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Even heating across a 4" x 4" PCB Heater

Discussion in 'Electronic Design' started by [email protected], Oct 19, 2012.

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  1. Guest

    Anybody have some experiences they would like to share regarding the designof film heaters? I am working on (with?) one that is heating to a modest 37C but needs to be plus or minus half a degree Centigrade across the 4" x 4" surface. My contention is that the trace (and therefore heat density) needs to be higher at the edges than the center.

    My other contention is that if you are going to design a heater a complete analysis of everything the heater is contact with is in order.

    My contentions remain contentious if not in actual contempt as of this writing ;-)
     
  2. Hmm, OK I would guess that for a thin plate (pcb?) the heat loss would
    not have much of an edge effect. So I'd try a uniform heater first.
    A heat spreader might help... can you put 2 oz. copper on one side?
    (Sorry, is this a pcb?) But we really need more info on the heat
    loss... is the plate in air, vacuum, is there some differential heat
    load from one side to the other?

    George H.
     
  3. Guest

    Perverse is a nice way of putting it ;-)

    The heater is part of a PCB that has other functions that require not having electrically conductive material in certain regions because there are proximity sensors for things moving in the z with respect to the x y of the heater.

    The heater sits below a 0.12" thick plastic plate which acts as a heat spreader but is really there to provide material for slots that engage rails onthe "thing" that gets heated. The "thing" is surrounded on three sides bymore plastic and a PCB on top that does "other stuff.

    We are getting +or- 1.3C in the regions of interest. Would prefer to use asingle heater. I say FEA (Finite Element Analysis) by hand or in SW is the next (should have been first) step)

    Thanks for having a look and sharing thoughts.

    EdV
     
  4. But perhaps a piece of aluminum oxide (Al2O3), see what Coorstek has
    in stock. Is this a 'one of' or a production thing?
    Maybe a nested thermal box 'thingie' would work.

    George H.
     
  5. Hey, I was just remembering these ads by (panasonic?) selling
    pyrolytic graphite sheets. That would make some nice thermal
    spreaders... something like 3x copper along the plane of the sheets
    (IIRC). I guess it might be too electrically conductive for the OP.
    (It'd be fun to play with some.)

    George H.
     
  6. Tim Williams

    Tim Williams Guest

    Seems like a bad idea: a few years of vibration later and it's
    vibratory-tumbled its way out of existence!

    I was hoping your story finished with "then soaked the sand with epoxy".
    Maybe you tried, and it didn't work? Low viscosity epoxy (usually with
    little to no filler of its own) does exist; vacuum would probably be
    required to fully impregnate it. But vacuum probably would've come in
    handy for the original problem (with potting alone), too.

    Tim
     
  7. Tim Williams

    Tim Williams Guest

    Have you tried aluminum core or heavy copper PCBs?

    Aluminum core kind of sucks in some aspects, but it's my understanding
    that manufacturers can make them with vias and multiple layers. Just
    maybe not cheaply?

    Tim

    --
    Deep Friar: a very philosophical monk.
    Website: http://webpages.charter.net/dawill/tmoranwms

    Anybody have some experiences they would like to share regarding the
    design of film heaters? I am working on (with?) one that is heating to a
    modest 37C but needs to be plus or minus half a degree Centigrade across
    the 4" x 4" surface. My contention is that the trace (and therefore heat
    density) needs to be higher at the edges than the center.

    My other contention is that if you are going to design a heater a complete
    analysis of everything the heater is contact with is in order.

    My contentions remain contentious if not in actual contempt as of this
    writing ;-)
     
  8. In the deep past I've mixed Al oxide with epoxy, (400 grit maybe?)
    but mostly just to thicken it up. (We dripped it over cheap connectors
    to make them leak tight... (10^-5 torr level))

    George H.
     
  9. Nico Coesel

    Nico Coesel Guest

    Nowadays there are potting compounds with better thermal conductivity.
     
  10. You do not need to pot it. A thermal pad and proper case thickness
    will homogenize and carry off the heat as best it can. Potting only
    reduces serviceability and also usually changes some of the circuit
    operational characteristics.
     
  11. Guest

  12. rickman

    rickman Guest


    Whoa! If you can get 30 W/mK out of solid alumina, wouldn't that work
    as a perfect heatspreader? Put the heatspreader between the heater and
    the constant temp unit and that should provide the heat uniformly as
    well as equalize the temperature of the unit needing the constant temp.

    I guess we don't know exactly what it is that needs the constant
    temperature. If it can be mated well to a solid block of alumina that
    sounds great. But then I've never heard of an alumina heat sink for
    computers so I expect there is something wrong with this idea.

    Rick
     
  13. rickman

    rickman Guest

  14. rickman

    rickman Guest

    So does that mean I'm wrong twice? lol

    Is Aluminum Nitride non-conductive? I know stainless is conductive.
    How do they use that as a substrate?

    Rick
     
  15. That is why it was mainly only HV insulative focus medium (LV stuff got
    thin "pads") Used mostly as a "thin" "transfer interface" for HV because
    it had a much higher punch through voltage than thin pads do, and had
    enough thermal capability for the task across a thin cross sectional
    thickness.

    A REALLY good one is simple "hard anodization" on Aluminum. It is
    electrically non-conductive, and quite good at passing thermal energy
    though its matrix. There are three standardized levels of it, and the
    thickest one can even do "HV" up to a certain voltage, but was never
    trusted for that due to the occasional pin-hole, and that won't do for
    mission critical designs.

    I'll bet those new skillet coatings would be pretty cool (or hot)
    interfaces. Maybe not.
     
  16. hamilton

    hamilton Guest

  17. Guest

    Thanks! I remember working with Minco in the late 80s on a "profiled" heater. I was sure they had a nice summary of the design process somewhere.

    EdV
     
  18. Guest

    Thanks much to all who have weighed in my topic. I think I have the info Ineed to move forward with a nice array of possible solutions.

    EdV
     
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