Mike,
the thermal conductivity of such a paste depends more on the granulometry of the
solids in it, than on the actual material.
Thermal compound is usually a mixture of finely ground thermally conductive
material, and some sort of oil or other thick liquid. The problem is that all
suitable liquids have very low thermal conductivity, compared to even a modest
ceramic material, let alone metals. So, whether the granules are alumina with a
thermal conductivity of 18 W/(m*K), or silver with 419 W/(m*K), makes little
difference when the voids between the granules are filled with an oil having a
thermal conductivity of 0.21 W/(m*K)!
And between silver and copper (372 W/(m*K)) there is essentially no difference,
when the voids are filled with that oil.
But it makes a lot of difference whether all granules are balls of the same size
(worst), or flakes of widely varying sizes having flat sides (that would be a
good filler). With those flakes, there is far less space between particles that
has to be filled with the liquid, and this strongly improves the thermal
conductivity of the compound.
I would think that manufacturers of premium thermal compounds have their own
technologies to make filler powder with carefully optimized particle size and
shape. It should be such that under pressure the particles arrange to squeeze
out as much liquid as possible, so that the final compound layer is mostly
solid, with only very small spaces filled by the liquid.
Also it would help a lot to have liquids with higher thermal conductivity, that
are stable enough to use them in thermal compounds. Water (0.6 W/(m*K)) is much
better than most other liquids, but of course it dries off quickly, so it's
useless in thermal compounds.
Cheap, lightly loaded thermal compounds typically have a thermal conductivity of
only around 0.3 W/(m*K), so it's only slightly better than using unloaded plain
grease. At least these are easy to apply and spread out, producing a thin layer.
Premium thermal compounds fal into the range of 3 to 5 W/(m*K) - some
manufacturers may claim even slightly more, and it's up to teh user to believe
that. But these premium compounds have lots of solids and little liquids, so
they are harder to spread out in a fine enough layer. Well applied, they are
indeed much better than the cheap compounds, but used in a casual way they might
not bring much improvement, because the layer ends up thicker.
Instead if you solder down the parts to the heatsink, you get a thin layer,
along with the thermal conductivity of solder, around 50 W/(m*K). This is very
much better than any other mounting system allows, but it's not always possible
to do, it gives no electrical insulation, and there can be problems coming from
stress cracking due to different thermal expansion of the parts.
Manfred
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