One method of getting the heat out is to design a hole (or holes) through the
board under the component which you can use to inject a thermal paste adhesive.
(A compliant adhesive like silicone would eliminate any z-axis expansion
concerns.)  If the thermal resistance of the adhesive isn't good enough at a
maximum thickness of .25 mm you may opt to solder a metal slug (solder plated
copper) under the component at the same time the component is soldered.  This
slug should also have a hole as the board does for thermal adhesive injection.
It has been my experience that although the component manufacturers have wide
tolerances on their spec sheets the actual parts are much more tighter within a
given lot.  If you inspect the components you should be able to select a
suitable slug thickness to compensate your components.

Jim Marsico

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