[SI-LIST] Re: Ground clearance at connector vias

Hi Scott,
        When you say it is "much better" are you saying that you can achieve a
larger reduction in capacitance than with two circular.
        Along the same lines would it not also be attained by a rectangular cut
out which maybe easier to attain in CAD than an oval which will cost many
segments.
        I would also like to add that it is only necessary to reduce the
parastic capacitance in line with the inductance of the pin/via to achieve the
correct impedance. Research in Backplane conducted at Lucent with upward of 30
layers has shown this to be acheivable with standard antipads for 50 ohm (100
ohm balanced) but not for 75 ohm.

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           /___  ____ |  jrobert@xxxxxxxxxxxxxxxxxxxxx
 Jim      __  / /___/ /  jgroberts@xxxxxxxxxx
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Scott McMorrow wrote:

> Christian,
>
> The recommendation is for an oval antipad that encompasses both vias of
> a differential pair, not just one.  And, yes, it is much better than two
> circular antipads around each via.
>
> best regards,
>
> scott
>
> --
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 2926 SE Yamhill St.
> Portland, OR 97214
> (503) 239-5536
> http://www.teraspeed.com
>
> Christian Schuster wrote:
>
> >Fabrizio and all the others:
> >
> >
> >
> >>I have a question regarding ground clearances for signal vias in a
> >>backplane connector, like the Molex/Teradyne HSD.  Connector and
> >>semiconductor manufacturers recommend using an oval ground clearance
> >>around the differential pairs, to minimize reflections.
> >>
> >>
> >
> >I completely agree that increasing antipad sizes is one way to reduce
> >parasitic via capacitiance. What makes me wonder is the fact that
> >oval shapes are proposed. Is that a mechanical/space requirement?
> >
> >If not: has anybody ever seen a difference in via transmission between a
> >circular and an oval antipad of the same area?
> >
> >
> >Regards,
> >
> >Christian Schuster
> >IBM
> >
> >
> >
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