Scratch-and-Peel Milling

Rather than spinning an engraver, use these cheap tungsten carbide scriber bits to scratch into the PCB copper.

Scratch milling into layout fluid using a scriber bit works really well, and might still be essential if you have a chip with 0.1 mm/4 mil spacing (see below). For most boards though, you can skip the whole etching step, because the copper will peel off by itself if you scratch contours very close to each other. In the video example below, the first pass was at -0.04 mm depth and just barely scratched into the copper. The second scratch, at the same depth and just beside it, causes the copper between the scratches to peel up and come cleanly off the board. Like this:

If you do a few passes, the copper comes off perfectly pretty much every time. This means having a board ready for assembly in as little as 10 minutes! Change your FlatCam ‘Gerber Object’ settings to a Tool Diameter 0.05 mm, 3 passes, and 10% pass overlap. You can go 4 passes if you prefer wider spacing. It takes a little longer to mill, but a little less time to solder. The passes will only happen where there is room, so it will still support chips with small pin spacing.

The Geometry object settings are the same as before, -0.04 mm depth for each pass, your machines maximum X,Y speed, 1 mm travel and don’t turn the spindle. I’m using a Nomad 883 witch is a very rigid machine – you may need to dial the speed down a bit if your machine loses steps here. I’ve done a number of boards at these settings now, and have yet to have a short – of course you may need to dial it in for your machine, but this should be a good starting point.

To recap, the process for scratch-and-peel milling is:

  • Place board face up in your jig, auto level, scratch and peel the front copper.
  • Turn the board over, auto level again, carve the back.
  • Leaving the board in place, load the drill file, but keep the auto level settings.
  • Probe the drill, then drill all the holes (you may need separate drill files if your drill lengths are different sizes).
  • Sand down both sides. I use 2000 grit sandpaper, but Scotch Bright scour pads work too. You just need to remove the dangling burrs.
  • Tin if you like (see the Etching and Tinning section), or just go straight to assembly.
This side was four passes, and took about 8 minutes. No etching. The accelerometer at the bottom has 0.2 mm traces.

Sometimes you have spacing so fine you only have room for a single scratch, and the copper will not peel this way. The solution here is to cover the board is either a few coats of Magnum Sharpie marker (thick tip), or layout fluid. Do this after auto leveling to be sure the bit doesn’t have trouble connecting to the surface. With this method you don’t need multiple passes, but it doesn’t hurt either. Once you are done, you will still need to etch the board (though there will be minimal copper exposed and it is already scratched, so this goes quite quickly).

Next: Soldering and Board Assembly

Alternate method requirement: Etching and Tinning