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

  • From: Aaditya K <aaditya.kandibanda@xxxxxxxxx>
  • To: "Rick Brooks (ricbrook)" <ricbrook@xxxxxxxxx>
  • Date: Thu, 16 Oct 2014 14:17:43 -0700

Thanks everyone for sharing valuable information.
To get to a conclusion, I will articulate my opinion (Intuition).

Assuming I am routing TX-RX lanes for USB 3.0 (5 Gbps). Considering the
following stackup:

TOP
GND_1
HS1
GND_2
POWER
HS2
GND_3
.
.
.

Instead of using USB 3.0 Standard A connector, if I am routing signals to a
SMP connectors.

Assumptions (for argument sake):

1) SMP is not edge connector.
2) Good quality Dielectric
3) Ignore coupling from other sources.
4) board size is 5"x5"

Question 1:

a) If I am routing from HS1 to TOP
b) GND_1 and GND_2 are stitched (not close to SMP signal pins).

My point is, it doesn't matter to have ground vias next to signal Vias.
Will it make a LOT OF DIFFERENCE if I route signal from HS2 to TOP under
same conditions?



Thanks
Aaditya

On Thu, Oct 16, 2014 at 12:13 PM, Rick Brooks (ricbrook) <ricbrook@xxxxxxxxx
> wrote:

> 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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