Introduction to Several Thermal Materials
Date: November 1, 2017 Source: Foreign Station Share: 
When it comes to cooling systems, many people immediately think of fans and heat sinks. However, they often overlook a crucial but less visible component: thermal materials. These materials play a vital role in ensuring efficient heat transfer between components. In this article, we'll explore the importance of thermal materials and discuss different types used in various power devices. Why do we need thermal materials? Some may assume that the surface of a CPU or the bottom of a heat sink is perfectly smooth, so no medium is necessary. This is a misconception. In reality, even the most precisely machined surfaces have microscopic gaps and unevenness. These spaces are filled with air, which has high thermal resistance. To reduce this resistance and ensure effective heat transfer, thermal materials are essential. Their primary function is to fill these gaps, increasing the contact area between the heat source and the heat sink. While heat conduction is one of their roles, enhancing the contact area is their most critical function. What are thermal materials? 1. Thermal Grease Thermal grease is the most commonly used heat transfer medium. It is made from silicone oil, thickened with additives, and processed through heating, decompression, and grinding. It has a semi-liquid consistency and is typically used within a temperature range of -50°C to 220°C. It offers good thermal conductivity, heat resistance, aging resistance, and water resistance. During operation, it becomes semi-liquid, filling the gaps between the CPU and heat sink for better thermal coupling. It is generally insoluble in water, not easily oxidized, and has some lubricity and electrical insulation properties. 2. Thermal Silica Gel Like thermal grease, thermal silica gel is also made from silicone oil. However, it contains a certain amount of viscosity in its chemical composition, giving it some adhesive properties. After solidification, it has a hard texture, and its thermal conductivity is slightly lower than that of thermal grease. There are two main types on the market: white solid and black solid. The white type is often used as a bonding agent between the heat sink and the heat-generating object. Its strong adhesion can make removal difficult during repairs. The black type has better heat dissipation and is easier to remove after solidification. Despite its lower thermal conductivity, it's recommended unless otherwise specified. 3. Thermal Silica Film Soft silicone pads with excellent thermal insulation and conductivity are used as alternatives to thermal grease. They have a thermal conductivity of about 2.6 W/mK and an anti-breakdown voltage of 4000 V. They are flexible, suitable for use between power devices and heat-dissipating aluminum sheets or machine casings, providing optimal heat conduction and dissipation. These materials are ideal for modern electronic applications and are easy to cut for automated production and maintenance. 4. Synthetic Graphite Sheets These are less common but used in low-heat generating applications. Made from graphite composite material, they offer excellent thermal conductivity and are suitable for cooling systems in chips and CPUs. For example, Intel’s early P4 processors used a graphite pad called M751. One advantage is that they are non-sticky and don’t leave residue when removed. Other thermal materials include aluminum foil thermal gaskets and phase change thermal gaskets, though they are rare in the market. Thermal materials also have performance parameters: 1. Thermal Conductivity The unit is W/mK, indicating how effectively the material conducts heat. Higher values mean better performance. 2. Working Temperature This determines if the material remains in a solid or liquid state. Thermal greases typically operate between -50°C and 220°C. 3. Thermal Resistance Coefficient This measures how much the material resists heat transfer. Lower values are better. 4. Dielectric Constant Important for preventing short circuits in certain CPUs. 5. Viscosity Indicates the thickness of the thermal grease. 6. Insulation Ensures safety and reliability in electronic devices. Comparative Advantages and Disadvantages of Thermal Silica Gel, Thermal Grease, and Double-Sided Adhesive Thermal silica gel sheets offer several advantages over thermal grease and double-sided tape: - Wide range of thermal conductivity and stability. - Reduced process requirements for heat sinks. - Excellent EMC and insulation performance. - Shock absorption and sound damping. - Easy installation, testing, and reusability. Thermal conductivity of thermal silica gel can be adjusted from 0.8 to 3.0 W/mK, making it stable and reliable for long-term use. Double-sided adhesives typically have a maximum thermal conductivity of less than 1.0 W/mK, which is not satisfactory. Thermal grease tends to crack at high temperatures, leading to instability and reduced performance over time. Thermal silica gel sheets can be customized in thickness, reducing the need for precise surface finishes and lowering production costs. They also provide better EMC protection and shock absorption compared to other materials. In terms of installation and reusability, thermal silica gel sheets are more convenient. Once applied, double-sided adhesives are hard to remove without risking damage. Thermal grease requires careful cleaning and is not easily removable, affecting test results and reliability. Advantages and Disadvantages of Thermal Silicone Sheet Adhesive There are two main types: double-sided and single-sided adhesives. Double-sided adhesives are useful for securing components without additional structures, but they reduce thermal conductivity. Single-sided adhesives help prevent misalignment during assembly, but they also lower thermal conductivity compared to double-sided adhesives. Regardless of the type, thermal conductivity will be lower than that of pure thermal materials, but they still serve an important purpose in specific applications. Label: A Brief Introduction to Several Thermal Materials
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