New designers and engineers may suffer from missing the forest for the trees. In this scenario, they become so laser-focused on solving a particular problem that they fail to account for how the solution fits into the overall design landscape. I recall a project where I spent ages pouring over datasheets for a pulse width modulator (PWM) to control a motor speed, only to figure out its implementation wasn’t necessary days later. The attempts to incorporate the PWM into the circuit needlessly complicated many design stages, and ultimately, reverting to an earlier iteration allowed my team and I to continue onward.
Faraday shields and cages in the enclosure also fit into this vexing framework. While they achieve what they set out to do, the enclosure-level solution operates over the whole board instead of exactly where needed. This issue can impact the performance of off-board communications and may require further design revisions to capture design intent accurately. Instead, PCB EMI shields – placed locally on EMI emitters or vulnerable circuitry – more precisely meet EMC.
Comparing Faraday Shields/Cages To Shielding Cans |
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General Purpose Enclosure Shielding Has Its Drawbacks
Electromagnetic interference (EMI) has increasingly become a concern within modern electronics due to the upward trend of device speeds and digitization of components. As waveforms have evolved from analog continuums to discrete wave packets with fast rise and fall times that model a Dirac delta function/impulse, transfer function roll-off has diminished with the growth of high-frequency devices. High-frequency harmonics, which used to decay to irrelevance (from an EMI perspective) quickly, now exert influence over a much wider bandwidth.
The appeal of higher frequencies is greater computing power, speed, and (historically) reduced power draw/heat generation, so while high-frequency devices complicate the board’s electromagnetic compliance (EMC), designers are more than happy to work with the downsides. The only other direct influence designers can exert on EMI is reducing the loop area of the offending circuits. This condition often stems from an overly circuitous return path due to split-plane traversal or other gaps in a reference plane. Finally, high current signals are also greater E/M emitters regardless of loop area, meaning even relatively mild frequencies can still manifest as EMI issues to surrounding components and traces.
EMC Past the Design Stage
It’s essential to underscore that EMC, past the design stage, addresses the symptoms, not the causes. Design teams can rectify the situation with a Faraday shield or cage within the enclosure that attenuates signal penetration in and out of the enclosure (cages may allow for undisturbed penetration of high-frequency signals depending on the wavelength and the size of the openings in the mesh). However, this solution arrives with some drawbacks that may rule it unfeasible for some operations:
- Weight – The added weight from a shielding enclosure may significantly cost aerospace operations, where shaving any nonessential weight is a primary objective.
- System performance – The shielding enclosure significantly attenuates signals going to and from the enclosed board; any off-board communications may suffer unless they can operate at a short enough wavelength to pass through the Faraday cage (which may further complicate the EMI issue the cage set out to solve).
- Post-design solution – Design teams can utilize models to determine the efficacy of a shielding enclosure pre-assembly, but testing won’t verify this until after PCB production. A shielding enclosure that cannot provide the requisite shielding without foregoing the board’s core functionality or performance necessitates a board revision. Revision is a potentially lengthy redesign process and a costly proposition post-prototype development.
How Localized PCB EMI Shields Hone in on EMI Issues
Shielding enclosures are system-level addresses of design-level issues, making them overreaching and inexact. A more precise approach would be to isolate any offending circuitry from its neighbors, effectively blocking EMC issues at the source. Fortunately, PCB shielding cans do just that, acting as localized shielding enclosures. Different cans allow for either a Faraday shield or cage at the component level, depending on the bandwidth under consideration. Furthermore, designers can utilize custom kits to prototype cans rapidly at the component level to align board performance to EMC quickly.
Shielding cans may also decrease the overall form factor of the enclosure by adding targeted shielding only where the circuit demands it. Provided the assembly has space for shielding cans (they add a small but non-negligible footprint to circuitry), this can prove to be a much more effective RF solution for many reasons:
- Assembly ease: Clips that mount cans to the board can easily integrate into automated assembly processes like pick-and-place and soldering. This step provides a twofold advantage to assembly. First, assembly saves time lost when manually enclosing the board in a Faraday shield and the associated hand soldering. Second, for high-volume production lots, both significantly accelerate per-board turnaround time while maintaining a more consistent level of quality.
- Testing speed: PCB shielding cans are sub-divisions of the board layout and, therefore, do not require the entire assembly to produce verifiable measurements. Designers can quickly transfer relevant sections of the board to a test board and incorporate changes (if any) to the prototype without waiting to complete the layout and assembly.
- Reworkability: The ability to modify a design through layout is valuable due to changes in component availability, circuit functions, and overall design intent. Shielding cans and mounts can be removed quickly and with minimal impact on surrounding board features.
Design for EMC? Your Contract Manufacturer Can-Do
PCB EMI shields offer design teams an additional tool for EMC. Shielding cans may be more effective than general enclosure shielding, depending on the board’s requirements. As in all things engineering, there are tradeoffs, and shielding cans are no exception. Design teams looking to ensure EMC before submitting boards to expensive laboratory testing will want to weigh every option available that minimizes impact on the design (cost, performance) while meeting the necessary compliance benchmarks.
As a team of engineers committed to building electronics, VSE can assist you with various services to bring your board from schematic to a product without sacrificing quality or design intent. With our valued manufacturing partners, we aim to deliver the highest-performing PCBs in a wide field of life-saving and life-changing applications.
