Additional question for Impedance Controlled Track

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Additional question for Impedance Controlled Track

Post by timchpi » Sat Jan 26, 2019 3:03 pm

Hello all:
I have manufactured my PCBs. However, prior to manufacturing, I was being told that the impedance controlled track I designed does not match up with desired value, and it is 10 ohm off comparing with calculated value, where they recalculated the desirable track width/gap to correct the error.
I failed to understand why.
But later on as i think back I came to remember that from a reply to my post previously the dielectric material of a 4-layer FR-4 PCB is not all FR-4 - something called "prepreg" - where I used the dielectric constant of FR-4 to calculate - and this is the only place I realize I might be wrong.
They are off to Chinese new year so they cannot answer my question in a while.
So my question:
Is the other prepreg having different dielectric constant the actual reason?
Does that mean I have to ask seller every time what is the dielectric constant between different layers?
Is there something else that might be wrong?

Any help apperciated

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