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

  • From: Hermann Ruckerbauer <Hermann.Ruckerbauer@xxxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Fri, 17 Oct 2014 18:37:55 +0200

Area and real estate ?
Many boards are very small and very dense having not the space to place
the "ideal" GND via (or any via at all).
The Routing space gets reduced ...

I guess there are always two sides of a medal ..

Hermann

EKH - EyeKnowHow
Hermann Ruckerbauer
www.EyeKnowHow.de
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Am 17.10.2014 um 17:24 schrieb Grasso, Charles:
> 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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