Double your pleasure, double your fun...

[Mark Hammer]

So, I took a break from making pedals, and distracted myself with some analog modular synth stuff that has been on the back burner for a while.  This digression was exacerbated by the discovery of some nifty "half-boxes" (four perfectly manicured plywood sides with no front or back) for $1.25@ at the dollar store that are a perfect size (7" x 7" x 3")for packing in a couple of modules (e.g., S&H + CV mixer + noise source, or 2xADSR + 2xLFO), in a way that lets one stack them or line the boxes up any way you want to optimize patching.

Part of this menagerie is inevitably going to be some of Ray Wilson's MFOS beasties, and many of those require double-sided PCBs (currently working on the Delayed Modulation Module).  I picked up some double-sided copper boards, and attempted to make my own (thought, trust me, I'd never attempt any of the bigger whole-synth boards).  It's a tricky business.

Initially, I thought it would be a simple matter of protecting one surface, doing a toner transfer on the other, etching that side, drilling, and lining up the toner mask for the top side by poking holes through the photo paper with toothpicks or push-pins, and inserting them into the corresponding holes.  It turned out that the top masks for the boards I was making were of the simpler variety, and the photo paper I was using didn't do such a great job of transferring all the little "floating" doughnut pads anyways, so I popped open a pack of old-school Radio Shack rub-on transfers I had (bought 'em all up when ours cleared out all that stuff a few years back) and just manually applied the pads and traces to those points on the top side that actually needed to be connected, omitting all the other pads that didn't provide continuity to anything.

Of course, when one goes to etch the top side, the bottom side needs to be protected from the etchant.  At this point, I have the board fully tinned on the bottom side, and all the holes are drilled.  So, I took some clear nail polish, and just painted the entire bottom, removing that coating once the board had been etched on the top side.

There are a couple of lingering challenges.  One is that I probably need a better way of protecting the side I don't want etched.  I'm reluctant to do it while it's still winter and the garage can't be opened and aired out, but I imagine I'll switch to a couple of schpritzes of clearcoat, once I can spray-paint again.  That'll be faster, and certainly give good coverage.  And please note that, however one chooses to coat the already-etched "first" side, you need to do any protective coating AFTER any attempts to heat-transfer a pattern to the other side.  Trust me, ironing a pattern on one side when there is anything that can soften in response to heat on the other side, does NOT end up pretty.

A second is what to do about components that are soldered to critical contacts on the component side of the board.  Resistors and ICs aren't too big of a problem, but caps can be an issue.  One doesn't want electros sitting way up, just to be able to get at their legs with an iron (without melting the plastic covering), so I bend the legs 90 degrees, and install them folded over.  For now, there has been space to do that, but I don't know the space will always be there.

For leads that are soldered on the top and bottom, I love the Small Bear pre-bonded stuff, since I can tin it, insert it with a couple of millimeters of exposed tinned wire on the top side showing, and solder it from both directions.  Still, requires a steady hand and better aim than if it was only soldered from underneath

A third challenge is that of keeping the "finger juice" off the board when it has to be handled so much, but I'm getting better at that.

It'd be nice to be able to just apply the mask to both sides at once, throw the board into an etchant bath, and then drill the thing, but this will remain a multi-step process for the foreseeable future.

[anchovie]

$1.25@ at the dollar store

You've been robbed!

[alparent]

I did some of MFOS boards myself. We should talk!

[italianguy63]

Mark--

I've been playing with etching enclosures, but the same applies.

Blue masking tape will protect--
Or regular ole spray paint.  It cleans of easy with laquer thinner after you are done.

For small areas, I have found Paint Pens to be very helpful.

[Mark Hammer]

I used masking tape, of both the painter's and normal varieties, and as much as I try to protect the edges of the board, the etchant tends to leak in from the sides.  Although it probably doesn't help that my current etchant bath is probably a little too well-used and I apply heat via the low setting on my heat gun.  Perhaps if I was using a fresher bath, such that the etch was faster, I might have fewer problems.

Alain, check your inbox.

[davent]

For masking i've used the clear/wide plastic packing tape, there's also the red tuck tape used as part of the vapor-barriering of buildings.



I've read of people doing double sided boards by taking the two images, aligning them face to face, tape them together along one edge, slip your board into the sandwich between the two images, secure the image to the board and transfer the two images, etch. Haven't tried it!

[Mustachio]

I used the clear packing tape on the last enclosure etch and it works good but I still had some leakage.

I do the same thing with the heatgun on the etchant Mark. +1 it really helps

I wonder if this Waffle Iron PCB press would do the trick .

http://www.ka6wke.net/finished-projects/pcb-press

[GGBB]

Ever though about trying sponge-etching?  This would greatly limit the exposure of the other side of the PCB to etchant - blue masking or clear tape will probably suffice.  An average pedal-sized board only takes a few minutes, and you don't need an etchant bath - just a sponge and a tablespoon or two of FeCl.

[greaser_au]

When I did this in the '80s I used to:
- cover the non-etch side with electrical tape
- only drill 2 or 3 holes for registration (and maybe 2 corners for each IC - I was using the Tandy/RadioShack rub down tapes as well).

david