[SI-LIST] Re: 2D vs 3D EM based signal integrity simulators

Hello Istvan--
I think you've touched on a very important aspect of all simulation
tools: they need to give you answers in a reasonable amount of time.
The art of designing a good tool is being able to provide accurate
answers quickly.

Your main concern seems to be with broken return paths.  Since you
mention HyperLynx, I thought I should clarify that HyperLynx does in
fact support the effect of layer transitions.  You can look at via
bypassing as part of the bypass analysis, as well as generate models for
the vias that include all the bypassing and effects of the planes.  You
can also view the via transition effects in time domain simulation using
the SI/PI Co-Simulation option.

As you indicate, trying to simulate all of these broken return path
issues can be very computationally intensive and is often not practical.
Mentor Graphics has a product called Quiet Expert which is aimed at more
qualitatively identifying these issues and eliminating them where
possible.  More info can be found at:
http://www.mentor.com/products/pcb-system-design/circuit-simulation/quie
t-expert/


--Pat


Patrick Carrier
Technical Marketing Engineer
High Speed Design
Mentor Graphics Corporation
Patrick_Carrier@xxxxxxxxxx
ph. (512) 425-3015 


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Istvan Nagy
Sent: Thursday, January 14, 2010 4:04 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] 2D vs 3D EM based signal integrity simulators

Hi experts,

I would like to ask your opinions about the following:
Which one is "better", using 2D or 3D electromagnetics computation based
signal integrity simulators?

The point is to get a voltage-time waveform at/inside the receiver, with
accurate enough electromagnetic modeling of the PCB interconnections. We
should get realistic waveforms even on boards without a perfect groung
plane. The non-perfect ground plane is the main reason why this topic
has been opened, on real product, the ground plane is never perfect for
all the signals on all the buses. For the following interfaces: 133MHz
PCI-X, DDR1-400MHz, DDR2, DDR3-1066MHz, SATA, PCI-express-2.5Gbps, 10Gig
ethernet. 
If there are splits in the reference planes, it forces the return
currents away from the traces cousing reflections, impedance change,
EMI, and crosstalk. We want to se the effect of these as well. If
someone is not an academic, but a practising design engineer, then
he/she knows that to have perfect reference planes (current return
paths) on a computer motherboard (or similar product) is mostly just a
dream. Some people say "dont route the critical signals over
discontinuities", but if we have 500 of these signals on a 160mm x 80mm
x86-SBC board, then we just can not make it. this is when we have to
simulate how bad it is for the signal integrity.

For 2D, I would mention Hyperlynx as an example. As far as I know, it
finds segments of the PCB trace structure where the cross section
geometry is constant (for example 2 traces 0.2mm gap for 22 milimeter
length, then they get closer to 0.15mm for another 10mm, so then the
program divides it to 2 segments), then runs a 2D field solver (meshes
the cross-section) to get per-unit-length parameters (maybe tline-Z0 or
R, L, G, C), then internally runs a time domain simulation using these
lumped parameters and the IBIS models of the buffer circuits to get the
final time domain waveforms. This segmentation does not deal very well
sith layer transitions, and the 2D computations (by their nature)
presume perfect reference planes.

For 3D, I would mention the Agilent ADS+Momenum macromodeling simulator.
It does not take segment-models, but meshes the whole 3D geometry and
runs a frequency domain field-solver to get a touchstone macromodel,
then we build a simulation circuit with this model and the IBIS models
to run the time domain simulation to get the voltage/time signal
waveforms.

Advantages, 2D:
-fast, we get results within a minute.
-it can use a lot higher density on the cross-section mesh, since it
only meshes the cross sections, and the problem-size is still lower than
it is for a 3D simulation. This leads to more accurate impedance and
skin-effect computations.
-we can simulate a full memory bus with 64 signals and get a timing
result spreadsheet.
-it runs on a normal desktop PC.

Disadvantages, 2D:
-does NOT model non-perfect reference planes: plane splits,
antipad-fields, layer transitions, stitching vias, decoupling capacitor
return paths...
-when a signal changes layer on a eg 14 layer board, the return currents
have to follow it to the reference planes of the new signal layer. this
can be modeled only in 3D simulation. The 2D simulator models a via with
lumped RLC elements. It presumes that the return current disappears from
the plane at the signal via and reappears on the other reference planes
by some magic. 
This obviously does not happen on a real board. Most of the cases we
just can not afford to have stitching vias at every signal via, so the
lack-of them should be modeled. The 3D simulators simulate this.

Advantages, 3D:
-it does exactly model non-perfect reference planes: plane splits,
antipad-fields, stitching vias, decoupling capacitor return paths...
-when a signal changes layer on a eg 14 layer board, the return currents
have to follow it to the reference planes of the new signal layer. this
can be modeled only in 3D simulation. The 2D simulator models a via with
lumped RLC elements. It presumes that the return current disappears from
the plane at the signal via and reappears on the other reference planes
by some magic. 
This obviously does not happen on a real board. Most of the cases we
just can not afford to have stitching vias at every signal via, so the
lack-of them should be modeled. The 3D simulators simulate this.

Disadvantages, 3D:
-slow, it may take days to get a result for a difficoult net. (eg. a
signal on a DDR3 DIMM memory fly-by address bus) -because of the memory
limitations of the available computers, we can not have very dense mesh
in the 3d structure. can it be dense enough at all, for example with a
server-PC with quad-Xeon + 24GB memory? If the cross-section mesh is not
dense enough, then the skin effect and impedance values may not be
modeled accurately.
-we can only model 1-2 traces at a time, even that takes hours/days.

It is another story that we can do 3D computation also on small
localised board areas, then chain these models for simulation. For
example only on a single via transitions to speed up our simulation. But
then it can not be applied to every problem. for example if we dont have
a stitching GND via-ring around every single signal via, then a the
return current flows out of our model... If we design a test vehicle
with one 10Gbps signal and SMA connctors then we can have stitching-via
ring around, but route 32 of these signals out from under a 40mm by 40mm
BGA next to a memory bus !

The timing is very tight on a DDR3-1066MHz bus or on a PCIe-kink, so
every little detail problem on the board may cause the system to fail.
They operate with almost zero margin when the board is well-designed.
Can we predict these with any of the methods? (2D or 3D)


regards,
Istvan Nagy

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