Estimating Printed-Circuit-Board Thermal Properties

Let’s say you have SOLIDWORKS Flow Simulation, with the Electronics Systems Cooling (ESC) database extension. And let’s assume you are are analyzing an assembly that contains a printed circuit board, (PCB), that has 6 layers – a Power and Ground layer in the middle, Signal layers on the outside faces, and a pair of conduction Via layers.

The ESC database contains a very useful abstraction object that allows you to enter the material properties of the board, (such as an FR4), and the conduction material (such as gold, nickel, or copper) – plus the thicknesses of each layer – and then it computes an aggregate, orthotropic thermal conductivity for the entire stack. This way you get the correct performance without having to build a detailed model of the board, and without having to mesh super-fine in the thru-thickness direction to resolve all the board layers.

The image below shows that the estimate requires you to input the Percentage Cover of each board layer that is layered with conductive material. But, how do you get THAT ? Often times, all you are working with is a CAD file of the traces, or a bitmap image or PDF file of each layer.

Estimating Printed Circuit Board Thermal Properties

First off, do NOT use your CAD system to try to read in all the trace paths and perform some survey/estimate – not if you can help it. The import will be slow, the image processing will be very cumbersome, and the data are very likely to be wire frames, with some assigned via width (or worse-yet, a “line font”) assigned. SOLIDWORKS is not tuned for optimum performance with such wire frame data.

Instead, you can use an online calculator to analyze the image of each layer of the board. My favorite is the one from wiseGEEK.com.

This site allows you to upload a picture image, and then it spits back out the averaged color value, and the percentage of the image that is either Cyan, Magenta, Yellow, or Black. Since your image of the conducive traces is black and white, the black value is your “Percentage Cover”.

Why is this my favorite calculator? Because not all PCBs are square. If you have an image of a board with an irregular outline, you can doctor the file to paint all the image outside the ‘real’ board, in a single color (like Cyan or Red). The Percentage of Black will still be correct for the overall image, and it will tell you the percentage of the board that is Cyan – you then simply discount the Black value by the remainder of that percentage.

For example, wiseGEEK finds that the average value of our ground-plane layer is 72% black.

Estimating Printed Circuit Board Thermal Properties

But the #2 (via trace) layer for the same board is only 35 % black.

Estimating Printed Circuit Board Thermal Properties

 

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