Hi,
   Actually the best thing you can do is get in touch with your board shop
and
allow them to direct you as to what line width should be used, based upon
your
board layer stackup, total board thickness and the impedance result you
want.

   They will make the calculations based upon the materials they will
actually use
to fabricate your board and your requirements.  If you, on the other hand,
dictate the line width, pre-preg material and thickness and the desired
impedance,
your shop may be required to purchase special materials based solely upon
your
calculations, costing you more money or modify line widths to meet your
impedance requirements.

Regards,
Brant McClelland
Design Center,
Solectron Texas
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> ----------
> From:         Hurst, Joe[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;Hurst, Joe
> Sent:         Monday, March 30, 1998 1:18 PM
> To:   [log in to unmask]
> Subject:      [TN] 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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