Subject:                              Time:  9:04 AM
  OFFICE MEMO         RE>High frequency effects of...         Date:  2/12/96

Richard --

The high-frequency effects of switching from a di-functional (FR-4 / GF) to a
tetra-functional (FR-5 / GH) base material, in itself should have no or very
minimal effect because the resin molecules are essentially the same.  More
small changes will be due to the fillers and other stuff added to the neat
resin for other requirements, such as to improve UV resistance.  One of the
other considerations is moisture absorption, as depending on the operating
environment, moisture absorption can have a serious effect on the variation of
dielectric constant (as a function of absorbed moisture).

In general, almost all epoxy-glass dielectric systems have about the same
dielectric constant, about 4.7-4.9 when tested at 1 MHz, and about 4.3 when
tested up in the 0.5-1 GHz.

For you information (in case you don't know), the rule of thumb, "first-order"
approximation of the dielectric constant (dk) of mixed dielectrics is
approximately the volume percentage time the dielectric constant of each
material.  In the case of epoxy (dk ~ 3.5+) and e-glass (dk ~ 6) and assuming
volume percentage is 50/50, then the effective dk is about 4.7.  Most organic
fillers have relatively low dk's and in small additions (volume percentage)
are not a significant effect.  However, water is a problem, it's dk is about
80, and a small volume percentage of absorbed moisture or moisture
condensurate on the surface of the assembly has it's maximum effect just where
you don't want it --  on the outer layers between conductors, where it coats
or is absorbed into polymeric coatings (solder resist or conformal coating) or
in the outer surface-layers of the base material.  In these locations, the
"surface" distributed capacitance is increased, which increases your
capacitively coupled cross-talk, and other electrical signal integrity noise
budgets.

If you need some more info. or help, feel free to send me a direct info. 
request and I'll be glad to do what I can to help you.

Ralph Hersey
Lawrence Livermore National Laboratory
e-mail:    [log in to unmask]
Phn.  510.422.7430
FAX   510.424.6886