Hello Ron,
I don't think that FR4 will work here.
Polyimide may be a better solution, or a PTFE based material.
If this is a digital board you really should be determining your 'frequency
of interest' based on the output edge rates of the devices used.
F=0.35 / Trise.
If analogue then your frequency of interest is 2.5GHz.

Apart from calculating it the long way, there are impedance calculators on
the web, such as,

http://www.westak.com/techcenter/imped

From a quick manual calculation, I don't think that you are talking about
stripline

_____gnd
--------sig
_____gnd

but more like microstrip

------sig
____gnd

A 0.010" microstrip trace 0.060" from the plane gives about 100ohms impedance.
The reduced width will alter the characteristic impedance of the trace, you
will need to calculate this to see the effect.
Note also that the dielectric constant of the material used will vary with
different laminate constructions and also temperature.

Phil.



At 09:10 9/09/99 -0400, you wrote:
>All:
>
>We are working a new design that needs to perform at signal speed of up to
>2.5 Gh.  The electrician has the following parameters for the design:
>
>*    Material - High Tg FR4.
>*    Edge coupled differential pairs .010" line/.010" space/.010" line in a
>stripline construction.  Impedance is 100 ohms.  Length of
>      these lines may be up to 12.0".
>*    1 oz copper for signal layers, 2 oz for plane layers.
>*    Because of skin effect concerns the only thing to be adjusted is the
>dielectric thickness.
>
>Space constraints may dictate that the line and space need to change to
>.0085" line/.0115" space/.0085" line.
>
>My question is will the reduced line width still work or will the skin
>effect increase the resistance to a point of disrupting signal integrity.
>Also, not beings an electrical person, how do I calculate.
>
>Thanks,  Ron Yanuszewski
>
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===============================================================

Phil Dutton C.I.D.
Senior CAD Technician
IPC Certified Interconnect Designer
Vision Abell Pty Ltd
Second Avenue, Technology Park,
Mawson Lakes.  SOUTH AUSTRALIA  5095

Phone :         (08) 8300 4400 (reception)
                (08) 8300 4481 (direct)
Fax :           (08) 8349 7420
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Internet Page:  http://www.vsl.com.au

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