
Re: Does anyone Know how to calculate the average propierties for a multilayer PCB?
Mike Pogue Dec 12, 2012 10:27 AM (in response to Manuel Galan)Manuel,
For static and vibration studies, I use SolidWorks composite shells. If you want to do this manually, you'd have to look at the section and weight the strength properties by their moment of area about the neutral axis. For weight, you can just take the average weighted by area.
You have no way to get zaxis heat transfer, though, through a SolidWorks shell element. So for heat, I have the attached spreadsheet. For xy heat transfer I use a weighted average of board layer properties. For zaxis heat transfer, I use a weighted average of the xy axis properties and the via properties. Then I create a custom material that has these properties. This is only useful if you have a multi layer board with a large number of vias that are reasonably spread out. One thing this technique will let you see is how good vias are for moving heat.

pwb and air gaps.xlsx 29.9 KB

Re: Does anyone Know how to calculate the average propierties for a multilayer PCB?
Manuel Galan Dec 12, 2012 10:49 AM (in response to Mike Pogue)Thank you Mike,
Could you say me where could I find and explanation for calculate manually the propierties, like you say in your answer: "you'd have to look at the section and weight the strength properties by their moment of area about the neutral axis" .
Best regards
Manuel

Re: Does anyone Know how to calculate the average propierties for a multilayer PCB?
Jerry Steiger Dec 17, 2012 8:17 PM (in response to Manuel Galan)Manuel,
You can calculate the moment of inertia of your PCB using the equivalent or transformed crosssectional area. The equivalent area of a Copper layer is equal to its actual area multiplied by the relative stiffness of the Copper and the FR4. If you assume that the Young's modulus for Copper is 17*10^6 and FR4 is 2.5*10^6, then you would mutiply the width of the Copper by 17/2.5=6.8. If the layer is only 50% Copper, then multipy its width by 3.4. You add up the moments of inertia of all of the layers using the parallelaxis theorem: I=I0+Ad^2, where I is the moment of inertia of the layer around the centroid of the PCB, I0 is the moment of equivalent moment of inertia of the layer around its own centroid, A is the equivalent area of the layer, and d is the distance between the centroid of the PCB and the centroid of the layer.
Jerry S.


Re: Does anyone Know how to calculate the average propierties for a multilayer PCB?
Ian Deller Jan 10, 2018 3:55 PM (in response to Mike Pogue)Hi Mike,
I know this post goes back awhile, but I'm having a similar issue and found this post helpful.
I am trying to use your attached excel sheet to calculate my boards conductivity in the zaxis. I noticed that some of the units in the calculations switch between standard and metric without converting. I am wondering if this is an error or if it is ok to ignore the missmatched units in this case.
I tried to update the spreadsheet with what I thought the calculations should be. I'll try to attach my version of the spreadsheet for you to review.

Re: Does anyone Know how to calculate the average propierties for a multilayer PCB?
Mike Pogue Jan 10, 2018 4:21 PM (in response to Ian Deller)I am not surprised that that old spreadsheet has errors in it. you should bring all of the units into agreement.

