Thermal Grease - Thermal Conductor Types

Thermal Conductor Types

Thermal greases use one or more different thermally conductive substances:

• Ceramic-based thermal grease has generally good thermal conductivity and is usually composed of a ceramic powder suspended in a liquid or gelatinous silicone compound, which may be described as 'silicone paste' or 'silicone thermal compound'. The most commonly used ceramics and their thermal conductivities (in units of W/(m ·K)) are: beryllium oxide (218), aluminum nitride (170), aluminum oxide (39), zinc oxide (21), and silicon dioxide (1). Thermal grease is usually white in colour since these ceramics are all white in powder form. These figures are for bulk material, not thermal grease.
• Metal-based thermal grease contain solid metal particles (usually silver or aluminum). It has a better thermal conductivity and is more expensive than ceramic-based grease.
• Metal-free thermal compound does not allow electrical conduction and therefore it eliminates the risk of short circuit and keeps the components safe. This type of thermal conductor is often used in computer systems to increase the thermal conductivity of the CPU or GPU. Besides, it is very easy to remove comparing with other types of thermal grease.
• Carbon based. There are products based on with carbon-based conductors, using diamond powder, or short carbon fibers, they have the best thermal conductivity and are generally more expensive than metal-based thermal grease.
• Liquid metal based. Some thermal pastes are made of liquid metal alloys of gallium. These are rare and expensive.
• Phase Change Metal Alloy (PCMA) is not a "grease" but another type of Thermal interface material. The design consists of a sealed alloy metal pad that needs to be "reflowed" under high heat (typically 90-100C.) The alloy on the inside of the seal will change phases, and fill all the micro-voids. Since this material is made of mostly metal alloy, the thermal properties of this interface material are very good.

All but the last classification of compound usually use silicone grease as a medium, a heat conductor in itself, though some manufacturers prefer use of fractions of mineral oil.

All these compounds conduct heat far better than air, but far worse than metal. They are intended to fill gaps that would otherwise hold air, not to create a layer between component and heatsink—this will decrease the effectiveness of the heatsink. Ideally perfectly smooth and flat metallic surfaces would not need heatsink compound.