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

Weston Beal (beal@galois.eng.hou.compaq.com)
Wed, 10 Dec 97 13:30 CST

...

] 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

I agree with the arguement above, but 'H' was not defined. It looks
like it is probably the height of the conductor. If this is true,
this equation does not take in to account the dielectric which is the
solder mask. That is the issue. An equation for our embedded
microstrip would need to include the height and dielectric constant of
the second layer (solder mask), which the above equation does not.
This is where Fred said that we are confusing the issues.

Regards,

-- 
Weston Beal                     Signal Integrity Engineer    
UNIX:beal@twisto.compaq.com     PC:weston.beal@compaq.com
"A little bit of knowledge is a dangerous thing"