RE: [SI-LIST] : Broadside Coupled Traces

Silbert, Steven F. (Steve.Silbert@viasystems.com)
Thu, 22 Apr 1999 12:25:28 -0400

I believe that I can address the PCB fabrication issues.

Edge-Coupled Stripline:

This differential impedance model consists of two traces of equal width
centered between two planes. It represents the most controllable
differential impedance model. The symmetry of the edge-coupled stripline in
all three axes results in maximum predictability. Considering that it is
shielded on both sides by planes, it is the most noise immune differential
model. Of importance to note with edge coupled differential pairs:

1. The distance between the traces is most easily controlled during
fabrication as the result of having all of the critical copper features
(trace and space) located on one side of a layer.
2. It is important to note that the circuits should be centered between
the two planes.

Broadside-Coupled Stripline:

This model has some benefits and liabilities. The pair is created by having
identical routing paths
for the two lines and placing them on adjacent layers. It is mechanically
similar to the dual stripline
model for characteristic (single-ended) impedance and makes constructive use
of the interplay between
overlapping circuits. This model is theoretically predictable but has poor
controllability in a
fabrication environment. The dielectric thickness variation causes signals
to have non-identical reference plane locations. Of importance to note with
broadside coupled differential pairs:

1. The dielectric between the two signals becomes critical. Natural
variation of this dielectric causes a large amount of variation in the
differential impedance along the entire trace and in localized sections of
the trace.
2. Layer to layer shift (variation in registration) causes variation in
the differential impedance because of a change in the overlap between the
circuits. This shift can be up to 2 mils if the broadside pairs are on the
same core. This shift can be up to 3 mils if the broadside pairs are on
different cores.
3. Also on the subject of overlap, the natural variation in the trace
widths on the two different layers of the broadside pair will cause a
reduction in the overlap, causing high variation in the resulting impedance.

If you must use broadside coupled pairs

1. Use WIDE lines (certainly greater than 10 mils) to cut down on the
impedance variation caused by registration and etch process variation.
2. Make sure that the reference planes are on layers adjacent to the
broadside pair traces.
3. Make sure that the two layers of the pair are on the same core.
This will help to reduce impedance variation caused by registration.
4. Try to require as wide an impedance tolerance as possible.

Please feel free to contact me with questions or concerns. Thanks.

Steve Silbert
Product Engineer
Viasystems Technologies
Phone: 804.226.5519
Steve.Silbert@viasystems.com

-----Original Message-----
From: mannand@wlgore.com [SMTP:mannand@wlgore.com]
Sent: Thursday, April 22, 1999 9:40 AM
To: 'si-list@silab.eng.sun.com'
Subject: [SI-LIST] : Broadside Coupled Traces

Can anyone outline the advantages and disadvantages of using
broadside coupled
vs. edge coupled differential traces? Is either one better from a
signal
integrity perspective ( less lossy? lower crosstalk?). Is it
easier to route
broadside coupled traces in high density applications? And what are
the issues
board manufactures need to deal with such as tolerances, trace
registration,
impedance control, number of layers, etc.? Any insight you can
provide would be
helpful. Thanks!

**** To unsubscribe from si-list: send e-mail to
majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
si-list, for more help, put HELP. si-list archives are accessible at
http://www.qsl.net/wb6tpu/si-list ****

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****