Tag Archives: thermal interface materials

Heat Spreading for Thermal Management

Proper thermal management is necessary for ensuring the performance and reliability of electronic devices. Conceptually, this is simple, starting with the transferring of unwanted heat from the source to a larger area for effective cooling by dissipation. However, an implementation may be a difficult task.

Devices that generate heat generally have surfaces that are not large enough or smooth enough. Therefore, they cannot efficiently transfer heat as their thermal impedance is not adequately low. Some devices may not have a planar surface, thereby increasing the challenge of thermal management. Moreover, the challenge can increase with the position of the component to be cooled. If the location of the hot component is deep within the system, extracting the potentially damaging heat may become further complicated.

Many applications depend on thermal greases and pastes for improving thermal conductivity. However, this can be tricky, especially as the coverage may be insufficient, and over-application may result in spillage onto circuit board traces causing short circuits. Another limitation is thermal greases and pastes can only move the heat perpendicular to the surface, and not laterally from the source.

Therefore, designers are now replacing thermal greases and pastes with a variety of TIMs or Thermal Interface Materials. These include fillers and heat spreaders for providing low thermal impedance. This is necessary for the effective transfer of heat while removing any concerns about PCB surface contamination.

TIMs can also meet specific system needs, as their structural design can allow the transfer of heat perpendicularly or laterally. Moreover, TIMs are available in a variety of thicknesses. This enables designers to match them to the requirement of specific applications. They can provide good reliability as they are mechanically stable at elevated temperatures, and they provide high electrical isolation. Furthermore, they are easy to apply.

Placing TIMs between the source of heat and a cooling assembly helps to improve the heat flow through better thermal coupling. Here, two factors improve the efficiency of the thermal coupling. First, TIMs have the ability to conform to surface irregularities. This eliminates pockets of insulating air that actually reduce the thermal conductivity of the interface. Second, TIMs have a high thermal conductivity that is necessary to effectively move heat from the source to the cooling assembly.

Würth Electronik offers TIM (blue) for filling in microscopic irregularities. These irregularities exist on the surfaces of components and cooling assemblies, reducing thermal coupling.

Apart from thermal conductivity, there are other concerns for selecting a specific TIM. One of them is the operating temperature—TIMs are available for different temperature ranges. Another is the distance between the mating surfaces.

Other concerns are whether the TIM needs compression for delivering the optimal amount of thermal transfer and whether the TIM has the withstanding capability for the compression pressure it will face. Würth Electronik offers TIMs with adhesive on one surface that enables mechanical fixing. TIMs may also have to provide electrical isolation.

TIMs made of synthetic graphite offer very high thermal conductivity. The WE-TGS family from Würth Electronik is a synthetic graphite heat spreader. It measures 297 x 210 mm and has a thermal conductivity of 1800 W/mK.

Why Is Thermally Conducting Paste Used?

No matter how highly polished a surface may seem, when seen under a microscope it will have some irregularities. When two such metal surfaces are put together, air within the irregularities prevents heat from flowing efficiently from one metal to the other. A heat sink is usually placed on a micro-controller to remove the heat generated within the CPU. However, this arrangement will not work in the way desired unless some filler material is used to replace the air in the gaps in the interface between an IC and its heat sink. Such filler materials are usually Thermal Interface Materials or TIMs and generally called as thermally conducting paste.

Thermal pastes are semi-solid materials with very high heat conducting properties. These are placed between two surfaces, usually between a CPU/GPU and its heat sink, to allow better heat conduction between the two. Irregularities on the two surfaces may trap air between them, leading to a loss in the performance of the heat sink, as air is a very poor conductor of heat. TIM, being more than 100 times a better conductor of heat compared to air, improves the performance of the heat sink. However, the heat conducting property of thermal paste is not as good as Aluminum (heat sink material) or Copper (IC material), and a thick layer of TIM might actually hinder the ability of the heat sink to conduct heat properly.

A magnified view of the cross-section of two mating surfaces shows the microscopic imperfections that can trap air as in a pocket. TIM would fill-in these areas, replacing air. If you could have two perfectly smooth and flat surfaces, thermal paste would not be required to aid in heat transfer. However, that being impossible to achieve, thermal paste is necessary to improve the flow of heat. Depending on the application, you could use one of three types of thermal pastes – Silicon based, Ceramic based or Metal based.

Silicon based TIMs are generally used in cooling kits. These are commonly known as thermal pads and stock heat sinks use them freely. However, other pastes have better heat conducting properties.

When you need a paste that can also isolate electrically – Ceramic-based TIMs are hard to beat. They consist of a thermally conductive paste with plenty of tiny ceramic particles, which prevent flow of electricity. Although not as good as the Metal based TIMs, the difference is only minimal.

Metal-based TIMs are the most popular. These have the best performance characteristics, which comes from the innumerable tiny metal particles present in the high thermally conducting paste. However, this paste has a disadvantage that it can also conduct electricity.

Apart from the above three types of TIMs, there are also thermal epoxies. Although these function in a manner similar to the thermal pastes described, they possess an additional property – they can attach the heat sink permanently. Therefore, thermal epoxies must not be used when you need to get the heat sink off an IC. In case you have to separate a heat sink from an IC bonded together with thermal epoxy, the best procedure is to place the assembly in a freezer. The cold makes the epoxy brittle and it comes off easily.