Most people are familiar with Benjamin Franklin’s famous kite experiment, but most may not know his early forays into electricity extended far beyond that. Franklin worked with early capacitor technology. He correctly hypothesized that electricity was not composed of two separate “electrical fluids” but a single “fluid” of a positive and a negative element.
His conventional current, or the flow of positive charge, is still used today even though the most common charge carrier is the negatively charged electron. To Franklin’s credit, the discovery of the electron wouldn’t occur for over a century after his death by J.J. Thompson.
However, like all good scientists, Franklin’s discoveries provided a stepping stone for later revelations. Michael Faraday, whose scientific contributions are too vast and monumental to touch upon in this setting, further described the transfer of charge from an initially charged object to the enclosing shell and that this charge existed on the outer surface of the shell. This forms the basis of what is now known as a Faraday cage. There are multiple uses for Faraday cages in electronics, among them a PCB Faraday cage to prevent outgoing and incoming EMI.
No matter where the originating E-field (in red) occurs, it draws the negative electrons (in yellow) of the conductive Faraday cage toward the positive end of the dipole. This, in turn, creates a secondary E-field (in blue) that is equal in magnitude yet opposite in direction to the original E-field, which causes cancellation. Note that the E-field lines should extend throughout the drawing, and visual elements are not drawn to scale for clarity.
Faraday Cage Operating Principles
The purpose of a Faraday cage is simple enough: by placing a conductive material around sensitive electronics, interior and exterior electric fields can be negated on the outer or inner surface of the cage. The mechanism of the Faraday cage occurs because an applied field causes the mobile electrons in the conductive material to migrate toward one end of the cage (the positive side of the dipole moment for an electric field). In doing so, the strength and orientation of the applied field are canceled completely by the induced field. Thus, effectively isolating the device from its surroundings, preventing interaction between the aggressor or victim lines of nearby devices.
There’s a small but important distinction between a Faraday cage and a Faraday shield. While they operate under the same principle, the former contains openings in the enclosure (usually in a hole-punched mesh pattern), while the latter is continuous.
As is always the case with engineered solutions, the Faraday cage is not a universal solution despite its invaluable role as an EMC tool. Here’s why:
- There is an additional cost associated with the design of a Faraday cage. Beyond normal enclosure considerations, the conductivity of the material, its thickness, and the size of its openings will determine the wavelength of signals that can enter or exit the cage.
- There are no hard and fast rules when designing a Faraday cage. The design may require multiple revisions and internal testing to assuage validity before submitting to expensive EMC lab testing. A lengthy redesign process can extend the turnaround time for a project, and since Faraday cages are ad-hoc fixes, this can lead to unforeseen delays.
- Surrounding the board with a shield inhibits airflow, which reduces convection and will need to be corrected secondary to the board’s EMC concerns to prevent poor service life of the board or a potential burn, smoke, or fire hazard.
- Many boards communicate externally to other devices with wired or wireless connections. The openings in the Faraday cage will have to balance EMI against the signal integrity of off-board I/O.
Since engineering is a give-and-take of performance among different parameters, there are ways to accentuate the critical attributes of the board while mitigating any detrimental effects.
What to Weigh When Building a PCB Faraday Cage
To devise a suitable PCB Faraday cage, designers will want to begin with some key points:
- Hole size: For good EMI prevention, openings must be approximately 1/10th of the smallest wavelength utilized (ideally smaller). Commonly, this is 2.4 GHz, which corresponds to a hole size of 1/4 in/~6/25 mm. Higher speeds mean a smaller hole to effectively block shrinking wavelengths. Of course, smaller openings can also mean reduced convection effectiveness, which may necessitate additional cooling measures.
- Cage thickness: The majority of charge is carried on the surface of a body, with an exponentially decreasing amount carried within the solid body. This property, known as the skin depth, is frequency-dependent, with the penetration depth decreasing as frequency increases and wavelength falls.
Typically, EMI is concerned with high transmission speeds (usually GHz range and up), which means that the Faraday cage does not need to be especially thick; this is a boon for mass-produced devices as it reduces the cost per unit. Conversely, slower RF signals that pose EMI problems will require thicker shielding that can better attenuate at those speeds.
- Grounding: Of all the electronics topics, EMI is arguably the most susceptible to designers following gut feelings lacking in rigor that have been passed down as best practices. Grounding does nothing for the effectiveness of a Faraday cage to stop signal penetration. However, it does prevent capacitive coupling, which may be worthwhile when dealing with low-frequency signals.
Additionally, grounding at multiple points of the design can transform a Faraday cage into an antenna due to its relatively large surface area, intensifying the very problem that the design team set out to solve.
It bears mentioning that a Faraday cage is ultimately a correction of a system-level design issue. Sometimes this is unavoidable, but the best solution may be a revision that addresses potential sources of EMI aggravation or susceptibility.
Your Contract Manufacturer Turns EMC into EZ
PCB Faraday cages play an important role in enclosure design, allowing engineering teams a chance to resolve EMI issues post-production quickly and relatively inexpensively. However, Faraday cages are much more than black boxes. Engineers need to weigh multiple factors that can greatly influence the effectiveness of the final product. Design intent must be maintained, but incorporating a Faraday cage can be less straightforward than many enclosures.
At VSE, we’re a team of engineers committed to applying our knowledge and experience to build electronics for our customers. Alongside our valued manufacturing partners, we build and house PCBs that serve a variety of life-saving and life-changing industries.
