From http://en.wikipedia.org/wiki/Thermal_grease
All these compounds conduct heat far better than air, but far worse than metal. They are intended to fill gaps that would otherwise hold air, not to create a layer between component and heatsink—this will decrease the effectiveness of the heatsink.
Excess grease separating the metal surfaces more than the minimum necessary to exclude air gaps will only degrade conductivity, increasing the risk of overheating.
Our Chinese factory is just sloppily brushing it all over the IC and the heatsink, and I think it's more harmful than helpful. Is there a recommended procedure for applying it? Are there machines for applying it quickly in mass production?
Best Answer
The best thermal transfer is metal-on-metal. Thermal grease is meant only to fill voids, not to act as an intermediary.
It should be applied thinly and evenly, then scraped off so that it only remains in the gaps. A properly trained line operator should have no issue with this.
I once had a production issue in an application where a heatsink assembly was being used as a low-voltage, secondary side DC path to an ORing diode. The operators were putting so much thermal grease on the assembly that instead of current passing from plate to plate directly, the only low-resistance path was through the 5 screws holding the plates together. Have you ever seen burnt screws before? Didn't think so...