RE: [SI-LIST] : Focusing on Parasitic Parameters

Abe Riazi (ariazi@anigma.com)
Mon, 30 Nov 1998 19:17:14 -0800

Andy,

Thanks for your reply.

At the present time, there exists a noteworthy problem, in simulation
field, associated with accurate modeling of vias.

In my opinion, there is a real need for simulation programs to possess
the capabiltiy to model both the via capacitance and inductance. For
example, I was recently involved in a high frequency (400 MHz)
simulation task of tight time margin (sub 100 ps). Practically, every
picosecond was precious and had to be accounted for. There was a
concern for simulation accuracy because the program we use (QUAD)
ignores the parasitic via inductance.

It seems to me that a PCB database plus other files (such as Global
Control File), which are utilized by a simulation program, contain
sufficient information to allow for modeling of via inductance.
Included in these files are information related to substrate dielectric
constant and thickness, number and types of layers (i.e. board stackup),
trace geometries, topology of the nets, and via dimensions.

Therefore, it is surprising (and undesirable) that simulation
algorithms omit via inducatnce and model a via as a purely capacitive
entity.

Regards,

Abe Riazi,
email: ariazi@anigma.com
>----------
>From: Andrew Ingraham[SMTP:Andrew.Ingraham@digital.com]
>Sent: Sunday, November 29, 1998 8:26 PM
>To: Abe Riazi
>Cc: 'si-list@silab.eng.sun.com'
>Subject: RE: [SI-LIST] : Focusing on Parasitic Parameters
>
>> ... Perhaps one conclusion implied by your
>>email is following:
>>
>> It is simpler to quantify parasitic via capacitance than via
>>inductance.
>
>This is certainly true. It is easy to determine capacitance as a static
>problem and typically it doesn't change much from its value at 0Hz.
>
>Determining inductance, on the other hand, requires you to define a
>closed loop. You will get different answers depending on how you choose
>the loop that includes your via. The inductive effect of the via may be
>imagined as being partly in series with the signal trace, partly in
>series with its return path, since the return current may need to spread
>out when reaching the via, to find enough capacitance to flow from one
>plane to another. It's an interesting 3D problem that doesn't easily
>shoe-horn into a simple 1D solution.
>
>Regards,
>Andy
>
>
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