[SI-LIST] Re: AW: Ground vias around signal via
- From: Douglas Smith <doug@xxxxxxxxxx>
- To: weirsi@xxxxxxxxxx, Lee <leeritchey@xxxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Sat, 18 Oct 2014 21:32:47 -0700
Hi Lee, Steve, and the group,
I have some data from a hand built board where I have two 9 inch paths on
theboard, one stays on the top and the other spends a third of its length on
the opposite side. The board has two ground planes about 40 mils apart and
would model either a path transitioning from the top to bottom of a four
layer board, or from top to bottom of a many layer board where I am only
modeling the top and bottom two layers.
I looked at several cases. Nearest gnd plane to gnd plane shorts (vias) at
~8cm (!), 30mm, 10 mm, and 5 mm from the signal vias. I looked from a few
MHzto 1.5 GHz. The board radiates 30 dB more when the path that transitions
layers is driven compared to the one that stays above the same plane with
the~8 cm spacings. The surprising thing to me is even with gnd-gnd vias only
5 mm away from the signal vias, the plane to plane voltage, and radiation,
was only reduced 6 dB compared to ~8 cm away at only ~300 MHz. The gnd-gnd
vias for this limited case need to be much closer to make a significant
difference to the emissions from this board. Have not looked at SI, which in
general I find less sensitive than emissions for things like this. I present
the full data at my seminars and in a short webinar from time to time. Too
much to post here.
Doug
University of Oxford Tutor
Department for Continuing Education
Oxford, Oxfordshire, United Kingdom
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On 10/18/14 7:31 PM, steve weir wrote:
Lee, where is this: "have been said to be needed" coming from? The various
papers and data out there tell us the sensitivities involved and therefore
how to determine in a given situation whether the vias that come from the
PDNalone are adequate or not. In order to resolve your experience with the
science one would have to evaluate the specific designs involved and see
whatthe science predicts versus measured data. If disparity results then we
have something that merits further investigation to see if our measurements
are off or there is a problem with the science. On the other hand we have
plenty of data from real correlated simulation, and measurement both that
backs up the science as sound. If you wish to challenge this well verified
science I would say that Clara's famous saying applies to you: "Where is the
beef?" Where is the measured data that contradicts what the science
predicts?The IBM paper, the SUN oracle paper, my presentation have all been
linked already. I have not seen you take to task the contents of any of
thosedocuments. Best Regards, Steve On 10/18/2014 11:21 AM, Lee 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
linksand 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
youare 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[3] Subject: [SI-LIST] Re: AW: Ground vias around
signalvia 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
answeras 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
highfrequency 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
budgetsare 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
enoughaway 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[4] ; scott@xxxxxxxxxxxxx[5] ;
Yishan.Li@xxxxxxxxxxxxxxxxxxxx[6] Cc: Gert.Havermann@xxxxxxxxxxx[7] ;
aaditya.kandibanda@xxxxxxxxx[8] ; 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[9] [mailto:si-list-bounce@xxxxxxxxxxxxx[10]] On
Behalf Of Lee Sent: Thursday, October 16, 2014 10:16 AM To:
scott@xxxxxxxxxxxxx[11]; Yishan.Li@xxxxxxxxxxxxxxxxxxxx[12] Cc:
Gert.Havermann@xxxxxxxxxxx[13]; aaditya.kandibanda@xxxxxxxxx[14]; 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
fortha rule, be prepared to offer the proof that the rule is valid as well
aswhere 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[15] Cc: Gert.Havermann@xxxxxxxxxxx[16] ;
aaditya.kandibanda@xxxxxxxxx[17] ; 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-referencedset of ground planes will have ground stitch vias that are
separated by no more than 1/10 the wavelength of the Nyquist frequency of
thehighest differential bit rate, or the bandwidth of the signal edge rate
ofthe 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
GHzlambda = 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
7.1ps In Dk = 4 ground vias should be separated by a maximum of 42 mil for
28Gbps 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
regionsto 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
orconnector 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[18] On Thu, Oct
16,2014 at 3:39 AM, LI Yishan <Yishan.Li@xxxxxxxxxxxxxxxxxxxx>[19] 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
seemsthe ground vias are not necessary. Best regards Li Yishan -----Original
Message----- From: si-list-bounce@xxxxxxxxxxxxx[20]
[mailto:si-list-bounce@xxxxxxxxxxxxx[21]] On Behalf Of Havermann, Gert Sent:
2014年10月16日 14:59 To: aaditya.kandibanda@xxxxxxxxx[22];
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
anyGND 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
thanshielding? -> Placing many stitching vias doesn't necessarily provide
anyshielding. 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
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Geschäftsführer: Dipl.-Kfm. Edgar-Peter Düning, Dipl.-Ing. Torsten
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Nachricht----- Von: si-list-bounce@xxxxxxxxxxxxx[23]
[mailto:si-list-bounce@xxxxxxxxxxxxx[24]] 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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