[SI-LIST] Re: VIA STUB and ANTIPAD
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: Newbie Pugal <pugal8686@xxxxxxxx>
- Date: Thu, 12 May 2011 00:24:52 -0400
Hi Pugal,
Yes, instead of backdrilling, you can switch layers from the
top towards the bottom. If you have enough traces that
can be safely placed on top layers, this is a free solution.
Otherwise we under-utilize the top layers
Your second question also relates to the first question. You
can optimize the via dimensions to get a matched via, and
if you can achieve a good matching, going through the via
from top to bottom will not hurt the signaling other than
adding a small delay. However, many times it is not easy
to achieve matched vias, because we dont want to perforate
the ground planes (which provide the return path) too much.
Smaller antipads are good for providing better return path
in dense via fields, but it will reduce via impedance. When
you hear the statement in your second question, it refers
to the dilemma between via impedance and plane perforation.
Regards,
Istvan Novak
Oracle
On 5/11/2011 11:57 PM, Newbie Pugal wrote:
> Hi All,
> I have two questions for you all,
>
>
> 1) Instead of backdrilling can we reduce the via stub by changing the routing
> layer near to the bottom layers? say we have 16 layer board and signals are
> routed in layer 4, instead of backdrilling from bottom to layer 5 shall we
> change the signal routing to layer 14 or 15? How much impact it will create
> on high speed signals with difference to backdrilling? does the long via
> create inductance effect?
>
>
> 2) It is mentioned to enlarge the antipad size of signal via for better
> signal quality, why?
>
> Thanks
> Pugal
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