Hi All -

One more shot at the filled thermal via query:

Most of us, by the time we're into using filled vias for dissipation of heat
from a component are already in between a rock and a hard place.  That means,
we're probably using a heat transfer adhesive to bridge the gap between
component body and circuit board.  It also means we may have designed in
thermal planes internal to the board to spread the heat.  And finally, you
will often find a thermal plane bonded  on the opposite side of the board
which is moving the heat to thermal rails on the unit housing.

All of these situations come about when one is faced with designing equipment
for severe environments, where the unit packaging prevents direct impingment
of cooling air on the parts involved.

Generally, with these circumstances, open vias really aren't an option.  In
most cases they have to be either tented or filled to prevent the risk of the
via being a trap for process residues.  In this case, the filled via tends to
be preferable, simply because almost any material is a better thermal
conductor than air.

As I noted in my earlier response, there are several good via fill materials
available from the various adhesive and circuit board products suppliers,
which offer a variety of heat transfer and TCE values.

Regards - Kelly

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