Thermal Paste Quick Guide

Thermal grease (also called thermal paste, heat paste, heat sink paste, thermal gel, thermal compound, heat transfer compound, or heat sink compound) is a kind of fluid substance, which originally has properties similar to grease. Thermal paste increases the thermal conductivity or heat transfer of the thermal interface by compensating for the irregularities on the surfaces of the components.

In electronics, this is often used to aid a component’s thermal dissipation with the use of heat sink.

Conductor types

Thermal grease employs one or more different thermally conductive substances:

Ceramic-based thermal paste has generally good thermal conductivity. It is usually made up of a ceramic powder that is suspended in a liquid or gelatinous silicone compound. This paste can be described as ‘silicone paste’ or ‘silicone thermal compound’.

The most used ceramics are beryllium oxide, aluminum oxide, zinc oxide, aluminum nitride and silicon dioxide. Thermal paste is usually white in color because these ceramics are all white in powdered form.

• Metal-based thermal grease use solid metal particles (typically silver or aluminum). This has better thermal conductivity but it is more expensive than ceramic-based thermal paste.

• Carbon based. There are thermal paste products which are based on carbon conductors. They use diamond powder or short carbon fibers. Carbon based thermal paste have the best thermal conductivity but they are more expensive than metal-based thermal paste and ceramic-based thermal paste.

• Liquid metal based. Some thermal pastes are composed of liquid metal alloys of gallium. These are quite rare and of course expensive.

Metal-based thermal paste and Carbon-based thermal paste use silicone grease as a medium. Silicone grease is a heat conductor in itself. Some manufacturers would use of fractions of mineral oil as an alternative.

Purpose of Thermal Paste

Thermal paste is used primarily in the electronics industries to help the heat sink in drawing heat away from semiconductor components such as an integrated circuit or discrete transistor. Thermally conductive paste can improve the efficiency of the heatsink by filling the air gaps that can occur when there are irregular surface on the heat generating component that is pressed against the irregular surface of the heatsink. The air is approximately 8000 times less efficient when conducting heat if we compare it to aluminum, which is common heatsink material. These surface imperfections usually arise from the limitations in manufacturing technology. Machining marks and casting irregularities can also cause sub-microscopic flaws which are not visible to the naked eye.

Thermal conductivity and the “conformability” (the ability of the material to conform on surfaces that are irregular) are the two important characteristics of thermal grease.

Both high power transistors, like the ones used in conventional HI-FI audio amplifier, and high speed integrated circuits, used in central processing unit (CPU), generate high amounts of heat requiring the use of thermal paste aside from heatsink. High temperatures can cause semiconductor parts to change their switching properties up to the point of failure. Overheating of the Central Processing Unit will causes logic errors as the amount of heat raises the electrical resistance on the very small circuits inside the CPU core.

Properties of Thermal Paste

In thermal compounds that contain suspended particles, the properties of the fluid itself may be the most important. The thermal conductivity of the thermal paste itself is closer to the value of the thermal conductivity of fluid and not on the suspended particles. Other factors that affect thermal conductivity are:

  • How well the thermal paste fills the gaps and conforms to the component as well as the heatsink’s irregular surfaces.
  • How well the thermal paste adheres to surface of both the heatsink and the component
  • How well it can maintain its consistency over the temperature range it was designed to operate
  • How well it can resists drying out and flaking over long periods of time
  • Whether the thermal paste degrades with oxidation or it breaks down as time goes by

The thermal paste should also be smooth and creamy; this enables us to apply it easily in a very thin layer.

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