Thermal paste

From left to right: Arctic Cooling MX-2 and MX-3, Tuniq TX-3, Cool Laboratory Liquid Metal Pro( Liquid Metal based), Shin-Etsu MicroSi G751, Arctic Silver 5, Powdered Diamond. In background Arctic Silver grease remover

Silicone thermal compound

Metal (silver) thermal compound

Metal thermal grease applied to a chip

Surface imperfections

Thermal grease (also called thermal gel, thermal compound, thermal paste, heat paste, heat sink paste, thermal interface material, or heat sink compound) is a kind of thermally conductive (but usually electrically insulating) adhesive, which is commonly used as an interface between heat sinks and heat sources (e.g., high-power semiconductor devices). The grease gives a mechanical strength to the bond between the heat sink and heat source, but more importantly, it eliminates air (which is a thermal insulator) from the interface area.

Composition

Thermal grease consists of a polymerizable liquid matrix and large volume fractions of electrically insulating, but thermally conductive filler. Typical matrix materials are epoxies, silicones, urethanes, and acrylates, although solvent-based systems, hot-melt adhesives, and pressure-sensitive adhesive tapes are also available. Aluminum oxide, boron nitride, zinc oxide, and increasingly aluminum nitride are used as fillers for these types of adhesives. The filler loading can be as high as 70–80 wt %, and the fillers raise the thermal conductivity of the base matrix from 0.17–0.3 W/(mK) up to about 2 W/(mK).

Filler properties

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Thermal conductivities

See also: List of thermal conductivities

For comparison, the approximate thermal conductivities of various materials relevant to heatsinks in W/(m·K) are:

• Air 0.024
• Water 0.58
• Thermal grease about 0.5 to 10
• Unbranded grease typically 0.8; some silver-and graphite-based greases claim about 9
• Aluminum oxide (surface layer on pure aluminium exposed to air) 35
• Zinc oxide 35
• Steel About 40, varies for different types
• Sodium Fluoride 132
• Aluminum 220
• Copper 390
• Silver 420
• Natural Diamond 2000

These figures vary slightly between sources, and depend upon purity, etc. of the material. Other units are sometimes used, obviously giving different numerical values.

These are bulk thermal conductivities; the thermal resistance of a particular interface (e.g., a CPU, a thin layer of compound, and a heatsink) is given by the thermal resistance, the temperature rise caused by dissipating 1 W, in K/W or, equivalently, °C/W. For example, a thermal pad of specified area and thickness will be rated by its thermal resistance. A typical value for a pad for a microprocessor is roughly 0.2 °C/W per square inch, dependent upon thickness and decreasing at high pressure.

An informal comparative test of thermal greases was made, examining the thermal resistance in °C/W for a heater simulating a processor, with a very thin layer of grease, rather than the bulk conductivity. The procedure used was explained in detail; a heater and sensor was used so that power dissipation and temperature were known accurately and consistently. A Thermaltake Volcano 6Cu+ heatsink was used, with a copper disc of 4 cm diameter in contact with the heat source, an area of 12.6 square centimetres (1.95 in²).

The temperature rise without any grease was 0.66 °C/W. Using all greases from standard types to silver-based ones gave results fairly close to 0.50 °C/W. A very thin layer of grease gave slightly better results than a thick one. Just about any wet paste produced similar results—toothpaste actually gave slightly better results, but of course would cause corrosion and dry out in hours if used in practice. Tapwater, before evaporating, gave excellent results: 0.41 °C/W - could be due to the heat used up in evaporating the water.

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'. Thermal grease is usually white in colour since these ceramics are all white in powder form. 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
  • silicon dioxide 1
• 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.
• Carbon based. There are products based on with carbon-based conductors, using diamond powder, short carbon fibers [1], or graphene oxide, 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 slightly heat conductor in itself (0.2 W/(m·K)), though some manufacturers prefer use of fractions of mineral oil (0.1-0.2 W/(m·K)).

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, because this would decrease the effectiveness of the heat transfer; ideally two perfectly smooth and flat metallic surfaces would not need heatsink compound.

Purpose

Thermal grease is primarily used in the electronics and computer industries to assist a heat sink to draw heat away from a semiconductor component such as an integrated circuit or transistor.

Thermally conductive paste improves the efficiency of a heatsink by filling air gaps that occur when the imperfectly flat and smooth surface of a heat generating component is pressed against the similar surface of a heatsink, air being approximately 8000 times less efficient at conducting heat than, for example, aluminum (a common heatsink material). Surface imperfections and departure from perfect flatness inherently arise from limitations in manufacturing technology and range in size from visible and tactile flaws such as machining marks or casting irregularities to sub-microscopic ones not visible to the naked eye. Thermal conductivity and "conformability" (i.e., the ability of the material to conform to irregular surfaces) are the important characteristics of thermal grease.

Both high-power handling transistors, such as those in an audio amplifier, and high-speed integrated circuits, such as the central processing unit (CPU) of a personal computer, generate sufficient heat to benefit from the use of thermal grease to improve the effectiveness of a heatsink. The need for heatsink compound can be minimised or removed by lapping the surfaces of the hot component and the matching heatsink face so that they are virtually perfectly flat and mirror-smooth. Computer overclockers, who increase computer speed by measures that increase heat production, resort to lapping and other extreme cooling methods such as water-cooling.

Properties

The metal oxide and nitride particles suspended in silicone thermal compounds have thermal conductivities of up to 220 W/(m·K). In comparison, the thermal conductivity of metals used particle additions, copper is 380 W/(m·K), silver 429 and aluminum 237. The typical thermal conductivities of the silicone compounds are 0.7 to 3 W/(m·K). Silver thermal compounds may have a conductivity of 3 to 8 W/(m·K) or more.

In compounds containing suspended particles, the properties of the fluid may well be the most important. As seen by the thermal conductivity measures above, the conductivity is closer to that of the fluid components rather than the ceramic or metal components. Other properties of fluid components that are important for thermal grease might be:

1. How well it fills the gaps and conforms to both the component's and the heat sink's uneven surfaces.
2. How well it adheres to those surfaces
3. How well it maintains its consistency over the required temperature range
4. How well it resists drying out or flaking over time
5. Whether it degrades with oxidation or breaks down over time

The compound must have a suitable consistency to apply easily and remove all excess to leave only the minimum needed.

Application and removal

Computer processor heatsinks utilize a variety of designs to promote better thermal transfer between components. Some thermal greases have a durability up to at least 8 years. Flat and smooth surfaces may use a small line method to apply material, and exposed heat-pipe surfaces will be best prepared with multiple lines.

Excess grease separating the metal surfaces more than the minimum necessary to exclude air gaps will only degrade conductivity, increasing the risk of overheating. Silver-based thermal grease can also be either slightly electrically conductive or capacitive; if some flows onto the circuits it can cause malfunctioning and damage.

Over time, some thermal greases may dry out, have reduced heat transferring capabilities, or set like glue and make it difficult to remove the heat sink. If too much force is applied the processor may be damaged. Heating the grease by turning the processor on for a short period often softens the adhesion. Another method to use can be by turning the heatsink slowly instead of lifting it up. It is recommended that thermal grease be re-applied with each removal of the heatsink.

Silicone oil-based thermal grease can be removed from a component or heatsink with an alcohol (such as rubbing alcohol) or acetone. Special-purpose cleaners are made for removing heatsink grease and cleaning the surfaces.

See also

• Computer cooling
• Phase-change material
• Thermally conductive pad
• Thermal adhesive
• List of thermal conductivities