[SI-LIST] Re: Impedance Uniformity
- From: Martyn Gaudion <martyn.gaudion@xxxxxxxxxxxxxxxxxxxx>
- To: mkauffman@xxxxxxxxxxxx
- Date: Tue, 07 Oct 2003 00:08:59 +0100
Hi Mike,
Outers are always more tricky when it comes to impedance, especially
if you have fine line traces. Plating can have an effect as can the type
of resist employed. Screened or curtain coated resist may be less uniform
than a sprayed on coating. However it is best to discuss with your
fabricator what can be achieved with their particular process.
Try these application notes:
Copper thickness, edge coupled lines and characteristic impedance
www.polarinstruments.com/support/cits/AP151.html
Modelling the effects of resist between surface traces:
www.polarinstruments.com/support/cits/AP169.html
Hope this helps...
Kind regards
Martyn Gaudion
Polar Instruments
www.polarinstruments.com
T: +44 1481 253081
M: +44 7710 522 748
F: +44 1481 252476
E: martyn.gaudion@xxxxxxxxxxxxxxxxxxxx
At 11:49 PM 06/10/03, you wrote:
>Can anyone provide guidance on expected impedance variations along the
>length of a differential grounded coplanar transmission line built with
>foil construction?
>
>I have made differential TDR measurements on several samples of a
>nominally 100ohm, 10" long line on prepreg, and besides seeing a
>difference in the average impedance, on some coupons, I see 'triangular'
>variations of >10ohms occurring over 500ps of round trip delay. Another
>sample, built with core construction showed no anomalous behavior - just
>a slight rise of impedance that I attribute to conductor resistance.
>
>I've been told by a board shop that for the outer layer, foil
>construction is cheaper than core construction, so I'd like to avoid
>core construction unless I have to.
>
>Have others seen this behavior?
>What specifications have you made on fab drawings to avoid it?
>
>Thanks,
>
>Mike Kauffman
>
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