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placing components above thermal vias

Discussion in 'Electronic Design' started by veeresh, Jul 2, 2007.

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  1. veeresh

    veeresh Guest

    Hi,
    Please guide me regarding thermal vias.

    Thermal vias are meant for dissipating heat to power/ground planes.

    Suppose if a transceiver sort of IC has a thermal slug, and thermal
    vias are provided in the pcb region below this, is it fine to place a
    plastic body component on the other side of the PCB below this
    transceiver?

    thanks and regards,
    Veeresh
     
  2. Unless you are providing a blast of air to that side of the
    board to cool the hot spot, it may actually be helpful.
    Most component materials, especially those with metal lead
    frames passing through them, are more thermally conductive
    than the air they replace. So their extra surface area just
    makes contact with the air through a larger boundary area.

    The exceptions would be components that are sensitive to the
    hot board, or components that add heat to that spot.
     
  3. It depends entirely on your circumstances.
    There can be a dozen factors involved, some of which are which are:
    - How is your boards mounted (vertical or horizontal)?
    - What layers connect to the thermal vias?
    - Are they blind vias?
    - Do you have airflow?, and where?
    - How much heat is your part dissipating?
    - What is the steady state temp on that side of the board?
    - What is the plastic body component?, and does it have a tempco that
    you care about?
    etc

    If your board got so hot that the plastic body on the component is
    your concern, then that is the least of your worries.

    But as a general guide, no, you wouldn't put a component over thermal
    via's.

    Dave.
     
  4. Robert Baer

    Robert Baer Guest

    Use your head!
    *Anything* placed on the other side will severely reduce convection,
    make conduction to a heatsink impossible, and will reduce radiant transfer.
    Might as well as forgo the thermal vias.
     
  5. Sorry, but that is not the purpose of a "thermal" via. In a thermal,
    part of the copper is removed to prevent the heat of soldering from
    dissipating into the plane(s) or other traces. This makes it easier to
    solder a leaded component to the circuit board.

    In addition, I have also been told that thermals allow the via to expand
    while soldering a lead into the hole thereby preventing damage to the
    circuit board.

    So if you want to pull heat away from a surface mount component, I would
    recommend you put solid vias to the appropriate plane(s) in the copper
    under the device's thermal slug. Some time ago I read an app note that
    suggested using small vias that would fill with solder and form a better
    thermal connection. Others advocate the use of large vias because they
    have more surface area to connect to the plane(s). YMMV.

    Also, as others have noted, you should not put components directly on
    the other side of the board as this will reduce the heat dissipation.
     
  6. Ah, no, you have your terminology a bit mixed up.
    You are talking about a "thermal relief" on a pad or via. That is
    completely different to a "thermal via" which is indeed designed to
    transfer the heat into a larger copper plane.
    That is precisely what a "thermal via" is.
    The exact terminology can vary of course, but it's the one I use and
    have most often heard used.

    It is clear the OP is talking about "thermal vias", not "thermal
    reliefs".

    Dave.
     
  7. veeresh

    veeresh Guest

    that's correct sir, thermal relief is like opening solder mask to
    expose copper in that area of IC's slug, and soldering it there. Since
    I don't have any air flow mechanism in the closed box, i thought heat
    would be dissipating to planes, so keeping a plastic body component
    should not affect much. But now I think it's better to leave that area

    ~veeresh
     
  8. Brian

    Brian Guest

    Poo poo on the answers you got so far. Many designers could use a
    lesson in thermal dynamics ;)

    The question is, what heat am I producing and how much of the copper
    plane do I need to cool it? The answer could be "none". Then blocking
    it, or even having it, is not an issue. On the other hand, I have used
    the heatsink from one regulator drawing very small current to help
    cool another running hot. One post here was correct, the part over the
    thermal may in fact REMOVE more heat rather than block it. So go back
    and ask these questions of your design.
     
  9. Could have sworn I asked the OP to ask himself all these sorts of
    questions in my first post...

    Dave.
     
  10. Jasen Betts

    Jasen Betts Guest

    the best solution seems to me to be one large plated hole
    backfilled with solderor possibly a soldered slug of copper.


    Bye.
    Jasen
     
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