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Thermal Relief Pad

Discussion in 'Electronic Design' started by [email protected], Apr 19, 2007.

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  1. Guest

    I'm having a PSU board laid out at the moment that is conducting some
    reasonable current (8A). The ground connections have been made to the
    ground plane with what i believe is called thermal relief - there
    isn't a full ring of copper around the hole to connect it to the
    ground plane, instead there are 4 thin tracks - to help when soldering
    the thing together. I have a couple of questions about this.

    1.) Does anyone have experience of soldering devices in without this
    thermal relief? If its not much worse then this is surely the way to
    go to ensure enough current handling / low resistance to ground.

    2.) If its a royal pain to solder is there a better alternative - for
    example thickening one of the spurs to the thickness of a track.

    Thanks

    Andrew
     
  2. Eeyore

    Eeyore Guest

    It is MUCH more difficult. There's a higher chance in particular of a cold/dry
    joint.

    If you're worried, alter the pattern of it. 8 Amps is nothing though.

    Graham
     
  3. John Larkin

    John Larkin Guest

    We "flood over" pads and vias all the time. It's no big deal. If the
    board is surface mount, or wave soldered, it's fine. Our boards are
    mostly surface mount, with a few thru-hole parts hand soldered after
    reflow. With a good assembler and a good Metcal iron, soldering the
    non-thermal pads is still no big deal.

    Thermal resistance and electrical resistance are inseparable. For most
    metals, including copper, the ratio is about 140,000 K/w thermal
    resistance per ohm electrical. So a typical thermal pad, with four
    spokes (1 net square of 1 oz copper) and maybe 0.5 milliohm of
    resistance, has a thermal conductivity of about 70 K/w to the plane.

    John
     
  4. Yes - This is the sort of job where a Metcal really shows how much better it is than most other
    irons.
     
  5. DJ Delorie

    DJ Delorie Guest

    Yup, best money I ever spent on a tool.

    Wouldn't oven reflow soldering be more tolerant of thermal-less pads?
    I would think that they'd heat up the solid copper as much as
    everything else, which avoids the cold joint problem. Same with
    hotplate reflow like I use; the ground planes are plenty hot by the
    time the paste melts.
     
  6. John Larkin

    John Larkin Guest

    But heating those flooded pads still slows things down. If a milliohm
    of electrical resistance is OK, the 140 k/w thermal resistance will
    make soldering a lot easier. You've just got to run the numbers.

    John
     
  7. Guest

    A milliohm shouldn't be a problem (0.064W / 0.008V drop), my concern
    was the current handling capacity of the spokes. After doing the
    calculations for the track width the spokes didn't seem to add up.

    Infact doing my own calculations on the spokes give each one to
    0.5milliohms (10mil x 10mil spokes @ 1oz), with 4 this then further
    reduces the resistance to ground. I guess the current carrying
    potential isn't really the issue as they are so short and therefore
    wont heat up.

    Thanks for all your replies.

    Andrew
     
  8. Re. current-carrying ability, remember that each end will be heavily heatsinked, so the temp rise of
    the spoke will be a lot lower than a piece of track of the same width.
     
  9. John  Larkin

    John Larkin Guest

    Again, approximate math is simple. a 10x10 mil spoke has 500 uohms
    resistance. Run one amp through it and it dissipates half a milliwatt.
    The thermal resistance from the center to the pad/plane is about 35
    k/w, and the average is about half that. So the center hot-spot temp
    rise is ballpark 10 millikelvins.

    John
     
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