Hello.

My PCB is stacked layer - core (doubled-side PCB) - stacked layer so 4 layers. There is prepreg between 1st layer and 2nd layer (also between 3rd and 4th layer).

I know dielectric constant of core which is normally 4.6 If it is FR-4. However, what is dielectric constant of prepreg?

My ground plane is 2nd layer so knowledge of this is important to make impedance-matched microstrip.

Some article said it is same to the core but is it normally true?

 

Prepreg should be the same material as core, but if you still have doubt ask your manufacturer of the PCB.