The Difference Between Thermal Conductivity and Thermal Impedance
Across industry, manufacturers often publish thermal conductivity in units of Watts/meter-Kelvin as well as thermal impedance in units of °C – inches2 / Watt on their datasheets. So, what is the difference between these two, and how should you consider them when selecting a TIM?
Thermal conductivity is a material property and describes the ability of the given material to conduct heat. Therefore, when a material's thermal conductivity is high, the material is a better thermal conductor. This property is independent of material size, shape or orientation in a homogeneous material, and because of this, thermal conductivity is an idealised value.
To understand thermal impedance, we must first understand thermal resistance and thermal contact resistance.
- Thermal resistance is another inherent thermal property of a material, and is the measure of how a material specific thickness resists the flow of heat. Since TIM thickness is directly related to the resistance, thinner TIMs transfer heat more efficiently than thicker ones.
- Contact resistance is specific to the interfaces where a TIM meets the heat generating component and the heat sink. In reality, neither of these components are perfectly flat or smooth, therefore these surface irregularities create micro-air voids when in contact with the interface material, reducing the effectiveness to transfer the heat (air is a very poor thermal conductor).
Therefore, the thermal impedance of a material is the sum of its thermal resistance and all contact resistances. When a material's thermal impedance is lower, the material is a better thermal conductor in that application. Based on this, it is understandable that factors such as surface roughness, surface flatness, clamping pressure, presence of adhesive, non-homogeneous, and material thickness all have large impacts on the material's thermal impedance. thus, thermal impedance is a better 'real world' thermal property as it accounts for more variables specific to the application.
In summary, when comparing different TIMs for a specific application, you can begin with thermal conductivity for general comparisons, but having thermal impedance versus pressure data will be far more accurate to your 'real world' conditions.
