Thermal pads serve as a crucial link between heating components and heat dissipation surfaces. They are flexible and elastic, enabling coverage of uneven surfaces. These pads facilitate the conduction of heat from individual components or the entire PCB to metal casings or diffusion plates, thereby enhancing the efficiency and lifespan of heat-generating electronic parts.
When selecting a thermal pad, several key considerations come into play:
Material’s Thermal Conductivity:
The more thermally conductive a material is, the lower its thermal resistance tends to be. This translates to improved heat conduction capabilities. Achieving higher thermal conductivity often involves increasing filler content in silicone gel and using powders like boron nitride, albeit with a higher cost. Thermal conductivity (symbolized as “k”) indicates a material’s heat conduction capacity and is measured in W/(m·K), representing watts per meter per Kelvin. Common examples include air with a low thermal conductivity of 0.026 W/(m·K), used more for insulation; water with a higher thermal conductivity of 0.6 W/(m·K), suitable for general thermal applications; and copper, an excellent conductor with a thermal conductivity of 400 W/(m·K), widely used in electronics for its exceptional heat conduction properties. Thermal conductivity is affected by factors like composition, structure, density, moisture, and temperature, categorizing materials as insulating or thermally conductive.
Thickness
The pad’s thickness should align with the design’s gap width. A recommended compression of 20% to 50% relative to the thickness brings it closer to the gap’s dimensions. For instance, a 2.0mm pad compressed by 25% aligns with a 1.5mm gap. This thickness ensures optimal gap filling without exerting excessive stress.
Material hardness and flexibility
Material hardness not only affects the compression ratio but is primarily concerned with the adhesion between the thermal pad and the heat sink or heat source during application. A softer thermal pad surface with lower hardness entails less surface tension, leading to better infiltration and integration with the adhering surface, minimizing any gaps and significantly reducing contact thermal resistance.
Material tensile strength
Adequate tensile strength ensures that the material is less prone to excessive deformation during assembly or potential rupture leading to gaps. Particularly for thermal pads with thicknesses around 1.0mm. Some manufacturers add a layer of glass fiber or silicone skin to some products to enhance tear resistance. While such structures simplify processing and assembly, they might increase the material’s intrinsic thermal resistance. Specifically, surface composite forms increase the thermal pad’s surface hardness, reducing its adhesion and infiltrating properties, subsequently raising contact thermal resistance.
Compression ratio of the material
The compression ratio refers to the material’s thickness change under different pressures, providing a convenient parameter for thermal designers in material selection. By considering the inherent gap size and assembly pressure settings, an appropriate thickness of thermal conductive material can be selected more efficiently.
Material compression deformation
Compression deformation mainly refers to the thermal pad’s ability to recover to its initial thickness after compression. Factors affecting its recovery include intermolecular forces (viscosity), changes or damage to the network structure, and molecular displacement. If the thermal pad’s deformation is due to molecular chain elongation, its recovery (or permanent deformation) is mainly determined by the thermal pad’s elasticity. However, if its deformation involves network damage and relative molecular flow, this part is irreversible, resulting in the thermal pad’s permanent compression deformation.
Oil permeation rate of the material
Softer thermal pads generally have lower curing degrees and are more prone to silicone oil seepage when heated. Silicone oil migration can not only contaminate components but also gradually increase the product’s hardness, leading to reduced adhesion and infiltration properties. Consequently, contact thermal resistance increases. Currently, common thermal pads on the market have an oil permeation rate of approximately 2.5% to 3.5%, and better products can control it between 2.0% and 2.5%.
Other Factors
Attributes like dielectric strength, fire rating, and electrical insulation capability should also be considered.
Understanding these aspects is crucial in selecting the right thermal pad to optimize heat transfer and efficiency within electronic devices and industrial applications.
