Subject: Leadframe models

From: [email-masked] (Bob Ward)
Date: Mon, 7 Mar 94 19:38:45 CST

Gentle ibis folk,

This is a worm.  That is, it is the kind BIRDS feed on.  It is in fact, Words
On Resolving a Matter.  It occurs to me that the BIRD on leadframe models is
getting hungry, thus I toss out a WORM.  There has been some traffic lately
bemoaning the fact that complete leadframe models get so large.  Yes they do.
But that is the cost of doing business.  Lets consider that leadframe models
become important to the arts and practices of signal integrity simulation only
when the metal structures on them are long enough to be considered transmission
lines.  That is indeed a high frequency, but there are significant amounts of
energy at those frequencies in todays digital signals, and there will be more
so as clock rates and edge rates get faster.  And be asured they will get 
faster.  Now if the relatively subtle effect of a quarter to a half inch of
metal is important, then it is worth modelling correctly.  Otherwise you get
warm fuzzy feeling of doing something great while you are actually and actively
shooting yourself in the foot.  No putting it up on the table and taking 
careful aim at it is more like it.  Yes the full representation of modern day
leadframes gets large, but a) the software that reads it doesn't care, or at
least shouldn't if it written right, and b) I really don't care how compactly
a model can be represented that gets me wrong answers.  I am fighting that very
battle here right now with one of our parts.  A stripped down model without all
the damping due to mutuals simulates to look artificially very bad, in fact 
virtually useless.  But the real silicon does not look anywhere near that bad.
I am preparing the full model for simulation, and following Jon Powell's rather
sage advice, comparison to actual physical measurement.  Bet it matches pretty
closely when tuned up right.  Let the record show that I do understand that 
there are first order and second order effects in this as in all simulation, 
but I also know from bitter experience how disastrous it can be to wave off an
effect because it is so bulky and tedious.  Caveat emptor and caveat vendor
both apply here!

Thanks,
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From: [email-masked]
Date: Tue, 08 Mar 94 10:38:51 -0500

Bob,

  Regarding your "worm" :

  I think _I_ am convinced we'll have to permit a full matrix of
couplings to be specified.  The BIRD will include that.  Look for it
"real soon," despite the fact that the "file inclusion" issue still
appears to be unresolved.

  One thing I'm not clear about from your message: you speak of
transmission line effects, and apparently advocate doing whatever it
takes in the model in order to capture them.  OK.  Does this mean
you're pushing for us to be able to specify arbitrarily complex
networks as the package model?  In general, the only way I know of
modelling a transmission line-LIKE structure with lots of 3-d bends
and tapers etc. is to cut the thing up into many series elements.
Now we've gone from a network with ONE series RL for each pin (plus
all the mutuals amongst them), into a network with MANY R's and L's
per pin, and a much larger thicket of mutuals to contend with.

  I'm willing to step up and write a BIRD for that, but I'd prefer
to do it with the sense that I had the "loving support" of the rest
of the forum.  Anybody else (semi vendors, simulation customers) 
care to comment?

--Eric

P.S. There's also a tie-in here with Ravender's "high-frequency" comment.
Are the nonlinear models (DC I-V curves) ready to step up to truly HIGH 
speeds, or do they require enhancements?  How can we decide?