From: S. Weir ([email protected])
Date: Mon Jan 22 2001 - 00:45:56 PST
Chris,
Both Ron and you make excellent points about the general problems of
broadside diff. pairs. There are some others as well, including thickness,
and moisture content variations. If you look back about 20 months or so in
the archives, I believe you will find Scott pointed pretty much all of
those problems in addition to what you and Ron have mentioned.
Regards,
Steve.
At 08:01 PM 1/21/01 -0500, Heard, Chris wrote:
>"SI list (E-mail)" <[email protected]>
Keeping Broadside pairs on Cores only is not practical in dense designs. A
20 layer construction only has 3 transmission line constructions available
for Broadside routing. Broadside 50 ohm constructions always end up thicker
than edge-coupled, which drives thickness up and total available layer count
in a given mechanical design down. Increased thickness makes manufacturing
engineers angry and less layers makes pcb layout folks angry.
If you don't have too many diff pairs, either way works. If your design has
a large quantity of diff pairs, broadside will make you crazy.
Broadside on backplanes is a bad choice for similar reasons. Cores aren't
available at every construction, increased thickness drives up plated
through hole capacitance which dominates connector signal integrity. Wide
lines with low skin effect are difficult to use because achieving 100 ohms
differentially requires thicker construction...and on it goes.
Chris
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