Alumina washers have the following significant features:
- High Thermal Conductivity: The thermal conductivity of alumina washers is as high as 20 W/(m·K) to 30 W/(m·K) at 20℃, much higher than that of ordinary thermally conductive washers (usually below 2.0 W/(m·K)). This high thermal conductivity makes them excel in heat dissipation applications for power devices, especially in thermal conduction between high-power MOSFETs and heat sinks. - High Temperature and Pressure Resistance: With a breakdown strength of up to 10 kV to 12 kV and a maximum operating temperature of 1600℃, alumina washers can stably operate in harsh working environments with high temperature, high pressure, high wear, and strong corrosion, meeting the requirements of various power products. - Long Service Life: The durability of alumina washers reduces the frequency of equipment maintenance, significantly enhancing the safety and stability of equipment operation. - Environmental Compliance: This material meets the EU RoHS environmental standards, ensuring no harm to the environment and human health during use.
II. Heat Dissipation Path
Despite the high thermal conductivity of alumina washers, due to their hardness and surface roughness, direct installation can create small gaps between the power device and the heat sink, increasing contact thermal resistance. Therefore, thermally conductive silicone grease is usually applied to both sides of the alumina washer to fill these gaps and reduce contact thermal resistance. The improved heat dissipation path is divided into two parts:
- Conduction Path: Power device (heat source) → thermally conductive silicone grease → alumina washer → thermally conductive silicone grease → heat sink. - Convection Path: Heat sink → ambient air (heat transfer is primarily by convection).
The main factors affecting thermal resistance include the surface flatness of the alumina washer, the thickness of the washer and the thermally conductive silicone grease, the thickness and shape of the heat sink, and the pressure of the fasteners. These factors collectively determine the thermal resistance under actual installation conditions.
III. Installation Process and Safety Design
The installation process and safety design of alumina washers are crucial in practical applications.
- Heat Sink Selection: Although alumina washers have excellent thermal conductivity, they are brittle and have poor resistance to bending deformation. Therefore, the flatness of the heat sink surface must be ensured to prevent the washer from shattering during installation. - Process Assembly Method: - Screw Fixed Method: - TO-247 Packaged Power MOSFET: The backside of this MOSFET is metal, while the rest is plastic. No special treatment is needed for installation. Just place the alumina washer coated with thermally conductive silicone grease between the MOSFET and heat sink, and fix it with screws. - TO-220 Packaged Power MOSFET: A plastic washer must be added to the screw to prevent the metal part of the MOSFET from contacting the heat sink through the screw and causing a short circuit. After adding the plastic washer, the creepage distance between the MOSFET metal part and the screw is ≥1mm. Thermal silicone grease also needs to be applied on both sides of the alumina washer to reduce contact thermal resistance. - Strip Fixed Method: This method is used for occasions with higher safety requirements and includes two types: horizontal and vertical pressing. When using this method, choose alumina washers without screw holes to increase the creepage distance and insulate the strip. - Thermal and Structural Design: Types of alumina washers include single and double washers. Double washers are often used for two parallel power MOSFETs. When designing, the PCB and structure must meet dimensional requirements. For example, for TO-247 packaged power MOSFETs, the center distance between two MOSFETs should be 19mm, and the hole distance on the heat sink should also be 19mm.
IV. Application of Alumina Washers in Electronic Loads
In an electronic load application, alumina washers demonstrated excellent performance. This electronic load has 16 channels, each with a working current of 10A, and each power MOSFET has a power dissipation of up to 32W. To ensure safe and reliable operation, the design employed four large aluminum heat sinks (150mm×120mm) and high thermal conductivity alumina washers.
- Installation Method: Since the product does not require enhanced insulation (the voltage difference between the internal circuit and the casing is 10V–65V), a screw mounting method was used to satisfy functional insulation. - Operating Temperature Test: Under normal temperature conditions of 25℃ with air cooling, the operating temperature of the power MOSFETs ranged from 45.3℃ to 66.3℃. Under high-temperature conditions of 65℃ with a temperature rise of 40℃ added, the operating temperature of the power MOSFETs ranged from 85.3℃ to 100.3℃, which is within their allowable operating temperature range (maximum 160℃). This confirms the excellent thermal conductivity of alumina washers in high-power applications.
