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Characteristic impedance in PCB

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Yes, I saw that it's the overall production cost. How do we compare their dielectric constant tolerances ? Which dielectric material has smaller tolerance of dielectric constant compared to FR4 ?


--- Quote from: joniengr081 on October 01, 2023, 10:27:12 am ---I was reading a comment which says that one problem with FR4 is that the characteristic impedance is not constant which is due to change or un-stable in dielectric constant. What is the tolerance level of dielectric constant in FR4 ?

--- End quote ---
A much bigger problem with FR4 is that the glass weaving pattern isn't constants which results in changes of the Er. So if you need to route differential pairs which have signals over a couple of GHz, you better make sure to run them at a 45 degree angle in respect to the weave pattern to even out the influences of the weave pattern.

You can Google and find the datasheets for Megtron 6 and Isola materials. We've used Megtron 6 and Isola 408HR at work. You are paying the price for a company (Panasonic or Isola) that is making a controlled material. Something like FR4 is not so controlled, and on top of that a PCB house may have a mix of material from different places. I you aren't paying for control, you don't get it.

I've used OSHPark for a board with Isola 408HR. They offer a 4 layer controlled impedance stackup for a low low cost (at least it seemed reasonable to me). Go to their site and compare prices of the various stackups. JCL does this as well. Both of these places are hobby friendly, and you can see the price differences.

There are also quality differences for FR4.
Some 10+ years ago there were experiments with different sort of weaves and spreading (pressing) the weaves to make them more even. But I don't know where those experments went and whether the results were sufficient.

AFAIK, they never rotate by fractional turns, and at that rarely if ever rotate at all.



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