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Heat effect on FR4?

Discussion in 'Electronic Design' started by ehsjr, Jun 11, 2010.

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  1. ehsjr

    ehsjr Guest

    I have a resistance circuit that dissipates ~1.2 watts. If I use
    11 one watt resistors in series (physically parallel) I can spread
    the heat out across the width of the board. I measured ~66C under
    each resistor at worst case ambient (~32C) in the test setup.

    What will happen to to the board over time (>10 years) of 24x7
    operation? (Resistors in contact with the board.) Does FR4 behave
    like wood that is exposed to temperature over time, slowly charring?

  2. TTman

    TTman Guest

    Mormal practise is to space the resistors off the board with ceramic
    beads...... then you don't have the problem. If you can put a copper pour
    all under the resistors that will help dissipate the heat evenly as well, as
    opposed to leaving localised 'hot spot' sources.
  3. Grant

    Grant Guest

    I'm inclined to put the resistors in parallel and with lots of copper
    under them. If they're those small body type 1W, the heat goes out
    through the leads, need copper there to spread the heat.
    Sorry, my crystal ball is bust.

  4. Tim Williams

    Tim Williams Guest

    C'mon, you must have some Consumer Electronics on hand! I see it all the
    time, discolored boards under diodes/resistors. Phenolic and epoxy both
    turn brown over time. I don't know how much this affects mechanical
    stability; it's not like it's crumbling under the hotspot.

    (Although I did once overload an ATX power supply, which had the result of
    turning the output filter choke into a heating coil, merrily charring the
    PCB to a combination of carbon and glass fibers.)

  5. Uwe Hercksen

    Uwe Hercksen Guest


    why do you want to use 11 one watt resistors to spread about 1.2 watts?
    If it is only 1.2 W, two or three resistors will do, if it is 12 W, 11
    of those resistors will not be enough. There are resistors for 4, 5 and
    11 W to be soldered in a FR4 board too. There are also resistors for
    chasis mount available for 25 or 50 W. If you think about 10 years of 24
    x7 operation, you should chose a robust solution.

  6. Phil Allison

    Phil Allison Guest


    ** So that is 110 mW dissipation per 1W resistor or 11% of rated.

    ** Probably nothing.

    I see old equipment all the time with resistors laying flat on the PCB that
    dissipate more than 25% of rated and there is not even a mark left on the

    You could space the body up a few mm from the surface for even greater

    ..... Phil
  7. ehsjr

    ehsjr Guest

    Thanks, Phil. I was hoping someone would answer based
    on experience with the long term effects, so your
    answer is particularily appreciated.

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