help with Thermal Impdeance vs. Thermal Conductivity

I was poking around on Bergquist's web page looking at various Sil-Pad that they manufacture and came across some information that I can't quite understand and was hoping that someone would help me out.

I'm looking for Sil-Pads for mounting the Mosfets for my Aleph-X to the heatsink. But, I can't tell which of these two products would be better for between the Mosfet and the heat sink.
Sil-Pad A2000 : Thermal Impedance = 0.32, Thermal Conductivity = 3.0
Sil-Pad 1500ST : Thermal Impedance = 0.23, Thermal Conductivity = 1.8

If you go by Thermal Impedance, it appears that the 1500ST is the better product, but if you look at Thermal Conductivity, it appears that the A2000 is the better product.

So which on would be better to use? The Aleph-X that I want to build will be near 150w into 4 ohms, so it looks like every last little bit of keeping those Mosfets cool is worth the effort.

Can anyone explain Thermal Resistance vs. Thermal Conductivity and indicate which product is a better choice?

Thanks!
Eric
 
Thermal impedance is the temperature difference across the pad per watt. Thermal conductivity is a property of the bulk material, so you need to factor in the dimensions of the pad to work out its performance.

Indeed, if you divide the thickness of the pad by the thermal conductivity to get the thermal impedance per area, you find that the thinner 1500ST is approximately 1.4 times better than the A2000, which is by no coincidence also the ratio between the given thermal impedances.

In conclusion, thermal impedance (usually it's resistance given, not impedance, but they will be the same for static measurements) is the important parameter, so the 1500ST wins.