Thermal compounds help improve themal efficiency of a heatsinks rough surface by 'filling' in the cracks with a highly heat conductive material but to have even better cooling the extreme modder makes sure that both surfaces are as flat as possible. The process of flattening out a cooler is called heatsink lapping.
Why lap a heatsink?
- Heatsink lapping or sanding can greatly decrease heat when over clocking.
- Heatsink lapping is relatively safe and if you have the correct sandpaper, it's free..
- I said its free and it reduces heat from the CPU. Do I need to say more?
Firstly a before picture, you can see in the reflection that the surface is curved (not flat) and on closeup, you can see little ridges where it has been machined. These ridges and troughs prevent good contact with the CPU. On the second picture, you can see the 'splatter' pattern, where the thermal compound makes good contact and where it makes poor contact.
This is not a how-to article, there are loads of good tutorials on this on the internet - we don't need another one. This is just my experience of it.
After about 10 minutes of sanding with 300 grit sandpaper, I used 600 for another 10, then moved to 1200 grit. Finally I used metal polish. When that was all done, I cleaned the surface with isopropanol to remove all trace of the polish (which is a very good heat insulator). The results:
Notice the reflection, how clear and undistorted it is. That is because the surface is like a mirror now. So shiny I can take pictures of the house opposite on the surface!
Performance wise it reduced my temperatures (as recorded by MBM5) from 49C idle to 48C, and 65C load to 60C. Well worth the effort in my opinion.