Saturday, 20 June 2015

Double sided PCB with toner transfer (chinese press-n-peel alternative) success!

Well, success, sort-of.
Maybe that should be a caveat on homebrew PCB manufacturing. Everything "sort-of" works. Or, at least, on this blog anyway, with all the short-cuts and penny-pinching and not-always-using-the-best-tool-for-the-job, but making do with the cheapest....

You get the idea. It's taken two evenings and a few sheets of copper board and toner transfer paper to get the result, but we got there in the end. As Steve might say, it's a bit pikey - but it works! Of course the way to get the best result would be to email the design files over to a PCB fab house, wait anywhere between 5 and 15 days, and try out the professionally produced, exceptional quality circuit board.

But that's not what home PCB manufacturing is about. At least, not here at Nerd Towers anyway! We're about getting something that works. Soldermask? Pah! So what if it makes it a nightmare to solder? That's part of the fun, right?!

Anyway, thanks to some suggestions from Jason (and later, Steffen) and after discovering some cool functionality in the Copper Connection software we use for making Gerbers, we managed to successfully etch a double-sided board which lined up enough to be usable. So in our eyes, that's a success, right there.

First up, getting rid of the printing problems:
It was actually quite straightforward. Instead of just dumping ExpressPCB to Gerber using Robot Room's Copper Connection, we downloaded the latest version and were delighted with the printing options.

The transfer etch option prints out two sheets (for a double-sided board) with the top-most layer mirrored. This means that after transferring, the top layer is the "right way around" and the bottom layer is flipped - exactly as it needs to be. No need for exporting to PDF then flipping in Inkscrape and trying to work out which layer needs to be flipped, and messing about to get it to print properly by converting to a bitmap and all that nonsense - just hit print and let the software do the thinking for you!

Jason's suggestion was to draw a border around the PCBs on both top and bottom layers (obviously making sure they're perfectly aligned in the drawing!). Then, from the carrier sheet - after printing - cut a template and place onto the double-sided copper board. Using the template as a guide, drill holes in each corner, outside of the border.

This does mean that the PCBs are in the centre of the copper clad board, with quite a wide border around their edges (something we try to avoid, in an attempt to reduce waste and get our designs as close to the edge as possible). For now, we'll just take the extra waste as a hit, in preference for nicely aligned boards!

After drilling the marker holes, place the press-n-peel on both sides of the copper clad board at the same time, carefully aligning the edges of the paper with the guide holes.

We're using paper-based masking tape to hold the paper in place; Jason recommended using an iron to "tack" the paper in place before sending through the laminator.

Laminate as normal, fixing the toner transfer paper to both sides of the copper clad board at the same time, and etch. Keep everything crossed until the very last bit of excess copper has gone, shut one eye, stick your tongue out to one side, chant a mantra and do a little dance. Then drill through the holes and hope they line up.......

Don't look too closely, else you'll see a lot of holes are off-centre. In fact, before drilling it was possible to see that the back and front didn't quite line up. So instead of drilling exactly in the centre of the holes on one side and potentially missing the centre of the pad on the other (or, worse still, drilling through the middle of an adjacent trace) we actually deliberately drilled off-centre. The result is a board that looks pretty crappy on both sides - but functionally does what it needs to do!

And there we have it. 
Success. Sort of.

Well, not perfect, but good enough to be usable.
And, as stated at the top of this post, for us, that means success.