Simcenter FLOEFD How to model non-aligned and flexible PCBs in Simcenter FLOEFD

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

Considerations for this approach:

• It only works if the in-plane conductivity is higher than the through-plane.
• It is valid for thin sheet-like bodies such as PCBs and graphite spreaders.
• When applying the contact resistance, consideration should be given regarding what surface (side) of the body it should be assigned to depending on the problem. There are also different options for applying it. The most convenient and error-safe one is the material/thickness option, in which the user simply has to define a solid material with isotropic conductivity equal to transverse ("through-plane") conductivity and specify the through-plane thickness if the resistance is only applied to one side of the body or half thickness if it is applied to both sides.

Example

Let’s consider the following flexible PCB with a thickness of 1.5 mm:

The thermal conductivity properties for this PCB are the following:

• Conductivity type: Axisymetrical/Biaxial
• Axial (Transverse) thermal conductivity: 0.3 W/(m*K)
• Radial (in-plane) thermal conductivity: 21.3 W/(m*K)

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:

1.Assign Isotropic (In-plane) conductivity to the entire flexible component.

From the material properties, the in-plane thermal conductivity in this example is 21.3 W/(m*K):

• In the Engineering Database, create a user-defined Isotropic material with the in-plane thermal conductivity value.

• Assign this material to the flexible PCB

2. Assign thermal contact resistances to the PCB sides (transverse conductivity)

From the material properties, the transverse conductivity in this example is 0.3 W/(m*K):

• In the Engineering Database, create a user-defined Isotropic material with the thermal conductivity value as the transverse thermal conductivity (0.3 W/(m*K))

• Assign the thermal Resistance to the top and bottom faces of the PCB. For the thermal resistance type choose “Material/thickness” and choose the material created with the transverse conductivity. For the thickness, set it equal to half the PCB thickness.

In this way, the PCB is assigned with a biaxial thermal conductivity even if the board is not aligned or if it is flexible.

 

Results

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).