This document describes the process of how to model not-aligned and flexible PCB's in Simcenter FLOEFD.
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For a flat and aligned PCB, a biaxial thermal conductivity (with one "through-plane" thermal conductivity value and one "in-plane" thermal conductivity) can be assigned to a block in order to represent the PCB, as in the following image:
The biaxial thermal conductivity can also be applied to non-aligned bodies, but in those cases the temperature distribution inside the PCB might not be correctly displayed in the results postprocessing, but the overall thermal performance of the PCB is still modelled correctly. On the other hand, the orthotropic thermal conductivity can only work with aligned thin flat bodies.
When using a biaxial thermal conductivity on a PCB not aligned with an axis or when the PCB is Flexible, there is an alternative approach which consists of assigning to the PCB cuboid an Isotropic material (with the In-Plane thermal conductivity assigned) + “solid/solid” and “solid/fluid” Contact Resistances, as represented in the following image
Let’s consider the following flexible PCB with a thickness of 1.5 mm:
The thermal conductivity properties for this PCB are the following:
This can be easily defined in a flat PCB, but for a flexible or not aligned PCB we have to do it in two steps:
From the material properties, the in-plane thermal conductivity in this example is 21.3 W/(m*K):
From the material properties, the transverse conductivity in this example is 0.3 W/(m*K):
In the following image we can seen the heat flux vectors using an isotropic material for the PCB (left) and the heat flux vectors using the isotropic + thermal contact resistance approach described in this article (right).