[SI-LIST] effects of stitching vias last attempt- without attachment
- From: "Eric Bogatin" <eric@xxxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Thu, 29 Nov 2012 07:24:55 -0700
Matthew-
In addition to the specific recommendations provided by others about the role
of stitching vias, I would recommend you follow my Rule #9: Never do a
measurement or simulation without first anticipating what you expect to see.
You can read a little more about this rule in the following two notes:
http://www.bethesignal.net/bogatin/bts218-bogatins-rules-engineering-p-679.html?cPath0
http://www.designconcommunity.com/author.asp?section_id&07
<http://www.designconcommunity.com/author.asp?section_id&07&doc_id%2916>
&doc_id%2916
If you have good engineering intuition about how the return currents flow when
signals change return planes, you will more easily be able to evaluate the
results from the simulation. You might also want to check out question #12 in
this blog post: http://bethesignal.net/blog/?pE2
And toward the end of this webinar,
https://www.bethesignal.net/bogatin/nma855-read-sparameters-like-book-p-684.html?cPath
, posted for free viewing, I show an example of the measured insertion loss
of a signal changing return planes, with no adjacent return via, and walk
through a simple interpretation of how to think about the return currents
propagating in the cavity mode of the planes. This will supplement what you
learned from Yuriyâs fine paper.
Note that when the diff signals are âin phaseâ itâs a common signal and
will behave exactly the same as two single end signals in parallel when passing
though the planes.
--eric
*******************************************************
Dr. Eric Bogatin, Signal Integrity Evangelist
Bogatin Enterprises
Setting the Standard for Signal Integrity Training
web site: <http://www.bethesignal.com/> www.beTheSignal.com
Blog: <http://www.bethesignal.com/blog> www.beTheSignal.com/blog
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e: <mailto:eric@xxxxxxxxxxxxxxx> eric@xxxxxxxxxxxxxxx
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Msg: #5 in digest
Subject: [SI-LIST] effects of stitching vias last attempt- without attachment
From: Matthew Severini <Matthew.Severini@xxxxxxxxx>
Date: Wed, 28 Nov 2012 13:08:52 -0500
Hello Experts,
I would like to start out by saying thank you all for the wealth of information
available on this list. I am about as green as it gets in this industry, and I
have learned more from this list in the last few months than I thought
possible. I have some results from a modeling project that I am involved in
that my team simply cannot wrap our heads around. The purpose of the project is
to investigate viability of both traditional and novel structures for high
speed serial lines. One of the things we wanted to look at was effects of
return path continuity. As an extreme case we built a few differential via
models without, any ground stitching vias.
The pictures below hopefully convey the stackups better than I can explain in
words. The first one breaks half the return path, the second one totally
breaks the return path, and the third one leaves the center plane totally
floating. Each one has a stripline differential pair on the bottom signal plane
coming into a pair of vias, which have a nicely tuned antipad, and transition
up to a stripline differential pair on the top signal plane.
Some care was taken to ensure the structure was pretty close to 100 ohm
differential. In normal construction all of the ground planes would be tied
together by a pair of PTHââ¬â¢s just outside this pair of vias, but we
wanted to see how badly we could break the structure by not including these.
The issue is that the signal integrity of this structure is not nearly as bad
as we thought it would be. We can understand that if the pair is excited
differentially, the signal can simply travel back the other wire, but when we
excite them in phase with one another, how could it not just look like an open
circuit? How are the return currents getting back? I (tried to
attach) a plot of insertion loss in all cases. It is encouraging that there is
some DC attenuation in the bottom two structures, but I would imagine it would
be worse than it is. I understand that the return path is not totally broken
for the (num_layers =2) case, but one would expect a lot of energy to be lost
because any currents in the external planes are either lost, or must be
radiated.
The first structure we built was (num layers=2):
G _______________________
S ____________
G _________ | __________
S ___________|
G ______________________
Then (num layers=3):
G _______________________
S ____________
G ________ | __________
S |
G __________ | __________
S ___________|
G ______________________
Then finally (num layers=4):
G _______________________
S ____________
G _________ | __________
S |
G __________ | __________
S |
G __________ | __________
S ___________|
G ______________________
Thank you so much for your help, I am trying to learn as much as I can, and any
intuition I can gain on this issue would be extremely helpful.
Thank you again,
Matthew Severini
Research & Development Engineering
Endicott Interconnect Technologies, Inc.
Phone: (607) 755-8119
Matthew Severini
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