Application of Thermal Conductive Silicone Grease in Modular Heat Dissipation System

      In the application of power semiconductor module cooling system, heat conduction silicone grease is usually used to transfer the heat generated by power devices to the radiator, and then the heat is dissipated by air cooling or water cooling. In practical application, many thermal design engineers pay more attention to the optimal design of radiator, but ignore the correct use of thermal conductive silicone grease. The correct use of thermal conductive silicone grease can not only improve the heat dissipation capacity of the power module, but also improve its reliability in the use process. So for a specific power module, how to choose a thermal conductive silicone grease? What kind of coating method is used? How thick is the coating? These are our common problems. Before answering these questions, let's first understand the role of thermal silicone grease in the power module cooling system.

The function of thermal conductive silicone grease in power module cooling system

      In the power module cooling system, the chip is the heat source, which transfers heat to the coolant (wind or liquid) through multiple layers of different materials. Finally, the heat is exported to the system through the flow of coolant, and each layer of material has different thermal conductivity. The power module base plate and radiator mostly use copper and aluminum and other metal materials, 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 their thermal conductivity is very good. Why use thermal conductive silicone grease with thermal conductivity of only 0.5 ~ 6W / (m.k) between power module and radiator?

      The reason is that when two metal surfaces are in contact, the ideal state is that the metal surface is in direct contact to achieve complete metal metal contact, but in reality, there is no direct contact between the two metal surfaces, and there are a large number of voids between the two metal surfaces on the micro level, as shown in Figure 1. These voids are filled with air, and the thermal conductivity of air is only about 0.003w / (m.k). The thermal conductivity is very poor. The use of thermal conductive silicone grease is to fill the air in these voids, while maintaining the existing metal to metal contact to achieve the optimal thermal performance of the system.

Thickness of thermal conductive silicone grease

     The thickness of thermal conductive silicone grease directly affects the thermal resistance from the module substrate to the radiator. The thermal conductive silicone grease should not be too thin or too thick, but should be controlled within a certain range. If the thermal conductive silicone grease is too thin, the air in the gap at the metal contact surface cannot be fully filled, and the heat dissipation capacity of the module will be reduced; if the thermal conductive silicone grease is too thick, the effective metal metal contact cannot be formed between the module base plate and the radiator, and the heat dissipation capacity of the module will also be reduced. Therefore, the thickness of thermal conductive silicone grease should be controlled in a range near the ideal value in order to achieve the optimal thermal conductivity from power module to radiator.

     Different module models have different requirements for the thickness of thermal conductive silicone grease. Each semiconductor module manufacturer will strictly test each module model according to the standard to get the appropriate thickness of thermal conductive silicone grease. This parameter will generally be marked in the installation instructions or technical instructions of each product. It should be pointed out that the module manufacturer usually gets the thickness value based on the test of a specific type of thermal conductive silicone grease. If other thermal conductive silicone grease with different characteristics is used, it needs to be retested to get the best thickness. Practical application experience shows that the thickness of thermal conductive silicone grease is generally 80-100um for modules with copper substrate and 40-50um for modules without copper substrate.

Coating method of thermal conductive silicone grease

      Under the existing process conditions, there are three main ways to coat the thermal conductive silicone grease: roller coating, screen printing and steel screen printing. Roller coating is the most traditional and simple way of coating, while screen and steel screen printing is the one with accurate uniformity and thickness control. The thickness of silicone grease is strictly controlled by the thickness of screen or steel screen and mesh size, and the printing process can ensure the uniformity of silicone grease. Wire mesh is suitable for applications with thinner silicone grease, while steel mesh is suitable for applications with thicker silicone grease.