RE: [IBIS-Users] Requesting for some help / information regarding the use of IBIS in SI simulation.

All,

 I have to agree with Scott on this. For the type of packages as outlined
by Scott, if one were to take the detailed model from a simulator and
decompose it to a RLC format, the best correlation between the detailed
model and the simplified model will occur when the simplified RLC is
treated as a distributed structure such as a transmission line. Like
Scott, we have proven this multiple time on a wide variety of package
styles (leadframe through 24+ layer packages). There is no disagreement
that a detailed model will be better, especially for high frequencies, but
we must work within the limits of what IBIS can support. The mode detailed
that package description is in IBIS, the better estimation we can make,
but using lumped data for high speed signals is just causing an issue
where one really will not exist. EDA tools need to make the best
estimation of the available data to predict performance.

Doug

Douglas Burns
Signal Integrity Software, Inc
6 Clock Tower Place
Maynard, MA 01754
978-461-0449 x14

 
-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf
Of Scott McMorrow
Sent: Wednesday, June 01, 2011 9:33 AM
To: bradb@sigrity.com
Cc: 'Ray Anderson'; 'ibis-users'
Subject: Re: [IBIS-Users] Requesting for some help / information regarding
the use of IBIS in SI simulation.

Brad

It is certainly your prerogative to disagree with me. In the context of
the original question, we are not talking about randomly distributed
RLC, but rather package connections from board to silicon, which is the
case in most IBIS models. Given that, we know the general topology.
Whether the package uses lead frame, 2 layer FC packaging, 2/4 layer
organic wire bond packaging, or multilayer buildup packaging, a single
path from die to ball looks remarkably similar to cascaded transmission
lines. Even bond wires look like transmission lines.

If I'm given a single RLC for an IBIS model of a conventionally packaged
silicon device , then I can guarantee you that the simulation will be
more faithful to measurements when the transmission line equivalent is
used, rather than lumped elements. Been there, done the measurements
many, many times. When multiple sections are specified in the IBIS .pkg
format, then there is no ambiguity, a non-zero RLC section length
defines a distributed element, and a zero length section defines a
lumped element. The simulator should model it as such.

Having done correlation of package modeling to measurements from DC to
40 GHz on a 12-layer build-up with near zero error, I'm well aware of
the issues involved with using the wrong modeling method, and poor
material modeling. I agree with you that a single RLC is an extremely
poor approximation of any complex package interconnect, or any
interconnect for that matter. However, given that a user is presented
with an RLC that was extracted with a quasi-static solver, as is the
case with most IBIS models, I maintain that the result will be closer to
reality when transformed into the distributed transmission line
equivalent. I've seen way too many cases where simulators used a lumped
RLC directly, grossly over predicting ringing at the die and in the
channel, and confirmed by physical measurements.

When we get into full-wave and hybrid full-wave AC modeling of packages
there are a host of other issues that we can discuss, such as accurate
dielectric modeling, accurate surface roughness modeling, as-designed
vs. as-built package geometries, +/- 15% impedance variation due to
manufacturing process and electromagnetic non-locality. But that is
for another time.

best regards,

Scott

Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

TeraspeedR is the registered service mark of
Teraspeed Consulting Group LLC

On 6/1/2011 1:53 AM, Brad Brim wrote:
> hello Scott,
>
> I don't agree with you. Such simulators (or model extractors which write
out
> such "postprocessed" model data) are no more assured accurate than
simply
> applying or writing out the base RLC data.
>
> Most package extractors, especially ones you may refer to as applicable
> "from the beginning", are based on static RLC simulation in which a
single L
> and C are extracted at exactly DC from two distinct simulations;
> magnetostatic and electrostatic respectively. One has NO CLUE how to
form
> the equivalent circuit given only the static lumped L and C computed
> independently. Of course, as asked and discussed by Ray, if the user has
> insight from the physical structure then a more broadband model MIGHT be
> generated, but there is no guarantee!
> One person assuming a PI circuit (with the C split equally in half - or
even
> unequally - and the L in the middle) and another person assuming a TEE
> circuit (with the L split and the shunt C in the middle) are no more
correct
> than the other. One has NO IDEA of the high frequency behavior of the
> circuit (either just above the quasistatic limit where the likely
spectral
> content is relevant or higher where it is less likely relevant) without
some
> form of AC high-frequency simulation or measurement. The behavior at
> "infinite" frequency in meaningless and to argue that a uniformly
> distributed multi-stage ladder network is valid has no more
justification
> than another person arguing it should me a lowpass network with all the
> inductance on the die side because of wirebond inductance. This argument
has
> blindly been accepted and argued for many years. It has been unjustified
and
> wrong, as you would say, "from the beginning".
>
> Want a valid model at higher frequencies, then extract AC information
from
> which to generate it. One's guess or intuitive argument without AC
> information to back it up is only a guess, as Andy stated. The classical
> uniformly distributed ladder network is a deceptively simple and
intuitively
> pleasing assumption that is easy to understand and unfortunately
accepted by
> way too many people in the industry. This supposed more accurate guess
has
> failed in too many cases and can even be worse than a simple totally
> unbalanced low pass filter assumption over the frequency spectrum of
> relevance.
>
> Many commercially available RLC extractors continue to make this
assumption
> today. Users are placed at significant risk because they falsely believe
the
> model is in some sense "broadband" or "wideband". It is a guess with no
> guarantee to be any better than any other assumed more distributed
> equivalent circuit for which there is no theoretical or physical
> justification.
>
> If one starts with AC data, assumes an equivalent circuit and *then*
selects
> element values in such to fit the data then there is reason to argue
this is
> a more valid model at higher frequencies. One cannot do this from only
DC
> terminal-based data. Not that it's difficult, it is impossible.
>
> best regards,
> -Brad
>
>
>> -----Original Message-----
>> From: owner-ibis-users@eda.org
>> [mailto:owner-ibis-users@eda.org] On Behalf Of Scott McMorrow
>> Sent: Tuesday, May 31, 2011 5:04 PM
>> To: Ray Anderson
>> Cc: ibis-users; Ray Anderson
>> Subject: Re: [IBIS-Users] Requesting for some help /
>> information regarding the use of IBIS in SI simulation.
>>
>> Since the beginning of IBIS, the most accurate simulators
>> would turn the RLC package parasitics into a transmission
>> line equivalent. In lieu of any additional information it is
>> the most reasonable thing to do, since packages can be
>> approximated as transmission lines.
>>
>>
>> Scott McMorrow
>> Teraspeed Consulting Group LLC
>> 121 North River Drive
>> Narragansett, RI 02882
>> (401) 284-1827 Business
>> (401) 284-1840 Fax
>>
>> http://www.teraspeed.com
>>
>> TeraspeedR is the registered service mark of Teraspeed
>> Consulting Group LLC
>>
>>
>> On 5/31/2011 7:59 PM, Ray Anderson wrote:
>>> If you are using extracted RLC data to manually set up the package
>>> model you can certainly make intelligent decisions on the
>> topology of
>>> the model and the distribution of the RLC parasitics.
>> However if you
>>> are depending on a simulator to read the data from a IBIS
>> .pkg format
>>> file (or the [PIN] data) and set up a model for you then
>> your results
>>> may differ from simulator to simulator.
>>>
>>> -Ray
>>> Xilinx Inc.
>>>
>>>
>>> -----Original Message-----
>>> From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On
>>> Behalf Of Andrew Ingraham
>>> Sent: Tuesday, May 31, 2011 4:48 PM
>>> To: ibis-users
>>> Subject: Re: [IBIS-Users] Requesting for some help / information
>>> regarding the use of IBIS in SI simulation.
>>>
>>> Yes.
>>>
>>> I am usually tempted to start with an evenly distributed
>> RLC model ...
>>> unless I have reason to think that the distribution is
>> otherwise. For
>>> example, wirebonds might have most of the inductance and
>> resistance,
>>> whereas the leadframe may have more capacitance, which
>> might influence
>>> one to imbalance those factors unequally between die end
>> and pin end.
>>> But without hard data to back it up, that is just guessing.
>>>
>>> Andy
>>>
>> --
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