[SI-LIST] Re: AW: Ground vias around signal via

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
  • Date: Fri, 17 Oct 2014 15:48:04 -0400

Stitch vias are needed when failure to stitch with ground vias causes an
unwanted resonance that is detrimental to signaling or causes within band
peaking of crosstalk.
To know that this is the case requires a significant amount of modeling and
simulation with tools that cost in the $70 range.

In lieu of the proper modeling and simulation environment I offer an
alternative, which is to stitch the ground planes in all directions within
1/10 lambda.

Scott
On Oct 17, 2014 7:08 PM, "Lee" <leeritchey@xxxxxxxxxxxxx> wrote:

> I am not suggesting they are never needed.  I am suggesting that
> postulating their need without evidence that they are needed is not good
> engineering. Most of the items discussed so far, are being done every day
> without the need for ground vias.  SO, I am asking for evidence of when
> they are needed.
>
> -----Original Message----- From: Rick Brooks (ricbrook)
> Sent: Thursday, October 16, 2014 12:13 PM
> To: leeritchey@xxxxxxxxxxxxx ; scott@xxxxxxxxxxxxx ;
> Yishan.Li@xxxxxxxxxxxxxxxxxxxx
> Cc: Gert.Havermann@xxxxxxxxxxx ; aaditya.kandibanda@xxxxxxxxx ; si-list
> Subject: [SI-LIST] Re: AW: Ground vias around signal via
>
> The counter argument is that anyone who says that GND stitching vias do
> nothing or are not needed under any conditions, is also suggesting a "rule"
> which should be proved for all possible cases.
> Honestly, I do not remember anyone saying on this thread that having close
> GND stitching vias is required in every conceivable place or scenario.
> I have certainly seen specific designs where the location and number of
> GND stitching vias made a big difference with signals containing even mode.
> Obviously, that does not mean they are a must have for every via on every
> board that is built or shipped.
> It also does not mean that I am willing to share the actual data, because,
> like others, there may be confidential aspects to it.
>
> I have also seen conditions where right angle bends on traces creates no
> visible problems.
> That does not mean you should use right angle bends, or that there aren't
> an infinite number of cases where right angle bends would be a disaster.
>
> As always, "it depends"
>
> This forum is for people to put forth their ideas and experience.
> I, for one, welcome their comments, with or without proof.
> cheers
>
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Lee
> Sent: Thursday, October 16, 2014 10:16 AM
> To: scott@xxxxxxxxxxxxx; Yishan.Li@xxxxxxxxxxxxxxxxxxxx
> Cc: Gert.Havermann@xxxxxxxxxxx; aaditya.kandibanda@xxxxxxxxx; si-list
> Subject: [SI-LIST] Re: AW: Ground vias around signal via
>
> This discussion seems to be terribly theoretical.  We all know that there
> are thousands, perhaps tens of thousands of assemblies shipped every day
> with differential signals as high as 28 Gb/S on them that work just fine
> without the need to put in "ground vias" nearby.
>
> What comes to my mind when I read discussions such as this is that old
> Burger King commercial  where Clara Peller asks "where's the beef?"
>
> For all of the postulated problems mentioned in these discussions, "where's
> the proof?"
>
> We do too much speculating on this forum and not enough proving!
>
> My position on this whole thing as well as many others that appear on this
> discussion group is, if you are going to put forth a rule, be prepared to
> offer the proof that the rule is valid as well as where it is valid.  If
> you
> are not prepared to do this, it is a disservice to those who are asking for
> advice to make such a posting.
>
> -----Original Message----- From: Scott McMorrow
> Sent: Thursday, October 16, 2014 3:29 AM
> To: Yishan.Li@xxxxxxxxxxxxxxxxxxxx
> Cc: Gert.Havermann@xxxxxxxxxxx ; aaditya.kandibanda@xxxxxxxxx ; si-list
> Subject: [SI-LIST] Re: AW: Ground vias around signal via
>
> Gert has good advice about the distance of the ground via to the signal
> via, but I would like to modify it a bit.  A well-referenced set of ground
> planes will have ground stitch vias that are separated by no more than 1/10
> the wavelength of the Nyquist frequency of the highest differential bit
> rate, or the bandwidth of the signal edge rate of the fastest single ended
> driver.
> So for some numbers in the English system.
>
> Let Dk = 4
> Tdelay = 170 ps/inch
>
> For DDR3/4 with 100 ps driver edge rates
> BW = .35/100ps = 3.5 GHz
> lambda = 285 ps
> 1/10 lambda = 28.5 ps
> In Dk = 4 ground vias should be separated by a maximum of 167 mil to
> maintain a good return path for these singled-ended DDR signals.
>
> For 10 Gbps
> Nyquist is 5 GHz
> lambda = 200 ps
> 1/10 lambda = 20 ps117
> In Dk = 4 ground vias should be separated by a maximum of  117 mil for 10
> Gbps signalling to maintain a good return path.
>
> For 28 Gbps
> Nyquist is 14 GHz
> lambda is 71 ps
> 1/10 lambda is 7.1 ps
> In Dk = 4 ground vias should be separated by a maximum of 42 mil for 28
> Gbps signalling.
>
> These recommendations apply to the region in proximity of the signal
> transition vias.  They serve to tie the ground cavities together, provide
> shielding for the power/ground cavities, eliminate resonances in the signal
> passband and first harmonic, and reduce via-to-via crosstalk.  If the
> signal transition is at the balls of a semiconductor device, or in the pin
> field of a connector, there are "usually" enough ground vias in these
> regions to meet these requirements.  If that is the case, then no
> additional grounds are required.  But, there are often cases that we
> encounter at Teraspeed Consulting where these rules are violated.  Here are
> some common areas to look at.
>
> Via transitions around dc blocking capacitors.
>
> Boards with outer layer buildup microvias, where drilled vias do not carry
> the package or connector grounds down through the board.
>
> Areas with asymmetric stripline crossing power splits on the distant plane
> side of the stripline (the stripline is close to ground.)
>
> In all these regions it is necessary to close the return path loop with
> ground vias as described above.  Use the above as a guide to current and
> future designs.
>
> best regards,
>
> Scott
>
>
>
>
>
> Scott McMorrow
> Teraspeed® Consulting - A Division of Samtec
> 16 Stormy Brook Rd
> Falmouth, ME 04105
> (401) 284-1827 Business
> http://www.teraspeed.com
>
> On Thu, Oct 16, 2014 at 3:39 AM, LI Yishan <Yishan.Li@xxxxxxxxxxxxxxxxxxxx
> >
> wrote:
>
>  Hi, Aaditya:
>>   According to 3D simulation, ground vias around signal via deeply effect
>> high frequency impedance. If your signal is low speed signal, it seems the
>> ground vias are not necessary.
>>
>> Best regards
>> Li Yishan
>> -----Original Message-----
>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>> On Behalf Of Havermann, Gert
>> Sent: 2014年10月16日 14:59
>> To: aaditya.kandibanda@xxxxxxxxx; si-list
>> Subject: [SI-LIST] AW: Ground vias around signal via
>>
>> Aaditya: My opinion is, if we have a proper return path,  they are not
>> necessary.
>>
>> -> As Wolfgang said, GND vias are needed for proper return path because
>> without any GND via there will be no proper return path. You don't have to
>> put multiple of those around the signal via. with proper placement one is
>> enough even for very high speeds.
>>
>> Aaditya: Any situations they are useful other than shielding?
>>
>> -> Placing many stitching vias doesn't necessarily provide any shielding.
>> That’s an old Myth. Take a look at waveguide filters. The placement of
>> screws into the dielectric (air) looks and functions very similar to a PCB
>> where the energy also travels in the dielectric. Stitching vias can act as
>> a filter, meaning that some frequencies are not shielded but guided to the
>> outside.
>>
>> Aaditya: How will they help? When do we need them?
>>
>> -> As already said, use GND vias to provide a proper GND return. as a rule
>> of thumb there should be a return via within the range of 1/8 wavelength
>> (Nyquist) to the signal via.
>>
>> BR
>> Gert
>>
>>
>> ----------------------------------------
>> Absender ist HARTING Electronics GmbH, Marienwerderstraße 3, D-32339
>> Espelkamp; Registergericht: Amtsgericht Bad Oeynhausen; Register-Nr.: HRB
>> 8808; Vertretungsberechtigte Geschäftsführer: Dipl.-Kfm. Edgar-Peter
>> Düning, Dipl.-Ing. Torsten Ratzmann, Dipl.-Wirtschaftsing. Ralf Martin
>> Klein
>>
>> -----Ursprüngliche Nachricht-----
>> Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>> Im Auftrag von Aaditya K
>> Gesendet: Mittwoch, 15. Oktober 2014 19:42
>> An: si-list
>> Betreff: [SI-LIST] Ground vias around signal via
>>
>> Hello Experts,
>> I have a question on ground vias placement around signal via.
>>
>> My opinion is, if we have a proper return path,  they are not necessary.
>> Am I correct?
>>
>> Any situations they are useful other than shielding?
>>
>> How will they help? When do we need them?
>>
>> Please help.
>>
>> Thanks
>> Aaditya
>>
>>
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