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How to choose the right thermal grease?


In the application of power semiconductor modules, thermal grease is usually used to conduct the heat generated by power devices to the radiator, and then dissipate the heat through air cooling or water cooling. In practical applications, many thermal design engineers pay more attention to the optimal design of heat sinks, while ignoring the correct use of thermal grease. As everyone knows, the correct use of thermal grease can not only improve the heat dissipation capability of the power module, but also improve the reliability of the product.

1. The role of thermal conductive silicone grease in the cooling system of power modules

In the heat dissipation system of the power module, the chip is the heat source, and the heat is exported to the system through the coolant through multiple layers of different materials, and each layer of material has a different thermal conductivity. Metal materials such as copper and aluminum are mostly used for power module substrates and heat sinks. The thermal conductivity of copper is about 390W/(m.K); the thermal conductivity of aluminum is about 200W/(m.K). The thermal conductivity of these metal materials is very high, which means that their thermal conductivity is very good, so why use thermal grease with a thermal conductivity of only 1~6W/(m.K) between the power module and the heat sink?

The reason is that when two metal surfaces are in contact, the ideal state is that the metal surfaces are in direct contact to achieve complete metal-metal contact, but in reality there is no direct contact between the two metal surfaces. gap, as shown in Fig. These gaps are filled with air, and the thermal conductivity of air is only about 0.003W/(m.K), and the thermal conductivity is very poor. The use of thermal conductive silicone grease is to fill the air in these gaps, while maintaining the existing metal-metal contact. To achieve the best thermal performance of the system.

2. Thickness of thermal grease

The thermal conductive silicone grease can neither be too thin nor too thick, but should be controlled within a certain range. If the coating is too thin, the air in the gap at the metal contact surface cannot be fully filled, and the heat dissipation capacity will be reduced; if the coating is too thick, effective contact between the module substrate and the heat sink cannot be formed, and the heat dissipation capacity will also be reduced. Therefore, the thickness of the thermal grease should be controlled within a range near the ideal value to achieve optimal thermal conduction performance from the power module to the radiator.


3. How to apply thermal conductive silicone grease

Under the existing process conditions, there are three main ways to apply thermal conductive silicone grease: roller application, screen printing and steel screen printing.

Roller application is the most traditional and easiest application method, which is simple and low-cost, and is suitable for small batch production.

Screen printing and stencil printing are application methods with precise control of uniformity and thickness, which are suitable for mass production. Wire mesh is suitable for thinner grease applications, while steel mesh is suitable for thicker grease applications.

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