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Using a PCB as a heatsink

Discussion in 'Electronic Design' started by Mike Noone, Feb 14, 2007.

  1. Mike Noone

    Mike Noone Guest

    Hi - I'm going to be making a board that has a large number of high
    power (1W) LEDs. I'd ideally like to use the PCB as a heatsink to get
    rid of all the heat. How much surface area/watt do I need to allow?
    Thanks!

    -Mike
     
  2. Boris Mohar

    Boris Mohar Guest

    How about
    http://www.alibaba.com/catalog/11083603/Aluminum_Based_Copper_clad_Laminate_Sheet.html

    If things get really hot?



    Regards,

    Boris Mohar

    Got Knock? - see:
    Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca

    void _-void-_ in the obvious place
     
  3. Eeyore

    Eeyore Guest

    How many ? Use some simple logic and work out how hot your pcb is going to get.
    Hint: 10W is a lot in an enclosed space.

    Graham
     
  4. MassiveProng

    MassiveProng Guest

    Make small PCBs for each LED (1 sq cm ea or less), and mount those
    above the driver board like a third of a cm or more with pins or stiff
    wires, and the airgap will do a lot of cooling with a bit of flow.
    Leave both sides of the tiny boards fully cladded on one or both sides
    (of course negate shorts), and leave them bare ie no mask so they
    radiate their heat better. Heavy gauge copper or SPC wire (solid)
    would allow some heat conduction as well.
     
  5. Hi Mike,

    Power leds (1W and above) definitively need an aluminium subtrate (SMI type
    PCB, ie PCB with the base epoxy replaced by aluminium), and this SMI needs
    to be efficiently fitted on an appropriately calculated heatsink. Even
    Alumium PCB alone is not sufficient, especially if you have several LEDs...
    A good reference is Luxeon's "Thermal Design" application note available
    here : http://www.luxeon.com/pdfs/AB05.pdf

    If you can't design your own SMI board then the only reasonnable solution is
    to by "level 2 LEDs", meaning LEDs already mounted on a small SMI, and to
    fix these small SMIs on a good heatsink.

    Cheers,
     
  6. Eeyore

    Eeyore Guest

    All good advice but you have to get the heat out of the enclosure eventually.

    I unintentiomally left some paprework on top of my digital 'set top box' yesterday
    and was astonished to see how warm it got. I had some picture breakup too which
    alerted me to it.

    The other important thing is to check how hot it gets *after several days* !

    Graham
     
  7. John  Larkin

    John Larkin Guest

    Bare copper has a thermal infrared emissivity near zero; it's hard to
    find anything that radiates worse.

    John
     
  8. Mike Noone

    Mike Noone Guest

    Probably about 10 maximum. It won't be too enclosed though...
     
  9. RHRRC

    RHRRC Guest

    Since this will obviously not work why go to all that trouble just to
    destroy some leds?

    You will need an mcpcb mounted to a heatsink.
    Do the sums and work out the required theta J to A. Thence with the
    knowledge of the led theta J to C the 'heatsinking' ability of the
    mounting is revealed.
    Look at the mcpcb data then calc the heatsink required.
     
  10. MassiveProng

    MassiveProng Guest

    I do not agree with the number you "selected" to use, but yes, any
    polished surface will have a lower emissivity. I forgot to mention
    grit blasting the PCBs to make the finish matte.


    Bare copper with a matte finish has a higher emissivity than any
    type of copper under an insulative blanket of solder mask.

    Heat conducts through the device leads or package, to the PCB copper
    and substrate, and then radiates to air. If it has an added layer of
    solder mask, that radiation will be less.
     
  11. MassiveProng

    MassiveProng Guest

    There is also conduction cooling through the large leads mentioned
    into the main PCB as well as their radiating surfaces. This works.

    One can also buy a bit of sheet copper, and solder a little U shaped
    piece onto each PCB, or across all them, linking them together.

    For a mere LED. Yes... it will work.
     
  12. MassiveProng

    MassiveProng Guest

    That's funny. Looks to me like exactly what these guys did with one
    of their one watt LEDs... you know Luxeon...

    They even put it in a little plastic tub.

    http://www.amazon.com/TerraLUX-LED-Replacement-Maglite-Flashlights/dp/B00068VAP0
     
  13. John  Larkin

    John Larkin Guest

    Do you really grit blast pcb's?
    I'd have to try that. I'm a bit dubious. But radiation is a minor heat
    dump mechanism at the sorts of temps that pcb's usually run at.
    Heat doesn't raidiate to air, it conducts and convects to air. Heat
    radiates *through* air.

    And a solder mask adds a minute amount of thermal resistance, not
    enough to worry about.

    John
     
  14. Eeyore

    Eeyore Guest

    I'd have thought so.

    How about chemically blackening the copper ? Or a black solder resist?

    I'm sure the main mechanism for heat removal will be conduction/convection. In that
    respect a few holes in the pcb will allow extra air flow.

    I'd have thought so myself.

    Graham
     
  15. MassiveProng

    MassiveProng Guest

    If its function is to be used as a thermal radiator.

    With HV, we have even grit blasted the shiny surface of HV caps. and
    diode packages. It makes the surface grab potting compound better.

    Potting detachment is a main cause for multiplier stage failure in
    HV supplies.

    But for this thermal application, yes, bare PCB (no mask) and if the
    foil is to shiny, a bit of blast makes it matte, or a bit of 320 grit
    emory cloth.

    Both before any population takes place.
     
  16. MassiveProng

    MassiveProng Guest


    Most of it at this level will/would get handled by thermal
    conduction cooling via the leads into surrounding substrates.

    That is why I think this would work so well. It has two soaking
    elements instead of one, and if they were all on the main PCB the
    whole mass could warm and actually degrade the thermal abatement
    desires. PCB substrates are fiberglass and epoxy, and don't move
    thermal energy very fast.
     
  17. MassiveProng

    MassiveProng Guest

    Heat radiating from a hot surface conducts into the air. Using the
    single term radiates to air is fine. IR does the work either way,
    from within the hot medium. How do you think heat conducts to air
    down at that little boundary layer? Does it shake hands with the
    surface? No. IR is the thermal engine of ALL matter. We wouldn't be
    here otherwise.
    It DOES matter when what WE are after is a maximized system for
    removing heat.

    Shall we discuss micro-air gaps under large IGBTs? Next thing,
    you'll be telling me that doesn't matter either. There is an entire
    industry that says you're wrong.
     
  18. MassiveProng

    MassiveProng Guest


    With emissivity the apparent color is not as important as the
    surface quality.

    We painted our black body calibration sources with a matte black
    paint and got near .98 plus emissivity. Average Human skin is also at
    ..98

    Both are due to surface texture. The bare aluminum ingot has a smooth
    surface, and some of the IR generated inside when it gets heated get
    reflected back inside. When it has the matte finish, it has many many
    more angles to radiate from the surface at. They are little tiny
    scratches that add up to more actual surface.

    IR and a smooth, polished , flat surface do not get along. It
    reflects back inside the medium, and any that does radiate only
    radiates from the surface in a perpendicular ray. The matte surface
    allows the heat in the medium to radiate away from its surface at
    several angles of incidence. A denser IR "flux" emanates.
     
  19. MassiveProng

    MassiveProng Guest


    In this case, if anything, s bunch of plated vias would assist in
    the conduction cooling process, but not much on convection in a box
    with no forced air. With forced air, it would add a little.
     
  20. MassiveProng

    MassiveProng Guest

    A blanket is a blanket is a blanket. Period.
     
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