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Thermal pads on PCBs

Discussion in 'Misc Electronics' started by dave madden, Oct 3, 2005.

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  1. dave madden

    dave madden Guest

    I have an audio amp in a 20-pin DIP, and pins 11-20 are all grounded;
    the datasheet says to hook them to a large area of copper as a heat
    sink. My PCB design software leaves an annulus around each pad in a
    ground plane, connecting the pad to the plane with a "+" trace. The
    manual says that's a "thermal" connection, and that if you put copper
    all the way around, the thermal conductivity away from the pin is so
    high that you can't solder it.

    How hard is it, really, to solder something like that? If I'm trying
    to take heat away from the chips, then a full connection sounds like
    what I want. But if I can't solder it, I'll be an unhappy camper.

  2. CWatters

    CWatters Guest

    Otherwise known as a "Thermal Relief".
    Is this going to be a volume product or just a one off?

    It is possible to hand solder but you need a soldering iron with a large
    thermal mass - a big lump of copper. The actual wattage is less important
    but big irons usually come with a good rating. It will probably need to be
    big enough that you heat and solder all the gnd pins at once.

    It might be worth leaving an area near the pins without solder resist so you
    can place the iron on the heat sink pad, wait a few seconds and then slide
    it over to heat the pins.
  3. Gerard Bok

    Gerard Bok Guest

    Your PCB software is probably expecting a production environment.
    There, equal amount of heath is applied to each solder joint.
    That requires the thermal standoff as your software puts in
    automatically or the joints may fail.
    It's not hard to solder. It's hard to keep both the chip and the
    board within the survivable temperature area :)
    The copper area acts as a heath spreader. Both during operation,
    where it is desirable and during production. Where it requires
    quite an amount of extra heath to get a proper joint.

    If this is a one-off situation, you may solve the problem quite
    the other way: attach a heathsink to the top of the package :)
  4. JeffM

    JeffM Guest

  5. Jasen Betts

    Jasen Betts Guest

    with a big enough soldering iron anything can be soldered.

    I'd expect you to want 50W or more (100W?) for that sort of application..

    if you only want to do it once get a cheap soldering iron from an auto
    parts or hardware retailer.

    If you plan to do many invest in a soldering gun which will give better
    control over the heat output.

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