Re: [SI-LIST] : stackup impedance.

Eric Bogatin ([email protected])
Wed, 17 Sep 1997 11:30:54 -0700


I have found over the years that this type of problem- modeling the
impedance of a PCB stack up, or the reverse, designing the stackup of a
PCB, never goes out of style. As controlled impedance boards get more
popular, more and more folks are asking this same question.

There are a number of analytical approximations for characteristic
impedance in use these days in board fab shops. If you ask 10 engineers,
you will probably get back 11 different equations. If you look through the
literature, the problem is that there are too many approximations- how do
you know which is the right one to use? and under what assumptions is it
accurate? and how accurate is it?

A simple way around this problem is to go the route of a field solver. They
typically handle arbitrary cross section geometry and the accuracy of the
tool can be checked against special geometries. For transmission lines, a
2D solver is just fine. The input is the geometry of the stack up with the
material properties. The output is the characteristic impedance of the
traces and cross talk coefficients, or a complete SPICE deck.

I used to be a fan of analytical approximations. They let you create
spreadsheets and perform tradeoff analysis really easily. However, in the
last few years, there has been such a rapid increase in the speed of
computers and the efficiency of field solver code that I now have switched
my favor to using field solvers. There are some field solvers that also
export spreadsheets, and allow you to do the same trade off analysis as
approximations, but with confidence in the accuracy. In fact, I was so
impressed with the field solvers from Ansoft, I joined the company!

I recommend that those of you who ever need to perform characteristic
impedance extraction from a stack up, seriously look at using a field
solver rather than searching for the "right" analytical approximation.
There is a new product from Ansoft, Maxwell EZ2D Calculator, that is as
easy to use as a calculator in extracting line parameters of a stack up.
There are also field solvers from Applied Simulation Technology, Pacific
Numerix and OpTem Engineering, among others, worth looking at.

If anyone would like info on the Ansoft parasitic extraction tools, please
contact me off line, or check our web site.

--eric bogatin

of At 01:59 PM 9/17/97 +0800, John Lin - TAO wrote:
>Does anybody have idea about how to calculate the impedance of
>stackup with following structures.
>S-S-G-P-S-S ( 6 layers).
>G-S-S-S-S-P (6 layers).
>I checked books and tried to find equations to calculate impedance.
>However, I cannot find any.
>Best Regards,
>CAE Engineer of EDA Department
>Digital Equipment Corp. Taiwan Branch
>Email: [email protected]
>TEL: 1-886-3-3900000 ext. 2565
Eric Bogatin
Product Manager, Signal Integrity Products
Ansoft Corporation
4675 Stevens Creek Blvd Suite 208
Santa Clara CA 95051-6374
voice: 408-261-9095 x13
fax: 408-261-1245
email: [email protected]
pager: 888-775-1138
web site:

"In God we trust, all others, show your data"