Thursday, 3 October 2013

Laser etching PCBs update. DO bother. It's awesome

After a successful evening etching circuit boards at BuildBrighton last night, Steve insisted on issuing a retraction to a statement made earlier this year: creating etching masks for PCBs with a laser cutter can be a quicker and easier than using press-n-peel - especially for enormous great big boards.

So why were we eching A4 sized circuit boards at 10pm last night anyway?
We'd already decided that a custom-made PCB was the way forward with our board game idea - now it was time to prove it!

Using full A4 sheets of press-n-peel was a non-starter: with so much ink on the board, getting a consistent, clean etch would be almost impossible - as a large quantity of toner melts, the entire blue sheet starts to float on a great big puddle of liquid ink, resulting in blurred traces and smudged filled planes with just the tiniest bit of movement during the heat transfer. We've done A5 sized boards before now, using press-n-peel, and may one in three worked well enough to get a clean etch. Trying to effectively make four of these boards in  one go just seemed like trouble! So it was time to give lasering another go...

We used the same Halfords Matt Black car paint as before and set the BB laser cutter into action. It took about 30 minutes to fully etch the paint off a full A4 sized copper clad board:

The important step that we'd missed last time, was giving the board a generous wipe over with some white spirit, to clear away the vapourised paint off the exposed copper tracks. Cleaning the board thoroughly before etching allowed the ferric chloride to work much more quickly on the copper. Also unlike last time, we used a PCB etching tank, complete with heated ferric chloride and an aerator (bubbler) to help improve etching times (and remove spent ferric off the surface of the board, without having to constantly dip the board in and out of the brown sludge)

The result was a pretty impressive, super-sized PCB (the edges have yet to be cut to size, but that's a job for once everything else is in place.

We did consider making a double-sided pcb, but lining up two sets of traces on one single board, with so many vias (this board has about 94 vias) seems quite tricky. Normally on a homebrew double-sided board there are three or four places where getting things lined up is critical, so you can use large pages and give yourself a little bit of "wriggle room" should things slip slightly. But because we couldn't be exactly certain of getting the laser to etch the opposite side in exactly the right place on the reverse (and because it takes so long to etch) we decided to make a separate pcb out of much thinner copper board and simply glue it to the underside of this board, once aligned in a few places.

Our thinner copper board wasn't quite A4 sized, so we put two sheets together and made sure the join was nowhere critical in the top layer design. Then it was time to set the laser going and discuss the days events over a brew - it would be another 30 minutes before the next board would be ready!

With the second boards cleaned, and dipped in the ferric chloride, we pretty soon had all our copper etched and ready for drilling.

checking the etch quality by holding the board up to a bright light - any non-etched or badly etched parts would be easily visible.

The question now was whether everything lined up correctly!
The PIC microcontroller sat in the right place and lined up with it's own pads, so things were looking quite promising to begin with...

So now it was time to put the boards together and see if the whole idea actually worked. Obviously drilling the boards separately would be time consuming, so we thought it best to place a couple of pins in  opposite corners and get the boards lined up, back to back, then drill through both layers, at the same time, for each pad.

The few locating pins lined up beautifully on both sides of the board (one pin was about 0.5mm out but that's a level of deviation we can work with, on 2mm pads). The boards were pressed firmly together and the pins soldered on the underside.

The resulting board is starting to look very promising indeed.
In the photo you can see the hand drill used. Originally we put a 1mm bit into a Dremel, but this does tend to grab a bit when drilling. Because we don't want even the slighted bit of deviation from the centre of each hole (to give us the best chance of hitting a usable part of the pad on the other side) we resorted to drilling each hole by hand.

With 88 squares (it's an 11" x 8" board) and a few extra power/connector vias, there are a total of 96 vias on this game board. So far we've drilled six. Tomorrow night's BuildBrighton meet-up is going to involve a lot of drilling!

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