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, a
10 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?
John
