Here are some equations that might help you out:
Assume steady heat flow:

(1) R= t/k

R=Thermal Resistance (m2.C)
K= Thermal Conductivity (W/m.C)
     Copper ~ 370
     Solder ~ 70
     FR-4 ~ 3
t= Thickness (m)

(2) q/a= delta T/Sigma R

Q= Heat Flow (W)
Delta T= Temperature differential (C)
Sigma R= Sum of Thermal Resistance
A= Area (m2)

With these two equations you can estimate the heat dissipation.

Hope this helps
Michael Yuen
 ----------
From: TechNet-request
To: TechNet
Subject: Re: Using ground plane as a heatsink
Date: Friday, August 23, 1996 8:32AM

     Hi,
        I don't have any hard numbers for heat dissipation for you, but I
     hope this helps.  I've done this a number of times at different
     companys and it seems to work.  Provide a solid copper area as big as
     possible under the component to be heatsinked on the external layers.
     Remove solder mask where ever the component touches the heatsinked
     plane.  If possible duplicate this plane on all layers.  Stitch the
     multiple planes together with vias, (I've used .023 dia and .033 dia
     with equal success).  Put the vias on a .100" staggered grid if
     possible.  (tip of the hat to Ralph Hersey here).


                                       x        x        x
     x= via hole
                                 - x       x        x
                                 |
                               .10     x        x        x
                                 |     |
                                 - x   |   x        x
                                   |   |
                                  -|   |-.050"


      If possible make via connections to the planes solid ie. no
     thermals.  If you are using forced air cooling you might use a few 
 larger holes to allow air circulation.


                                Best of luck,
                                        Rob

     ROBERT BUTTERWORTH
     ADC VIDEO SYSTEMS
     WALLINGFORD, CT 06492
     [log in to unmask]

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