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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