1) It is always valid to omit very small parasitics.
2) Just because the results differ between your "large" parasitic
RLC model and T line model does not mean that using RLC models is wrong.
3) Molded plastic packages with wirebond to leadframe connections have
no T-line sections.
4) Any package can be modeled by either RLCs or T-lines as long as
models
are constructed correctly including mutual or crosstalk elements.
While part of some multi-layer ceramic and plastic packages do have
sections
that may be easiest to model with T-lines, most packages have elements
that
easily translate to discrete elements. Example, an inductor for the
wirebond
loop. In discrete element modeling it is easy to error by not properly
accounting
for the return path.
One should use their knowledge of package construction and desired
bandwidth
to partition the package and resulting model. As an example, most
partition
the model in segments > 1/10 Tr but may start with segments ~1/2 Tr.
Often
there are obvious partion points: Wirebonds in one segment, traces or
leadframe
in another segment, and perhaps the IC solder balls can be ignored.
Aubrey Sparkman
Daniel.Adar@ecitele.com wrote:
> I made some spice simulations of package parasitics effect.
> I compared the RLC model of IBIS with an ideal Transmission line (with
> zo=sqrt(L/C) and Td=sqrt(L*C).
> In both cases I drove the package model (RLC or T line) by a pulse with various
> rise time, Tr.
> I'd like to share with you the results and conclusions:
> 1. For very small parasitics (whereTd<<Tr) the RLC gave similar results to those
> of the T line, but there was no need in any model since the effect was
> negligible.
> 2. For larger parasitics there is a significant difference between the two
> models.
>
> My conclusion is that referring the package parasitics simply as R,L and C is
> incorrect.
> It should always be modelled as a T line or skipped for small values!
>
> Does anyone like to comment on that?
>
> Daniel Adar
> e-mail: Daniel.Adar@ecitele.com
>
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