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

  • From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
  • To: "Gert.Havermann@xxxxxxxxxxx" <Gert.Havermann@xxxxxxxxxxx>, "Lee " <leeritchey@xxxxxxxxxxxxx>, "scott@xxxxxxxxxxxxx" <scott@xxxxxxxxxxxxx>, "Yishan.Li@xxxxxxxxxxxxxxxxxxxx" <Yishan.Li@xxxxxxxxxxxxxxxxxxxx>
  • Date: Fri, 17 Oct 2014 15:24:36 +0000

Surely there has been sufficient research published that quantifies the use of 
ground
vias for single ended, differential pairs(as recently as DesignCon 2013)  and 
cavities?
Besides - what's the downside?


Best Regards
Charles Grasso
Compliance Engineer
Echostar Communications
(w) 303-706-5467
(c) 303-204-2974
(t) 3032042974@xxxxxxxxx
(e) charles.grasso@xxxxxxxxxxxx
(e2) chasgrasso@xxxxxxxxx

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Havermann, Gert
Sent: Friday, October 17, 2014 1:47 AM
To: Lee ; scott@xxxxxxxxxxxxx; Yishan.Li@xxxxxxxxxxxxxxxxxxxx
Cc: aaditya.kandibanda@xxxxxxxxx; si-list
Subject: [SI-LIST] AW: Re: AW: Ground vias around signal via

Lee,
When talking about stitching vias I'm not always talking about complete plane 
stitching with via patterns. The rule of thumb is just about creating at least 
one GND return path for a signal transition, and I don't think that anyone of 
us needs proof that changing signal layers without a GND return path is big 
trouble. Thus there must be a GND via somewhere when signals switch Layers and 
reference planes.
You are correct in regards to via pattern stitching. There is no general proof 
that these are always necessary, or even beneficial.
Usually there are many GND vias in each design, and if they are not too far 
away from the signal via then there is a decent GND return and you don't need 
any additional vias. If the GND vias are well distributed (which is the case in 
90% of the Designs), then you usually don't need to stitch the complete plane 
using via patterns. But that’s also just a rule of thumb.

My two cents here are:
The question wasn't specific enough to create a single complete answer. The 
best answer then is to give guidance and provide some insights that allow 
engineers to think into the right direction. That’s what we all did.

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: Lee [mailto:leeritchey@xxxxxxxxxxxxx]
Gesendet: Donnerstag, 16. Oktober 2014 19:16
An: scott@xxxxxxxxxxxxx; Yishan.Li@xxxxxxxxxxxxxxxxxxxx
Cc: Havermann, Gert; aaditya.kandibanda@xxxxxxxxx; si-list
Betreff: Re: [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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