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

Fred Balistreri (fred@apsimtech.com)
Wed, 10 Dec 1997 10:13:21 -0800

Doug Brooks wrote:
>
> 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.
>
> THIS REFERENCE AND FORMULA ARE FULLY DISCLOSED IN THE HELP FILE.
>
> 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
> President
> UltraCAD Design, Inc.
> www.ultracad.com
There seems to be confusion in this discussion. The original problem
was whether Solder mask influences microstrip impedance and in what
direction. Several responses were correct. The impedance will drop
somewhat when soldermask is applied. The reasons given were correct.
The microstrip turns into an embedded microstrip. Where once the
copper was exposed to air er=1, now its exposed to surroundings,
alas solder mask. Based on measurements solder mask would seem to
have a greater dielectric constant than air. Thus the impedance goes
down not up.

Your discussion about E'r is confusing the issue. Embedded microstrip
impedance will be less than pure microstrip assuming the material the
material its being embedded in has a dielectric constant greater than
air. I would say that in this day and age thats a pretty good
assumption.

Best Regards,

-- 
Fred Balistreri
fred@apsimtech.com

http://www.apsimtech.com