5
\$\begingroup\$

It is a well known fact that high speed signal would have return current path on the plane right beneath it's PCB trace.

Suppose we have 4 layer stackup, SIG-GND-PWR-SIG with dielectrics CORE-PREPREG-CORE respectively. Difference between core and prepreg is that core has cooper filled on both sides of dielectric and prepreg is just full dielectric, no copper on any side of the material.

Why wouldn't return current flow on the top side of CORE between SIG-GND since there is a full cooper plane on top side of CORE?

\$\endgroup\$
8
\$\begingroup\$

Difference between core and prepreg is that core has cooper filled on both sides of dielectric and prepreg is just full dielectric, no copper on any side of the material.

Since you just sandwiched the prepreg between two cores, which are themselves clad with copper, the prepreg now has copper on both sides of it.

Why wouldn't return current flow on the top side of CORE between SIG-GND since there is a full cooper plane on top side of CORE?

High frequency return currents for layer 1 (SIG) will mostly flow on layer 2 (GND). Just how you want it.

High frequency return current for layer 4 (SIG) will mostly flow on layer 3 (PWR). Making this work well likely requires having good bypassing near all source and load locations for high-speed signals.

Return currents for PWR will indeed flow in the GND layer, because those two layers are separated by just the few mil of prepreg. If there also large ground fills on layer 4 you will also get substantial return currents there, even more so if the core layers are much thinner than the prepreg layer.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.