Subject: comments on BIRD 31.1

From: "David Fogel" <[email-masked]>
Date: Thu, 11 Apr 1996 16:38:47 -0700 (PDT)

To:  [email-masked]

Subj.: Mated connector model, BIRD 31.1

Regarding the proposed BIRD31.1 specs, I have the following questions/comments

1. Power and ground pins assignment will affect L,C values

1. Ground (and power) pins assignment will affect both the self LC values
of a pin and the LC coupling matrix, based on the position of each pin
relative to the ground pins. 
Consider the following three connector pins example:

                o      o      o
                ^      ^      ^
                |      |      |
               gnd     1     2(or gnd)

The pins are 50 mils apart.

When only the center pin is a signal and the other two are ground I get:

SINGLE CONDUCTOR LC:
____________________

 i    j      Lij      Cij      Ze     Zo       Se     So      Fwdx   Rvsx  
from  to   (nh/in)  (pf/in)  (ohms) (ohms)   (ns/ft)(ns/ft)   (s/s)  (v/v) 
--------------------------------------------------------------------------
  1   1    10.537    3.066    58.63    -       2.16    -        -     -
;

and when only one side pin is a ground I get:

DUAL CONDUCTOR LC:
_________________

 i    j      Lij      Cij      Ze     Zo       Se     So      Fwdx   Rvsx  
from  to   (nh/in)  (pf/in)  (ohms) (ohms)   (ns/ft)(ns/ft)   (s/s)  (v/v) 
--------------------------------------------------------------------------
  1   1    15.880    3.066    88.36    -       2.16    -        -     -
  1   2    10.903    1.502   130.90  33.01     2.46   1.81    0.303  0.597
  2   2    22.247    2.188   123.78    -       2.16    -        -     -
;

Which clearly demonstrates the fact that you can not assign LC values to
connector pins just by their physical location in the connector. You also
need to know their position relative to the  ground pins.

This can be accomplished by depicting several most likely to be used signal-gnd
configurations and providing the LC matrices to all of them. In a dual
row connector you may end up with about 10 different combinations. 
This kind of information is precisely what's needed for high speed 
simulations and will make the modeling effort worthwhile from system 
engineering point of view.

2. Another level of complication in a mated model is the plug-in daughter
card. The fingers themselves are part of a PC board. How long are those
fingers in the mated model? is there any definition for it? 
The electrical characteristics can be affected by the presence (or absence)
of a ground plane in between the daughter cards' top and bottom layers (the
finger layers). How is this going to be reflected in the model?

3. Another issue is measurements and verifications of the model itself.
Since we are talking about relatively small LC values, the test set-up 
can greatly affect the measured LC values.
You may build a very elaborate test fixture at your facilities and obtain 
very precise figures. But can every vendor or user do the same?
How do we decide which values are the right ones. I believe that we need a
well defined measurement methodology to be included as part of the 
specs.

4. Another method for model development is to use the mechanical drawings and 
feed the physical dimension to a field solver. If precise mechanical dimensions, 
including the ER of the connector body, can be provided, then a field solver can 
be used to extract the LC matrices. This can be an alternative to the above methods.
The issue here is which field solver to use? 2D? 3D? from which CAE vendor?
etc. 

David Fogel

---

From: Herrmann Hank <[email-masked]>
Date: Fri, 12 Apr 96 15:22:00 E

To: David Fogel & IBIS Forum

David raises good questions.  My responses are inserted in his text below, 
set off by "+++++" for easy location.

Hank Herrmann
Technical Staff Member
AMP Incorporated, Electromagnetic Technology
M.S. 106-14
P.O. Box 3608
Harrisburg, PA 17105-3608

            Phone:  717-986-5534
                 FAX:  717-986-5643
   INTERNET:  [email-masked]

 ----------
From: owner-ibis
To: ibis
Subject: comments on BIRD 31.1
Date: Thursday, April 11, 1996 4:38PM

To:  [email-masked]

Regarding the proposed BIRD31.1 specs, I have the following 
questions/comments

                o      o      o
                ^      ^      ^
                |      |      |
               gnd     1     2(or gnd)

SINGLE CONDUCTOR LC:
____________________

DUAL CONDUCTOR LC:
_________________

This can be accomplished by depicting several most likely to be used
signal-gnd

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ From a pure academic perspective, you are correct, but there is another 
way.  We at AMP have devised proprietary methods that enable us to develop 
SPICE models that behave correctly without having defined the ground 
locations.  You get the right behavior by just connecting the model to the 
actual circuit wiring pattern.  There are full series and mutual parameters 
in the model for every line of the connector.  Obviously, I can not share 
those methods but I feel that they are applicable and should be permitted by 
IBIS.  If they are not permitted, there will have to be an overwhelming mass 
of specifically modeled configurations for each connector.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ This is exactly my point for the need to define model boundaries.  We have 
definitions for each case.   For card edge connectors, the model includes 
the recommended pads (as shown on the AMP customer drawing). There are 
separate models for boards with and without ground planes where those are 
common practices.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ This is a real challenge.  EIA/JC-15 has been struggling with this for 
packages for some time now.  Capacitance is easy compared to inductance 
measurements.  You can not measure a package or connector to get the 
inductances that are in the model.  All measurements find the effects of all 
series and mutual inductances in the entire set of current loops.  The 
values in the model are the partial parameters that work together to behave 
correctly.  I do agree that some defined validation process should be sought 
but I suggest that it should measure behavior, not partial parameters.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

4. Another method for model development is to use the mechanical drawings 
and
feed the physical dimension to a field solver. If precise mechanical
dimensions,
including the ER of the connector body, can be provided, then a field solver
can
be used to extract the LC matrices. This can be an alternative to the above
methods.
The issue here is which field solver to use? 2D? 3D? from which CAE vendor?
etc.

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ I believe there are at least three major problems with this approach. 
 First, I know we do not want to release manufacturing level drawings or 
files to the world.  I think other companies would feel the same way. 
 Second, field solvers are expensive, slow, require considerable expertise 
to use correctly, and generate very large model files.  Also, there are many 
common structures that can not be analyzed directly with 2D solvers and the 
3D solvers are really difficult to use correctly.  Finally, how do you know 
the model is good?  Validation is another expensive and time consuming 
engineering specialty, as is modeling.  Bottom line, the suppliers should 
provide validated models.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

David Fogel

---

From: [email-masked] ( Bob Ross)
Date: Fri, 12 Apr 96 11:57 PDT

David, Hank, and IBIS Team:

Here are some comments on David's points.  David, you have raised some
good questions, and some of my responses may be controversial.
However, I believe they are generally supportive and complimentary
to Hank's comments which I just received.

1.  One of the underlying assumptions is that the IBIS mated model
captures the electrical contents of the connector without reference
to any external connections.  I believe that the BIRD31.1 data can
be entered with that assumption.  There are two further assumptions:

a.  The contents of the BIRD31.1 structure can be reformatted into a
Spice Nodal Syntax.

b.  It is up to the simulator companies to correctly identify the 
rail connections (ground and power) and either do the analysis
correctly or to transform the model internally with that assumption.
Simulators vary in analysis capability and in purpose, so how this
is done may differ and may be more a function of the type of analysis
being done.  Predefined rail connections may apply for certain classes
of mated models.  But as mating components extent to thousand(s) of
contacts, this becomes impractical for the general case.

In the case of a Spice simulator, the rail connections can be applied
to the Spice connector model.  To my knowledge, Spice Connector models
(which include coupling between pins) do not rely on predefined rail 
connections.  Therefore, for defining the IBIS data base for a mated 
models,  we should assume that the rail definition is external to the
mated model.

2.  The mated model applies only to the "mated" configuration.  So the
part of any finger that extends into the connector is included in the
mated model, and the part outside should be part of the circuit board
physical layout.  I believe designer's assume that 1/2 of the finger
is inside the connector socket for optimal tolerance purposes.  You
have raised a fundamental question regarding how the analysis program
should treat this situation and how any extending reference planes for
the fingers themselves would be accounted in the matrix mated model.
My presumption is that the mated model would be provided assuming 
the suggested finger design configuration or separate models would
be provided for suggested variations.

3.  I have some brief thoughts the measurement methodology question.  I do
not believe any stated measurement defined extraction can be defined that
would work in all cases because an "ideal" test setup cannot be built.
Therefore, I would rather see crisp definitions of the parameters that
are needed which can be derived from any source - Spice equivalent structures,
field extractors, measurements, etc. 

4.  Field solver extraction is an expected option.  Which one is used
may depend on the connector itself and what specification detail (such
as "edge rate") is required in the model.  

For a closing comment, you have touched on some very difficult issues.  With
respect to mated models one key concept is that the purpose is to provide
a MODEL that is suitable for the analysis that is being done.  It will NOT
be a perfect replication of all aspects of performance - just as any 
model is not a perfect representation of any particular device.  So the
representation must reflect a reasonable compromise between complexity
and accuracy that makes it suitable for the intended purposes.  The
questions you raise are useful to nail down the definition and scope of
the data contents of the mated model itself.

Bob Ross
Interconnectix, Inc.

> Date: Thu, 11 Apr 1996 16:38:47 -0700 (PDT)
> From: "David Fogel" <[email-masked]>