How to improve the performance of thermal grease

March 07, 2024

How to improve the performance of thermal grease

Thermal conductive silicone grease is also known as heat dissipating paste, thermal conductive paste, which is made of thermal conductive silicone grease compound with silicone as the main raw material and materials with excellent heat resistance and thermal conductivity. Thermal grease is a kind of material used to fill the gap between the CPU and the heat sink, which is also called the thermal interface material. Its role is to conduct the heat emitted by the CPU to the heat sink, so that the CPU temperature can be maintained at a stable level, to prevent the CPU from being damaged because of poor heat dissipation, and to extend the service life. It is often used for heat conduction and heat dissipation of electronic components such as power amplifiers, transistors, tubes and cpus, so as to ensure the stability of electrical performance of electronic instruments and meters.

In thermal management applications, even two surfaces with a very smooth surface will have a gap when they come into contact with each other, and the air in the gap is a poor conductor of heat, which will hinder the conduction of heat to the heat sink. Thermal grease is a material that can fill these gaps and make heat transfer smoother and faster.

Performance parameters of thermal grease

Coefficient of heat conduction

The heat conduction coefficient of thermal grease is basically the same as that of the radiator, and its unit is W/mK, that is, the heat conduction power when the temperature difference of the column with a cross-sectional area of 1 square meters is 1 Kelvin (1K = 1℃) along the axial distance. The larger the value, the faster the heat transfer rate of the material, the better the thermal conductivity.

Thermal resistance coefficient

The thermal resistance coefficient indicates the obstruction effect of an object on heat conduction. The concept of thermal resistance is very similar to resistance, and the unit is similar to it (° C /W), that is, the temperature difference between the two ends of the thermal conductivity path when the object's continuous heat transfer power is 1W. The lower the thermal resistance is obviously the better, because under the same ambient temperature and thermal conductivity 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℃/w, and the excellent one can reach 0.005℃/W.

Operating temperature

Due to the characteristics of silicone grease itself, its operating temperature range is generally -40 ° C ~125 ° C. The working temperature is an important parameter to ensure that the thermal grease is in a solid or liquid state. If the temperature is too high, the thermal grease fluid volume expands, the distance between molecules is far away, the interaction is weakened, the viscosity decreases, the temperature decreases, the fluid volume shrinks, the distance between molecules is shortened, the interaction is strengthened, and the viscosity increases, both of which are not conducive to heat dissipation.

viscosity

Viscosity is a measure of fluid viscosity, refers to the resistance of the fluid internal resistance to flow, expressed by the ratio of the shear stress and shear rate of the fluid, the measurement of viscosity, expressed in many ways, such as the unit of dynamic viscosity is pa · second/millipa · second. For thermal grease, the viscosity is 40,000 ~ 250,000 millipa · second, has a good tiling, can be easily laid around the surface of the chip under certain pressure, and ensure a certain viscosity, so that the excess silicone grease will not flow after extrusion.

Dielectric constant

Dielectric constant is used to measure the performance of an insulator to store electrical energy, and refers to the ratio of the capacitance between two metal plates when the insulating material is the medium and the capacitance between the same two plates when the air is the medium or vacuum. Dielectric constant represents the degree of polarization of the dielectric, that is, the ability to bind the charge, the greater the dielectric constant, the stronger the ability to bind the charge.

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

Oil dispersion

Oil dispersion is to guide the amount of silicone oil precipitation after the thermal grease radiator is kept at 200℃ for 24 hours, and it is an index to evaluate the heat resistance and stability of the product. The silicone grease grease on the white paper to observe, you will see the phenomenon of oil seepage, oil separation is high, oil separation is obvious; Or open the container containing silicone grease for a long time, the silicone grease with large oil separation, and the obvious oil separation phenomenon is seen on the surface of the silicone grease or around the container.

Key points to improve the performance of thermal grease

Because the thermal conductivity of thermal grease is mainly improved by the filler, the selection and processing of the filler is very critical! According to the research, in the preparation process of thermal grease, the heat dissipation of the product can be improved from the following ways.

01 Control the ratio of thermal conductivity filler

By using suitable filler with large and small particle size, the viscosity of silicone grease can be controlled, and the purpose of increasing the filling amount can be achieved, so as to improve the thermal conductivity. This is because the combination of multiple particle sizes can make the size of the thermal conductivity packing more tightly packed, forming more thermal conductivity channels. And the small particle size filling in the gap between the large particle size can achieve a higher filling ratio, forming a closer packing.

In addition, the addition of small particle size fillers can also fill the gap between the thermal conductive fillers, which can not only reduce the thermal resistance of the thermal conductive silicone grease, but also reduce the oil climbing tendency of the silicone oil, so that the oil seepage rate of the thermal conductive silicone grease is lower.

02 Packing snap treatment

Snap treatment refers to the use of screening means to screen the filler, remove the large particle size of the filler, control the maximum particle size. The particle size distribution of the filler can be well controlled through the clamping treatment of the filler. When the filler with a concentrated particle size and narrow distribution is used, the gap of the filler can be more easily filled, the grading effect of different particle sizes can be achieved, and a good thermal conductivity pathway can be formed, the contact thermal resistance can be reduced, and the thermal conductive silicone grease with lower thermal resistance can be obtained.

03 Control the viscosity of silicone oil

The combination of 500 mPa·s and 100 mPa·s polydimethylsiloxane can reduce the oil permeation rate of silicone grease under the premise of high filling and low viscosity, and help to reduce or avoid the occurrence of oil powder phase separation. In addition, by increasing the addition amount of small particle size powder, reducing the interface thermal resistance of silicone grease, it can also effectively improve its thermal conductivity and oil permeability, obtain a thixotropic thermal grease with ultra-low thermal resistance, and use its rheological properties to improve the vertical cracking resistance and aging resistance of thermal grease.

04 Kneading process temperature

By using the appropriate kneading temperature and time, it can ensure that the filler is firmly coated by silane coupling agent, and achieve stable and uniform modification of the powder, which can not only reduce the viscosity of the system, but also achieve good gap filling between the fillers, and obtain thermal conductive silicone grease with excellent thermal conductivity and printing performance.

peroration

In short, to obtain excellent thermal grease need to consider many factors, such as the thermal conductivity of the filler reflected in the thermal grease is not only related to the thermal conductivity of the material itself, but also closely related to the amount of filling in the system, the degree of packing tightness and the degree of infiltration of the filler and silicone oil.

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