FR4 -- er vs frequency relation

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Eugen_E

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I know that for FR4 the εr value decreases with frequency.
I want to measure the εr at low frequency by measuring the capacitance of a sheet of FR4 and then estimate εr at 1 GHz. -- this would be more precise than using the "standard" value of 4.6 .
Do you know the actual relation for εr vs frequency ?

Thank you
 

fr4 is not dielectric controlled, so every board you get will be different from the last. Also, it is anisotropic due to the large fiberglass threads embedded in the epoxy. So...good luck with that
 
Thanks for the answer.
I use FR4 below 1 GHz and I wanted to measure the sheet of FR4 I will use. This is not for large scale production.

I found a general answer that applies to all dielectrics -- the capacitance should be measured at two frequencies, and εr and loss tangent can be estimated for any frequency.

[URL="https://www.sigcon.com/Pubs/news/4_5.htm[/URL]
 

Eugen_E said:
I want to measure the εr at low frequency by measuring the capacitance of a sheet of FR4 and then estimate εr at 1 GHz. -- this would be more precise than using the "standard" value of 4.6 .
Click to expand...

Nice idea, but it is more complicated than that. FR4 is not homogeneous, and not isotropic. The glass fibre and the resin have different εr, and the effective εr for vertical E-field (=> resonator length) is different from the effective εr for horizontal E-field (=> coupling between closely spaced lines). For practical application, we usually talk about some effective εr, but technically that isn't exact.

Unfortunately, there is no simple way to extract the frequency dependent parameters from one DC-measurement and one measurement at 1GHz. You better measure at your frequencies of interest (e.g. ring resonator).
 

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