit.
I also want to create stencils for solder paste, but using Easel and Carvey. I am using ExpressPCB to make my circuit boards. Their software can export a .DXF file, and I can use TurboCAD to convert that to .SVG. Easel is able to import the resulting SVC successfully. I have not tried carving yet and am looking for some advice on that: material to use (and where to obtain), hold down methods, and bit choice. The PCB is about 110mmx150mm.
In other projects, I have had good success with double sided carpet tape, and mentioned by others in this thread. I was thinking an engraving bit, like I have used for lettering would be a good choice. I have some very small parts on this PCB, for example needing paste pads pattern of 0.165x0.53mm on 0.35mm centers, but others are more reasonable. I like the idea of using metal, if possible.
Suggestions and comments appreciated.
Robert