Technical Tidbit - May 2003, Signal Paths and Ground/Power Planes in a PWB

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Abstract: In 4 layer printed wiring boards, practical layouts require paths to cross from the top signal layer to the bottom signal layer. Pathsthat exist on both the top and bottom signal layers affect the emissionsand immunity characteristics of a circuit board. Results from a test of radiatedEMI are presented and discussed.

Figure 2 shows the voltage induced in the path that penetrated the copperplanes. A peak voltage of over 200 mV was recorded along with a ringing frequencybetween 400 and 500 MHz. The amplitude of the induced noise is a substantialfraction of the noise margin for some of today's fast logic. Potentially such a waveform could disrupt sensitive circuits..

Figure 3 shows the induced voltage in the path that wasrouted over a solid copper plane. The amplitude reached a peak of about 100mV with very little ringing. Such a noise would have a lower risk of signalcorruption compared to that in Figure 2.

Figure 3. Noise Induced into Path Routed Over a Solid Plane

One way to think of the increased EMI in Figure 2 isthat the inter-plane impedance between the copper planes can develop a voltagein response to the radiated EMI. This voltage is in series with the returnpath of the signal line as the line crosses from being over the bottom planeto the top plane. If the path stays over a solid plane, there is no opportunity for this to happen. For board stack-ups with closer power-ground plane spacing, the effect would be proportionally smaller, yielding lower observed EMI amplitudesthan shown here.

Another similar experiment is described in the paper "ESD Immunity in System Designs, System FieldExperiences and Effects of PWB Layout "(~950K),where an IEC 61000-4-2 ESD generator is discharged to the same board at 3kV (about the human level of threshold of feeling an ESD event). The paththat dives through the planes receives over 2 Volts of interference whereasthe path that stays over a single plane receives only about 300 mV of interference.This is a greater difference than noted above, possibly due to the increasedenergy at lower frequencies in the hand metal ESD waveform of the IEC generatorcompared to the radiated field from small metal ESD. The lower frequencyenergy would more closely match resonances in this board. Since currentswere injected directly on the board in the paper, different coupling mechanismsare possible as well..

As one might expect, emissions from the two paths are different as well.Data presented in my seminars show that the diving path has significantlyhigher emissions, as much as 30 dB, than the straight path at several frequencies.

A similar experiment to this one, for paths crossing a break in a groundplane, was described in the February 2003, Technical Tidbiton this site. The effect of crossing a break in a ground plane is strongerthan passing through planes as evidenced in the data in that article.

Conclusion: The data shows that passing through copper planeson a circuit board can lead to lowered immunity to radiated and conducted EMI. In general,any board feature that disrupts the signal return path in a plane, will causelowered immunity to external EMI.

Shorting the edges of the planes together (directly or through bypasscapacitors) and other techniques are discussed, and shown as experiments, inmy seminars. These techniques can reduce the amount of EMI induced in boardfeatures such as diving through multiple ground or power planes.