[SI-LIST] Re: AW: Ground vias around signal via
- From: "Orin Laney" <olaney@xxxxxxxxx>
- To: <scott@xxxxxxxxxxxxx>, "'Lee'" <leeritchey@xxxxxxxxxxxxx>
- Date: Mon, 20 Oct 2014 11:20:24 -0700
Let's give it a rest, guys. This is starting to sound like high school.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Scott McMorrow
Sent: Monday, October 20, 2014 11:07 AM
To: Lee
Cc: steve weir; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: AW: Ground vias around signal via
Lee
well then, I do not accept that systems you have worked on do not have issues.
Please show me and provide documentation. I've already provided a link to a
paper that discusses the issues with imperfect power and ground systems in
packaging. The results also apply to PCBs.
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 Sat, Oct 18, 2014 at 4:24 PM, Lee <leeritchey@xxxxxxxxxxxxx> wrote:
> I'm still from Missouri and won't accept justifications that are hidden.
> Til I see a real failure I'm not on board with what is being said.
>
> -----Original Message----- From: Scott McMorrow
> Sent: Saturday, October 18, 2014 12:47 PM
> To: Lee Ritchey
> Cc: steve weir ; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: AW: Ground vias around signal via
>
> 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@
>> freelists.org
>> ]
>> >> On Behalf Of Havermann, Gert
>> >> Sent: 2014å¹´10月16æâ€â€Ã‚Â¥ 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:
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>> >> 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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>> --
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