[SI-LIST] Re: Antipad dimensions
- From: Karthik Raj Guruchandran <karthik.guruchandran@xxxxxxxxx>
- To: bertsimonovich@xxxxxxxxxx
- Date: Tue, 9 Mar 2010 16:40:14 +0000
Many thanks to all who replied, both to the list and personally. I
appreciate your time.
Hope i can post some useful (??) findings at some point.
Cheers!
Karthik
On Tue, Mar 9, 2010 at 12:25 AM, Lambert Simonovich <
bertsimonovich@xxxxxxxxxx> wrote:
> Karthik,
>
> For high speed design in the GHz regime, excess via capacitance is the
> issue. The distributed capacitance associated with the via anti-pads of
> each
> reference layer over the length of the vias that affect the its impedance.
> The closer the antipad is to the via barrel, the higher the capacitance and
> the lower effective via impedance. The lower impedance causes an impedance
> mismatch and translates into noise in the form of reflections and
> potentially lower BER performance.
>
> Usually, the practice is to make the anti-pad as large as possible without
> compromising routability of other nearby traces. I.E. any traces routed
> near
> the via that needs a good reference plane for return current. More often
> than not, the component footprint will determine maximum antipad
> dimensions.
> If there are no mechanical restrictions, there will usually be a sweet spot
> where increasing the antipad size beyond a certain diameter diminishes any
> further advantage. In any event, a 3D field solver is needed to determine
> this sweet spot if the design is that critical. Often oval anti-pads are
> used when there are routing restrictions for example backplane connector
> pinfield footprints.
>
> Finally, if your design has long via stubs (not a good idea for muti-Gb
> links), this excess distributed capacitance in the stub section will
> translate into a higher effective dielectric constant, and tend to lower
> the
> 1/4-wave resonant frequency caused by the length of the stub. Resonant
> frequencies at or near 1/2 the bit-rate will destroy the received eye.
> Minimizing via stubs through back-drilling is a common practice and is the
> best thing you can do to improve channel response even more so than
> maximizing the anti-pads.
>
> Regards,
>
> Bert Simonovich
> Backplane Specialist and Founder
> LAMSIM Enterprises Inc.
> http://lamsimenterprises.com/index.html
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On
> Behalf Of Karthik Raj Guruchandran
> Sent: March-08-10 12:16 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Antipad dimensions
>
> Hello all,
> What would you consider to be the most important factors that would have an
> impact on antipad clearance dimension (in vias), apart from fabricators
> capability? In other words, what do we need to consider to determine
> antipad
> clearance dimension?
>
> I am trying to put together an article on the effect of antipad dimensions
> on energy transfer (by looking at s-parameters) and see some interesting
> simulation results.
>
> Thanks,
> Karthik
>
>
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