sci.electronics.basics

>>>Sil-pads are garbage. Even with huge mounting pressure, they never
>>>meet the specified thermal conductivity. And they're thick, so thermal
>>>resistance is high.

I never drilled through the AlO2 pad as the Dremel bits were wayyyy
too expensive to even consider using, but I got some testing done.

I couldn't test the case temperature of the FET thru the AlO2 pad but
I did measure the drain lead temperature (as it exited the case) and
using that, along with case temp. w/o the pad, derived the approx.
junction temperature for the FET with the pad and without.

It appears that the TO-247 Aavid AlO2 insulators I'm using have
approx. degrees-C/W thermal resistance using a good grease. This
is about what a K-10 Silpad gives me. Not good.

Double checking my results by using the thermal conductivity of AlO2
(from Aavid) and that of a K-10 pad verifies the results above.

AlO2 conductivity = W/m-K, 80 mil thickness
K-10 pad cond. = W/m-K, 6mil thickness

AlO2 pad is times thicker than K-10.
AlO2 pad is times more thermally conductive.

The numbers balance out almost perfectly.

I would love to try AlN as it's, IIRC, about 15-20 times more
thermally conductive than AlO2. Gotta' find it for a decent price
though (<<<$5/pad for TO0264) or hardcoat anodizing wins. I can't
afford the thermal penalty of the pads so far.

John