Joe -

     The question that you need to answer first is how accurate does the
impedance of the trace need to be.  If +/- 20% is accurate enough for your
application, you can use the numbers derived manufacturer's
frequency/relative permittivity graph and be done with it. This should give
you a value of between 4.4 at 1MHz and 4.0 at 1000MHz. If you want to be
more accurate than that allows contact your fabricator and have them supply
you with TDR derived  numbers based on processed coupons.  This may better
than half the tolerance required on your calculations.  Beyond that may not
be possible, most of the Fabricators I have worked with will only guarantee
the accuracy of the boards to +/- 10% .

Jeffrey A. McGlaughlin
PWB Designer
Battelle Memorial Institute
Columbus Ohio


        -----Original Message-----
        From:   Hurst, Joe [SMTP:[log in to unmask]]
        Sent:   Tuesday, March 31, 1998 8:56 AM
        To:     [log in to unmask]
        Subject:        [DC] FR4 Relative Permittivity

        Hello Folks:


        Since I often design PWBs having controlled impedances, the question
        often arises as to what value to use for the relative permittivity
(Er)
        of FR4 material.

        1. Some consultants have said to use 4.8 for outside layers and 3.5
for
        inside layers.
        Impedance calculations based on these numbers give results closely
        matching actual measurements.

        2. Another source has said that due to the pressing process the Er
        around the traces on inside layers actually is 0.5 less than the Er
of
        the FR4 material being used.

        3. Others insist that an Er of 4.5 be used for all calculations.

        Considering that these materials and processes have been around so
long
        the entire issue must have been settled by someone.    What are
other
        people using in these calculations?

         Thank You
        Joe Hurst

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