**From:** Dima Smolyansky (*[email protected]*)

**Date:** Mon Jul 17 2000 - 09:34:36 PDT

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RE: [SI-LIST] : inductance extracted by ansoft SI3DHello:

There is always, of course, the way of the TDR.

If the via is so long compared to system rise time that it needs to be considered a distributed elements, TDR extracts the Z and Td quite nicely.

If it is necessary to compute L and C of the via separately, extending the JEDEC publication JEP-123 from packages to other elements, one can do it, as long as one is able to create appropriate test structures beforehand. R is a separate issue, but R is typically a small number, millohm one hopes, is it not? In that case, it is best measured with a DC meter, which can provide an accurate value.

There are also TDR techniques for computing partial or loop inductance values, depending which is required.

Thanks,

===================

Dima Smolyansky

TDA Systems, Inc.

11140 SW Barbur Blvd., Suite 100

Portland, OR 97219

(503) 246-2272

(503) 246-2282 (fax)

(503) 804-7171 (mobile)

http://www.tdasystems.com

The Interconnect Modeling Company(TM)

----- Original Message -----

From: Hassan Ali

To: si-list

Sent: Monday, July 17, 2000 7:47 AM

Subject: RE: [SI-LIST] : inductance extracted by ansoft SI3D

* > > 2.For a via through several power and ground planes, does the SI 3D
*

* > > consider the effects of those planes when doing the extraction?
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* > >
*

* >
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* > For this I believe you need a full wave solver such as their HFSS. Edge
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* > rate (frequency content) & geometry really are the factors. If you edge
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* > rate is slow compared to the geometry, then the complication of an
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* > additional solver MAY not be necessary. But, since we MAY not know all
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* > those rules of thumb & guidelines, take no chances & use a full wave because
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* > you probably do have edge rates which are "fast". Your investment in
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* > understanding a refined full wave solver will be worth it.
*

As to the original question, AFAIK (as far as I know) Ansoft SI Q3D is NOT capable of computing via parasitics in consideration to individual planes. You see, SI Q3D considers the entire via structure comprising of the signal traces connected to the via, the via barrel (the plating on the via hole), and all the pads at different layers, as ONE conductor. ALL the ground planes are considered connected hence they make ONE conductor. In that case, the self L and R values computed for the via structure are for the ENTIRE via structure as mentioned above (i.e. not just for the via barrel), and the capacitance to ground is with reference to ALL the ground planes (i.e. you don't get separate values for capacitance with reference to EACH individual ground plane). That information is not very useful for critical SI analysis. And unfortunately, I don't know of any software tool that can accurately compute separate parasitics. Any suggestions?

To illustrate further the problem in question, suppose I want to include via parasitics for a signal that goes from the top pcb layer to an inner signal layer, then I need to include via parasitics of only that portion of the via that gets into the path of my signal i.e. not the parasitics of the entire via structure. Any body knows how to do that with the presently available tools?

As to the capabilities of HFSS, I think many people make wrong assumptions on how full-wave field solvers can help us (SI engineers). First of all HFSS would NOT spit out via parasitics! Using your various signal traces as "ports", HFSS can accurately compute scattering (S) parameter matrix for all the ports. These S-parameters are computed for each propagation mode of interest (e.g. TEM mode) and indeed takes into account the electromagnetic (EM) field interactions of all the structures in the geometry of the problem (e.g. for a via, all the conductors, power and ground planes).

That is well and good, but the problem is that you CANNOT (easily) separate individual interactions in terms of R, L, and C parasitics. The only method I know of is to find a lumped-element equivalent circuit (which may not be unique) and use a microwave circuit simulator (like Touchstone, Libra, ADS, MDS, Ensemble, SuperCompact, APLAC, etc.) to optimize the R, L, and C, values to make the equivalent circuit have the same S-parameters as the original 3D structure. This is a painful process and at best not accurate and reliable. This is because, at high frequencies, all the parasitics are distributed and therefore cannot (easily) have an accurate lumped-element equivalent. Am I too much of a pessimist here? Any ideas of what works best?

Regards.

Hassan.

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