[SI-LIST] effects of stitching vias last attempt- without attachment
- From: Matthew Severini <Matthew.Severini@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- 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 PTH’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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