FR4 is not considered a high frequency/controlled impedance material.  Even
within the same brand and grade of material, there is a reasonably wide
range of impedance value that can result.
If you are high enough in frequency, your wavelength is small enough that
the varying amounts of glass fiber/resin in any one area can affect how your
circuit works in that area.  And one board will work fine while another will
not, even though they may have come from the same panel of FR4.
If your circuit can work over the whole impedance range, then specifying it
the way you mentioned is fine.  If you need tighter control over impedance,
you need to go to something other than FR4.  Generally, they don't recommend
using FR4 over 1GHz, although we have used it higher.

-----Original Message-----
From: Franklin D Asbell [mailto:[log in to unmask]]
Sent: February 22, 2005 1:21 PM
To: [log in to unmask]
Subject: Re: [TN] FR4 board material


I would think an exotic material (Teflon, Rogers, etc) would be best suited
for controlling RF signals.

Material specifications may be the same between the US and Asia, I'm
thinking the differences are in how each continent interprets them

The material is only one factor in controlling impedance, stack-up design
and copper weight (starting and process) are additional factors to be
considered.

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