Some advertise as much as 2 W/mK of thermal conductivity. Free air is no more than 0.03 W/mK at usual temperatures. Air convection I think will be the same in both the potted and unpotted cases. Does that mean that potting with such epoxy helps with heat dissipation, and the thicker the epoxy is the cooler the device will run?
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The answer is highly dependent on the package(s) being cooled, geometry, installation configuration, etc. The 0.03 W/m*K figure doesn't take convection into account, which will become the dominant form of heat transfer if there is adequate room for it to occur (at normal electronics operating temperatures). As the gap increases, the thermal compound will become limited by the thermal gradient inside the compound while air cooling will more closely approximate free-air convection conditions. You will probably have to perform a detailed analysis to determine which is better. There are of course exceptions, but thermal potting compound should be chosen primarily because you need potting for your application. If you just need to get rid of heat, there are plenty of other options that are easier to work with - thermal pads, thermal grease, heatsinks, heatpipes - that don't make your device impossible to repair and don't expose you to e.g. triethylenetetramine.
