PCB Construction

The circuit boards in my original OGEE synth were made in a hodge podge of ways - some hand drawn with a resist pen, one wire wrap (the sequencer), and most on 4 holes-per-pad protoboards with power supply busses I got from Radio Shack. Looking at these now, I'm a bit embarrased!

CAD

The main difference from my construction possibilities between 1981 and now is the advent of PC's and PCB CAD programs. I use Isis for schematic capture, and Aries for PCB layout. These are part of the Proteus Lite package from LabCenter. I chose this package after looking at a lot of them because I wanted unlimited (or at least what for my purposes would be unlimited) devices and interconnects, at a decent ($100 or less) price. The Proteus Lite package cost $50 together.

What are the limitations? The schematic capture cannot output netlists, and the PCB layout cannot input netlists. The original package could not export Gerber files but I upgraded to permit that for another $30.

PCB manufacture

The best source for PCB making materials I have found is Circuit Specialists. They have chemicals, pre-sensitized positive resist boards, etchant tanks, exposure lights, you name it.

I tried a couple of methods for doing PCB's from artwork, including the iron on transfer type - which was totally unsuccessful for me. I finally hit on this method that works rather well:

  • Print foil patterns on transparency with laser printer - positive image. Toner needs to be fairly fresh to get a dense image. Touch up any light traces on transparency with a permanent marker
  • Cut presensitized boards to size. Double sided boards: Position transparency pattern, drill holes through alignment marks
  • Use pre-sensitized positive photo resist PCB's. Expose with a flourescant lamp for 10 minutes. Develope photo resist
  • Touch up any breaks in resist patterns with permament marker
  • Etch with Ammonium Persulfate in heated etchant tank. Circuit Specialists has one for $40 that comes with heater and the bubbler - well worth it. Ammonium Persulfate is much nicer than Ferric Chloride - doesn't stain, and turns a nice pretty blue when it gets full of copper
  • Remove resist with steel wool. Check for unwanted bridges between traces and pads, fix with an Exacto knife
  • Drill with #65 (.035") drill bit. I started with one of those Craftsman drill presses that you load a hand drill into. It worked, but if later on I bought a real drill press off of ebay, for not much more than what I paid for the Craftsman press
  • File burrs down, buff with steel wool
  • Tin PCB's with TINNIT solution. This stuff works pretty good and is cheap. You put the crystals in a Pyrex pan, dissolve in hot water, then put it on your stove on low heat to keep it hot. Put the PCB's in the bath, they are coated in about 1 minute. Rinse the board in household ammonia (the only smelly part), then in water

    • Stuffing the PCB's

  • Print out a copy of the top silk + top copper that I use as a reference
  • Since my process doesn't do plated thru holes, I use a wire soldered on both sides. I use a special circled crosshairs symbol I place over all my vias so I can see them easily on my reference copy. Mark them with a highlighter as I solder them
  • I usually solder components in this order: sockets (I use sockets for most of my IC's), resistors, caps, diodes and transistors, connector header

Testing

After soldering, I take a continuity checker and make sure my power supply traces aren't shorted to anything. I also check between IC pads.

For og2, the connectors to the PCB are 2x13 pins on 0.1" centers. These can plug right in to a standard IDC ribbon cable. I have a jig that allows me to plug a ribbon cable into a solderless breadboard and attach the other end to my PCB. Then I can plug essential front panel components (pots, inputs and outputs, switches, etc) into the breadboard. For later modules (og3, Lockbox Synth) I changed to Molex crimp connectors on 0.1" centers.

I do another continuity check to make sure I've got the power supplies wired right. Then I power on. Note that at this time all my sockets are still empty. I check that I get the right voltages at the socket pins. Power down, insert the IC's, and power up for a smoke test.

Test the operation of the module. My main tool for this is my oscilloscope. If you are serious about DIY you need one. ebay is a good source. I got my original scope there - a 35MHz B&K dual trace, for $150. That was three years ago, there are a lot more scope listings now and therefore cheaper prices. A while later I upgraded to a Tektronix 435 dual 60MHz, also from eBay.

©2002 Scott Bernardi