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PCB Power Trace Widths, Ground Planes, and Routing

Discussion in 'Electronic Design' started by Apparatus, Dec 25, 2004.

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  1. John Larkin

    John Larkin Guest

    I recently did a couple of boards using some fine-pitch 456-ball BGAs,
    Xilinx FPGAs. We needed 8 layers, 6 mil traces, and 10 mil via drills
    to get all the signals out. It might have been possible in 6 layers,
    if the Vcc-io and Vcc-core had shared a layer and we'd worked at it a
    lot longer, but it would have been really nasty.

  2. John Larkin

    John Larkin Guest

    Modern. Funny, I bet I've written a few million lines of code and at
    least a hundred manuals, and I've never learned to type.

  3. Most mispellings won't pass by a compiler. Tragically, some will.
  4. Mac

    Mac Guest

    We only designed CompactPCI boards. CompactPCI is a spec put out by the
    PICMG ( It specifies the thickness of
    all cards, regardless of whether they are master cards (like the ones
    I worked on) or peripheral cards.
    Yeah, the cost wasn't a big issue. But we had an upper limit on trace
    impedance that was hard to meet with a 14-layer, 0.062" board. We had to
    use 4 mil traces on all the inner layers, IIRC.

  5. Did this once. Worked great until it came to EMI susceptibility.
    Never got as far as emissions. The board was mounted in a
    fiberglass enclosure and that didn't help, but neither did all
    the conductive spray paint and rf-gasket we could stuff into
    the blighter. Went to 4-layers, just added Vcc and gnd planes
    with thermal reliefs to the power vias, and it worked a charm.

    FWIW, I would advise some caution if you are planning a grid
    ground plane on a 2-sided board with a real (not an 8051...)

    2-sided PC & AT motherboards were common on Taiwanese clones.
  6. Nico Coesel

    Nico Coesel Guest

    Still a problem with the power on the 2 sided board I guess. I made
    quite some combined audio / digital PCBs. Never had problems while
    using 2 layer boards even when the CPU was running its software from
    external memory and have several microphone inputs on the same board.
  7. Paul Burke

    Paul Burke Guest

    Very important. Ideally you want every signal trace to have a ground
    return parallel to it all the way, on the next layer down. You also want
    the power to be distributed without significant static or dynamic
    voltage drops.

    4 layers achieve this more closely than ever you can with 2, but I've
    had very good success with the following approach:

    Route good thick (0.1") power traces for VCC and ground in the form of
    grid patterns. Ensure plenty of decoupling caps, you may also want to
    insert ferrites in the supply to some very busy ICs.

    Route your signals, bearing in mind what you will do with the next step.

    Flood fill all the area that remains with ground.

    Use vias to stitch together all the ground fill and tracks. Examine
    closely to optimise the connectivity of the ground "plane"- many routes
    are better than fewer.

    Iterate over the last stages until you are fed up of the whole thing and
    want to get a life.

    Paul Burke
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