Fundamentals of thermal resistance

The electronic device industry has seen impressive growth in recent years as devices become more compact and possess higher processing capabilities. However, despite the changing landscape of electronic devices today, the challenge of managing heat effectively continues to hinder the performance and reliability of these systems.

A key concept in thermal management is thermal resistance, which determines how efficiently heat is dissipated from heat-generating components. Understanding thermal resistance helps engineers design heat sinks and electronic devices with better performance and reliability.

Figure 1: Thermal resistance networks determine how heat can be effectively dissipated from electronic devices. Source: Raimundas/Adobe Stock

What is thermal resistance?

Thermal resistance is a measure of a material's ability to resist the flow of heat. It is analogous to electrical resistance but in the context of heat transfer. Therefore, the lower the thermal resistance, the more efficiently heat is transferred through the material. Thermal resistance can generally be calculated using:

Where ΔT is the temperature difference across the material (°C or K) and Q is the heat transfer rate (W) through the material. While this formula is typically used to calculate the overall thermal resistance, it is worth noting that different forms of heat transfer occur in thermal management systems, including conduction, convection and radiation. Each of these modes of heat transfer contributes to the overall resistance to heat flow.

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Components of thermal resistance

1. Conduction thermal resistance

Conduction is the transfer of heat through a solid material. The thermal resistance due to conduction can be calculated using:

Where e is the thickness of the material (m), Fig9 is the thermal conductivity of the material (W/m.K), and A is the material cross-sectional area through which heat flows (m²). Materials with high thermal conductivity (such as aluminum and copper) typically have low conduction resistance. This makes them perfect for use in heat sinks and other thermal management systems. In addition, increasing the cross-sectional area through which the heat flows can significantly reduce the conduction thermal resistance.

2. Convection thermal resistance

Convection is the transfer of heat between a solid surface and a fluid (such as air or liquid) in motion. The thermal resistance due to convection is calculated using:

Where h is the convective heat transfer coefficient (W/m²·K), and A is the surface area of the solid exposed to the fluid (m²). Engineers typically enhance convection heat transfer (or reduce convection thermal resistance) by increasing the surface area of the solid. For example, the use of fins and extended surfaces on a heat sink reduces the convection thermal resistance. Engineers also reduce convection thermal resistance by improving airflow around the heat sink using fans, blowers and forced liquid cooling. This approach significantly increases the convective heat transfer coefficient, thereby reducing the convective thermal resistance.

3. Radiation thermal resistance

Radiation is the transfer of heat in the form of electromagnetic waves. Although radiation is often less significant in most thermal management applications, it can be important in high-temperature environments or where other forms of heat transfer are minimal.

Calculating total thermal resistance

In practical thermal management systems, thermal resistances are often combined in series and parallel configurations, similar to electrical resistances.

Series thermal resistance

Figure 2 shows a composite wall with three different materials, each with its thermal conductivity and thickness. The thermal resistances are connected in series and represented for both conduction and convection. In this case, heat flows sequentially through each resistance. The total thermal resistance is the sum of the individual resistances, as shown below:

Figure 2: Series thermal resistance Source: Ellande/CC[SA][4.0]

A series thermal resistance network is common in thermal management devices in electronics, where layers of materials with different thermal conductivities are used to dissipate heat away from sensitive components and prevent overheating.

Parallel thermal resistance

Figure 3 shows two materials with different thermal properties in a composite structure, where the configuration indicates that the thermal resistances are in parallel. When thermal resistances are in parallel, heat can flow through multiple paths simultaneously. The total thermal (R_{th, total}) is found by taking the reciprocals of the individual resistances, as shown below:

Figure 3: Parallel thermal resistance Source: Temitayo Oketola

Parallel thermal resistances are common in systems where heat can bypass some barriers by flowing through alternate paths. For example, electronic component enclosures may use parallel paths (through the casing and thermal pads) for heat dissipation.

Conclusion

Thermal resistance is an important concept in thermal management systems helping engineers effectively dissipate heat from heat-generating components in electronics. While this article presents the basics of thermal resistance, several other factors must be considered when designing thermal networks for devices. For instance, the type of working fluid, thermal gradients, flow rates and pumping power demands are all essential factors affecting the overall performance of thermal management systems. Therefore, it is recommended to contact thermal management device suppliers to discuss application requirements.