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

Doug Brooks ([email protected])
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.

THIS REFERENCE AND FORMULA ARE FULLY DISCLOSED IN THE HELP FILE.

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,
unintelligible.

Doug Brooks
President

At 08:22 PM 12/9/97 PST, you wrote:
>
>Text item:
>
>CAUTION!!!
>
>I just tried UltraCAD's program (out of curiosity) to see what results it
would
>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
for
>impedance) and found out that the two configurations will give identical
resuts
>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
likely
>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
HSPICE to
>verify this conclusion.
>
>Instead, I called UltraCAD to find out what is wrong, but all I could do is
>
>Intel Corporation
>===========================================================================
=====
>
>UltraCAD also has a useful freeware calculator. Find it at
>The formulas and their sources are included in the help
>file.
>
>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
>>being responsible for some of the impedance drop.
>>
>>Perhaps someone from Polar would like to comment, especially about the
>>source for their equations?
>
>
>and may be ignored unless there are problems.
>
>***IF THERE ARE PROBLEMS SAVE THESE HEADERS***.
>
>Precedence: bulk
>Sender: [email protected]
>Subject: RE: [SI-LIST] : Does solder mask reduce trace impedance ?
>From: Doug Brooks <[email protected]>
>To: [email protected]
>Date: Tue, 09 Dec 1997 10:12:34 -0500
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