Companies designing PCBs to be produced at volume and released to the open market must comply with a broad set of regulations. This includes environmental regulations, regulations regarding important exports, consumer safety regulations, and EMC regulations. Among all these areas, the one set of requirements where designers most often fail compliance is, without a doubt, EMC regulations. Often rolled into EMC failure as a root cause is a defect in signal integrity or power integrity, especially in high-speed digital PCBs. In general, however, the most common EMC failures we have found exist at the system level and are usually related to grounding.
It is interesting that so many companies run the hamster wheel of EMC failure, then they come to an outsourced PCB design services firm like NWES to debug the problem. Many companies simply plan for an EMC testing failure as part of their project budget, hoping that their testing lab and test engineering resources can identify the root cause of the problem. Based on the many projects we have seen over the years, we decided to compile this list of common EMC failures.
List of Most Common EMC Failures in PCBs
I decided to scour our past projects, design notes, and internalized knowledge from the team to compile this list. These failures are known to be observable in high-speed digital PCBs, simpler microcontroller PCBs, power systems, RF and wireless designs, and HDI PCBs. In some of the IC substrate projects we have worked on, the substrate can exhibit the same EMI problems as HDI PCBs and high-speed digital PCBs. The moral of the story is: no one is immune to an EMI problem, and the right PCB design expertise is needed to address most of the common system-level EMC failures.
Below are mechanisms that produce both radiated emissions and conducted emissions failures. I have tried to break out each of these to list the mechanism by which noise is radiated or injected into the system, resulting in the compliance failure.
- Radiation caused by fast-switching circuits when ground planes are absent
- Inadequate coplanar ground or absence of a dedicated ground plane
- Lack of a consistent ground reference in routing, leading to increased radiation
- Using multiple disconnected grounds for unnecessary signal isolation
- Emission or reception of radiation due to improper shielding of cables and connectors
- Radiation from ferrites in a high-bandwidth digital PDN
- High-frequency radiation caused by an incomplete return current path
- Floating metal sections emitting or receiving radiation
- Noise arising in return paths connected to AC mains inputs, which may necessitate a PFC circuit
- Switching noise generated by a digital PDN supporting high-speed processors
- Absence of board-level shielding for certain circuits, increasing susceptibility to EMI
- High impedance in return paths between the shield, chassis, or earth
- Failure to guide ESD currents away from vulnerable components
- Attempting to isolate fast power rails with a high-resistance ferrite, producing radiated emissions from strong transients
- Attempting to connect disconnected grounds with ferrites, possibly leading to high-frequency radiated emissions from the isolated ground region
- Overlapping (capacitively coupled) power rails that reference each other and radiate emissions
- Failure to maintain ground near clock traces from piezoelectric crystals, particularly on vertical transitions
- Noise from switching circuits coupled into I/O lines on a connector, appearing as excess noise on a cable
The above points are not an exhaustive list, but they address a broad set of problems that can arise in PCBs and their external connections to power or other systems. PCB designers work hard to ensure the design is both manufacturable and performs to its electrical requirements and specifications, and a big part of that is ensuring low EMI so that a design can comply with EMC regulations. PCB design experts understand these facts and the context around when these guidelines should or should not be applied. The context stems from one important electrical characteristic of signals in your electronics system: edge rate (for digital signals) or frequency (for RF signals).
It All Starts with Edge Rate and Frequency
The frequency range within which your board operates, or the edge rate at which switching circuits such as digital logic are operating, will collectively determine the EMI spectrum you could expect from your PCB. These factors also determine which of the common EMC failures you might expect if you violate the design guidance in the above list.
What makes edge rate and frequency so important? These are two sides of the same coin, linked by Fourier transforms. The reason these two factors determine the level of radiation from an electronic device comes from fundamental electromagnetic theory. When your designs are at risk, make sure you work with a design firm that understands signal integrity and power integrity, as well as their linkage to EMI/EMC thanks to signal edge rate and frequency. There are very few firms like NWES that focus on advanced designs where these factors are incredibly important, so reach out to us to help you get started on your next project.
Whether you’re designing high-speed PCBs for mil-aero embedded systems or a complex RF product, you should work with a design and development firm that can ensure your product will be reliable and manufacturable at scale. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology, including power systems for high reliability applications and precision control systems. We’ve also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we’ll make sure your design is fully manufacturable at scale. Contact NWES for a consultation.
