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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