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Heatsink for DPAK

Discussion in 'Electronic Design' started by Pasquale, May 28, 2008.

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  1. Pasquale

    Pasquale Guest

    Hello to everybody,
    anyone of you knows how to fix an Heatsink for DPAK on PCB ?

    Thank you,
    Pasquale.
     
  2. Jim Yanik

    Jim Yanik Guest

    or maybe you could solder it to a copper HS and solder that to the
    PCB....if the leads will still reach. you could rout a hole in the pcb for
    the copper HS and glue it in flush with the PCB.
     
  3. Eeyore

    Eeyore Guest

    You mount it on a large copper area. Ideally on both side of the PCB
    with some decent size vias joing the two.


    Graham
     
  4. Pasquale

    Pasquale Guest

    Thank you very much.
    I'm gonna do a copper area large enough in order to fix the DPAK and the
    Heatsink for DPAK. Do you think is it ok ? I have a MOSFET DPAK where the
    drain should lies on the copper area. What do you think is the best way
    to solder it ? Remember that I should solder it by hand.


    Thank you,
    Pasquale.
     
  5. You may have a bit of trouble soldering this by hand. They are designed for
    reflow soldering.
    What equipment do you have available?

    BTW, have you done the thermal calculations to figure out what size heatsink
    you actually need?

    Dave.
     
  6. Pasquale

    Pasquale Guest

    You may have a bit of trouble soldering this by hand. They are designed
    Actually I don't have really good equipment (I have the stuff to solder
    by hand, "regular" equipment), for sure I don't have a reflow solder
    station. I know that is gonna be difficult but how can I manage to solder
    it by using "regular" equipment ? I need suggestions in order to get the
    best result with the equipment I have.
    Not at all. I've seen some heatsink and I though to take the best one. I
    know that it's not good choice but I don't have any idea about the
    calculation. Could you help me ?


    Thank you,
    Pasquale.
     
  7. Eeyore

    Eeyore Guest

    Slightly tricky as they're normally soldered using paste. However if you do
    it carefully you may be able to 'flow' some solder under the package but not
    that won't work with leadfree solder because it has no surface tension to
    talk of.

    Graham
     
  8. I wsa under the impression that lead-free solders have a slightly
    higher surface tenson than classic tin-lead solders, which, judgeing
    from the context, may have been what you meant anyway.
     
  9. Tim Shoppa

    Tim Shoppa Guest

    True, that's what they're designed for, but for a part with a pad more
    than a few mm by a few mm (for example the TO-252 has a pad 6.5 x 6
    mm, quite big by most SMT standards) I find that a plated-through hole
    big enough for my soldering iron tip lets me do a very decent job of
    getting the part attached to the board. I'm sure there's some loss off
    cooling ability because there's no copper under part of the pad but
    I'm not pushing the limits.

    Tim N3QE
     
  10. Pasquale

    Pasquale Guest

    I'm really interested in what you have done. Could you, please, send me
    some pics or something in order to understand better ? Or some links
    where I can check your procedure ?


    Thank you,
    Pasquale.
     
  11. Tim Shoppa

    Tim Shoppa Guest

    No pictures handy, but: just put a plated-through hole underneath the
    thermal pad in a rectangle of copper. To solder the device to the
    plated through hole, turn the board over, stick the soldering iron
    through the hole to heat up the device and the board, when it gets hot
    enough to melt solder all around, stick in the solder and flood the
    hole. Solder will wick between the device and the copper pad.

    I used this for a low-scale production AD9954 DDS a few years back.

    Having the board pretinned helps a bit if you aren't used to it.

    Having the hole of a good size for your soldering iron tip really
    helps. If not, you won't directly heat the devices thermal pad, just
    the board.

    Tim.
     
  12. Hammy

    Hammy Guest

    I had a similiar question a while ago heres what I did. It's not ideal
    but it worked.The poor mans reflow method.

    All I do is put a dab of solder on the draintab of the mosfet,and some
    solder on the drain portion of the footprint.Press the mosfet down and
    heat the drain tab at the same time.Keep going around heating the
    drain tab that extends above the case,and the drain part of the
    footprint.I used a small insulated screwdriver to keep pressure on the
    FET,while heating.

    It took a few tries to get it down, but once you get the feel for
    it,it does the job in a resonable time.Just dont overdo the solder.

    Good luck
     
  13. Pasquale

    Pasquale Guest

    No pictures handy, but: just put a plated-through hole underneath the
    Thank you very much. I'll try your method. What about the heat ? I'm
    trying to do some calculation if I need a heatsink or not. How did you
    manage that ? I've see that I should use the Rthjc, Rthja and so on but
    maybe a bigger copper pad will work. What do you think ?


    Thank you,
    Pasquale.
     
  14. First you need to know how much power your device is going to
    dissipate. This value will depend on how it's used in your circuit.
    If you don't know how to calculate this far then I suspect you may
    have more to worry about than whether or not you can solder a
    heatsink!

    Dave.
     
  15. Pasquale

    Pasquale Guest

    I know, more or less, how much power my device will dissipate. For the
    MOSFET should be something like Pd = Rds_on*I^2. For this reason I'm
    trying to understand how to choose the right heatsink,


    Thank you,
    Pasquale.
     
  16. Eeyore

    Eeyore Guest

    If it's switching at high speed you also need to add the switching losses.

    Graham
     
  17. The device should have a degC/W figure for dissipation into ambient (no
    heatsink).
    If that temperature rise is acceptable in your application then no extra
    heatsink is needed. In practice because the device will be soldered to the
    board, it will have some extar heatsink already. If that temperature rise is
    not acceptable then you have to start looking at thermal resistance figures
    of the package and heatsink.
    Generally, you can get a ballpark figure by simply taking the heatsink
    degC/W figure and ignoring the thermal resistance losses (junction to case)
    in the package etc. The heatsink I quoteed is 25degC/W, not marvelous, but
    it helps.

    Often with DPAK packages, a PCB heatsink is enough, and there are various
    ways to improve this with layout. But PCB heatsinks can be harder to
    calculate a figure for.

    Dave.
     
  18. Pasquale

    Pasquale Guest

    Then I think it's gonna be something like that :

    fs = PWM freq.
    tc_on = switching time off-on;
    tc_off = switching time on-off;
    T = PWM period;
    t_on = time when the PWM in high;

    P = 1/2*(Rds_on*I^2)*fs*(tc_on+tc_off)+Rds_on*I^2*(t_on/T);

    Correct ?


    Thank you,
    Pasquale.
     
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