I did not know what type of heatsink you are using and my suggestion may
not be a good one. However, you can it as a reference.

Using the bottom layer, construct a aluminium plate with holes drilled
larger than the signal vias. This will provide complete isolation to the
signal vias and eliminate the need for thermal isolation by mean of
isolative material currently used. This aluminmum plate can then be
thermally glued onto the PCB surface using thermal adhesive.

Using a single adhesive material would removal the isolative material
and gaskets and would prevent migration between different materials over
time. I am not a process gal but hope this method would work well.

Regards - Wee Mei

Genny Gibbard wrote:

>  We have a small PCB with SM components mounted on one side and a
> heatsink mounted against the other side over the entire surface of the
> PCB.  The heatsink is grounded, so we use a thermally conductive,
> electrically isolative thermal interface material in between (0.5mm
> thick), to protect signal vias.  We also use some strips of
> electrically conductive gasket in strategic areas to provide good
> electrical grounding between the two.Migration of some substances out
> of the thermal material onto the PCB over time has caused performance
> problems on random, not all, units.I was wondering what other methods
> might exist for heatsinking a PCB, possibly permanently bonding the
> metal heatsink to the PCB in some way.  Any sources of information for
> alternatives is appreciated.  We cannot change the construction
> materials of the original PCB, which is a multilayer (4 layer) FR4,
> but we may be able to bond an additional layer onto the PCB, in place
> of the gasketing we use now.Thanks in
> advance,Genny---------------------------------------------------
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