Heat is generated whenever current flows through a resistive element, no matter whether it’s an incandescent bulb, a wire, or a semiconductor junction. Excess heat degrades performance and can cause permanent damage or even device failure. Heat management and disposal is a concern for any designer working with high-power semiconductors, from MOSFET power drivers to high-speed digital devices such as graphical processing units (GPUs) or microprocessors (MPUs).
Thermal paste is a key element in heat management. A standard means of controlling the temperature of a semiconductor transfers the heat through conduction away from the heat source to a heat sink that dissipates it safely to the surrounding environment. The heat sink typically attaches directly to the surface of the heat source – GPU, MPU, or power device – and is constructed of a material that has a high thermal conductivity. The thermal conductivity of a material is denoted by λ and measured in watts per meter-kelvin (W/m·K): it’s a measure of the material’s ability to conduct heat. The mechanical design of a heat sink typically contains numerous fins to provide the most efficient heat dissipation within a given volume; common materials for heat sinks include aluminum alloys 6060 and 6063, with high λ values of 166 and 201 W/m·K respectively.
How does thermal paste work?
In a heat management design, it’s important that each stage in the conduction path is as thermally efficient as possible to maximize the heat transfer between the source and the sink.
The mating surfaces between these two assemblies appear smooth to the naked eye, but manufacturing tolerances cause tiny imperfections that can clearly be seen under magnification. Mechanically attaching the sink and source together will leave air gaps, but air is a poor conductor of heat with a λ value of about 0.025 W/m·K. Any air gaps or spaces in the thermal conduction path reduce thermal efficiency and heat dissipation.
Thermal paste is a thermally conductive compound that is applied between the heat source(s) and sink to eliminate air gaps. Thermal paste is called by numerous names: heat sink compound; thermal transfer compound; thermal gel; thermal grease; and so on. Most standard thermal paste formulations are electrical insulators.
What is thermal paste made of?
Thermal paste consists of a base matrix and containing a thermally-conductive filler. Typical matrix materials are epoxies, silicones, urethanes, and acrylates. Fillers include carbon micro-particles, aluminum oxide, boron nitride, zinc oxide, and aluminum nitride. The filler content can be up to 70–80% by mass and raises the thermal conductivity of the base matrix from 0.17–0.3 W/m·K up to about 4 W/m·K, depending on the formulation. Metal fillers tend to have higher thermal conductivities.
Each formulation is designed for a particular use case; regardless, the final result must be easy to apply, fill gaps effectively, conduct heat well, and remain stable for thousands of hours without drying out or flaking.
M.G. Chemicals’ Type 860 is a good example of a silicon-based silicone heat transfer compound. It uses zinc oxide as the filler to achieve a thermal conductivity of 0.66 W/m·K. The product is electrically-insulating and non-corrosive with an operating temperature range of -40 to 200 °C.
If higher performance is required, silver-based compounds can have a thermal conductivity of 3 to 8 W/m·K or more. These consist of micronized silver particles suspended in a silicone medium. However, metal-based thermal paste can be electrically conductive and capacitive and can cause malfunctions or damage if it flows onto electrical circuitry.
The thermal formulations with the highest performance (and highest cost) consist almost entirely of liquid metal, usually a variation of the alloy Galinstan™ -- a mixture of gallium, indium, and tin. Metal pastes can attain thermal conductivities greater than 13 W/m·K but are difficult to apply evenly and have the greatest risk of causing malfunction due to spillage. In addition, gallium is highly corrosive to aluminum, so they cannot be used with aluminum heat sinks.
What to look for in a thermal paste
General guidelines for selecting thermal paste include:
- A balance of cost vs. thermal conductivity for the required application.
- Low viscosity during application using the preferred manufacturing method: dispensing, stencil printing, screen printing, etc.
- Stable homogeneous mixture for consistent thermal performance.
- Compliance with industry standards such as RoHS and REACH.
Some formulations come with limitations on their use. For example, zinc oxide is classified as a marine pollutant by the International Maritime Organization; this imposes restrictions on the shipment of bulk quantities of material. Zinc-oxide free formulations are available. Silicone compounds have high chemical and thermal stability and outstanding lubrication properties but can cause contamination of bond surfaces in some applications. Many formulations are silicone-free: Aavid’s Sil-Free 1020, for example, uses a silicon-free synthetic grease, and MG Chemicals’ 8627-85ML Super Thermal Grease III is both zinc-oxide and silicone free.
Thermal paste is available in several packages. Syringes offer the most flexibility, with the ability to utilize the same product package for production and field requirements. Cartridges can accommodate either manual, automated or silk-screening equipment. The delivery system allows dispensing of the material, while protecting the integrity and exposure level of the unused portion. Bulk purchases have the lowest unit cost for large scale production facilities.
Many products offer a wide range of options. The Type 860 paste mentioned above is available in pouches, jars, and tubes in sizes ranging from 1.7mL to 470mL.
Tips for applying thermal paste
As a conductor of heat, thermal paste is better than air, but not as good as metal, so it is important to use the minimum amount needed to fill the air gaps. If an excessive amount is used the thermal paste will act like an insulator since its thermal performance is inferior to that of the metals it is connecting.
The conductive paste performance depends heavily on surface preparation. Improperly prepared contact surfaces can degrade the paste’s stability, conductivity, and lubrication characteristics. Before applying the paste, the two surfaces should be cleaned with a non-oil based cleaner such as isopropyl alcohol; the paste should be spread in a thin layer.
The application pattern is usually determined empirically to provide a good coverage of both surfaces to be thermally connected while minimizing excess use. Popular choices include dots, circle, and “X” patterns.
Alternatives to thermal paste
Thermal paste may not be the best solution for every application and other options are available. For a reusable solution, a thermal pad consists of either a silicone or silicone-free base material filled with thermally conductive aluminum oxide or boron nitride. A thermally conductive adhesive tape combines thermal conductivity with mechanical attachment. And thermally conductive polyimide films have high resistance to flow and compressive stresses. Thermal foam and ceramic inserts are other options, each with their preferred application.
Conclusion
Thermal paste plays a key role in managing and disposing of excess heat from semiconductors. Designers have a choice of options at different price & performance points. Paying attention to the characteristics of each formulation and following good application practices are keys to success.
