[SOLVED] Dielectric layer thickness question: sum of several layers?

[jackyzhangsh]

Hi,

If in a region on the PCB board, over several layers we intentionally remove copper so that there is only dielectric FR4 in the area, then how should we calculate the thickness of this FR4 region?



First, if there is no copper, would FR4 layer over and immediately beneath the area expand to fill the area? I guess it is a must, because eventually board undergoes pressing, and evidently it is impossible for the FR4 layer above and beneath the empty copper region to touch each other. However, in this situation the density of the FR4 would also somehow changed, so would there be a significant change to its dielectric constant?

Second, when we are trying to calculate the thickness of the dielectric layer in this situation, should we calculate the copper thickness, even if it left empty and have been filled by expanded FR4? A problem which seems somehow related to this question is the calculation of “stripline” impedance, for which the thin copper is sandwiched by dielectric layers. I have checked several sources, some (as shown in the lower left) does include copper thickness in their formula, and some (as shown in the lower right) don’t.

Could someone help with this question?


Jacky

 

[enjunear]

When you press the boards, the dielectrics will touch and it will look like a single piece of dielectric to EM waves. The minor discontinuities in the dielectric will barely affect the dielectric constant.

As for the dielectric thickness, you should look at the overall thickness dimensions placed on the PCB and use that to help you estimate the thickness of the combined FR4 region. In most cases, you have so much tolerance slop in your overall thickness that the thickness of the copper layers is inconsequential. Run your transmission line impedance calculator for both cases and split the difference... that will get you pretty close to the nominal value (which will vary from board to board, so aim for the middle of the range).

 

[jackyzhangsh]

This is solved. Thanks!

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This is solved. Thanks!