[SI-LIST] Re: High-Speed GHz differential signals routed Broadside Coupled

I can't totally concur with the previous poster.  Using a well known 2D
solver
I show a variance of only about 1.5% with the shift that was mentioned.

One *attempt* to reduce this shift is to make sure that the traces are
etched on a
core.  However, your fabricator may prefer to use the trace/plane pair as
the core
for etch balance.  Contact them and find out.  If you can use the
trace/trace
as a core you can reduce the shift.

That said the following produced a 100 ohm diff line.

Plane to trace 1: 6 mils
Trace 1 to trace 2: 14 mils (yuck)
Trace 2 to plane: 6 mils
Trace 1,2 thickness: 0.7 mils each (1/2 oz Cu)
Trace 1,2 width 5.5,5.25 (trapezoidal in nature)
Er: assumed constant @ 4.0

Traces aligned: 100 ohms
Traces shifted 2.5 mils (ea): 101.34

If you think about it the trace to plane return doesn't change.  Only the
field
between traces *stretches* just a hair which is why you should NOT see
such a drastic change in impedance.  A while back I sat down with a top tier
fabricator to go over this and their results matched what I am presenting.
Their
SI guys had intentionally produced shifts in broadside pairs to see the
impedance
change (that everyone was whining about) and it just wasn't enough to worry
about.

However... I use edge coupled diff pairs in 95% of the work that I do.

I will be glad to give you the fabricator's contact info -- OFFLINE!

Bill

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Javier DeLaCruz
Sent: Thursday, January 30, 2003 11:37 AM
To: doug.hopperstad@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: High-Speed GHz differential signals routed
Broadside Coupled



Doug,
        I've run a variety of simulations on this in the past=20
but a significant hurdle presented itself.  While broadside=20
differential coupling appears to improve routability in=20
substrates/PCBs, it is sensitive to layer alignment.  Layer-
to-layer registration on substrates and PCBs are too loose=20
to be able to get a repeatable differential impedance. =20
        I had, in the past, run 2D simulations of these=20
broadside coupled traces, and the space between the traces=20
was about double the space to ground respectively.  In the=20
following diagram, the layers comprised of "G"'s are the=20
reference layer (ground usually) and the "S"'s are the=20
differential signals.  The space between the "S" layers would=20
need to be a much thicker core layer.  You would not be=20
able to use double prepreg nor a core+prepreg since the=20
thicknesses can vary easily.  This issue can be overcome as=20
described, but the alignment issue remains.

GGGGGGGGGGGGGGGGGGGGGGGGGGGGG
          SSSSSSS

          SSSSSSS
GGGGGGGGGGGGGGGGGGGGGGGGGGGGG

        When the same simulations are run at the extremes of=20
the layer alignment windows, which can be as much as +/- 5mils on=20
PCBs, you will see that you cannot maintain a repeatable=20
impedance.  This is illustrated below.  This is why, I=20
assume, that this technique is not commonly used in the=20
industry.  If a technology emerges for substrates or PCBs=20
that have much tighter tolerances on these layer=20
registrations, and variable layer thicknesses, then the=20
practice may re-emerge. =20

GGGGGGGGGGGGGGGGGGGGGGGGGGGGG
            SSSSSSS

        SSSSSSS
GGGGGGGGGGGGGGGGGGGGGGGGGGGGG

Thanks,
Javier DeLaCruz

-----Original Message-----
From: Doug Hopperstad [mailto:doug.hopperstad@xxxxxxxxxx]
Sent: Wednesday, January 29, 2003 9:58 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: High-Speed GHz differential signals routed
Broadside Coupled



I am looking for feedback if anyone has tried or is using Broadside =3D
coupling for 100 ohm differential GHz signals? And if so, what trace =3D
width and lengths are being used?=3D20

Thanks for your time
D.M. Hopperstad
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