Fred,

I've personally done boards beyond 1Ghz with standard high Tg FR4
material. The high Tg (>140) was for Z-axis stability. The board was
exclusively 50 ohms impedance, and included a test coupon on the board
itself. One thing we did was ask for impedance testing to be done to
200db levels instead of the standard 20db. This would show any and all
discontinuities along the traces.

My recommendation is to talk with your fab shop and determine what
exactly they can do for you (technical input), and find out if they can
fabricate using GETEK material (much like FR4, should be no additional
costs) which has a lower and much more stable dielectric constant. My
fab shops was dieing to give me GETEK boards, but this particular
engineer wasn't familiar with it, and couldn't evaluate it properly.

But then again, he did get boards at 1.2Ghz using standard FR4 material.
Go figure?

Good luck,
Mitch Morey
Sr PCB Designer
NSI Communications
San Diego CA 92121
(619)657-5338
[log in to unmask]

>>> Fred Watt <[log in to unmask]> 05/21/98 07:25am >>>
        Has anybody had any problems with FR-4 at 100 to 200 MHz?  I
have several boards from several fabrication shops in which the signal
loses amplitude above 80-100 MHz.  The boards are controlled
impedance (50 Ohms) with 5 mil traces and 5 mil dielectrics.  The traces
are about 6.5" long.
        I would appreciate any comments on this subject.  The supplier of
the material says that FR-4 should be good to 400 MHz.

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