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How to Set Anisotropic Thermal Properties?

This knowledge base article explains how to set anisotropic thermal properties in Conjugate Heat Transfer Analysis.

Assigning realistic material properties is important to achieve accurate simulation results. Isotropic material properties mean that the properties are independent of the direction. Default materials in the SimScale library are isotropic. Whereas anisotropic means that the properties are direction-dependent.

A typical printed circuit board (PCB) is made up of layers of plastic (typically FR4), with copper layers in between. This component is unique as it is not a single type of material, but for simulation purposes, it is treated as such. PCB is characterized by anisotropic thermal conductivity properties, so it is very important to take this into account for calculations. The in-plane thermal conductivity is usually very high and around 20 W/m-K, but the through-plane is usually below 1 W/m-K. If you are interested to learn how to calculate anisotropic thermal conductivity values in PCBs, check this article.


Find the thermal conductivity values of your particular PCB board. Note your thermal properties by considering an imaginary local coordinate system. In this example, we used the following electronics device model:

description of global coordinate system and local coordinate system
Figure 1: Global coordinate system vs local coordinate system

Once you decide the local coordinates, define the orientation of the PCB concerning the global coordinate system. While Unit vector 1 should define the direction of the surface normal, unit vector 2 should define any orthogonal direction.

anisotropic model settings for conjugate heat transfer simulation
Figure 2: Anisotropic model settings for conjugate heat transfer simulation.

To make it more clear, the following shows two different scenarios, and how to apply the correct settings:

defining anisotropic thermal properties material for conjugate heat transfer
Figure 3: Scenario 1 – Defining anisotropic material for conjugate heat transfer simulation.
defining anisotropic material for conjugate heat transfer simulation for angled local coordinate system
Figure 4: Scenario 2 – Where the local coordinate system is angled relative to the global coordinate system.

Expected Outcome

To show how different the results can be, the simple electronic device model is tested with isotropic and anisotropic PCB materials. Anisotropic PCB material has a high resistance along the surface normal. This slows down the heat transfer, therefore a higher temperature gradient is observed. On the other hand, it has a high thermal conductivity in the other two directions, thus heat is transferred towards the edges more evenly. Thus, the temperature difference between the minimum and maximum points is lower than in the second case.

As shown below, there is a high-temperature gradient at the bottom of the PCB, right below the CPU. Since thermal conductivity is isotropic, heat is transferred to every side evenly, thus high heat flux increased temperature in this region.

simulation results modeled as anisotropic in simscale post processer
Figure 5: Simulation results with PCB material modeled as anisotropic PCB.
post processing results with pbc material modeled as isotropic in simscale platform
Figure 6: Simulation results with PCB material modeled as isotropic.

Last updated: July 7th, 2020

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