PCB Copper Thickness Versus Rth - is this graph correct?

Discussion in 'Electronic Design' started by Klaus Kragelund, Jan 9, 2014.

  1. Hi

    I have a PCB, 50mm x 50mm, and I need to optimize cooling of the PCB. A number of dissipating components are spread out on the PCB to produce an uniform temperature across the PCB.

    Currently I am using 0.5 Oz PCB thickness, but it is possible to increase that to 1 Oz.

    So I was looking for a graph of the thermal resistance on a certain area ofPCB versus the copper thickness. My initial feeling would be that the increase of the copper thickness would be insignificant with respect to the Rth..

    Found this graph, figure 3 on page 2:

    http://www.iaasr.com/wp-content/upl...roduced-by-surface-mount-components.-Rev1.pdf

    Increasing the copper from 0.5oz to 1oz would reduce the thermal resistancefrom 260K/W to 180K/W

    But, is this valid. If we take the example of a single hotspot device in the center of the board, the increased thickness would reduce the thermal resistance from the device to the rest of the board, so the temperature would be close to uniform.

    If on the other hand, with a PCB with decreased copper thickness, I have a number of devices spread evenly on the PCB and dissipating individually thesame amount of power, the heat would then also be uniform. But the transfer of the heat to the surroundings are convection and conduction, and these should not be affected by the thickness of the copper layer.

    So, for the actual design, evenly spaced components would not benefit from thicker copper thickness. Is this a valid assumption?

    Cheers

    Klaus
     
    Klaus Kragelund, Jan 9, 2014
    #1
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  2. Klaus Kragelund

    RobertMacy Guest

    On Thu, 09 Jan 2014 03:55:55 -0700, Klaus Kragelund
    <> wrote:

    >> ...snip to keep Aioe happy

    > If on the other hand, with a PCB with decreased copper thickness, I have
    > a number of devices spread evenly on the PCB and dissipating
    > individually the same amount of power, the heat would then also be
    > uniform. But the transfer of the heat to the surroundings are convection
    > and conduction, and these should not be affected by the thickness of the
    > copper layer.
    >
    > So, for the actual design, evenly spaced components would not benefit
    > from thicker copper thickness. Is this a valid assumption?
    >
    > Cheers
    >
    > Klaus


    Rule of thumb for heat dissipation of free standing surface, no fan is
    1 C rise per watt over 100 sq in area.

    That kind of implies that thicker copper, which is in series with your
    copper/PCB to air transfer, doesn't make a lot of difference.

    But gut feel is that thicker copper also gives you some thermal mass,
    which might save a marginal part during a 'spike' of dissipation.
     
    RobertMacy, Jan 9, 2014
    #2
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  3. On Thursday, January 9, 2014 5:55:55 AM UTC-5, Klaus Kragelund wrote:
    > Hi
    >
    >
    >
    > I have a PCB, 50mm x 50mm, and I need to optimize cooling of the PCB. A number of dissipating components are spread out on the PCB to produce an uniform temperature across the PCB.
    >
    >
    >
    > Currently I am using 0.5 Oz PCB thickness, but it is possible to increasethat to 1 Oz.
    >
    >
    >
    > So I was looking for a graph of the thermal resistance on a certain area of PCB versus the copper thickness. My initial feeling would be that the increase of the copper thickness would be insignificant with respect to the Rth.
    >
    >
    >
    > Found this graph, figure 3 on page 2:
    >
    >
    >
    > http://www.iaasr.com/wp-content/upl...roduced-by-surface-mount-components.-Rev1.pdf
    >
    >
    >
    > Increasing the copper from 0.5oz to 1oz would reduce the thermal resistance from 260K/W to 180K/W
    >
    >
    >
    > But, is this valid. If we take the example of a single hotspot device in the center of the board, the increased thickness would reduce the thermal resistance from the device to the rest of the board, so the temperature would be close to uniform.
    >
    >
    >
    > If on the other hand, with a PCB with decreased copper thickness, I have a number of devices spread evenly on the PCB and dissipating individually the same amount of power, the heat would then also be uniform. But the transfer of the heat to the surroundings are convection and conduction, and these should not be affected by the thickness of the copper layer.
    >
    >
    >
    > So, for the actual design, evenly spaced components would not benefit from thicker copper thickness. Is this a valid assumption?
    >

    Hi Klaus, To my mind what's important is how the heat is being removed from the pcb. Is there some thermal connection to the outside world? (like brass standoffs.) Or is it just cooled by air conduction/convection? In theformer the thickness of the copper would help... where if it's just air cooling, and approximately uniform temperature across the pcb already, then thicker copper won't do much.

    George H.
    >
    > Cheers
    >
    >
    >
    > Klaus
     
    George Herold, Jan 9, 2014
    #3
  4. Klaus Kragelund

    Tim Williams Guest

    The defining quantity is the spacing of said components relative to the
    lateral diffusivity (i.e., how far sideways along the board the heat will
    spread out).

    I believe it's around 3cm for 2oz copper (ah, such wonderful juxtaposition
    of units :) ), so putting equal-dissipating components on a grid of around
    6cm center-to-center (note a triangular mesh allows maximal packing) will
    be about optimal between copper/board thickness and utilization. Such
    spacing will allow about 2W per component.

    Use proportionally smaller spacings for thinner material.

    Not necessarily smaller for thinner foil only, but let's see. FR-4 is
    0.81 W m^-1 K^-1 while copper is 400; the average board is 1600um thick.
    0.5oz copper is 17um, or say 34um total (double sided). The conductivity
    per square of copper is 0.0136 W K^-1, and of FR-4, 0.0013 W K^-1. So
    even for thin plating, it's still true that copper dominates the lateral
    conductivity.

    Tim

    --
    Seven Transistor Labs
    Electrical Engineering Consultation
    Website: http://seventransistorlabs.com

    "Klaus Kragelund" <> wrote in message
    news:...
    Hi

    I have a PCB, 50mm x 50mm, and I need to optimize cooling of the PCB. A
    number of dissipating components are spread out on the PCB to produce an
    uniform temperature across the PCB.

    Currently I am using 0.5 Oz PCB thickness, but it is possible to increase
    that to 1 Oz.

    So I was looking for a graph of the thermal resistance on a certain area
    of PCB versus the copper thickness. My initial feeling would be that the
    increase of the copper thickness would be insignificant with respect to
    the Rth.

    Found this graph, figure 3 on page 2:

    http://www.iaasr.com/wp-content/upl...roduced-by-surface-mount-components.-Rev1.pdf

    Increasing the copper from 0.5oz to 1oz would reduce the thermal
    resistance from 260K/W to 180K/W

    But, is this valid. If we take the example of a single hotspot device in
    the center of the board, the increased thickness would reduce the thermal
    resistance from the device to the rest of the board, so the temperature
    would be close to uniform.

    If on the other hand, with a PCB with decreased copper thickness, I have a
    number of devices spread evenly on the PCB and dissipating individually
    the same amount of power, the heat would then also be uniform. But the
    transfer of the heat to the surroundings are convection and conduction,
    and these should not be affected by the thickness of the copper layer.

    So, for the actual design, evenly spaced components would not benefit from
    thicker copper thickness. Is this a valid assumption?

    Cheers

    Klaus
     
    Tim Williams, Jan 9, 2014
    #4
  5. On Thursday, January 9, 2014 4:24:51 PM UTC+1, John Larkin wrote:
    > On Thu, 9 Jan 2014 02:55:55 -0800 (PST), Klaus Kragelund <>
    >
    > wrote:
    >
    >
    >
    > >Hi

    >
    > >

    >
    > >I have a PCB, 50mm x 50mm, and I need to optimize cooling of the PCB. A number of dissipating components are spread out on the PCB to produce an uniform temperature across the PCB.

    >
    > >

    >
    > >Currently I am using 0.5 Oz PCB thickness, but it is possible to increase that to 1 Oz.

    >
    > >

    >
    > >So I was looking for a graph of the thermal resistance on a certain areaof PCB versus the copper thickness. My initial feeling would be that the increase of the copper thickness would be insignificant with respect to the Rth.

    >
    > >

    >
    > >Found this graph, figure 3 on page 2:

    >
    > >

    >
    > >http://www.iaasr.com/wp-content/upl...roduced-by-surface-mount-components.-Rev1.pdf

    >
    > >

    >
    > >Increasing the copper from 0.5oz to 1oz would reduce the thermal resistance from 260K/W to 180K/W

    >
    > >

    >
    > >But, is this valid. If we take the example of a single hotspot device inthe center of the board, the increased thickness would reduce the thermal resistance from the device to the rest of the board, so the temperature would be close to uniform.

    >
    > >

    >
    > >If on the other hand, with a PCB with decreased copper thickness, I havea number of devices spread evenly on the PCB and dissipating individually the same amount of power, the heat would then also be uniform. But the transfer of the heat to the surroundings are convection and conduction, and these should not be affected by the thickness of the copper layer.

    >
    > >

    >
    > >So, for the actual design, evenly spaced components would not benefit from thicker copper thickness. Is this a valid assumption?

    >
    > >

    >
    > >Cheers

    >
    > >

    >
    > >Klaus

    >
    >
    >
    > It's complex.
    >
    >
    >
    > The surface area of the board is the convective path to the air.
    >
    >
    >
    > Copper pours and planes spread the heat out from a component. Spreading thermal
    >
    > resistance is usually important. A surface-mount resistor or transistor can get
    >
    > very hot if it can't spread the heat laterally into the board surface.
    >
    >
    >
    > The thicker the copper, and the more un-interrupted planes, the better the
    >
    > lateral heat spreading.
    >
    >
    >
    > 1 oz copper has a sheet thermal resistance of about 70 K/watt. That's thetheta
    >
    > from opposite of a square of copper foil of any size.
    >
    >
    >
    > Example: a 1206 resistor with normal pads and traces.
    >
    >
    >
    > https://dl.dropboxusercontent.com/u/53724080/Thermal/V220_1206/1206.txt
    >
    >
    >
    > https://dl.dropboxusercontent.com/u/53724080/Thermal/V220_1206/DSC06287.JPG
    >
    >
    >
    > https://dl.dropboxusercontent.com/u/53724080/Thermal/V220_1206/IR_0056.jpg
    >
    >
    >
    > The resistor is a hot spot, because the heat doesn't spread laterally very well.
    >
    > Theta would be much lower if the pads were bigger, or if there were thermal vias
    >
    > to other-layer copper pours or planes.
    >
    >
    >
    > So just physically spreading out parts doesn't solve the hot-spot problem.. Lots
    >
    > of copper is the best lateral heat spreading mechanism on a PC board.
    >


    I have the resistors spread out and all components have as much copper as possible to provide lateral heat spreading:

    https://www.dropbox.com/s/y55jw3urqgr5329/900mW into 12x 1206.pdf

    Cheers

    Klaus
     
    Klaus Kragelund, Jan 10, 2014
    #5
  6. On Thursday, January 9, 2014 4:46:46 PM UTC+1, George Herold wrote:
    > On Thursday, January 9, 2014 5:55:55 AM UTC-5, Klaus Kragelund wrote:
    >
    > > Hi

    >
    > >

    >
    > >

    >
    > >

    >
    > > I have a PCB, 50mm x 50mm, and I need to optimize cooling of the PCB. Anumber of dissipating components are spread out on the PCB to produce an uniform temperature across the PCB.

    >
    > >

    >
    > >

    >
    > >

    >
    > > Currently I am using 0.5 Oz PCB thickness, but it is possible to increase that to 1 Oz.

    >
    > >

    >
    > >

    >
    > >

    >
    > > So I was looking for a graph of the thermal resistance on a certain area of PCB versus the copper thickness. My initial feeling would be that the increase of the copper thickness would be insignificant with respect to theRth.

    >
    > >

    >
    > >

    >
    > >

    >
    > > Found this graph, figure 3 on page 2:

    >
    > >

    >
    > >

    >
    > >

    >
    > > http://www.iaasr.com/wp-content/upl...roduced-by-surface-mount-components.-Rev1.pdf

    >
    > >

    >
    > >

    >
    > >

    >
    > > Increasing the copper from 0.5oz to 1oz would reduce the thermal resistance from 260K/W to 180K/W

    >
    > >

    >
    > >

    >
    > >

    >
    > > But, is this valid. If we take the example of a single hotspot device in the center of the board, the increased thickness would reduce the thermalresistance from the device to the rest of the board, so the temperature would be close to uniform.

    >
    > >

    >
    > >

    >
    > >

    >
    > > If on the other hand, with a PCB with decreased copper thickness, I have a number of devices spread evenly on the PCB and dissipating individuallythe same amount of power, the heat would then also be uniform. But the transfer of the heat to the surroundings are convection and conduction, and these should not be affected by the thickness of the copper layer.

    >
    > >

    >
    > >

    >
    > >

    >
    > > So, for the actual design, evenly spaced components would not benefit from thicker copper thickness. Is this a valid assumption?

    >
    > >

    >
    > Hi Klaus, To my mind what's important is how the heat is being removed from the pcb. Is there some thermal connection to the outside world? (like brass standoffs.) Or is it just cooled by air conduction/convection? In the former the thickness of the copper would help... where if it's just air cooling, and approximately uniform temperature across the pcb already, thenthicker copper won't do much.
    >


    I has only limited contact to the enclosure, regretfully

    Cheers

    Klaus
     
    Klaus Kragelund, Jan 10, 2014
    #6
  7. Klaus Kragelund

    Tim Williams Guest

    "John Larkin" <> wrote in
    message news:...
    > Theta depends on the part size, too. If you dump heat into, say, a
    > circular
    > patch on an infinite metal sheet, theta depends on the patch area. Theta
    > goes to
    > infinity as the contact area goes to zero. Getting the heat out locally,
    > close
    > to the part, is often the bottleneck.


    Yes. Or, since you "can't do equations", ;-)

    For surfaces with no surface heat dissipation (lateral heat spreading
    only), the thermal resistance between concentric cylindrical surfaces is:
    Rth = ln(r2 / r1) / (2 pi sigma_th)

    Which of course diverges for r1 --> 0.

    When the surfaces dissipate heat linearly with temp difference (true of
    solid conductors, but a poor approximation of actual convection or
    radiation), solutions take the form of the complex Bessel function (i.e.,
    T(r) = c1 * J_0(i*c2*r)). A closed form solution (albeit in terms of the
    Bessel function) is left as an exercise for the student. ;-)

    Tim

    --
    Seven Transistor Labs
    Electrical Engineering Consultation
    Website: http://seventransistorlabs.com
     
    Tim Williams, Jan 10, 2014
    #7
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