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How to choose a thermal pad


Thermal pads are used to fill the air gap between a heat-generating device and a heat sink or metal base, and their flexible, resilient characteristics allow them to be used to cover very uneven surfaces. Heat is conducted from discrete components or the entire PCB to the metal housing or spreader, thereby improving the efficiency and lifespan of heat-generating electronic components. In the use of gaskets, pressure and temperature are mutually restrictive. As the temperature rises, after the equipment has been in operation for a period of time, the gasket material will soften, creep, and stress relax, and the mechanical strength will also decrease. The pressure of the seal is reduced.

So, how to find the right thermal pad among the many specifications? Mainly from the following aspects to consider:

1. The choice of substrate

There are three common polymer materials as the matrix for thermal pads, silicone, polyurethane and acrylic resin. The latter two are generally also called silicon-free thermal pads. Silicone thermal pads inherit the characteristics of silicone materials and are widely used as a type of thermal pad. However, there is a disadvantage of silicone oil precipitation, which cannot be used in some occasions (such as optical equipment, hard disks, etc.). The main advantage of silicon-free gaskets is that there is no silicone oil precipitation, but the disadvantages are also obvious, including slightly poor temperature resistance and high hardness.

2. The choice of thermal conductivity

What kind of thermal conductivity gasket should be selected depends on the application environment and requirements. The first is to look at the heat generation of the element, and the second is to design the thickness of the gap, the expected temperature reduction and the heat transfer area. Based on these, the area thermal resistance is estimated according to the Fourier equation, and then the required product can be determined according to the thickness thermal resistance curves of gaskets with different thermal conductivity.

3. The choice of structure

Generally speaking, the physical strength will be improved after adding reinforcement materials, but some thermal conductivity will be sacrificed. If the specifications are relatively large, it will have little effect on thick products, but it will have a certain impact on thin (<1mm) products. Gaskets without reinforcement materials will elongate and crack in severe cases. The gasket of the material is strong and does not undergo dimensional changes.

4. Choice of thickness

The thickness of the gasket generally needs to be selected according to the designed gap width. It is recommended to compress 20-50% of the thickness to be close to the gap thickness. For example, if the thickness of the gap is 1.5mm, a product of 2.0mm can be recommended, because the product of 2.0 is compressed to the same thickness as the gap after being compressed by 25%. A product of this thickness can ensure that the gap is filled without causing excessive stress.

The hardness of the product has a great influence on the compression performance. Under the premise of ensuring the physical strength, it is recommended to choose products with low hardness. In addition to the reason of smaller stress, the interface affinity of the gasket with low hardness is also better, and the interface thermal resistance is lower.

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