Generally most boards do not have the same conditions that Jack is
describing to the group. This is a specialized application and I would
recommend doing a thermal model of the board before making any assumptions. 
We use a program called ICEPACK 

(http://www.ansys.com/products/icepak/default.asp ) 

along with our Solidworks 3D model of the board assembly which is a direct
output from the Altium Designer board design program and we can plug thermal
temp and conductivity values into the board assembly and predict what the
temps will be at any given ambient in any enclosure we design. 

The challenge facing Jack is predicting what the total load of heat will
raise the temperature to hoping of course that it does not exceed the Tg of
the board material. 


Bill Brooks 
PCB Design Engineer, C.I.D.+
Tel: (760)597-1500 Fax: (760)597-1510
Datron World Communications, Inc.
Vista, California

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