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Heat dissipation for small leaded components

Discussion in 'Electronic Components' started by Tim Shoppa, May 29, 2006.

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  1. Tim Shoppa

    Tim Shoppa Guest

    OK, I don't know too much about heat dissipation.

    For semiconductors that are designed to take heat sinks I can do the
    arithmetic of adding thermal resistances from the semiconductor
    datasheets + thermal resistance of the thermal pad + thermal resistance
    of the heat sink just fine.

    For some medium-power surface mount components (semis with tiny body,
    big leads) I know that dissipation is done primarily by conducting heat
    away from the device via the leads and/or a thermal pad on the bottom
    and then it is radiated via the PCB. I know this because the
    datasheets/appnotes all emphasize PCB layout and copper thickness etc.
    to achieve the rated dissipation.

    But for small leaded components (say 1/8W resistors or TO-92's without
    heatsinks) it must be the case that the bulk of heat dissipation is
    done not through the tiny package, but that most of the heat is
    dissipated through the component leads into the PCB which then does the
    radiating. Yet... the datasheets give no guidance into how lead lengths
    or PCB layout reduces dissipation ratings.

    To some extent things must be self-compensating: If you use longer
    leads (reducing conduction to the PCB) then the device is presumably
    sitting in free air (away from a hot PCB) and the body itself is going
    to be better at dissipating heat. But I'm looking for basic arithmetic
    with thermal resistances to turn common sense into something
    quantifiable.

    Back in the late 90's, HP had some decent appnotes with summaries
    (including the basic thermal arithmetic and example PCB layouts)
    related to dissipation in T1-3/4 LED arrays but I cannot locate these
    appnotes anymore. I am looking for something similar that discusses
    numbers (maybe with arithmetic examples) for other small leaded
    components (say 1/4W and 1/8W resistors, TO-92's, etc.) The best I can
    find is recommendations to derate by factors of a few to several when
    you have a lot of these components in close proximity. Nothing talks
    about PCB/copper layout.

    Any hints?

    Tim.
     
  2. Very few, in industrial (mass production), the PCB layout is usually
    given to software, so safety measures are essential.
    Then if hickups develope you take the cirquit and do enviromental tests
    with IR detection to see what gives trouble.
    On mission critical designs you do those tests as part of development
    and it is VERY costly and time consuming.
    It is usually a specialised branch of engineering, those that seat
    around the fireplace and smoke silently pipes through long hours of
    overtime.

    Have fun

    Stanislaw
    Slack user from Ulladulla.
     
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