Impact on Radiated Emissions of Printed Circuit Board Stitching

Dr. Gary Hauss-mann 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 produces, in addition to confining layer-channel 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.

Marty Matthews, Franz Gisin Silicon Graphics Inc.

2011 N. Shoreline Blvd., M/S 946 * Mountain View, CA 94094

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 with- stitching, 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.

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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 planes (for example on an external layer). With this 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.

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Figure 2: Cross Section of FDTD .\lodel -Top: Open Edges. Hottom: Stitched Edges

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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, Black - Without Stitching

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Figure 5: Frequency Domain Results of Electric Field Magnitude in Space Near Board-With Trace, Without Stitching

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Figure 6: Frequency Domain Results of Electric Field Magnitude in Space Near Board-With Trace, With Stitching

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