Additional question for Impedance Controlled Track

This is the space to ask all your burning tech questions and discuss the Skills Bright Sparks competition. We are here to help!
Post Reply
timchpi
Posts: 30
Joined: Wed Aug 01, 2018 11:19 am

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?

Thanks
Any help apperciated

Post Reply