The PCB stackup is a big part of electromagnetic compatibility (EMC) and it fits within the “80-20” rule of SI/PI/EMI (signal integrity, power integrity, electromagnetic interference). Experienced PCB designers know that 80% of your most common EMI problems are related to only 20% of your design, and one of the key factors underlying SI/PI/EMI success is the PCB stackup. From the materials selected, to the layer thicknesses, to the layer assignments, PCB stackups play a major role in EMC success.
Success in EMC is not just about getting the layer assignments correct. Instead, the stackup is involved in all subsequent design decisions that influence EMC. This is why it is so important to match the stackup design to the end product. All too often, new designers and experienced engineers alike don’t learn the best approach until an EMC failure occurs. By investing the time up front to properly design and validate your PCB stackup, you can ensure your product meets EMC requirements and avoid costly redesigns.
What’s Impacted by the PCB Stackup?
The PCB stackup is more than just an arrangement of copper and insulators, and the strategic placement of layers is about much more than routing. Placement of layers in the stackup, selection of dielectric thicknesses, and assignment of layers with an eye toward noise, SI, and PI can prepare your design for EMC success by eliminating or suppressing common noise challenges.
Radiated EMI
Every AC signal generates radiated EMI, it’s just a matter of whether the EMI signature exceeds emissions limits. It’s no secret that edge rates in digital logic continue to get faster, and faster-edge-rate signals create more radiated emissions. This is coupled with the fact that power regulators supporting these components are running at higher frequencies with higher edge rates, pushing EMI into the higher frequency range. These factors have forced PCB designers to rethink their stackup and routing decisions in order to suppress EMI.
The PCB stackup forms the foundation for suppressing and preventing EMI in many types of designs because it influences the routing strategy that will be implemented in the PCB layout. The routing strategy is also related to the grounding strategy, and indeed the regions allocated to ground must be planned in the PCB stackup. Planning ground in a PCB stackup is the most effective tool designers have against these EMI issues:
- Radiation from switching signals and circuits, such as digital circuits or power circuits
- Crosstalk into nearby traces—essentially near-field radiation which is received as noise in a nearby trace
- Ensuring galvanic isolation without turning copper pours or planes into strong dipole radiators
- Isolation between signal layers as the layer count increases and more signals are routed in the PCB
Conducted EMI
EMI conducted into or out of an electronic device can become a source of radiated EMI, leading to radiated emissions failure. In some products, the conducted EMI itself also exceeds allowable limits, requiring a change in the design or additional circuitry to remove the source of conducted EMI. However, the PCB stackup can also impact conducted EMI through self-capacitance, mutual capacitance, self-inductance, and mutual inductance with respect to external conductors.
Noise in power and signal lines does not just radiate, it can also conduct to an external system, such as through a cable. The stackup helps address some fundamental noise sources that lead to both radiated and conducted EMI, such as power-line transients (supply bounce), simultaneous switching noise (ground bounce), and capacitive coupling of noise to signal lines (e.g., common-mode noise).
EMI Susceptibility
Part of susceptibility is measuring the vulnerability of a device to noise induced by external electromagnetic fields and currents. This is tested to ensure compliance with EMC regulations; a device must demonstrate immunity to stray fields. Stackup design factors that help to control radiated EMI can also improve susceptibility.
Conducted EMI susceptibility is also impacted by the stackup in multiple ways:
- Conductive path management with strategic ground placement on multiple layers helps provide pathways for ESD to prevent damage to components
- Capacitive coupling resulting in conductive EMI can be reduced by providing internal pathways for critical conductors or modifying their self-capacitance to ground
- Allowance for earth connections in power systems enables filtering circuits that remove conducted EMI coming into a product via the power cable
- Maintaining stable power for digital I/Os and power rails keeps their noise low, reducing conducted noise on certain cables and connectors, possibly without the need for filter circuits
Put Your Electronics on Solid Ground
PCB design, like architectural design, begins with a good foundation. Make sure your designs have a solid foundation by following the best PCB stackup design guidelines.
To put yourself on solid ground, download our free PCB stackup guide. This guide focuses on how stackup design decisions impact the generation of EMI and susceptibility to external fields, both of which are tested directly under global EMC regulations. When you’re ready to evaluate your PCB layout for potential EMI problems, DENPAFLUX offers on-demand EMC expertise for modern electronic companies.
To see how DENPAFLUX can help, contact us to schedule an appointment.
