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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