What Is a Thermal Resistance and Thermal Impedance？

When selecting thermal interface materials or heat sinks, you will find suppliers use thermal resistance and thermal impedance to characterise material performance. These values are different in different models. Are you confused by them? This article provides a detailed guide to thermal resistance and thermal impedance. This article will tell you what they are and how to select the appropriate material based on the data. You can make an informed choice after knowing them.

What is a Thermal Resistance

Thermal resistance refers to the ability of thermal interface materials and heat sinks to impede heat transfer at steady-state conditions. You can calculate it by simply dividing the temperature difference and the heat flux, following Fourier’s law equation as follows.

Qth =  △T/Q

– Where Qth indicates the thermal resistance,

– △T is the temperature gap between the heat source and the heatsink

– Q is the total heat flux going through the material

Following this equation, it is typically measured in K/W, and this denotes the temperature change per unit power during steady-state heat transfer. Materials exhibiting higher thermal resistance typically demonstrate lower heat transfer efficiency and poorer heat dissipation performance. Both thermal resistance and thermal conductivity determine the performance characteristics of thermal interface materials such as thermal pads and thermal pastes.

What is a Thermal Impedance

Basically, thermal impedance includes the thermal resistance of the material itself and the contact thermal resistance on both mating surfaces. Thermal impedance also denotes the capacity to impede heat transfer, but compared with thermal resistance (measured under steady-state heat transfer conditions), it reflects the temperature change per unit power during power fluctuations(in °C-in²/W or °C-cm²/W). As the heat generated by your heating elements in operational equipment is not constant, thermal impedance is more practical and important for your thermal design on a system basis.

Difference Between Thermal Resistance and Thermal Impedance

Measurement Method

The measurement of thermal resistance is simpler, as it does not follow the change in thermal power and provides a reference based solely on heat transfer performance under steady-state conditions. Thermal impedance, however, requires more complex calculations but reflects a material’s response behaviour across different frequencies based on various factors, including its intrinsic heat capacity, contact area, clamping forces, wettability, etc. If you require more stringent parameters, thermal impedance provides additional information for your design reference.

Representative Performance

Thermal resistance and thermal impedance provide the performance characteristics of thermal interface materials under static and dynamic conditions. Should you be more cautious about the heat dissipation capability of thermal interface materials during prolonged and stable operation, the thermal resistance value is better reflected. Conversely, thermal impedance better reflects a component’s capacity to withstand loads during transient events (such as start-up, switching, or pulsing).

Applications

When choosing thermal interface materials and heat sinks, you should consider both values. However, one may be prioritised depending on the uses. For example, when you are selecting thermal management solutions for servers or lighting equipment, low thermal impedance results in more efficient heat dissipation. This is because such devices typically require fast thermal response to tackle the transient heat flux when powered on. Conversely, when selecting materials for high-power or power-varying equipment (such as automobiles or computers), low thermal resistance enables more stable heat dissipation and extends the operational lifespan of your devices.

Factors Affecting Thermal Resistance

Thermal Conductivity

Thermal interface materials and heat sinks made from different materials have different thermal conductivities, with higher thermal conductivity corresponding to lower thermal resistance. Materials such as copper and aluminium offer higher thermal conductivity than other materials, making them ideal for heat sink fabrication and significantly reducing thermal resistance during heat transfer. Graphene, a material with exceptionally high thermal conductivity, can be made into graphene thermal pads featuring extremely low thermal resistance, delivering superior heat transfer performance for you compared to conventional thermal pads.

Material Thickness

While the use of thermal interface materials and heat sinks can achieve greater thermal transfer efficiency than air, more is not necessarily better. Thicker materials bring higher thermal resistance, and thinner materials can achieve superior heat dissipation. Therefore, when selecting thermal pads, you should opt for the thinnest possible option. When applying thermal paste, you should ensure complete coverage whilst minimising the thickness.

Porosity

Air is a gas with high thermal resistance, impeding heat transfer. Consequently, porous materials or components with rough surfaces get exceptionally high thermal resistance. This is why thermal interface materials are so important. They effectively fill the gaps between heat sinks and component surfaces, increasing the thermal contact area and minimising thermal resistance. When applying thermal paste or thermal putty, you must not only ensure thorough coverage but also minimise air pockets to prevent air ingress.

Environment

Firstly, temperature differentials affect thermal resistance. So, environmental temperatures can increase or decrease thermal resistance by influencing the temperature gradient during heat transfer. Secondly, when you are selecting thermal interface materials for applications in humid, dusty, or high-pressure environments, it is advisable to opt for materials with higher thermal conductivity to balance the increased thermal resistance introduced by such conditions. Furthermore, adequate ventilation can significantly improve environmental conditions, thereby diminishing the impact of thermal resistance on heat dissipation.

Material Hardness

The hardness of materials also influences thermal resistance, primarily by affecting contact area and porosity. Softer materials conform better to components during use and require less pressure, thereby reducing thermal resistance. When installing harder materials, sufficient pressure must be applied to minimise thermal resistance.

Factors Affecting Thermal Impedance

Thermal impedance is also influenced by factors such as the material’s thermal conductivity,  material thickness, hardness and the roughness of the component. Moreover, selecting materials with higher thermal conductivity, thinner thickness, and greater softness, and employing them on surfaces with lower roughness, achieves the lower thermal resistance, thereby facilitating more efficient heat transfer. Additionally, thermal resistance is also affected by volumetric heat capacity and thermal diffusivity.

Volumetric Heat Capacity

The volumetric specific heat reflects the quantity of heat a material must absorb per unit temperature increase, typically relating to both the material’s density and specific heat capacity. When transferring heat generated by heating elements, materials with higher volumetric specific heat store more thermal energy before conducting it, thereby reducing the rate of heat transfer. Consequently, materials exhibiting lower volumetric specific heat possess reduced thermal resistance, enabling more rapid heat dissipation for you.

Thermal Diffusivity

Thermal diffusivity refers to the efficiency with which heat is transferred within a material after absorption. Particularly under high-load thermal management requirements, materials with high thermal diffusivity can distribute heat more uniformly across the entire thermal interface material and heat sink, thereby achieving satisfactory thermal efficiency.

How to Minimise the Impact of Thermal Resistance and Thermal Impedance on the Working of Heat Dissipation?

Minimise the impact of air

Whether components are flat or feature rough surfaces, air occupies the space between them and the housing or heat dissipation equipment, resulting in high thermal resistance. You must replace this air with a reliable thermal interface material. This not only reduces thermal resistance but also enhances thermal conductivity. Furthermore, positioning your operational equipment in a well-ventilated environment minimises the impact of ambient factors such as temperature and humidity on thermal resistance and thermal impedance.

Minimise Contact Resistance

On the one hand, the surface smoothness of components and heat sinks is important for reducing thermal resistance and thermal impedance. When applying thermal interface materials, in addition to ensuring precise cutting, it is also important to prevent curling, bubbles, and stacking to achieve minimal porosity. On the other hand, alongside using thermal interface tape for securing, using fasteners such as screws or nails for other materials enhances contact integrity.

Selecting the Right Heat Sink and Thermal Interface Material

Firstly, you must select materials with lower thermal resistance and thermal impedance. Secondly, their type is also crucial. For instance, thermal paste can achieve satisfactory thermal efficiency for complex shapes and uneven surfaces. Furthermore, the shape, thickness, and size of heat sinks and thermal interfaces should ideally match the component dimensions to maximise contact area and prevent localised overheating.

Regular Replacement

Heat sinks and thermal interface materials ageing over longed use (e.g., thermal paste drying out, thermal tape losing adhesion). This increases thermal resistance and thermal impedance. Therefore, regular maintenance is essential to replace materials and maintain high thermal conductivity. Furthermore, thermal interface materials should not be reused.

FAQs

What is the Thermal Resistance of the Thermal Pad?

The thermal resistance values are different depending on the material and thickness of the thermal pad. Common silicone thermal pads exhibit thermal resistance values of approximately 0.15–0.45 °C·cm²/W, while carbon fibre thermal pads typically range from 0.05–0.15 °C·cm²/W.

Is Thermal Resistance or Thermal Impedance More Important?

This depends on your application requirements. If your application operates under stable loads, thermal resistance and thermal conductivity are the most critical selection criteria. If your application experiences frequent load fluctuations, thermal impedance is a more preferred criterion to be considered as the top priority.

Is Lower Thermal Resistance Always Better?

When selecting thermal interface materials, you should determine your heat dissipation requirements and choose the appropriate value based on our recommendations. Whilst a lower thermal resistance may achieve higher heat transfer efficiency, it will also increase the budgetary burden.

Do the Thermal Resistance and Thermal Impedance of Materials Change Over Time?

Yes, thermal interface materials have varying lifespans. Thermal pads typically last approximately 5 to 10 years, whilst thermal paste generally lasts around 12 months. Materials exceeding these periods will deteriorate, potentially leading to increased thermal resistance and thermal impedance. You’d better replace them with new materials periodically.

Does a Thinner Material Have Lower Thermal Resistance?

For a given thermal interface material, the thinner the layer, the lower the thermal resistance. However, in practical applications, you cannot only pursue low thermal resistance without considering the degree of surface coverage. If the material fails to fully cover a rough surface, it may instead result in high porosity, thereby increasing thermal resistance.

Final Thoughts

Once you know thermal resistance and thermal impedance, you can better select more suitable materials according to product specifications. GALLOP INNOTEK offers one-to-one consultation support and can provide customised TIM and heat sink solutions tailored to your application. You may contact us immediately.