I quote from IPC-D-317, Design Guidelines for Electronic Packaging
Utilizing High-Speed Techniques, p 22
5.5.2 Embedded Microstrip Line .... The equations for embedded
microstriplines are the same as in the section on (uncoated) microstrip,
with a modified effective permittivity..... the effective permittivity
can be determined as in sction 5.2
Section 5.2 (equation 5.17 on p 17) gives this relationship as
E'r = Er[1 - exp(-1.55H1/H) ]
if H1 becomes infinite, the exp term goes to zero and E'r becomes Er
Therefore, according to this reference, which I relied on for the
calculator, the results ARE THE SAME for microstrip and embedded microstrip
if the thickness of the coating is very thick.
THIS REFERENCE AND FORMULA ARE FULLY DISCLOSED IN THE HELP FILE.
To Continue -------------------------
E'r = Er[1 - exp(-1.55H1/H)]
if H1 > H then the exp term is <1
Therefore E'r < Er (which makes sense, because it will be between
the Er of the material below and Er of air, which is 1. So, it will
be between 1 < E'r < Er
Now Zo is an inverse function of the square root of Er
So if E'r goes down, Zo will go up (not down as Arpad alleges)
I rely, for my source, on the referenced IPC manual
I believe you all will find our little calclator (AND its Help
file where all this is disclosed and referenced) a useful
addition to your tool set.
UltraCAD Design, Inc.