[SI-LIST] : Embedded microstrip calculations, Ultracad Calculator

Doug Brooks (doug@eskimo.com)
Thu, 11 Dec 1997 09:10:40 -0500

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.


I am happy to make this defense of my calculator, but our e-mail
addresses is clearly available at the same place the calculator is,
and I guess I would appreciate a private criticism before a public one!
Arpad did (apparently) call, but the message was, unfortunately,

Doug Brooks
UltraCAD Design, Inc

At 08:22 PM 12/9/97 PST, you wrote:
>Text item:
>I just tried UltraCAD's program (out of curiosity) to see what results it
>give me, becuase knew it already that covered traces have lower impedances.
>I was shocked to find out that according to UltraCAD the embedded microstrip
>line came out with a higher impedance than the uncovered one, in which the
>conductor is surrounded by air.
>Knowing that this is incorrect, I started to play with the numbers (solving
>impedance) and found out that the two configurations will give identical
>if the hight of the dielectric above the conductor is very large (or infinite)
>for the embedded case. From this, I concluded that the equations for the
>microstrip line with air above the conductor must be incorrect, and most
>are the equations which HSPICE calls "sea of dielectric" (DLEV=0).
However, due
>to lack of time I didn't compare the numerical results of UltraCAD and
>verify this conclusion.
>Instead, I called UltraCAD to find out what is wrong, but all I could do is
>leave a message to which I didn't get a response yet.
>Arpad Muranyi
>Intel Corporation
>UltraCAD also has a useful freeware calculator. Find it at
>and follow the links to the calculators.
>The formulas and their sources are included in the help
>I suspect that the problem is four-fold:
>1. As has been pointed out, there is a slight embedded microstrip
> effect here (see the calculator for this effect.)
>2. Er is a function of frequency, so at the frequency of interest,
> Er might be mischaracterized
>3. Er might also be mischaracterized simply because it often is not
> exactly what you expect it to be or what it is spec'd at
>4. Manufacturing processes cause a variation that is hard to control.
> We have found variations as much as 3 or 4 ohms ALONG A GIVEN TRACE
> and especially between boards in the same production run. I have found
> that the practical accuracy for impedance vs spec is about 10%.
>See the results reported in the article "The Effects of Vias on PCB
> Traces" PCB Design Magazine, 8/96 for some real world, controlled examples
> of how much variation there can be.
>At 02:35 PM 12/9/97 -0000, you wrote:
>>Polar Instruments, UK, have a useful little calculator program (public
>>domain) that allows you to check this effect. Their web site is
>> http://www.polar.co.uk/
>>I've put John's figures into this, and it shows about a 5 ohm drop due
>>to the mask.
>>The calculator also confirms Kenneth Willis' comments about overplating
>>being responsible for some of the impedance drop.
>>Perhaps someone from Polar would like to comment, especially about the
>>source for their equations?
>Text item: External Message Header
>The following mail header is for administrative use
>and may be ignored unless there are problems.
>Precedence: bulk
>Sender: owner-si-list@silab.Eng.Sun.COM
>Subject: RE: [SI-LIST] : Does solder mask reduce trace impedance ?
>From: Doug Brooks <doug@eskimo.com>
>To: si-list@silab.Eng.Sun.COM
>Date: Tue, 09 Dec 1997 10:12:34 -0500
>Content-Type: text/plain; charset="us-ascii"
>Mime-Version: 1.0
>X-Mailer: Windows Eudora Light Version 1.5.2
>X-Sender: doug@mail.eskimo.com (Unverified)
>Message-Id: <199712091810.KAA16354@mail.eskimo.com>
>Received: from doug.ndip.eskimo.net (doug.ndip.eskimo.net [])
> by mail.eskimo.com (8.8.8/8.8.8) with SMTP id KAA16354
> for <si-list@silab.Eng.Sun.COM>; Tue, 9 Dec 1997 10:10:05 -0800 (PST)
>Received: from mail.eskimo.com (mail.eskimo.com [])
> by saturn.sun.com (8.8.8/8.8.8) with ESMTP id KAA17722
> for <si-list@silab.Eng.Sun.COM>; Tue, 9 Dec 1997 10:10:22 -0800 (PST)
>Received: from saturn.sun.com (saturn.EBay.Sun.COM [])
> by Eng.Sun.COM (SMI-8.6/SMI-5.3) with SMTP id KAA01827
> for <si-list@silab.Eng.Sun.COM>; Tue, 9 Dec 1997 10:10:36 -0800
>Received: from Eng.Sun.COM by silab.eng.sun.com (SMI-8.6/SMI-SVR4)
> id KAA16608; Tue, 9 Dec 1997 10:11:58 -0800
>Errors-To: si-list-approval@silab.Eng.Sun.COM
>Received: by silab.eng.sun.com (SMI-8.6/SMI-SVR4)
> id KAA16612; Tue, 9 Dec 1997 10:12:06 -0800
>Received: from silab.eng.sun.com (silab.Eng.Sun.COM [])
> by Eng.Sun.COM (SMI-8.6/SMI-5.3) with SMTP id KAA25020;
> Tue, 9 Dec 1997 10:26:09 -0800
>Received: from Eng.Sun.COM ([]) by mercury.Sun.COM
>ddr) with SMTP id KAA16761; Tue, 9 Dec 1997 10:26:18 -0800
>Received: from mercury.Sun.COM (mercury.Sun.COM [])
> by thalia.fm.intel.com (8.8.6/8.8.5) with SMTP id KAA04388
> for <Arpad_Muranyi@ccm.fm.intel.com>; Tue, 9 Dec 1997 10:32:55 -0800 (PST)
>Received: from thalia.fm.intel.com (thalia.fm.intel.com [])
by fmm
>ail.fm.intel.com (8.8.5/8.7.3) with ESMTP id KAA11746 for
>intel.com>; Tue, 9 Dec 1997 10:32:05 -0800 (PST)
>Return-Path: owner-si-list@silab.Eng.Sun.COM