Hallo Raghunathan,

yes, Steve is totally right. If you could have a look at the joint between chip case and heatsink without applying thermal grease, you would notice that only the 'hills' of both surfaces directly touch. And heat transfer will only occur, where both surfaces directly touch! At regions, where no direct contact between both surfaces occurs, a very thin air layer exists. This air layer, which is filling the 'valleys', does NOT provide any relevant heat transfer, because this air layer shows a heat transfer resistance which is several orders of magnitudes greater than that of the bare joint!!

Now, what about the heat transfer resistance of thermal grease?
The performance of thermal grease nearly ranges in the middle between the bare joint and the air layer. So, you would improve the heat transfer properties, when filling the air gaps with it. But you would heavily worsen situation, if you apply so much, that the 'hills' of both surfaces are separated, that they stop contacting each other. If this occurs, means if you apply too much thermal grease, then heat transfer resistance will heavily increase!!

So, you must only apply such a small portion of thermal grease, that the 'valleys' among the 'hills' are filled with it, means that the air in the 'valleys' is driven out. But the 'hills' of both surfaces must still touch each other!

Steve focused on a very important characteristic, which is nearly unknown, even under professionals! How often you can find the lines: "And don't forget to apply enough thermal grease...", which makes you believe 'the more the better'? But just the opposite is right.

Apply a thin, virtually translucent smear, as Steve wrote, just hits the point...


Bye,
Kai