Hi KC Chan

I believe 'effective dielectric constant' is also a function of where a
trace is in a PWB layer. The closer to the surface the trace is the more
the dieletric of air (1.0) is effective in the propagation of signals.
Traces burried between two ground planes "see" the dieletric of the FR-4
material only. IPC-2141 has a nice discussion of this as well as dieletric
constants of various PWB materials (including FR-4) as a function of
frequency and the formulas to compute effective dieletric constant for
various PWB configurations.

Ray Klein

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