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Any problem parallel traces on both sides of board carrying same signal?

Discussion in 'Electronic Basics' started by Michael Noone, May 31, 2005.

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  1. Hi - I'm working on a board that will be switching a fairly high current (I
    think maybe 5A) 120V AC line. I'm trying to design the board to handle as
    much current as possible. I'm making all of the traces carrying this signal
    as wide as possible. I was struck with an idea though - would it be OK to
    mirror all these traces? In other words - have a duplicate of each trace
    directly below each trace on the bottom of the board? Being that they're
    the same signal I don't think there will be any issues - but I thought it'd
    be best for me to check first. Thanks,

    Michael J. Noone
     
  2. Chris

    Chris Guest

    Hi, Mike. There's no potential between the traces if you mirror them
    so that should be safe. Try less expensive solutions first, though.
    Specify 2 oz. copper and solder coat. These are a lot cheaper than
    getting a double sided board, and will boost your safe current rating.

    Obviously you're increasing the trace heat sink surface area, so you'll
    be increasing power dissipation for a given trace temperature.
    However, if you've got significant heating with both traces, you'll
    tend to hotbox the circuit board between, causing discoloration and
    tending to delaminate the traces from the board. That might be a bit
    of a problem.

    I would tend to have some doubts about your solution being that much
    better, but it should help some. I would guess the positives would
    somewhat outweigh the negatives, especially if there's a fan or it's
    not buried in still air. Mounting the board vertically rather than
    horizontally will also help if there's no air circulation.

    Sorry that doesn't give you a good answer, but if you give more
    information, you might get a better answer than yes and no. What's
    your trace width, and how long does it run? Are you expecting elevated
    environmental temperatures, or is this running in free air at room
    temperature?

    Good luck
    Chris
     
  3. mike

    mike Guest

    Don't forget to worry about how you get the current into and out of the
    trace. For thin copper, the current density right at the connection pin
    can get high enough to cause long-term heat problems. Might need to
    think about multiple input points on the trace. I see a lot of power
    supplies and TVs where the solder joint cracked over time.
    mike

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  4. John Larkin

    John Larkin Guest


    No problem. But 5 amps isn't a lot of current for a pc board.

    John
     
  5. Hi - the traces will be 35um copper (1 oz) - I think about 3/8" of an inch
    wide. Is there a good way to calculate how much current that can safely
    take, and if I need a dual trace on the bottom of the board? Thanks,

    Michael
     
  6. The relay that I'm using has two pins for each contact, so that should help
    somewhat with this. I also thought I'd make the pads very large and then
    use a much larger than normal amount of solder, giving more surface area to
    the connection.

    -Michael
     
  7. Unfortunately I don't have control over the copper thickness - it will
    have to be 1 oz. For other reasons (primaryily due to a RF receiver
    needing a groundplane) the board has to have two sides anyways, so
    making it two sided is a non-issue.
    It will be mounted vertically
    I believe trace width will be about 3/8". I'm making the final layout
    later today so I'll know then. It will be run inside a home, so normal
    environmental temperatures for the most part. It will not have much air
    circulating over it though, and a fan is not an option, unfortunately.
    Thanks,

    Michael
     
  8. Chris

    Chris Guest

    Hi, Mike. There are trace width calculators all over the net. Here's
    one I've used before:

    http://www.circuitcalculator.com/4pcb/trace_width_calculator.php

    It says that you should be OK with single side, 1 oz. copper at over
    ..108" outer trace width (10 degree C rise in temp over ambient being
    the unofficial standard for the start of deep doodoo). As long as you
    don't have an internal trace, that is.

    Take these results with a grain of salt -- your results may vary.
    However, I can unequivocally guarantee you have nothing to worry about
    conducting 5 amps with a 3/8" wide 1 oz. single copper trace. You
    won't need to double up, for sure.

    It's great you're being cautious about these things -- that's a good
    sign, and you're headed in the right direction.

    Good luck
    Chris
     
  9. John  Larkin

    John Larkin Guest

  10. colin

    colin Guest

    I would be surprised if you run into problems with 5 amps at 120v, i worked
    on smpsu designs ages ago wich handled 15 amps on single sided pcbs,
    although in such cases its not too clever if you have long runs as cross
    regulation becomes an issue, i measured the voltage acsross a wire link once
    at 50 mv.

    Colin =^.^=
     
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