Impact
on Radiated
Emissions
of Printed
Dr. Gary Hauss-mann
Silicon Graphics Inc.
2011 N. Shoreline Blvd., M/S 946
Mountain View, CA 94094
Board Stitching
Marty Matthews, Franz Gisin
Silicon Graphics Inc.
2011 N. Shoreline Blvd., M/S 946
* Mountain View, CA 94094
Abstract The use of closely-spaced vias, or “stitching,” is
used to reduce radiated emissions from the edge of a printed
circuit board (PCB). However, this stitching, by confining the
electromagnetic noise to the PCB internals instead of allowing
it to radiate, causes undesirable effects by coupling to internal
traces and vias. Internal traces connected to outside traces and
components provide a second path For radiation, in addition to
the edge radiation that stitching is intended to suppress.
Introduction
In multilayer printed circuit boards a significant amount of
radiation occurs around the edges of the board. A common
technique to reduce the amount of radiation along the edge is
to “stitch” together the ground planes within the board using
closely-spaced vias along the outer edges of the board.
Stitching along all edges of the PCB, connected to the ground
layers, forms a faraday-cage like structure out of the board.
This structure is intended to affect electromagnetic noise that
is propagating in the channel between board layers, by acting
like a short at the end of the radial transmission lines formed
by the ground planes. At the same time it confines this
channel energy, the stitching is not intended to affect signal
energy on internal board traces and vias. (see Figure 1).
Figure 1: Ground Plane Stitching Along Bottom Edge of a PCB
This paper shows the secondary effects that PCB stitching
confining
layer-channel
addition
to
produces,
in
electromagnetic noise. The net reduction in emissions is
dependent on a number of factors, including: the size and
shape of the printed circuit board, the number of tracesand
circuits that exist above the ground planes, and the frequencies
at which the circuits operate. The internal reflection produced
by the PCB stitching transforms the enclosed layers into a
resonant chamber, resulting in board resonance highly
dependent on the board geometry and internal trace layout.
0-7803-5057-X/99/$10.00 © 1999 IEEE
Circuit
This resonance, as well as simple reflections from the stitching
line, can couple to internal board traces, increasing the overall
signal crosstalk. Noise energy coupled to these internal traces
can also travel to and radiate from an external board trace.
Internal
Board Reflection and Resonance
Referring to Figure 2, an FDTD simulation of a gaussian pulse
shaped source located at the center of an 82 mm by 82 mm
multi-layer board will propagate outward from the source
between two ground layers. [S] Simulations are performed
both with and without stitching around the board periphery,
allowing for comparison of board edge radiation. Simulations
are also performed with and without a trace running from the
board internals to an external signal layer, to examine the
effects of stitching on crosstalk and trace radiation. Noise
energy is injected via a gaussian voltage source at the PCB
center.
A visual display-a density plot-of the internal electric fields
propagating within the board, for a board with and without
edge stitching, is shown in figure 3. When- it reaches the edge
of the PCB, a portion of the energy will radiate outward. The
density plots show a marked reduction in edge radiated fields
for the board that contains stitching; when a fine stitching pitch
is used, almost all of the energy is reflected back into the
board. The same plots demonstrate an increase on the
reflected and propagating fields that remain within the PCB
layers, setting up a sharp resonating pattern in this example
because the sample board is a perfect square.
With internal vias and traces, as well as a non-square board
shape, the resonant frequencies will not be as distinct. By
running additional simulation with a trace between the layers,
coupling to an outside layer (shown in figure 2), we can
examine the board resonances as seen from a trace. Figure 4
shows voltages found on this trace in frequency domain,
comparing the sample board both with and without edge
stitching. Referring to this figure, one can see that withstitching, the induced voltage is not only higher, but also
“rings” much longer, indicating that the resonances in the
printed circuit board continue to couple energy into the trace
long after the initial signal pulse is finished. Figure 5 shows
the FFT of the time domain pulses. As expected, the
resonances -of the voltages on the trace are approximately 6 10 dB higher than without the stitching.
793
This reflected energy picked up by internal traces increases the
overall crosstalk coupling inside the board. This increase in
crosstalk degrades the signal integrity of critical clock and data
signals, especially at board resonance frequencies.
Effect on External Trace Radiation
Since the traces that pick up the reflected signals also connect
to integrated circuits that are located above the ground planes,
the RF energy that normally would have left the board along
the edge now leaks out via’the integrated circuits. As a result
of this leakage, the net emission reduction from the stitching is
not as great as it first appears. While many references exist for
reducing EM1 using guard traces as a means of EM1
containment from individual traces, little research has been
done in the effectiveness of stitching around the periphery of a
PCB as an alternate containment method for all traces. [l] - [4]
If a trace is added to this structure, the radiation properties of
the PCB change dramatically. A portion of the trace is routed
between the two planes, and a portion is routed above the
With this
planes (for example on an external layer).
configuration, the energy reflected back by stitching will
couple onto the trace, propagate down the trace to the outside
layer, where it will re-radiate. This energy will dominate at
those frequencies where the printed circuit boarfj is resonant.
Figures 2 shows the trace structures that were modeled using
LC FDTD.
The simulations done for this paper examine the PCB radiation
by placing small electric field probes suspended around the
main PCB board. These probe locations are shown in figure 2
as small grey rectangles suspended in various positions around
the PCB. The field magnitudes for a stitched and non-stitched
board are shown in figures 5 and 6. The magnitude for various
positions shown around the PCB indicate that, while the edge
radiation has dropped dramatically, emissions from the top of
the board due to the external trace are barely effected by the
presence of stitching on the PCB periphery. Comparison of
the radiated field strengths seen above and below the bottom
of the board clearly delineate the external trace on top of the
board as a significant cause of board emissions in this
example.
Summary
This paper examines the effects of periphery stitching on PCB
radiation properties. Numerous FDTD full-wave simulations
were performed of a PCB model, consisting of two parallel1
plates connected with or without periphery stitching, and with
or without a trace. The trace runs from the board interior to a
termination on the exterior of the board.
Simulation results examine the noise propagating between
board layers. Fine stitching will almost completely reflect this
noise back to the board, providing one method of mitigating
board emissions. This reflected noise easily couples to an
internal trace, and the signal on this trace shows the increase
in PCB resonance caused by adding stitching.
Simulations examining the direction and severity of board
emissions demonstrate that stitching can dramatically reduce
side emissions from the PCB edge. At the same time, interior
board noise can also travel to and radiate from an exterior
trace, shown by comparing side, top, and bottom emissions
from a PCB with and without stitching.
These simulation results suggest that the effectiveness of
stitching can be compromised by leakage through signal
traces, and can exacerbate signal crosstalk by reflecting noise
back to the board signal traces. Other methods, including
coarser stitching and localized return vias/guard traces, may be
just as effective as fine stitching in reducing overall board
emissions.
~~
*
References
[I]
Dheena Moongilan, Bell Laboratories, “Grounding
Optimization Techniques for Controlling Radiation and
Crosstalk in Mixed Signal PCBs”, 1998 IEEE
International Symposium on Electromagnetic
Compatibility.
[2] D. Scott Britt, David Hockanson, Fei Sha, James L.
Drewniak, Todd H. Hubing, and Thomas P. Van Doren,
“Effects of Gapped Ground Planes and Guard Traces on
Radiated EMI”, 1997 IEEE International Symposium on
Electromagnetic Compatibility.
[3] M. Feliziani, E. Latini, S. Liotta, F. Maradei, “Layout
Optimization in Nonuniform Transmission Line
Configurations to Reduce Radiated Emissions and
Crosstalk”, 1995 IEEE International Symposium on
Electromagnetic Compatibility.
[4] S. Van den Berghe, F. Olyslager, D. De Zutter, J. De
Moerloose, W. Temmerman, “Power Plane Resonances as
a Source of Delta-I Noise and Influence of Decoupling
Capacitors,” IEEE Electromagnetic Compatibility
Symposium Proceedings, pp. 145-148, Austin, TX, 1997.
[5] “LC FDTD Home Page”, http://lc.cray.com.
794
-
1005
Figure 2: Cross Section of FDTD .\lodel -Top:
100
125
Open Edges. Hottom: Stitched Edges
150
175
Simulation Time Step
200
Figure 3: Effects of Stitching Along the Edges of a Printed Circuit Board
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Figure 4: Frequency Domain Results of Voltage on Trace, Light Grey - with Stitching,
795
Black - Without
Stitching
I
I
probe under board probe above trace -+probe at board edge -E-
0
5e+08
le+09
1.5e+09
Ze+09
2.5e+09
Frequency
Figure
5: Frequency
Domain
Results of Electric
Field Magnitude
3e+09
3.5e+09
4e+09
4.5e+09
5e+(
(Hz)
in Space Near Board-With
Trace, Without
Stitching
probe’under
boakd --+probe above trace -t-probe at board edge -
40
20
B
z
5
73
0 0
iz
0
5
:
iLi
-20
-40
0
5e+08
le+09
1.5e+09
2e+09
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Frequency
Figure
6: Frequency
Domain
Results of Electric
Field Magnitude
796
3e+09
3.5e+09
4e+09
4.5e+09
5e+(
(Hz)
in Space Near Board-With
Trace, With Stitching