Sunday, 5 June 2016

Laser cutting copper clad boards for irregular-shaped PCBs

About a hundred years ago we played about using a laser cutter to etch PCBs (although, more accurately, to laser away an etch-resist mask, and then etch the usual way, in ferric chloride). And it worked really well. In fact, when we can, we use the "laser-etch" method in preference to press-n-peel.

But laser cutting copper clad board?
Actually cutting shapes?

Yup, that's the idea.
Except, of course, that a laser can't cut through copper. But what we though was that perhaps we could laser cut a mirror image of the shape required, from the back of the board - so if we stuck to resin-bonded-paper (rather than FR4/fibreglass) boards, we should be able to laser cut the paper into any shape.

Then, when the board is etched, and the copper laminate holding the pieces together on the front is etched away, all the individual parts come loose and swish about in the bottom of a tank of ferric chloride (quite how to retrieve them, we haven't yet worked out).

FR4 boards are no good for this. Fibreglass is incredibly resilient to burning/cutting with lasers. But SRBP (synthetic resin bonded paper) or FR2/FR1 boards should be ok. Maybe....

First up we tried some FR2 from Rapid Electronics.
It burned really well, but didn't actually cut all the way through.


On the reverse of the board, we cut a simple circle, and then some "break lines" to help the board to fold in the right places, so we could run a scalpel blade around the circle, to cut through the copper laminate.


But in places, the laminate didn't cut properly, and left sections of copper laminate on the waste piece (ripping it away from the circular disk piece we wanted to keep).

Then we tried some non-FR-rated board from http://www.jprelec.co.uk


This stuff has a white, rather than a cream/ivory coloured backing. And when you laser cut it, it really stinks. Worse than leather. Worse, even, than lasagne! And the laser cuts through the paper perfectly well. It's just a shame that there's what appears to be a glass matt between the paper and the copper laminate.

Not a problem, if you only cut straight lines - the fibreglass layer snaps cleanly and leaves a nice, sharp edge.


But when we tried more exotic shapes (in this case, another, simple circle) the paper backing came away, but the fibreglass sheet remained perfectly intact. In fact, with this trial, we managed to separate the paper board not just from the copper laminate, but left the entire glass-fibre layer in place too (where we actually wanted to cut a round shaped PCB)


So there we have it.
Laser cutting copper clad boards doesn't work.
But not for the reasons you might expect (the laser bouncing off the copper surface) but because the SRBP paper board actually contains layers of glass fibre to reinforce it.

Great news if you're looking for cheap, but durable, alternative to FR4. Rubbish if you want to use a laser cutter to shape your copper clad boards before toner transfer/etching. 

It looks like the messy old desktop CNC is going to get a workout again soon.....