Mike,

Assuming .062 thick, the spreadsheet calculations on thermal vias .012
[0.3mm] to farside plane using 1 ounce clad plated .002 with .001 holes; with
and without solder don't show much difference.  Same configuration using
conductive hole fill CB100 render same result.  Unless your taking the heat
somewhere (aluminum plate, copper plate, planes) you have nowhere to conduct
it to.
There are several PWB substrate solutions.  Barrel plate vias with standard
plate gives a max thick wall without copper plugging, conductive fill and
plate the surface flat, Dielectric solutions such as Thermagon (10X thermal
improvement over FR4 and the like).  But, you still need to conduct the heat
to chassis.

As to the suggested assembly procedure, it may work but not on all cases.  In
the RF configurations I am familiar with there is a plane farside that
interfaces with hardware and that flatness is important.  The reflow
procedure will render uneven surfaces built up with solder.
An added temp excursion always makes me wince.
Added assembly time, adding an entire reflow operation, seems to the straw
that breaks the camels back (more of a 2 x 4 sized straw).

I suggest you have a sit down with your thermal guy, buy him a coffee... but
make sure it's hot.  Feel free to contact me, we have a full time Thermal
Engineer on board that just loves this stuff.  Hope this rambling gives you
some insight.

Boston Brad
781 8585 0783

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