On Thu, 12 Apr 2018 13:31:11 -0700, Jeff Liebermann
wrote:

What's the grain diameter? If it's larger than the depth of the
cracks and crevasses in the aluminum extruded heat sink, then you're
taking a step backwards. The bulk of the heat is passed by metal to
metal contact.

For power transistors, there is no metal to metal contact at all, if
there is a mica insulator in between the transistor and the heat sink.
Somehow that thermal paste transfers the heat thru that mica.

Going way back, I think my first encounter with power transistors were
the audio power output transistor in car radios. I remember working on
one and got that grease on my hands and I had to ask the teacher what
that was. He handed me a tube of it and told me to make sure the mica is
not cracked and make sure to apply enough of that grease so it oozes out
when I tighten the screws. Then he explained the purpose of it. That was
in the 1960s.

Since then, I've probably used more on CPUs than I ever did on
transistors. But the CPUs dont have an insulator on between.