Scratch Milling PCBs

Hi, I’ve had some good results milling PCB’s by ‘scratching’ with the Nomad rather than actual milling. I use a cheap scriber bit and scratch through a layer of Sharpie ink (and then etch). There are tradeoffs (the etching step), but I’ve found it a very consistent way to get small traces, down to 4 mil or so. and it’s very fast.

If anyone is interested I’ve written up my process here: http://debreuil.com/blog/making-circuit-boards/

It is still a work in progress, but I’d love to hear any thoughts and tips you may have with this. I’m really amazed how versatile the Nomad is, such a rock solid machine. Thank you for it :).

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Thank you for sharing, this is a neat technique!
Very clean homemade PCBs, kudos.

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@Debreuil I really appreciate all the time and effort you put into this series of blog posts describing your process! Some really useful concepts here. I especially like your approach to placing smd passives in through holes.

A few of questions after reading through your writeup:

  • Looks like you’re using sharpie or dykem as your etch resist. Are you using it as a solder mask too?
  • Is simply scratching through the etch resist enough to get a clean etch, or do you have to score the copper as well?
  • You seem to be drilling your through-holes only after your scratch-etch operations. Wouldn’t it make more sense to do a tool change on the machine and drill your holes during your first setup, so as to take fewer risks with registration?
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Agreed.

@Debreuil Thanks for sharing!

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@koolatron Thanks for having a look :).

I am using a Magnum Sharpie (very wide tip) as a etch resist, but not as a solder resist – tested that and the solder will burn through it. I only got Dykem yesterday, it is weirdly hard to get in Canada, so I need to test that more. It takes longer to dry, but might work as a resist. I’ve used dry photoresist sheets as a solder resist, but it is a little time consuming, and a little tricky to line up perfectly on both sides. I didn’t find it that much better for soldering tbh, but it did look much nicer – need to experiment more there. I also have been experimenting with 3D printer UV resin. The easiest way there is just add a bit to the board and scrape it into the etched grooves with a plastic – I find a few playing cards taped together work well as the scraper. Then 5 seconds in the UV lamp and it is ready. That resin does seem to make it easier to solder, but you need to be sure no resin stays on the surface. In the end though, I’ve gotten a bit more used to soldering these kinds of boards and now just tin them and don’t use a solder resist. I’d love to find a fast easy way to get that right though. I’m getting a laser printer soon, if the registration is consistent I think blasting away paint or enamel might work well.

I generally try to scratch a bit into the copper, but not into the FR4 board. 0.03 to 0.04 mm seems to work well, enough to widen the track a bit, but still lots of copper left. I’m surprised how well the Nomad goes through a bit of copper, basically at max speed. I’ve also done double contours in FlatCam, that makes soldering much easier, and can clear the copper completely away even (to the point you hardly need to etch). That does make the spacing a bit wider though.

You are right I can drill before etching, that is mostly me being superstitious about the etching removing some copper around the holes. I need to do some comparison test with this, but I’m sure you’re right and the difference will be negligible. The registration is really precise though, the rivet nuts hold the board on their OD, so re-registering to drill is quite precise (assuming I put the board back in the correct way up that is ;).

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Robin, this process is revolutionary in the traditional sense. +1

I can see that your process requires a really flat mounting arrangement that the Nomad just naturally provides. Having said that, it does seem to minimize that requirement compared to the pcb trace milling method.

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@CrookedWoodTex Thanks! :slight_smile: And yes, the Nomad really helps with being able to mill a level bed on the wasteboard, though I find it still needs to autolevel with a probe. At least the results are much more consistent with that.

I updated the probe page - I’m sure many here have done this mod already (most of the info was gleaned from these forms). Given that is just coexisting with the probe switch, it might be nice in future version to expose a jack or something to plug a probe into. This is such a good machine for PCBs, but autoleveling really makes the process routine – and I know it can be a nerve wracking making even simple mods like that.

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Small update - if the scriber is set up for multiple passes with a tiny offset, the copper between the passes peels off. This is pretty reproducible, zero issues on six boards now. That means no etching and very little manual intervention beyond changing drill bits.

Getting 15 minute boards now, really works well on the Nomad at least :). http://debreuil.com/blog/scratch-and-peel-milling/

scratchAndPeel240x236

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Awesome! I threw away my etching tank years ago (too messy for my taste), this looks like a fantastic way to make one-off prototypes.

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Yeah, totally agree! I was always trying to minimize the etching step, but it was a pain no matter what. Also required babysitting, so the time it took was time wasted.

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What kind of reliability do you get with this method? (in terms of quality of isolation between tracks / risk of shorts ? I would imagine “tearing out” the copper might result in minuscule shreds of metal potentially bridging the two sides ?). That said, one has to check the circuit whatever the method, so…

So far I haven’t had a single short or cut. I’ve done three and four passes, four gives a bit of extra room to solder. I think it works because the first pass (close to the trace) doesn’t do any removal. If the contours would go from outside to in it probably would peel off the traces.

There are definitely copper hairs hanging on after the mill, I used 2000 grit sandpaper or a scrub pad to remove those. Most come off in the mill, but the remaining ones tend to only be clinging by one end and come off pretty easily. Also the drill holes sometimes leave a ‘chad’ that comes off in this step. There is room for scrubbing as I don’t do any prep, so even the oxidation layer is still on it.

Even when I was etching, there would sometimes be shorts or over etched cuts - maybe that will still eventually happen now, but the traces themselves look much cleaner now.

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Oh, and I wanted to ask a more general question too: could you use that scratcher pen in a spring-loaded holder of some kind, and remove the autoleveling step altogether ? This is a naive question, I guess a spring-loaded holder would have more play than the precision you need for those tiny traces.

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Oh, that would be a great option I think. When I was just scratching I looked at using center punch mechanisms, but couldn’t find something that worked. Wegstr has diamond engraving bits that are spring loaded, I imagine those would work. A little pricey but I can’t imagine the diamond wearing out any time soon :). I’ll try ordering one, I was going to get via rivets there anyway.

Another option might be putting the springs in the work holder. I’ve tried scratch only boards with the PCB fastened to the top of the rivet nuts (I drilled the holes too small), and it worked ok. The board naturally flexed a little in the center, but that didn’t effect the results. Maybe even something like a weak spring washer around each post, and then the PCB holes slightly oversize to allow up and down travel. The only question is the side to side potential motion of the board, and if it affects the drilling (those bits are very sensitive to lateral motion). I should be able to try this with things on hand, I will let you know.

Thanks for the ideas! I would love to get rid of the auto level step too, not just because of the machine mods, but it’s also the only reason I use bCNC as well (which is a great program in the end, but hard to get used to).

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A diamond-drag bit for PCBs, now that is a cool option, and one I could see myself using.

CNCjs has an autoleveling add-on too, which may be forth a try. I never used it myself, just mentioning this as a possible option to look into. I believe EstlCam supports autoleveling too.

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