Sometimes called thermal paste or thermal interface material (TIM), thermal grease is a thermally conductive material used to connect a heat source to a thermal spreading device. With its profound performance and flexibility, you can find thermal grease at use in a variety of thermal settings, including heat sinks. Thermal grease improves a system's overall thermal conductivity effectiveness.
How to Apply Thermal Grease on a CPU
You'll often see thermal grease in use in computing devices. More specifically, the grease resides in the central processing units (CPUs), as these electronic components create large amounts of thermal energy that must be removed to perform properly. Thermal grease, which is designed to be electrically non-conductive, is applied directly to the casing (thermal spreader) of the CPU package. These casings, which are the first mode of transportation of thermal energy away from the semiconductor die, are thermally conductive. However, the casing protects the semiconductor material and integrated circuit from the thermal grease and other external factors.
Next, a heat sink, thermal spreader, heat pipe, or vapor chamber―or a custom configuration including all four devices―connects to the thermal grease. Thermal grease's goal is to reduce air pockets between the semiconductor chip's heat spreader and the thermally conductive material that moves thermal energy away from the heat source.
What Does Thermal Grease Do?
Air, which happens to be a great thermal insulator, inhibits heat transfer from a heat source to the thermal management device. Thermal grease's liquid properties allow it to penetrate the microchasms in both the surface of the heat source and the surface of the thermal spreading accessory.
Without the use of thermal grease, a heat source in direct contact with a thermal accessory would develop an array of air pockets between the two devices. These air pockets would directly reduce the thermal management system's thermal conductivity. Using thermal grease eliminates these air pockets, thus improving the subsequent thermal management accessories' effectiveness. But it's not just about improving efficiency. Without thermal grease, operating the heat source normally risks damaging the internal semiconductor die and integrated circuit.
Thermal Grease Properties: Thermal Paste Chemical Composition
Thermal grease's electrically-insulative, thermally-conductive nature allows manufacturers and builders to utilize it in almost all electronic applications that require connectivity between thermal management devices. However, achieving these properties requires specific chemical compositions. The two main compounds that make up thermal grease―and most thermal connection devices―are:
- A polymer base commonly called a 'matrix'
- Liquid or micronized metal filler
The ratio between these two classes of ingredients dictates the grease's thermal and electrical conductivity properties. Based on the ratio, each grease mixture will have its own benefits and implications.
Thermal grease that contains high concentrations of metal compounds, such as WLPG-10, which boasts 10 W/m.K, will have significantly higher thermal conductivity than thermal grease with lower concentrations of metal. Depending on the polymer's electrical conductivity properties, a high metal-concentration thermal grease will also have higher electrical conductivity properties, which can increase the risk of electrically shorting the heat source or circuit. The trade-off for a design that utilizes a lower metal-concentration thermal grease is a lower thermal conductivity, such as 251G, which has a thermal conductivity of 0.765W/m.K.
How to Use Thermal Grease: Heat Sinks
You can often find thermal grease bridging the small gaps between an IC heat spreader and a heat sink. However, high-thermal-conductivity greases are most commonly found in a constant liquid state, which requires the components that they serve to be adhered to each other using physical connectors or mechanisms. One common attachment configuration utilizes a spring and screw mounting system that surrounds the heat-generating chip. The heat sink screws into the mounting system or PCB until it makes contact with the chip. In order to avoid over-tightening and ruining the contents of the IC, the springs enable firm contact between the chip and the heat sink. Manufacturers also spread thermal grease between the chip and the heat sink to maximize contact between the two devices. In the image below, you can see a heat sink being mounted to a chip using two screws, two springs, and threaded holes on the PCB.
Thermal Glue: Is Thermal Paste Adhesive?
Depending on the polymer chemistry of the liquid matrix, thermal grease can even act as an adhesive. Some manufacturers refer to thermal grease a "thermal glue" to boast its adhesive capabilities. These thermal adhesives take on many forms, including a solid-form tape intended to be a quick solution for adhesion in low thermal conduction applications. These adhesives can also be more traditional glues that go on in liquid form and, when dried, inherit adhesive properties.
One very popular example of a thermal adhesive involves the heat sink on the chip that powers the Raspberry Pi's single-board-computer. There are no mounting brackets for a heat sink on any Raspberry Pi model, which makes it necessary to use a thermally conductive heat sink. Another heat sink, also from Seeed, comes with a conductive adhesive already attached to the Raspberry Pi heat sink, making installation simple and fast.
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