Wednesday, 29 April 2015

Double-sided PCB fails

It feels like ages since the Nerd Unit Laser Cutter was fired up, so we put that right this weekend. There are a number of projects on the go (as always) but it was good fun to fire up the laser and etch some boards, instead of messing about with laminators, press-n-peel, and the frustratingly awful Brother laser printer (the old Xerox was a joy to make PCBs with - this Brother laser printer makes every board a hit-and-miss affair).

Anyway, one thing we thought we'd try was some double-sided boards.
It seemed easy enough in principle - firstly, etch our PCB designs onto some MDF and drill out a few holes.

This is just going to be a template for etching; the idea is to place the copper boards on top of the template and when we hit "etch" again, it should carve the same pattern in exactly the same place on the board.

So after etching one side of our double-sided copper board,we drilled the same holes as we had done in our MDF template. Then simply push some pins (we used 0.8mm copper wire) through both the board and the MDF, to get the reverse side to line up

Tape the copper board to the template, spray with matt black paint, etch the reverse pattern, and.... ta-da!

Not exactly successful. The holes are probably a good 3mm away from where they should be!
Undeterred, we went back to the trusty old method of press-n-peel.
There are a few articles online about how to make double-sided boards with press-n-peel, and many use this same approach:

The idea is to print the design twice, one flipped (and mirrored if necessary) with a line exactly half way between the two designs. This is where we're going to crease the press-n-peel. To ensure that we got a perfect line up, we cut our press-n-peel along the very top of the design, and made sure that, after folding, the two top edges were perfectly aligned.

Insert the (double-sided) copper clad board, affix in place and laminate to secure the image

This time the alignment was better - but still not perfect (or, to be honest, not even good enough to be used!)

[photo here]

So in a last-ditch effort, we scrapped all the fancy-shmancy ways of making double-sided boards and went back to the method we know works best - etching just one side at a time.
This involves placing one design on one side of the board (using the "traditional" press-n-peel method) coating the reverse with paint (to protect the copper during the ferric chloride etching) and etching the board as if it were a single-sided PCB.

Then we drill a few alignment holes and clean the back of the board...

...and manually align a second design onto this reverse side.

Alignment is still not perfect - in hindsight, we should have image-transferred and etched the boards before cutting them down to their final size, so that the masking tape holding the press-n-peel in place had a bit more material to stick to!

Although not perfect, at least the alignment of the holes means this board will actually be useable. So the last thing to do is coat the front side (the bit that's already been etched) with paint (to protect it from the ferric chloride during the second etching) and to etching the reverse of the board.

And there we have it. A double-sided PCB that sort-of lines up.
Today's lesson - if you want to make something as quickly as possible, slow down, take your time, and don't try to do anything clever.