What Is Thermal Grease?

What Is Thermal Grease?

Thermal grease is commonly known as thermal grease, also known as thermal paste. Thermal grease is made of silicone as the main raw material, adding materials with excellent heat resistance and thermal conductivity, and made of thermal grease-like compound for power amplifiers , transistors, tubes, CPU and other electronic components heat conduction and heat dissipation, so as to ensure the stability of the electrical performance of electronic instruments, meters and so on.

Thermal grease is a kind of material used to fill the gap between the CPU and the heat sink, which is also called thermal interface material. Its function is to conduct the heat emitted by the CPU to the heat sink, keep the temperature of the CPU at a level that can work stably, prevent the CPU from being damaged due to poor heat dissipation, and prolong its service life.

In thermal management applications, even two flat surfaces with very smooth surfaces can have voids when they come into contact with each other. The air in these voids is a poor conductor of heat and hinders the conduction of heat to the heat sink. Thermal grease is a material that can fill these voids and make heat conduction more smoothly and quickly.

Since thermal grease is a chemical substance, it also has relevant performance parameters that reflect its own working characteristics. As long as we understand the meaning of these parameters, we can judge the performance of a thermal grease.

Thermal conductivity

The thermal conductivity of thermal grease is basically the same as that of the radiator. Its unit is W/mK, that is, the temperature difference of a cylinder with a cross-sectional area of ​​1 square meter along the axial distance of 1 meter is 1 Kelvin (1K=1℃). heat transfer power. The larger the value, the faster the heat transfer rate of the material and the better the thermal conductivity. At present, the thermal conductivity of Novotel thermal grease can reach 5.2W/m.K.

Thermal resistance coefficient

The thermal resistance coefficient expresses the effect of an object on the conduction of heat. The concept of thermal resistance is very similar to that of resistance, and the unit is also similar (℃/W), that is, the temperature difference between the two ends of the heat conduction path when the continuous heat transfer power of the object is 1W. Obviously, the lower the thermal resistance, the better, because under the same ambient temperature and thermal power, the lower the thermal resistance, the lower the temperature of the heating object. The size of the thermal resistance has a great relationship with the material used in the thermal grease.

At present, the thermal resistance coefficient of mainstream thermal grease is less than 0.1°C/W, and the excellent ones can reach 0.005°C/W.

Operating temperature

Due to the characteristics of silicone grease itself, its operating temperature range is very wide. The working temperature is an important parameter to ensure that the thermal conductive silicone grease is in a solid or liquid state. If the temperature is too high, the volume of the thermal silicone grease will expand, the intermolecular distance will increase, the interaction will weaken, and the viscosity will decrease; if the temperature decreases, the fluid volume will shrink and the intermolecular distance will be reduced shortening, the interaction is strengthened, and the viscosity increases, both of which are not conducive to heat dissipation.

The working temperature of thermal grease is generally -40℃~200℃. We don’t need to worry about the working temperature of thermal grease, after all, it is difficult to exceed the temperature of CPU/GPU by conventional means.

Viscosity

Viscosity is a measure of fluid viscosity, which refers to the resistance to flow inside the fluid, expressed by the ratio of the shear stress to the fluid and the shear rate. There are many ways to measure viscosity. For example, the unit of dynamic viscosity is Pa. ·second.
For thermally conductive silicone grease, the viscosity is about 2500 Pa·s, and it has good tiling properties. It can be easily tiled around the surface of the chip under a certain pressure, and a certain viscosity is guaranteed, so that it will not become too thick after extrusion. Excess grease will flow.

Dielectric constant

Dielectric constant is used to measure the performance of insulators to store electrical energy, which refers to the ratio of the capacitance between two metal plates when insulating material is used as the medium to the capacitance between the same two plates when air is used as the medium or vacuum. The dielectric constant represents the degree of polarization of the dielectric, that is, the binding ability to charge. The larger the dielectric constant, the stronger the binding ability to charge.

Ordinary thermal grease uses materials with good insulation, but some special silicone greases (such as silver-containing silicone grease, etc.) may have a certain degree of conductivity. The dielectric constant of air is about 1, and the dielectric constant of common thermal grease is about 5.

Oil separation

Oil separation refers to the precipitation amount of silicone oil after the thermal grease heat-dissipating paste is kept at 200 ℃ for 24 hours, and it is an index to evaluate the heat resistance and stability of the product. Apply the silicone grease heat-dissipating paste on the white paper for observation, and you will see the phenomenon of oil seepage. If the oil separation is high, the oil separation phenomenon is obvious; or open the container containing silicone grease for a long time. Obvious oil separation is seen on the surface of the silicone grease or around the container.

There are many types of silicone greases on the market today, and different parameters and physical properties determine different uses. Thermal grease is a kind of high thermal conductivity insulating silicone material, which is almost never cured, and can maintain the grease state when used for a long time at the temperature of -40℃-200℃. Thermal grease thermal paste has excellent electrical insulation, excellent thermal conductivity, low oil separation (tends to zero), high and low temperature resistance, water resistance, ozone resistance, and weathering resistance. Thermal grease heat dissipation paste can be widely applied to various electronic products, between the heating body (power tube, thyristor, electric heating stack, etc.) and heat dissipation facilities (heat sink, heat dissipation strip, shell, etc.) in electrical equipment The contact surface acts as a heat transfer medium, and has the properties of moisture-proof, dust-proof, anti-corrosion and shock-proof. Thermal grease heat-dissipating paste is suitable for surface coating or overall potting of various microwave devices such as microwave communication, microwave transmission equipment, microwave special power supply, regulated power supply, etc. This kind of silicon material provides excellent heat-generating electronic components. Thermal conductivity.

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