Hey all;
The trick is to specify the material in your fab drawing, use the IPC
number for that material that is correct dielectric constant
for your other calcs. There's a chart listing the number with it's dielectric constant,
 this causes the fab house to use the correct material, not what they have laying
around and they don't have to tweak the stackup or the line and trace width's
on you...

> From [log in to unmask] Tue May 21 12:19 EDT 1996
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> From: [log in to unmask] (Brookhaven National Laboratory)
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> Subject: Re: Controlled Impedance - Dielectric Constant- Comment
> Date: Tue, 21 May 96 11:36: 7 EDT
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> Regarding dielectric constant, I have found that 4.5 is an average for FR4 material however if you check the data sheet information from supplier to supplier you will see a range of 4.2 to 4.9
> 
> Ron 
> 
>