Consult your PWB fabricators and ask them what is most commonly used.


> -----Original Message-----
> From: Hurst, Joe [SMTP:[log in to unmask]]
> Sent: Tuesday, March 31, 1998 7:56 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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