Over the years, heat has become a greater problem for computer components. As the speeds of the processors have increased, the amount of waste heat produced by the circuits has required a more active approach to dissipating the heat. In the early days of the Pentium CPUs from Intel, the only additional cooling required was an aluminum heatseat that was attached to the processor through a thermal tape or epoxy. Eventually this was insufficient to properly cool the processors so active cooling was applied by adding a fan to the heatsink to increase the rate of dissipation of the cooling solution.
One of the problems with transferring the heat between the processor and the heatsink has to do with the thermal interface. Neither the heatsink nor the processor has a completely smooth surface. This introduces air pockets that exist between the two materials and air has a very high thermal resistance or a poor heat conductor. Many modern double paned windows use this fact of air's poor heat conductivity as an insulator, but that is the opposite effect you want to happen with computer components. To help alleviate this problem, thermal compounds are used to fill in the gaps between the two surfaces. There are 4 types of thermal compounds used: thermal tape, thermal pads, thermal grease and thermal epoxy.
Thermal Resistance and Conductivity
All thermal compounds are rated on their ability to transfer heat between surfaces. The rating is referred to as the thermal resistance. This is the amount of heat that is absorbed by the compound rather than being transferred through it to the other surface. As a result, the lower the thermal resistance of the compound the more effective it will be at transferring heat between the chip and the heatsink. This is a very important factor when evaluating the potential performance of a thermal compound.
Another number that is thrown around by manufacturers is thermal conductivity. This is the opposite of resistance in that it shows how much heat energy can be transferred through the compound. Therefore, the more effective thermal compound will have a high thermal conductivity rating. Most manufacturers tend to not use this number, but you should be aware of it.
Another concern with the thermal compound is their thickness. Typically the thermal compound should have the least amount of material needed to form a thin layer between the chip and the heatsink while still allowing it to fill in all those gaps. If a thermal compound is put on too thick, it reduces the effectiveness of the compound.
Thermal tape is probably the simplest form of thermal interface for computer components. It is typically used when it is necessary to adhere a heatsink to a computer chip without any form of mounting clip or bracket. It looks very similar to many double-sided tapes that are found on the market for general consumer use except that it has been engineered to be more efficient at transferring heat. It still has the poorest thermal resistance out of all the compounds with a rating of .5 Cin^2/W.
A thermal pad looks very similar to thermal tape except that it does not have the same adhesion to the surfaces. Typically thermal pads are placed on the bottom of a heatsink surface and the chip, but another heatsink retention mechanism is required to prevent the heatsink from sliding off of the chip. The pad differs from the tape and from thermal grease in that it is based upon a paraffin base. The waxy substance is solid during low temperatures, but as a chip heats up, that paraffin base melts spreading the thermal transfer elements inside the pad into the gaps between the chip and heatsink. This is the most common form of thermal compound included with stock cooling solutions included with retail processors. The thermal resistance of a typical thermal pad is rated around .05 Cin^2/W.