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

Jim,

I'm not exactly sure how your research was performed.  However, I do 
agree that one must take into account the via spacing and the antipad 
sizing. The reason for placing an oval antipad (or two circular antipads 
connected by a rectangular region which works better in CAD systems) is 
to eliminate any parallel plate mode power injection into the plane 
cavities.  Yes, it is possible to create a narrow band impedance match 
with some coupling to the planes, but at the sacrifice of impedance 
mismatch at higher frequencies.  It is generally better to eliminate the 
planar power injection with larger antipads that encompass the entire 
differential via.  This will reduce return loss over a very large 
frequency range.  An optimization can then be performed with a 3D full 
wave field solver that uses the via spacing and antipad size as 
variables and seeks to minimize return loss.  An optimized design with 
an antipad region encircling both vias can be designed to have better 
broadband return loss than an optimized design with individual antipad 
regions around each differential via.  Now, for press fit connectors, 
the PTH via spacing  cannot be altered, but for surface mount 
connectors, there is a great latitude of spacing that is possible.


regards,

scott


-- 
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com




Jim G Roberts wrote:

>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.
>
>--
>Regards,    __________   James G Roberts
>           /___  ____ |  jrobert@xxxxxxxxxxxxxxxxxxxxx
> Jim      __  / /___/ /  jgroberts@xxxxxxxxxx
>         / /_/ /---| |   Room: BE436, Hilversum
>         \____/    /_/   Tel: +31 35 687 4308 Fax: 5976
>
>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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>
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>  
>




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