GRAPHENE APPLIED TO IGBT HEAT DISSIPATION

The advantage of graphene used in IGBT is high transverse thermal conductivity, but its disadvantage is poor longitudinal thermal conductivity. In addition, the heat dissipation of IGBT is system engineering. Graphene used in different parts (chip, DCB, baseplate, heat sink and the interface of these parts) has different effects and different requirements on graphene. For example, for baseplate or heat sink, it only needs to combine graphene with copper and aluminum, which can be used as composite materials, as well as surface in-situ growth of graphene, or coated with graphene. However, if it is used as a thermal interface material, graphene needs to be compounded into the existing interface material formula or redesigned.

1. Graphene is used to enhance secondary pathway heat dissipation

In the traditional packaging structure of IGBT devices, the heat of local hot spots on the chip is mainly transferred to the copper-clad ceramic substrate from top to bottom, and then to the outer substrate, and then to the environment through heat sink. Heat from the chip upward through the packaging resin and shell to the environment is the secondary heat conduction path. Due to the low thermal conductivity of the packaging resin, the heat conduction speed of the secondary path is slow, and most of the heat is transferred from the main path. Graphene film / coating can enhance the secondary pathway, thus improving the overall heat transfer efficiency. The higher the thermal conductivity requirements, the better. Graphene film placed on the surface of the chip can not heat well. This is because although the transverse heat dissipation efficiency of graphene film is very high; its longitudinal thermal conductivity is low, which can not achieve good heat dissipation efficiency. Therefore, the side of graphene should be close to the IGBT chip for side heat dissipation. As a kind of heat distributor, graphene based film can improve the lateral heat dissipation of local hot spots, and has high heat flux, which can greatly reduce the maximum temperature of chip surface. In addition, the graphene based films with micron thickness have better heat dissipation performance than those with Nano thickness.

2. Graphene is used in DBC interlayer to reduce thermal resistance

As mentioned in this paper, DBC copper clad laminate plays a very important role in the heat dissipation of the whole IGBT, which is not only conducive to the thermal effect diffusion and heat transfer of hot spots, but also conducive to the stability of IGBT structure. Therefore, it is of great significance to improve the thermal conductivity of DBC layer or reduce its thermal resistance. It has been reported that graphene is added to the middle of the original DBC layer to form a transition layer. The reduced graphene oxide was attached to the surface of the upper copper layer of the copper-clad ceramic substrate. At the same time, the graphene film was grown on the upper copper layer surface of the copper-clad ceramic substrate by chemical vapor deposition to obtain the graphene modified DBC board. The high transverse thermal conductivity of the DBC board can be exerted, and the maximum temperature of the chip can be reduced, so as to improve the service life of the module.

3. Graphene is used to strengthen the thermal interface materials

The interface resistance formed by the surface roughness of heat source and radiator limits the effective heat transfer. Inserting thermal interface material between the two solid contact surfaces of heat source and radiator is an effective method to improve the heat removal rate of devices. Traditional thermal grease, thermal pad and phase change material are all composed of matrix material and thermal conductive filler. The matrix material is usually silicone grease or polymer, and the thermal conductive filler is usually metal (such as silver, copper or aluminum) or ceramics (such as aluminum oxide, zinc oxide or boron nitride). As a two-dimensional carbon material, graphene is easy to composite with polymer, and can be used as a heat conduction path to achieve high thermal conductivity. At the same time, graphene has ultra-high specific surface area, which can reduce the amount of addition and improve the enhancement efficiency.

Generally, there are several preparation methods of graphene thermal interface materials, including solution mixing method, melt mixing method and in-situ polymerization method. The strengthening principles include: 1) after reaching the threshold value of graphene fillers, graphene is used as the main heat conducting medium; 2) through functionalization, covalent modification or non covalent modification, the interfacial gap between graphene and polymer is reduced, and the mechanical properties of graphene and polymer are improved Heat conduction. At present, many reported literatures show that although graphene as a filler can improve the thermal conductivity of the original system, but still can not meet the requirements of thermal conductivity higher than 5W / MK, which makes graphene has little advantage compared with traditional materials in a wide range of commercial applications. Although the research of graphene filled polymer composites has made many amazing progress in the field of thermal management, there are still some challenges to overcome and further research is needed in the future.

4. Graphene / metal for thermal expansion matching

The stability, life and safety of high-power IGBT are the basic requirements for the development of new composite materials. In the long-term use of IGBT, the temperature rise will cause the thermal expansion of the material, and if the physical properties of each layer of material do not match, it will lead to serious delamination or damage. Graphene can be used to adjust the coefficient of thermal expansion of materials. In order to eliminate the difference of coefficient of thermal expansion between materials (coefficient of thermal expansion: 1 A graphene / copper composite material was developed by the Institute of Nano Engineering and Microsystems, National Tsinghua University, Taiwan. The nano graphene sheet obtained by ball milling was electroplated with copper to obtain graphene / Cu composite powder to ensure its flake structure. Then, it was extruded with pure copper to obtain the thermal conductive graphene / copper composite with low CTM, Its CTM value is only 5.3 ppm / K when 0.2% graphene is added, and the thermal conductivity of the composite is not affected (the thermal conductivity is still 390W / MK), so the service life of IGBT module can be greatly improved.