Low leakage diodes -- some stories.

Discussion in 'General Electronics Chat' started by RichardO, Jul 22, 2015.

  1. RichardO

    Thread Starter Well-Known Member

    May 4, 2013
    I have uploaded a picture of the PCB that used 64 low leakage diodes.

    Here is a little history:

    The board has 64 channels made up of 8 IC's, 8 SIP resistor packs, 64 diodes, 64 transistors, 3 ribbon connectors and a handful of other parts.

    The original design was a hand drawn schematic on 4 sheets of 8 1/2" by 11" paper. The first page was the control circuit, the second page was 16 of the 64 channels, and the 3rd and 4th pages had a note the said something like "build 4 copies of the 16 channels with the variations shown in the table". After doing the drawings, the engineer left for an extended trip to Europe. Arrrrg.

    Once the connector pinouts were added to the schematic, the documentation was given to Steve, the PCB layout guy. He hand-taped the layout at a 4 times actual size using black tape and pads on several sheets of clear Mylar -- no computers used here.

    The board size (about 5.5" long) was determined by the largest size allowed to fit into a hand held case. The control and 64-channel boards were the same size and "hinged" together by a 50 conductor ribbon cable.

    When the Steve came in with the layout it was really tight. Note in the picture that the diodes are partially under the transistors. (I fact, to replace a diode the transistor must be removed first). I commented how few vias he had used. He replied that he did not have room for more vias than that! I believed him.

    I literally could not sleep at night thinking that the pinout of the 50-pin cable was wrong. I re-checked that circuit at least 4 times without finding an error but I was still not comfortable. When the boards were finally put together they worked showing my sleepless nights were unfounded. To this day, my gut says that the boards should not have worked together.

    Even with the parts packed as tightly as they were, the PCB had to have 4 layers. In that era, 4-layer boards were somewhat exotic. In our area there was only 1 company that did 4-layer boards and their lead-time was months -- not good. After a number of calls, the decision was made to ship the original, one-of-a-kind 4-layer board artwork from Colorado to a board house in California. As you can imagine, this was quite scary.

    When the board house in California looked at our artwork the decided they could not do the board after all -- but the did know of a nearby company that could and hand delivered our valuable artwork to them. The second company made all of our 4-layer boards.

  2. Roderick Young


    Feb 22, 2015

    There was a quote to the effect that what is impossible to one generation, is merely difficult to the next, and trivial to the generation after that.

    A hobbyist could draw up that schematic today, route it on their desktop, and send Gerbers to any board house without ever leaving the room. It would automatically match the schematic, no trace-by-trace verification needed. Surface mount components would make things smaller, too.
  3. Lestraveled

    Well-Known Member

    May 19, 2014
    I still have a few rolls of Bishop Graphics tape that I used to make PCB artwork a long time ago. Now I use it when I wind small transformers. We have come so far.
  4. RichardO

    Thread Starter Well-Known Member

    May 4, 2013
    ...Get the customers requirements... do design... order the parts... get the board assembled... send unit to customer... submit invoice... get paid... change out of pajamas...

    Yes, the world sure has changed.