PCB layout engineers have a difficult job.  Long gone are the days of simply worrying about electrical connectivity.  Now they must also worry about the energy stored in the electromagnetic fields that permeate and surround their designs.  It’s worth taking a moment to remember some college physics before jumping into your next design.

Electromagnetic Fields and Charges

Charges interact with the electromagnetic fields that surround them.  Changes in the charge configuration cause the electromagnetic field to rearrange itself. The changes in the electromagnetic field radiate outwards at the speed of causality (often called the speed of light). There is always some delay in field rearrangement based on the distance from the charge and the universe’s fundamental laws.  But undergraduate physics coursework usually focuses on steady-state conditions rather than the transients, which is a pity since the transformation is the cause of some fascinating phenomena.

You might recall that physicists represent the electromagnetic fields that surround conductors with field lines. Electric field lines from isolated charges radiate outwards equally in all directions like the spokes on a bicycle wheel, while magnetic field lines encircle the charge like the rubber tire on the rim.  Electric field lines radiate to and from charges; magnetic field lines encircle the direction of charge motion.

This image shows the electric field lines (blue lines) and the direction (colored arrows) a test charge would experience a force if placed at arbitrary locations for a theoretical positive and negative charge. 

It is important to remember that electric field lines do not exist; they are just visual representations that indicate the direction a test charge will travel if placed at arbitrarily chosen points in space.  Individual lines are unimportant, but the line-to-line spacing is important since closely spaced lines indicate a high potential energy gradient.

Electromagnetic Interactions

Charges interact with electromagnetic fields.  Not only does changing charge configurations affect the nearby electromagnetic field but changing electromagnetic fields affect charges.  Electric potential differences along the length of a conductor give rise to electromagnetic field disturbances that create displacement currents in surrounding conductors.

When charges travel at a constant speed, their surrounding electromagnetic field lines are uniform at a given distance.  When a charge accelerates, perturbations in the field lines travel outwards from the charge at the speed of causality in that medium.  For most situations electronics engineers will encounter, changes propagate through the conductor and nearby space at anything from 50-100% of c (≅3⨉10⁸ m/s).

During acceleration, the electric field lines that lie primarily antiparallel to the direction of motion don’t significantly change.  But slightly off-axis from the direction of motion, the field lines begin to warp.  The field lines compress in some areas and expand in others.  The artistic renderings below visually demonstrate that the electric field is no longer homogenous at specific distances around the charge.

This artistic (not mathematically rigorous) image demonstrates a charge accelerating to travel near the speed of causality in the surrounding medium and then stops.  Field lines should never overlap, but they can compress into small areas near each other.  Where the lines are very close together, there is an area of high electromagnetic field energy corresponding to a wave crest.

When charges accelerate quickly, as happens during a fast transition from logic-low to logic-high, the electric field compresses in the direction of acceleration, and a wave of electromagnetic energy emanates outwards from the charge.  As the charges accelerate back and forth, as in an alternating current or a series of logic transitions, the crests radiate outwards in alternating directions as well.

This artistic (not mathematically rigorous) animation of a point charge sinusoidally accelerating along a line shows the accumulation of field lines in the direction of movement.

The electric field animations above only illustrate a single aspect of the radiating electromagnetic fields.  The magnetic portion of the electromagnetic field is also present, but it is difficult to illustrate both clearly.  Below is an animation of the cross-sectional magnetic fields that might surround a physical dipole antenna.

The animation above shows a cross-section of magnetic field iso-lines as they radiate outwards from a sinusoidally oscillating current in a collection of Hertzian dipoles that make up the vertical center conductor (black rectangle).  When the charges change direction, the isolines do not cross; they break and reconnect with lines from the other end of the conductor. 

Applications to PCB Design

The electromagnetic field has the greatest intensity in the area immediately next to the radiating conductor.  If you can place another conductor in close physical proximity to the radiating conductor, it is possible to absorb a great deal of the radiating energy and keep it from becoming unwanted electromagnetic interference.  In a printed circuit board design, the best way to absorb the energy is to run a parallel conductor on an adjacent layer or immediately next to the radiating element.  That includes both in-layer and inter-layer conductors.

This 3D render of a VIA (red) shows multiple signal return vias (blue) that allow a single, uninterrupted path for a conduction current.

Routing Proper Signal Return Paths

Electronics and Electrical Engineering professors drill a mantra into the heads of their first-year students: “Every signal needs a return path.”  Unfortunately, rote memorization does not lead to the critical understanding of the underlying concepts of conduction and displacement currents.  Students tend to view a “ground plane” as the universal return path that all signals take.  While that might be a designer’s intended return path, there are no laws of physics that preclude any other nearby conductor from becoming the return path for a given trace.

If a properly designed return path is not present, electromagnetic energy will radiate out into space to excite a current in any conductor it encounters.

The image below illustrates a four-layer stackup.  Version “A” illustrates a rather pervasive design decision.  It assumes signals on outer layers and ground and power planes on the two inner layers (signal-power-ground-signal).  Version B assumes that signals/power exist on outer layers and return/reference layers are on internal layers.  (signal/power-return/reference-return/reference-signal/power).

Version A is an exceedingly common and exceedingly poor design practice.  Engineers can route the path for conduction currents.  But displacement currents determine their path based on the laws of physics, not on the layer name in the stackup manager.  If there isn’t a clear, low impedance return path, the energy will radiate into other nets and create noise, or it will radiate out into space, where it creates electromagnetic interference.  There is simply no way for a signal on layer one to couple to a distance layer 3 with a copper plane on layer 2.

A better layout practice uses the outer layers as mixed power/signal layers and keeps the two interior layers for a ground reference and signal return.  With this stackup choice, signals will generate displacement currents in adjacent layers.  And with plenty of vias stitching the copper pours together, there is little chance of unintentionally creating an antenna.

Close coupling

There are two common ways to create a PCB, termed “foil” and “cap” construction.  In foil (also known as sequential) construction, fabricators start with a single copper-clad central core and progressively add layers of prepreg and foil to create the final design.

In “cap” construction, fabricators start with several copper-clad cores and sandwich prepreg between the various cores.

But the thickness of the core or prepreg layers is entirely adjustable based on the raw materials chosen for the job.

The fabrication methods (cap vs. foil) and the stackup specifications create conduction layers with various thicknesses of dielectric separating them.  Ensure that you are designing a stackup that intentionally routes return paths on the nearest adjacent layer.  Remember, displacement currents will find a path based on physics, not based on what you named the layer in your design software.

For best results, route return paths on a copper conductor are immediately adjacent to the signal layer.  Never route a high-speed return path through a conductor to a non-adjacent layer.  In a six-layer board with a signal on layer two, possible return paths are layer one and layer three, and whichever layer is physically closest to layer two is the preferred choice.  But the signal return will follow the lowest impedance path.  You should never route a return for layer two on layer four unless there is a copper void on layer three.  Electromagnetic fields will not couple to a far away conductor by coupling through a nearby conductor — they will simply couple to the nearby conductor.

Routing Vertical Returns

Routing a PCB is like playing an advanced version of the computer game “snake,” every routed trace makes it more difficult to route the next.  Eventually, single-layer routing becomes impossible, and an engineer must shift the net to another layer to avoid collisions with existing traces.  Even engineers who know how to route return paths on adjacent nets often forget to create a low impedance vertical return path between their current and new layers.  Without at least one dedicated return via, the intended return path will be non-continuous, forcing the energy to either find a return path the engineer didn’t imagine or radiate out into the environment.

Summary

All charges interact with the electromagnetic fields that surround a conductor.  When charges accelerate, they create disturbances in the electromagnetic field that radiate outwards.  By intentionally routing continuous conductive return paths, most energy is immediately captured and never becomes radiating emissions.  As an engineer, if you develop a better understanding of the underlying physics, you’ll be able to design boards that pass electromagnetic compliance testing and are less susceptible to electromagnetic interference.