Introduction to Several Thermal Materials
Date: November 1, 2017 Source: Foreign Station Share: 
When it comes to cooling systems, most people immediately think of fans and heat sinks. However, they often overlook a crucial but less visible component—thermal materials. These materials play an essential role in ensuring efficient heat dissipation from power devices. In this article, we’ll explore different types of thermal materials and their functions. Feel free to share your thoughts and experiences regarding the thermal management of various electronic components.
Why are thermal materials necessary?
Some may believe that the surface of a CPU or the bottom of a heat sink is smooth enough, so no thermal medium is needed between them. This assumption is incorrect. Even with precision machining, it's impossible to achieve a perfectly flat surface. Gaps and voids will still exist between the CPU and the heat sink, filled with air. Air has a high thermal resistance, which significantly reduces the effectiveness of heat transfer. To overcome this, thermal materials are used to fill these gaps, increasing the contact area between the heat source and the heat sink. Their main function is not just to conduct heat but also to enhance the effective contact area, which is vital for optimal thermal performance.
What are thermal materials?
1. Thermal Grease
Thermal grease is the most commonly used thermal material. It is made from silicone oil, combined with thickeners and other fillers through heating, decompression, and grinding processes. It has a semi-liquid consistency and is typically used in a temperature range from -50°C to 220°C. Thermal grease offers good thermal conductivity, high-temperature resistance, aging resistance, and water resistance. During operation, when heated, it becomes semi-liquid, filling the gaps between the CPU and the heat sink, thus improving heat transfer efficiency. It is generally insoluble in water, resistant to oxidation, and provides lubrication and electrical insulation.
2. Thermal Silicone Gel
Like thermal grease, thermal silicone gel is made from silicone oil but includes additives that give it some viscosity, making it slightly adhesive. After solidification, it has a hard texture and lower thermal conductivity compared to thermal grease. There are two common types: one is white and solid, while the other is black and brittle. The white type is often used as a bonding agent between the heat sink and the heat-generating component. While it offers strong adhesion, it can be difficult to remove during repairs, leaving residue on the contact surfaces. The black type, however, is easier to clean due to its brittleness, though it still has lower thermal conductivity than thermal grease.
3. Thermal Silicone Film
Soft silicone pads offer excellent thermal conductivity and insulation. They have a thermal conductivity of around 2.6 W/mK and a breakdown voltage of 4000 V. These materials are ideal for replacing thermal grease, especially in applications where flexibility and ease of installation are important. They can be cut to size, making them suitable for automated production and maintenance. Thicknesses range from 0.5mm to 14mm, allowing them to fill gaps and provide damping, insulation, and sealing. They meet modern electronic industry standards and are widely used in miniaturized and ultra-thin devices.
They also meet UL 94V-0 fire resistance standards and comply with EU SGS environmental certifications.
4. Synthetic Graphite Sheet
This type of thermal material is less common and is typically used for low-heat applications. Made from graphite composite materials, it has excellent thermal conductivity and is used in electronic chips, CPUs, and similar components. For example, Intel once used a graphite pad called M751 on the bottom of P4 processors. The advantage of this material is that it is non-sticky and doesn’t leave residue when removed, making it easy to handle without damaging the CPU or heat sink.
Other thermal materials include aluminum foil thermal gaskets and phase-change thermal gaskets (with protective film), but these are less common in the market.
Thermal materials also have key performance parameters:
1. Thermal Conductivity
Thermal conductivity is measured in W/mK. Higher values mean better heat transfer. Mainstream thermal greases typically have a thermal conductivity greater than 1.134 W/mK.
2. Operating Temperature
Thermal materials must remain in a stable state within a certain temperature range. Most thermal greases operate between -50°C and 220°C, which is suitable for typical electronic applications.
3. Thermal Resistance
Thermal resistance measures how much a material resists heat flow. Lower resistance means better performance. It is influenced by the material’s properties and design.
4. Dielectric Constant
For CPUs without metal covers, the dielectric constant is important to prevent short circuits. Most thermal greases are insulative, but some silver-based ones may conduct electricity.
5. Viscosity
Thermal grease typically has a viscosity around 68, affecting its application and performance.
6. Insulation
Comparative Advantages and Disadvantages of Thermal Silica Gel Sheets vs. Thermal Grease and Double-Sided Adhesive
Thermal silica gel sheets offer several advantages over thermal grease and double-sided adhesive:
* Wide range of thermal conductivity and stability * Reduced process requirements for heat sink design * Excellent EMC and insulation performance * Shock absorption and sound reduction capabilities * Easy installation, testing, and reusability
Thermal silica gel sheets can be customized for thermal conductivity, ranging from 0.8 to 3.0 W/mK or more, and maintain stable performance over time. In contrast, double-sided adhesives typically have lower thermal conductivity, and thermal grease tends to degrade over time, reducing its effectiveness.
In terms of structural design, thermal silica gel sheets allow for adjustments in thickness, reducing the need for highly precise surfaces and lowering manufacturing costs. They also provide better electromagnetic compatibility (EMC) protection and are easier to install and reuse.
Thermal silica gel sheets are also known for their shock-absorbing and sound-dampening properties, which are not present in thermal grease or double-sided adhesive. This makes them ideal for sensitive electronic environments.
Advantages and Disadvantages of Thermal Silicone Sheet Adhesive
There are two main types of thermal silicone sheet adhesives: double-sided and single-sided. Each has its own pros and cons.
Double-sided adhesive is useful for securing components without additional fixing structures. However, it may reduce thermal conductivity and lead to less efficient heat transfer.
Single-sided adhesive is beneficial for mounting thermal silica sheets, preventing positional shifts during assembly. While it may slightly reduce thermal conductivity, it is generally better than double-sided adhesive.
Regardless of the type, thermal conductivity is always compromised to some extent. However, thermal silicone sheets remain a reliable choice for many applications.
Tags: Introduction to Several Thermal Materials
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