The thermal grease is basically composed by silicone and zinc oxide, but there are more elaborate greases that can even contain ceramic and silver – noble materials that promise more efficiency in heat transmission.
Its price can range between USD 2.00 and USD 32.00, in the case of greases that have silver in their composition.
Thermal grease can be found in many kinds of packages and usually come with the cooler. In Figure 3 you see some kinds of packages.

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Figure 3: Many kinds of thermal greases.
There are also coolers which have some material applied from factory, it’s the case of coolers that come with the processor (“in-a-box”) or even some coolers sold alone found in the market.
In the case of coolers that come together with the processor and coolers of recognized brands, the quality of the material applied is usually good and both AMD and Intel recommend the use of those compounds.

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Figure 4: Cooler in-a-box for Pentium 4 with thermal compound.

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Figure 5: Cooler in-a-box for Athlon 64 with pre-applied grease.

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Figure 6: Cooler in-a-box for AMD socket 462 processors with thermal compound.
All the compounds above are good quality ones and have better efficiency than regular thermal greases. If you bought a super thermal grease with silver, of course you should remove the original compound and apply the new grease instead.
But the use of those compounds “from the factory” has some inconveniences. The first one is that they can be used just once, i.e., if you remove the cooler for some reason, you’ll have to clean the old compound and apply the original one again – and it’s hard to be found – or apply thermal grease instead. Another very common problem is that the compound ends up sticking the heatsink to the processor, making cooler removal difficult.
In the case of Intel socket 478 and AMD sockets 754 or 939 processors it’s very common to pull the cooler and take out the processor fixed to the heatsink, something that ends up damaging the processor in most cases. The tip here is to use a hairdryer to heat the heatsink a little, in order to melt the thermal compound and then remove the heatsink with lateral movements. But don’t use the hairdryer too much.
There are also cheaper coolers hat come with a graphite square or thermal tape similar to gum, that are terrible heat conductors. In this case you should remove those compounds and apply thermal grease instead.

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Figure 7: Cooler for Socket 7 with graphite compound you should remove.
Many people believe that the more thermal grease, the better. But a lot of grease turns out to be insulating, not to mention that grease in excess ends up dropping and reaching motherboard contacts. Remember that most greases don’t conduct electricity, but there are greases that take metal in their composition, which turn them into conductors, and this may cause short circuit and destroy your equipment.
We will show in details how to apply thermal grease on several kinds of processors.