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

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
  • Date: Sat, 18 Oct 2014 15:47:52 -0400

lee
i've had to work on quite a few designs where i've had to do triage.
 recently, the most common problem has been failure to provide decent
return path vias.  there are many cases where they are not required,
because they are inherent in the pin assignments for bga and connectors.  l
cannot reveal confidential customer information, but you may certainly read
the designcon paper linkni posted.  much of the information in that paper
is based on simulations that were correlated to measurements.

i'm very confident in the problems inherent with cavity resonances,
crosstalk, and lack of ground vias.

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 Sat, Oct 18, 2014 at 2:21 PM, Lee <leeritchey@xxxxxxxxxxxxx> wrote:

> Maybe I am old fashioned, but I like to draw on experience when I can.  I
> have participated in the design of a couple dozen PCBs with hundreds of 10
> G
> links and some with 28 G links.  All work great without any "ground" vias
> that have been said to be needed and had none of the dire problems alluded
> to in this thread.  No EMI problems, no SI problems, they just work.
>
> As  I said before, where is the proof for all these things that have been
> mentioned?  And, don't reply with there has been plenty written on the
> subject unless you are ready to cite those documents for us all to read.
>
>
> -----Original Message-----
> From: steve weir
> Sent: Friday, October 17, 2014 2:21 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: AW: Ground vias around signal via
>
> Lee, the body of hard scientific data establishing the effects of Vss to
> Vss vias is very well established.  As to the practical issue when a Vss
> to Vss via should be added, I there is no simple fits all sizes formula
> or guideline.  As with when and where to apply power bypass the generic
> answer as to where to apply Vss to Vss vias is:  "Wherever they are
> necessary".  It's counter productive to turn a board black with bypass
> capacitors and its counter productive to madly drill Vss vias everywhere.
>
> Additional Vss vias over and above the PDN vias should be added when:
>
> 1) Structural  resonances can be excited by by signal energy to create
> excessive EMI.
>
> 2) Cross-talk would be excessive due to too much signal to signal coupling.
>
> 3) Loss and or jitter would be excessive due to excess channel
> discontinuity resulting from structural resonances.
>
> Figuring out when the above are really going to happen takes time,
> effort, and capable tools.  Circumstances where these sorts of problems
> are more likely to occur have been discussed:
>
> 1) There are many signal penetrations into a cavity with high frequency
> content.   Lots of energy coupling through common impedance and in
> particular common impedance that has resonances within the signal
> bandwidth is an invitation to trouble.  Adding vias in appropriate
> locations can reduce the extent that signal energy spreads through the
> cavity, and raises the frequency of structural resonances.
>
> 2) Loss and distortion budgets are tight on individual signals. While
> differential signals greatly reduce even mode currents, they do not
> eliminate them.  Mode conversion is a particularly evil beast and as
> particularly demonstrated in the IBM paper, location of Vss vias can
> have a marked effect good and bad on mode conversion.
>
> 3) High frequency signal penetrations are electrically far enough away
> from cavity stitch to support standing waves within the the working
> signal bandwidth.  Scott has offered a very conservative criteria of
> lambda / 10.  That is not a universal hard stop.  It is a point that we
> shouldn't go very far beyond on assumption alone.
>
> After a board has been fabricated we don't want to be in a position to
> have discussions like this bit of 39 year old dialog:
>
> Andrel:  " You didn't know we were in trouble?"
> Freytag: "No."
> Andrel: "That's not good."
>
> Best Regards,
>
> Steve.
>
>
>
>
>
> On 10/16/2014 3:49 PM, Lee 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.:
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> >>
> >> -----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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