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

  • From: "Lee " <leeritchey@xxxxxxxxxxxxx>
  • To: <scott@xxxxxxxxxxxxx>, <Yishan.Li@xxxxxxxxxxxxxxxxxxxx>
  • Date: Thu, 16 Oct 2014 10:16:04 -0700

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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