Thermal Management Materials for Electronic Components - Application of Thermal Conductive Silicone Resin Thermal Conductive Ceramic Gaskets

Thermal conductive ceramic gasket is a material with high thermal conductivity, mainly composed of alumina (with an alumina content of over 96%), with a pure white appearance and a hard texture. It is mainly used for heat transfer and electrical isolation between power devices and heat sinks. It is related to power devices (such as power MOSFETs, power transistors, etc.), aluminum heat sinks After the PCB board is tightly combined, it has excellent sealing performance, achieving the ideal effects of dust prevention, waterproofing, thermal conductivity, and insulation. It can also adapt to harsh working environments such as high temperature, high pressure, and high dust, improving the safety and stability of equipment operation. The article analyzes the performance points of ceramic gaskets and discusses their safety regulations, processes, structural design, and other aspects in the application of power products. Finally, its application in electronic loads (specialized loads for module power supply high-temperature aging) is presented.

Performance characteristics of ceramic gaskets

Ceramic gaskets have the following characteristics:

• The thermal conductivity of ceramic gaskets (20 ℃) can reach up to 20 W/(m-K)~30W/(m-K), which is much higher than that of ordinary thermal gaskets. Therefore, it has been widely used in power devices with very demanding heat dissipation requirements. At present, the thermal conductivity of most thermal pads is below 2.0 W/(m-K), and the Begus SIL-Pad2000 series with higher thermal conductivity is only 3.5 W/(m-K);

• High temperature and high pressure resistance. The breakdown strength of ceramic gaskets ranges from 10kV to 12kV, with a maximum allowable temperature of 1600 ℃. They can adapt to harsh working environments such as high temperature, high voltage, high wear, and strong corrosion, meeting the application requirements of power products in various occasions;

• Long service life. It can reduce the frequency of equipment maintenance, improve the safety and stability of equipment operation;

• Compliant with the EU RoHS environmental standards.

Ceramic gasket heat dissipation path

Ordinary thermal pads are composed of soft dielectric materials that can fill the small gaps between power devices and heat sink surfaces, reducing their contact thermal resistance. The ceramic gasket is composed of hard alumina with a certain roughness on the surface. If directly assembled, there will be many gaps between the power device and the ceramic gasket, as well as between the heat sink and the ceramic gasket, which seriously affects the heat dissipation efficiency and greatly reduces the performance of the heat sink, even rendering it ineffective. Therefore, when using ceramic gaskets as thermal conductive materials, it is necessary to apply thermal conductive silicone grease on both surfaces to fill the small gaps between the ceramic gasket and the heat sink, and between the ceramic gasket and the power device, reducing their contact thermal resistance.

After installing ceramic gaskets, the thermal resistance of power devices to ambient temperature is mainly composed of thermal conductivity silicone grease thermal resistance, ceramic gasket thermal resistance, thermal conductivity silicone grease thermal resistance, and heat sink thermal resistance. Its heat dissipation path is divided into two parts:

⑴ Power device (heat source) → thermal grease → ceramic gasket → thermal grease → heat sink (heat transfer is mainly conducted);

⑵ Heat sink → ambient air (heat transfer mainly by convection).

Figure 1 shows the thermal resistance model and heat dissipation path of the power device. The thermal resistance factors affecting power devices mainly include the surface flatness of ceramic gaskets, the thickness of ceramic gaskets and thermal grease, the thickness and shape of heat sinks, and the pressure of fasteners. These factors are also related to actual application conditions, so the thermal resistance between power devices and heat sinks will also depend on actual assembly conditions.

Installation process, safety regulations, and heat dissipation design

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Precautions for radiator selection

Compared with thermal pads, ceramic pads have the following defects (to be noted when selecting heat sinks): the material of ceramic pads is hard but brittle, with poor resistance to bending and deformation. In cases where the surface flatness of the heat sink is very poor, they are prone to breakage during installation. So when using ceramic gaskets as thermal conductive components, it is necessary to require the manufacturer to control the surface flatness of the radiator to ensure that this indicator is within the allowable range.

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Process assembly method (analyzed using power MOS transistor as an example)

Ceramic gaskets, power MOSFETs, and heat sinks involve process and safety issues during installation, which will be introduced one by one below.

1

Screw fixation method

After installing the power MOS tube, due to the limited creepage distance between the screw and the metal part of the power MOS tube, the screw fixing method can only be used for functional insulation occasions (the heat sink is not connected to the ground of the shell), and cannot be used for strengthening insulation occasions (the shell is used as a heat sink, and the safety distance requirement is relatively large), otherwise the safety requirements cannot be met.

(1) Fixed TO-247 packaged power MOS transistor with screws

The power MOS tube packaged in TO-247 only has metal on the back heat dissipation part, and the other parts are made of plastic. Therefore, when fixing the power MOS tube, do not do special treatment. Instead, clamp the ceramic gasket (coated with thermal conductive silicone grease on both sides) between the power MOS tube and the heat sink, and fix it directly with screws to meet the functional insulation requirements, as shown in Figure 2 (a) and (b). The creepage distance between the metal part of the power MOS tube and the screw is 1.3 mm~1.5 mm (determined by the shape of the power MOS tube from the manufacturer).

(a) Back view of power MOS transistor packaged in TO-247

(b) Screw fixed TO-247 packaged power MOS transistor

(2) Fixed TO-220 packaged power MOS transistor with screws

When fixing the power MOS transistor packaged in TO-220, a plastic pad should be added to the screw (as shown in Figure 3) to prevent the metal part of the power MOS transistor from contacting the heat sink through the screw and causing a short circuit. After adding plastic pads, the creepage distance between the metal part of the power MOS transistor and the screw is ≥ 1mm. Ceramic gaskets also need to be coated with thermal conductive silicone grease on both sides to reduce their contact thermal resistance.

Fixing method of pressure strip

When using pressure strips to fix power MOS transistors, it is mainly used in situations with high safety requirements. It has two fixing methods: horizontal pressure and vertical pressure. Figure 4 (a) shows a schematic diagram of fixing the power MOS transistor (TO-220 or TO-247 package) using vertical compression, and Figure 4 (b) shows a schematic diagram of fixing the power MOS transistor (TO-220 or TO-247 package) using horizontal compression.

(a) Schematic diagram of fixing power MOS transistor using vertical pressure method

(b) Schematic diagram of fixed power MOS transistor using transverse compression method

Process design should pay attention to the following three points:

① When using pressure strips to fix power MOSFETs, ceramic gaskets without screw holes should be selected, otherwise it will greatly reduce the creepage distance from the power MOSFET to the heat sink;

② The size of the ceramic gasket must meet the creepage distance requirement from the power MOS transistor to the heat sink;

③ Due to the fact that the pressure strip is directly connected to the heat sink through screws, an insulating sleeve must be added to the pressure strip to increase the creepage distance between the power MOS tube and the heat sink.

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Precautions for thermal and structural design

The commonly used types of ceramic gaskets include single piece, double piece, etc. Double ceramic gaskets are mainly used for parallel installation of two power MOS transistors. Figure 5 shows the various parameters and size dimensions of dual ceramic pads in PCB and structural design (taking TO-247 packaged power MOS transistor as an example).

(1) The distance between the two openings of the dual ceramic gasket is fixed, so in PCB design, the center distance between the two MOS transistors should be kept at 19mm; When designing the structure, the distance between the openings of the two MOS transistors on the heat sink is 19mm.

(2) If two power MOSFETs have high power consumption and the heat sink is installed at the bottom of the PCB, the edge of the MOSFET should not be too close to the edge of the heat sink. The size of this size should be determined based on the results of thermal simulation analysis, otherwise the heat dissipation effect will be greatly compromised.

Application of Ceramic Gaskets in Electronic Load

A certain electronic load (module power supply high-temperature aging dedicated load) has a total of 16 circuits, each with a working current of 10A (each circuit uses a power MOS transistor as the electronic load), and the power consumption of each power MOS transistor is as high as 32W. Effective heat dissipation measures must be taken to ensure the safe and reliable operation of the power MOS transistor. Due to the high power consumption of the entire system, four 150mm x 120mm large aluminum heat sinks were used (the manufacturer cannot process a single long aluminum heat sink, so four aluminum heat sinks were used instead), located at the bottom of the PCB. The power MOS transistor is selected as IRFP150P (TO-247 package), which is installed horizontally on the bottom shell heat sink. The thermal conductive material selected is a ceramic gasket with high thermal conductivity (ordinary thermal conductive gaskets cannot meet the design requirements). Since the product does not require reinforced insulation treatment (the voltage difference between the internal circuit and the shell is 10V~65V), the power MOS transistor is fixed by screws to meet functional insulation requirements.

Table 1 shows the operating temperature of the electronic load at room temperature of 25 ℃ and air cooling conditions. According to the data in Table 1, the operating temperature of the power MOS transistor is between 45.3 ℃ and 66.3 ℃. The electronic load requires operation at a high temperature of 65 ℃, and an additional temperature rise of 40 ℃ is required to determine that the operating temperature of the power MOS transistor is between 85.3 ℃ and 100.3 ℃. The temperature rise of all power MOS transistors (with a maximum operating temperature of 160 ℃) is within the allowable range. The ceramic gasket has excellent thermal conductivity and can meet the heat dissipation requirements of high-power power products.