Re: [SI-LIST] : Effective Dielectric

D. C. Sessions (dc.sessions@tempe.vlsi.com)
Mon, 23 Mar 1998 14:36:10 -0700

dfall@thin-film.com wrote:
>
> I have a question regarding the effective dielectric of two layered
> materials (such as ceramics, etc.). For example, say I have two materials
> with dielectric constants of 4 and 7. When a signal passes through these
> two materials, what is the effective or total dielectric constant that it
> sees. Any help on this matter is greatly appreciated.

Think of it as two caps in series, with a virtual plate at the
material interface. 1/C = (t1/e1) + (t2/e2), so

(t1+t2)/e' = (t1/e1) + (t2/e2)
e' = e1*e2*(t1+t2)/(t1*e2 + t2*e1)

Of course, this only applies if the layers are close together
relative to the size of the conductor; if the space near the
conductor is dominated by one material or the other (eg a
75-micron trace with 150 microns of FR4 and a few light-years
of air and vacuum above it) then the farther dielectric
has less effect.

-- 
D. C. Sessions
dc.sessions@tempe.vlsi.com