Central processing unit (CPU) chips are the “brains” of a computer, responsible for performing most of the calculations required to run a computer system. Anybody who has owned a computer knows that they can run hot. Most computers use a fan and heatsink to keep the processor from overheating. To aid the transfer of heat from the processor to the heatsink, a suitable thermal interface material must be used. One reason for this is that the CPU surface is not perfectly flat. Air is a poor thermal conductor and the gaps between the processor and heatsink will negatively affect the heat transfer.
Thermal pad and thermal paste materials are two different options to provide this thermal interface. What is the difference between them and what are the advantages of these different materials?
Thermal Pad
Thermal pad. Source: HenkelThermal pads are pre-formed phase-change materials, meaning they have the ability to change their physical characteristics. The pads are firm at room temperature and cut to the same size as the CPU surface, making them easy to apply. They soften when temperatures rise to conform to both the processor and heatsink surfaces and fill any air gaps. As a general rule, thermal pads are not as effective at removing heat as thermal paste, however, some CPU manufacturers do specify thermal pads with some of their processors.
To install a thermal pad, begin by mounting the pad to the bottom of the heatsink in the area that will directly come in contact with the CPU. Remove the protective film from the pad and then mount the heatsink onto the processor. Never reuse a thermal pad.
Thermal Paste (Thermal Grease)
Thermal paste. Source: Master Bond Inc.Thermal paste is just another name for thermal grease, thermal compound or heat paste, and is the most common thermal interface material. The paste is applied directly to the surface of the CPU and is an effective way to avoid gaps between the processor and heatsink surfaces. Thermal paste usually comes in a tube or a syringe and, while easy to apply, it can be messy and difficult to fill larger gaps. Another challenge when using thermal paste is applying the correct amount. Too little or too much paste can lead to problems. Too little paste can leave gaps and too much paste can leak, creating a mess or contaminating other components.
Not all thermal paste is the same. Low-quality compounds can dry out or leak, reducing the heat transfer efficiency. Thermal paste also has a shelf life and the product should not be used if it is past its expiration date.
After the components have been cleaned properly, most experts recommend applying a bead the size of a BB (slightly smaller than a pea) to the middle of the CPU. Other techniques include applying the paste in a thin “X” shape or in two lines. However, the single bead is the most popular method. To spread the paste, place the heatsink on the CPU and press straight down to spread the paste. You may carefully move the CPU in a rubbing motion to spread the grease if desired. Another common technique is to spread the paste with a rigid flat surface, such as the edge of a credit card.
Conclusion
Thermal paste and thermal pads each have their advantages and disadvantages.
Before applying any thermal interface material, the surfaces of the heatsink and processor should be cleaned with a wipe or lint-free cloth and a mild solvent such as denatured alcohol. Failure to clean the surfaces properly to remove dirt or debris will compromise the effectiveness of any thermal interface material. If a heatsink or CPU is replaced, remove any old residue from reused components and apply a new layer of thermal paste or a new thermal pad. Thermal pads and thermal grease should never be used together; select one or the other.
If the CPU manufacturer suggests a specific thermal interface material then it is best to follow that recommendation. Thermal paste is more effective than thermal pads in most cases, however it is messier and more difficult to apply correctly. Thermal pads also have a shorter lifespan and may need to be replaced over time.
Characteristics of thermal pads and thermal pastes
Reference Information:
Thermal Compounds and Thermal Interface Materials products and suppliers on IEEE GlobalSpec Engineering 360
Thermal Interface Material Comparison: Thermal Pads vs. Thermal Grease from AMD (.pdf)
