Suppose we have a 6-layer pcb, FR4, 10 mil dielectrics for the sake of argument. Ground plane is Layer 3.
L1 traces are simple microstrip, 20 mils above the plane, so something like Appcad gives the trace impedances, about 87 ohms for a 10 mil trace.
If the L2 trace were pure microstrip, 10 mils wide above a 10 mil diel with no overlay, the impedance would be 66.5 ohms.
But the L2 traces are buried microstrip. They are 10 mils above the ground plane but have 10 mils of epoxy glass on top. So the upper dielectric lowers the impedance.
I coded an equation from a Motorola appnote, but like most simple equations of this sort, it's not very good. For the situation above, a10 mil buried trace, it predicts an impedance *above* 66.5 ohms!
Wadell's book suggests that the buried impedance will be about 0.85 of the non-overlay impedance, so my L2 trace will be maybe 56 ohms. Is this about right?
Anybody have or know know of a calculator app that does buried traces?