re: [SI-LIST] : Re: Embedded microstrip calculations, Ultrac...

fabrizio zanella (fabrizio=zanella%eng%[email protected])
Thu, 11 Dec 97 7:48:01 -0500

Arpad, I fully agree with your comments.
Using simple formulae for calculating the characteristic impedance of
transmission lines just does not work. There are some cases where the
come close to reality, but for most cases it is much more accurate to use
the boundary
element method, method of moments or finite element method to calculate the
self inductance
and self capacitance of the signal conductor.
(this is based on my past experience at a backplane company)
So I also suspect there is a problem with the UltraCad formulas used.
Fabrizio Zanella
Sr. Design Engineer
Hardware Engineering
_/ _/ email:
[email protected]
_/_/_/_/ _/ _/ _/_/_/ _/ voice: (508) 435-1000 Ext.
_/ _/_/ _/_/ _/ _/ Fax: (508) 435-8949
_/_/_/_/ _/ _/ _/ _/ _/_/_/
_/ _/ _/ _/ EMC Corporation
_/_/_/_/ _/ _/ _/_/_/ 171 South Street
Hopkinton, MA 01748-9103
Original Text
From: "Arpad Muranyi" <[email protected]>, on 12/10/97 2:01
To: [email protected]@EMCHOP1[<[email protected]>],
[email protected]@EMCHOP1[<[email protected]>]

SUBJECT too long. Original SUBJECT is
[SI-LIST] : Re: Embedded microstrip calculations, Ultracad Calculator

---------------------- Original Message Follows ----------------------

Text item:


First, I want to apologize for the "public criticism". For one, I didn't
to criticise. My intention was to alert the innocent user, because I am
not convinced that the results of your impedance calculator is correct.

I am not a numerics guy, so I cannot comment on the equation itself. Your
explanation seems to be right, but something seems to be wrong with the
equation. If you check your results, you will understand what I mean.

microstrip in air embedded microstrip

H = 5.0 mil H = 5.0 mil
T = 1.4 mil T = 1.4 mil
W = 10.0 mil W = 10.0 mil
Er = 4.7 Er = 4.7
H1 = 11.4 mil

Z = 40.7275095 Ohms < Z = 41.1925677 Ohms

If I change H1 to a large number, such as 10,000,000.00 the impedance
will both be 40.7275095 Ohms.

This implies that a microstrip with air cover layer has the same impedance
the same microstrip conductor with an infinitely high dielectric cover,
which is
simply NOT true. Something is clearly wrong in the calculator and this is
all I
wanted to report in my first EMAIL.

Again, I want to apologize for the public response, but I feel that the
users of
this program need to know about this. Please do not take it personally.

Arpad Muranyi
Intel Corporation

I apologize, but I should have carried this argument one step
further .........................

To repeat:

I quote from IPC-D-317, Design Guidelines for Electronic Packaging
Utilizing High-Speed Techniques, p 22

5.5.2 Embedded Microstrip Line .... The equations for embedded
microstriplines are the same as in the section on (uncoated) microstrip,
with a modified effective permittivity..... the effective permittivity
can be determined as in sction 5.2

Section 5.2 (equation 5.17 on p 17) gives this relationship as

E'r = Er[1 - exp(-1.55H1/H) ]

if H1 becomes infinite, the exp term goes to zero and E'r becomes Er

Therefore, according to this reference, which I relied on for the
calculator, the results ARE THE SAME for microstrip and embedded microstrip
if the thickness of the coating is very thick.


To Continue -------------------------

Now, if
E'r = Er[1 - exp(-1.55H1/H)]
if H1 > H then the exp term is <1
Therefore E'r < Er (which makes sense, because it will be between
the Er of the material below and Er of air, which is 1. So, it will
be between 1 < E'r < Er

Now Zo is an inverse function of the square root of Er

So if E'r goes down, Zo will go up (not down as Arpad alleges)

I rely, for my source, on the referenced IPC manual

I believe you all will find our little calclator (AND its Help
file where all this is disclosed and referenced) a useful
addition to your tool set.

Doug Brooks
UltraCAD Design, Inc.

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Cc: Arpad Muranyi <[email protected]>
Subject: Embedded microstrip calculations, Ultracad Calculator
From: Doug Brooks <[email protected]>
To: [email protected]
Date: Thu, 11 Dec 1997 09:27:52 -0500
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