Dear George,

FR4 and pre preg are a mix of Glass and resin, the Er of glass is around
6 and the Er of resin around 3, the Er of the pressed board will be somewhat
dependant on the process at your PCB vendor.  I would recommend you
talk with them and if they are a knowledgeable shop they should be able to
help you get good yields at your target impedance.

Its also worth noting that if you are working with differential structures we
have noticed the effective Er can change depending on the trace separation
and the position of the glass layer.

On inner layers with edge coupled differential structures the and closely
spaced
traces the majority of the field travels between the traces and usually
this space
will be resin filled.  So the effective Er will be closer to that of resin
than of glass.

Again the best recommendation I can give is to speak with your PCB vendor.

You can get a good idea of how impedance and prop delay will alter with Er if
you download CITS25 field solving impedance calculator from Polars
  web site..  www.polarinst.com

Kind Regards
Martyn Gaudion
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At 13:49 11/05/00 -0400, you wrote:
>Can somebody tell me how Er of the FR4 layer correlates with Er of the
>prepreg layer?
>
>If we specify Er=3.8, does the board shop have special low-Er prepreg to go
>with it?
>Or is there just one standard prepreg that everybody uses?
>
>thanks,
>
>-George

Kind Regards

Martyn Gaudion - Marketing Manager
Polar Instruments Ltd.
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Tel: +44 1481 253081
Fax: +44 1481 252476
www.polar.co.uk
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