[SI-LIST] Re: Impact of gap on stripline trace
- From: bdempsey85@xxxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 24 Sep 2003 23:49:48 +0000
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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