Depends on via sizes and quantities (totally exposed thermal plane preferred)? Also, as you were
proceeding, even the next layer down (through prepreg, laminate, and core material) provides a
thermal blanket. Obviously, this inhibits thermal transfer but, depending on size and numbers of
vias, "forces" thermal transfer through the vias to the extent maximum transfer is effected.

Another way to do the deed is to make bare the MLB edges to the extent the internal heat sink is
exposed to air or a thermal transfer interface. The interface would be a "card" guide, as an
example. Exposing, say, a .125" perimeter of bare copper would ensure more effective heat sinking
and thermal transfer. In some avionics designs, the method is called a "cold" wall.

Earl Moon

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