[SI-LIST] Re: Impact of gap on stripline trace

All,

Be sure to check the articles written by Doug Smith (a regular contributor to 
this list).  His articles on this subject are excellent reading.

A link to one in the series on the effects of splits can be read here:
http://emcesd.com/tt2002/tt120102.htm

Regards,
Bill
> Several years ago I built a test board similar to Frank's example.  There
> was a line on one side and a 0.100" gap between two different ground planes
> on the other.  I built it so that I could place shorts between the two
> ground planes at various distances from the signal trace.  With no short
> there was a large inductive spike as the signal passed over the gap as
> viewed on a TDR.  As I moved a short or cap closer and closer to the trace
> the inductive spike both lowered in amplitude and shortened in time.
> Finally, when the short was right below the trace there was a minimum
> discontinuity on the TDR trace from the gap.  If I had widened the short the
> discontinuity likely would have disappeared.
> Lesson learned, as long as you have a short return path for the ground
> currents you should not have significant impact on your signal.  If the
> return path is long then you will see an significant impact.  If the planes
> couple to each other the amount of coupling between the two planes will
> dictate the discontinuity seen by the signal.
> 
> Tom Dagostino
> Teraspeed Consulting Group LLC     Teraspeed Consulting Group LLC
> 2926 SE Yamhill St.                Device Modeling Division
> Portland, OR 97214                 13610 SW Harness Lane
>                                    Beaverton, OR 97008
> http://www.teraspeed.com           503-430-1065 
> tom@xxxxxxxxxxxxx 
> 
> 
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Loyer, Jeff
> Sent: Wednesday, September 24, 2003 3:01 PM
> To: fdunlap@xxxxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Cc: Loyer, Jeff
> Subject: [SI-LIST] Re: Impact of gap on stripline trace
> 
> 
> Hi Frank,
> My answer to your questions is conjecture based on my understanding of =
> the physics, and on some experimentation done for a scenario not exactly =
> like yours, but with some similarities.
> 
> What is the impact of the gap?
> I think there are at least 3 issues here:
> 
> 1) The path the return current must take as it transitions from:
>    a) 2 ground planes to=20
>    b) 1 ground plane plus 1 power plane to
>    c) 2 ground planes again
> I believe the inter-plane capacitance between the ground and power =
> planes will play the most significant role in determining the impact to =
> Signal Integrity and EMI.  If the spacing between the plane below the =
> trace and the one above the trace is small enough that there is a lot of =
> inter-plane capacitance, AND the gap is small, I would expect negligible =
> impact.  The return current will find an adequate path between the =
> different reference planes through the plane capacitance.  I would =
> expect this to be the case for typical stripline topologies, with =
> 15-20mils separating the planes.
> If the spacing between the top and bottom planes is large and the plane =
> capacitances are small, the return current will find a substantial =
> discontinuity at the gap, causing signal integrity and EMI problems.
> It's also possible that the entire length the trace spends traversing =
> under the power plane is insignificant, relative to the risetime (though =
> it doesn't sound like this is the case in your scenario).
> I've pasted a synopsis of previous work concerning an issue similar to =
> this (return path with various reference planes) below, under the solid =
> line.
> I don't know how effective capacitors between the planes ("stitching" =
> caps) would be.  For short risetimes, the effects of caps is diminished =
> by their associated parasitics.  I would rather rely on inter-plane =
> capacitance.
> 
> 2) The discontinuity of the gap itself.
> I would typically assume this is insignificant, since the gap will =
> probably be about 10mils, and that's only significant (using risetime/6 =
> rule-of-thumb and 160pS/in Tp) for a risetime of less than ~10ps.
> 
> 3) Another exotic effect - the gap becomes a waveguide
> I have heard of traces passing over a gap having substantial amounts of =
> energy propagating along the gap, causing much more significant =
> crosstalk between adjacent traces than would be present without the gap. =
>  I believe the information I saw was for microstrip; I don't know how =
> much difference your scenario would be.  I don't have any details I can =
> share about this - perhaps someone else has seen publications describing =
> the effect?
> 
> ________________________________
> 
> I found that, when TDR'ing a stripline trace that was referenced to both =
> power and ground, I got the same impedance whether decoupling caps were =
> populated or not. Actually, instead of a cap, I physically shorted power =
> and ground pins together at the launch point to keep even the parasitics =
> of a capacitor out of the equation.  What I found was that, for the =
> stackup (5mil
> trace 7 mils above ground and 7 mils below Vcc), I saw no substantial =
> difference, regardless of whether I measured:
> (1) with the probe referenced to GND,
> (2) referenced to VCC, and=20

> (3) with GND and VCC shorted together (at the launch).  =20
> 
> Also, TDR'ing between the two planes shows a dead short.
> 
> The risetime was ~50pS (a TEK TDR), and I even slowed the risetime down =
> to 400pS, no change.  I'm pretty sure rise-time is not a factor.
> =20
> FURTHER INVESTIGATION: I wondered if, by definition of this symmetrical =
> stripline, there isn't enough capacitance between the planes that the =
> return current has a low impedance path to the reference plane.  I.E., =
> TDR'ing between the 2 planes shows a dead short - no need for external =
> caps (or a shorting bar, in my case).
> 
> This worked until I thought of the case of asymmetrical stripline - =
> would the impedance measured depend on which plane you were referenced =
> to?  So, I built myself some crude asymmetric stripline (using a TDR =
> characterization board from TEK as a starting point).
> 
> I took a microstrip trace (20 mils above ground plane) and added a layer =
> of Kapton tape (2.5mils thick) over it, with a
> sheet of copper over that.  This turned the microstrip into a stripline, =
> with the 2nd plane floating.  I TDR'ed the trace relative to Gnd, then =
> relative to the floating plane, and with the planes shorted together at =
> the source (again, relative to Gnd and the floating plane).
> 
> I then added another layer of Kapton tape between the trace and the =
> floating plane, and repeated the measurements.
> 
> I did this until I had 8 layers of Kapton tape between the trace and the =
> floating plane.
> 
> Granted, this was a pretty crude experiment and there were clearly some =
> measurement errors, but some things were pretty obvious.
> 
> Findings:
> 1) Regardless of the Kapton thickness, the lower impedance measured =
> (referenced to Gnd or the floating plane) was approximately the same as =
> that as when the planes were shorted together.
> 
> 2) With thin dielectrics (in the range that we typically use, < 7mils), =
> the impedance was approximately the same regardless of which plane was =
> used as a reference, and whether they were shorted together at the =
> source.
> 
> Conclusions:
> 1) When TDR'ing stripline, it probably won't matter which plane we use =
> as reference.  If in doubt, I would TDR relative to whichever plane was =
> closest to the trace.  If still not convinced, I would short the 2 =
> planes together at the source.
> 
> 2) I would ensure that, when using stripline with both power and ground =
> planes, the trace is closer to ground than power.  This is assuming the =
> signal is routed relative to ground elsewhere.
> 
> 3) I believe that a correct model for what I'm seeing is - it's the =
> parallel combination of Trace-to-Plane1, Trace-to-Plane1, and =
> Plane-to-Plane impedances that makes up the final impedance for a trace, =
> relative to either Plane1 or Plane2.
> 
> -----Original Message-----
> From: Frank Dunlap [mailto:fdunlap@xxxxxxxxxxxxxxxxx]
> Sent: Friday, September 19, 2003 6:46 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Impact of gap on stripline trace
> 
> 
> Consider a stripline signal trace that passes over a gap between a GND
> plane and an I/O PWR plane. The stripline is covered above by a GND
> plane.
> =20
> What is the impact of the gap?  Is it totally unacceptable for the trace
> to cross this gap (there is a continuous GND plane on the other side of
> the signal trace), or are there "speeds (edge rates)" for which the gap
> may be okay?  If there are some "speeds" for which it is okay, how does
> one determine those acceptable speeds?
> =20
> Does scale matter? In other words, if the gap is not acceptable for
> feature sizes common in a PCB, might the gap be acceptable at the scale
> of feature sizes common inside high-speed IC packages?
> =20
> Regards,
> =20
> Frank
> =20
> -----------|  |-----------
>            |  |
>            |  |
>            |  |
>  GND       |  |  I/O PWR
>            |  |
> ------------------------------
>           SIGNAL TRACE
> ------------------------------
>            |  |
>            |  |
>            |  |
>            |  |
>            |  |
> -----------|  |-----------
> =20
> =20
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