Heat dissipation is an important factor in any electronic device or computer system. If components are allowed to overheat, the results can be catastrophic as they experience thermal runaway and eventual total failure.
Proper ventilation in many cases may be adequate, but for components such as high output power transistors used in power amplifiers, and a computer system’s processing units, more aggressive heating solutions are employed.
For high power transistors, a heatsink is typically used. The size and design of the heatsink are such that it can efficiently cool the component, maintaining it within operating tolerances. This is known as a passive cooling solution.
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In the case of a computer’s CPU and GPU, a heatsink alone is not enough, as these components reach extremely high operating temperatures, and so a fan is also used, blowing air across the heatsink fins. This is known as an active cooling solution.
In both cases, no matter how large or well designed the heatsink, no matter how much air flows across the fins of a heatsink, the weakest point in the heat dissipation cycle is the point of contact between the component (the heat source) and the heatsink.
Heatsinks are typically made of aluminum, which has a thermal conductivity of 205 W/mK, and some employ a copper base which has an even higher thermal conductivity of 400 W/mK. Thermal conductivity can be defined as the rate at which heat is transferred by conduction through a unit cross-section area of a material.
When two metal surfaces come into contact, as in the case of a heatsink and the metallic Integrated Heat Spreader (IHS) of a CPU or GPU, imperfections on both metal surfaces leave air gaps that greatly reduce thermal conduction. Air has a very low thermal conductivity of 0.02 W/mK, working more as an insulator than a conductor.
Types of Thermal Pastes
To remedy this, thermal paste is applied between the two surfaces to help fill any voids. Common thermal pastes typically offer a thermal conductivity of between 2 W/mK to 20 W/mK. This is still very low compared to the thermal conductivity of aluminum or copper, but much better than air. For this reason, only a very thin layer of thermal paste is ever applied, sufficient to fill the air gaps.
The thermal conductivity of a thermal paste is determined by its composition. Thermal paste, also known as thermal interface material (TIM), thermal compound, heat sink compound, and heat paste, comprises two main components, a polymer base (commonly called a matrix), and a thermally conductive filler.
Typically, the polymer base is a high-performing thermosetting epoxy resin, offering high mechanical and adhesion properties, with thermal stability. However, epoxy resins have very low thermal conductivity (approximately 0.2 W/mK), and so it is the filler that gives thermal paste its thermal conductivity, with some pastes containing up to 80% filler by weight.
There are several types of thermally conductive filler used, the most common being boron nitride, aluminum nitride, alumina, zinc oxide, ceramic, copper, and silver.
Learn more: Does Thermal Paste Expire?
Silicon Based Thermal Paste
Silicone based thermal pastes are low cost and very easy to apply (they do not easily flow into unintended areas), and for this reason have been very popular. Their thermal conductivity however, is not as high as other types of thermal paste.
They comprise a silicone oil base with a powdered metal oxide (typically zinc oxide) as the thermo-conductive component. One drawback is that the silicone oil can separate and ooze from the thermal paste (known as capillary flow), causing both solderability issues and dewetting (when a conformal coating will not evenly cover the surface it is being applied to due to improper mixture of the materials).
Dewetting can also occur while the thermal paste is in the tube, and in order to avoid this, some manufacturers advise rotating the tube every few months.
Silicon based thermal pastes are usually pale gray in color, non-curing, have a thermal conductivity range of 1 to 14 W/mK, operating temperature range of -58°F to +400°F, and dielectric strength of 18 kV/mm. Their shelf life is 4 to 6 years.
Metal Oxide Thermal Paste
There are some thermal pastes referred to as metal oxide thermal pastes, but are in essence still silicone based, although the silicon content may be far less than in standard silicon-based thermal pastes. The main ingredient is usually a metal with very high conductivity, such as silver, but may also include carbon and other metal oxide compounds.
Metal oxide thermal pastes offer better thermal conductivity over standard, silicon-based compounds while offering negligible electrical conductivity. They are easy to use and (according to manufacturers) recommended for high performance applications.
Liquid Metal Thermal Paste
Liquid metal based thermal pastes (or just liquid metal thermal pastes) are among the most popular with overclockers, since they are the most effective, due to the fact that the compound is almost entirely made of metal, usually gallium.
Gallium is a soft metal with a low melting point and very high boiling point. When combined with indium (another soft metal), the melting point falls from 30°F to -2°F so that the thermal compound remains a liquid at room temperature, while its boiling point remains high at 2370°F.
Because of this high boiling point, little to no evaporation takes place. Liquid metal thermal pastes also have a very high thermal conductivity at around 73 W/mK, much higher than any other thermal paste. However, liquid metal thermal pastes have some drawbacks.
Being all metal, they are electrically conductive, and coupled with the fact that the paste is a liquid, it must be applied with care so that it does not spill over onto the pins of components or tracks, causing short circuits. This makes liquid metal thermal pastes more difficult to apply, and manufacturers usually include special instructions.
A further drawback is the fact that gallium, the main component, reacts with aluminum to create an aluminum alloy that crumbles at the touch. Liquid metal thermal pastes can therefore not be used with aluminum heatsinks. This is not so much of a problem since most of the better CPU and GPU coolers use heatsinks with a copper baseplate.
Ceramic Based Thermal Paste
One of the problems with silicone based thermal pastes is that they can dry out. They also tend to have a high percentage of silicone to filler content, giving them on average, a lower thermal conductivity compared to the other types of thermal paste.
Ceramics, like boron nitride, have gained momentum in the manufacture of thermal pastes, providing high thermal conductivity, a low coefficient of thermal expansion, are resistant to corrosion and erosion, while providing electrical insulation. Considering the production cost of various fillers, ceramics are considered to be an excellent filler.
Boron Nitride is an advanced synthetic ceramic material available in solid and powder form. It falls into the category of ceramic fillers, offering excellent thermal conductivity and electrical insulation. It is the most commonly used ceramic filler in thermal pastes, but other fillers include aluminum oxide, aluminum nitride, beryllium oxide, and zinc oxide.
Ceramic based thermal pastes have a typical thermal conductivity of 10 W/mK and offer long term stability over a temperature range of 5°F to 400°F. Ceramic based thermal pastes are sometimes also referred to as silicone-free thermal paste.
Carbon Based Thermal Paste
Another relatively recent candidate to gain momentum in thermal pastes is carbon fillers, providing high thermal conductivity, mechanical strength and stability, and durability. Common carbon fillers include graphene, graphite, carbon nano-tube, and carbon nano-fiber.
Carbon based thermal pastes are metal-free and so not electrically conductive. They are easy to apply with a long lifespan (8 years). The thermal conductivity of carbon fillers ranges from 8 W/mK to 35 W/mK, and operating temperature range of -60°F to 300°F, making them an excellent thermal paste.
Conclusion
There is a large number of thermal pastes on the market, all claiming to provide efficient heat transfer and improved heat dissipation. One value that gives a general idea of how effectively a thermal paste will perform, is thermal conductivity.
Thermal conductivity describes the ability of a given material to conduct heat. The higher the thermal conductivity, the better. However, this is not the only measure of a thermal pastes effectiveness.
The reciprocal measure of thermal conductivity is thermal resistance, which is a measure of resistance to the flow of heat through a material of a specific thickness. When applied to thermal paste, this implies the thinner the layer of thermal paste applied, the less resistance will be offered and hence, the more efficiently it will work.
Apart from the thermal resistance of the actual thermal paste itself, is the contact resistance offered at the point of contact, where the thermal paste meets the heatsink and heat source (e.g. a CPU’s Integrated Heat Spreader).
The thermal paste’s thermal resistance coupled with the contact resistance provides the thermal impedance. The lower the thermal impedance, the less the resistance to heat transferring from one material to the other.
Factors that affect thermal impedance include surface micro-structure (rough, undulating, even, flat), thermal paste density and composition (a non-homogeneous paste is likely to offer greater impedance), and the layer thickness of the thermal paste applied.
Thermal impedance is therefore a more exact performance indicator of thermal transfer, as it accounts for more factors specific to the application. However, it is not always easy to find thermal resistance values in the specifications of many thermal pastes, while all manufacturers provide thermal conductivity.
Relying solely on the value of thermal conductivity, it is obvious that liquid metal thermal pastes offer the best results. However, because of the nature of this thermal paste, namely that it is a liquid, its application is not straightforward, and certainly not for the inexperienced.
Carbon based thermal paste is probably the next best choice, and in fact could be recommended for most applications, especially considering its excellent thermal conductivity, high durability, and ease of application. Furthermore, it is not electrically conductive if that is also a concern.
Ceramic based thermal pastes are also worth consideration, having many of the advantages of carbon based thermal paste but with slightly lower thermal conductivity.
Obviously, there are going to be differences (sometimes vast), even between the same type of thermal paste from various manufacturers, and it is always worth reading through the specifications and data sheets to gauge the appropriateness of a thermal paste for the intended application.
