PCB DESIGN
4 Areas for Controlling EMC in PCB Design
EMC for PCB design is anything but black magic.
by Ralf Brüning
Electromagnetic compatibility (EMC) problems are often responsible for redesign cycles during the PCB design process, but once engineers and designers understand the basics, they see there’s nothing mystical about it.

EMC is the branch of electrical engineering and physics that deals with the unintentional generation, propagation and reception of electromagnetic waves (in the E and H fields). These can cause undesirable effects in electronic devices, including functional interferences, malfunctions, or even physical damage.

Generally, two fundamental aspects are considered. First, the emission referring to the unwanted generation of electromagnetic energy and its transmission to the sinks, along with the necessary countermeasures to reduce such emission. Second, the respective susceptibility to interference relating to the operation of electrical/electronic equipment (or components) that become “victims” of unintended electromagnetic interference (EMI).

To simplify, EMC is the ability of electronic systems to function in a common electromagnetic environment without interfering with or being affected by other systems.

Know the potential EMI sources. On a PCB, various potential sources of interference can cause a variety of possible effects:

  • Signal and power integrity (conducted emission).
  • Radiated emission.
  • Immunity to radiated and conducted emissions.
  • Electrostatic discharge (ESD).

Often unnoticed by PCB designers, the ribbon cable on a PCB connector, for example, physically forms the arm of a dipole and thus creates a parasitic antenna. In this case, current and voltage peaks occurring during the switching process of the active components in the power supply can lead to an excitation of this parasitic antenna. This activity can result in an increased radiation pattern.

In addition, the signal shapes in digital signaling are, in theory, ideal rectangles. In reality, they don’t exist in such a form. Instead, the signals are created by adding various sine waves containing high-frequency content. These signals are more or less distorted and disturbed while traveling from driver to receivers. The resulting voltage peaks of the reflections and crosstalk will also have a negative effect on the EMC behavior.

Integrating EMC-compliant design into product development. EMC-compliant design is crucial to the success of a product. Products gain approval for customer deployment only by complying with EMC regulations of the specific target market or application (for example, the medical or automotive industry). Problems often only arise during the prototype testing phase, however, often due to a lack of properly integrated EMC verification procedures in the design process.

Several options exist for managing EMC in the design process and detecting problems at an earlier stage. The first step is the systematic definition and use of design constraint processes, especially for signal and power integrity issues.

Tool-supported EMC design reviews ensure adherence to relevant EMC guidelines. Rules in some tools can also be prioritized on a per-user or per-design basis. The circuit designer can classify EMC-relevant signals for such checks early on during the schematic design process. The selected EMC rules suitable for an application are then applied during the PCB design phase.

Direct integration into the CAD process (2-D and 3-D) and automatic generation of reports in the form of DRC checks – familiar to any PCB design engineer – simplify the workflow. These reports should contain images, progress status, or approval information. This information can also be stored in the design data for the joint work on EMC aspects during design.

The rules implemented will contain recommendations for various design issues that enable non-experts to solve signal integrity, power integrity, and electromagnetic compatibility problems. No additional software is necessary to validate the identified potential EMC issues.

For an EMC-compliant PCB design, it is essential to consider the following four areas:

  1. Identify and evaluate parasitic antennas. Work out where parasitic antennas could form on the PCB. Parasitic antennas are developing electrical or magnetic monopole or dipole structures.
  2. Recognize and account for the current return paths. An electric current inevitably returns to its source. Therefore, visualizing the return paths and the return loops is critical. Depending on the application, the returning current runs along either the path with the lowest impedance or the path with the lowest resistance. To select a correct return path, do not wire lines across slots if possible, not even in differential pairs.
  3. Understanding various coupling effects. Coupling paths between the source and the sinks can occur depending on parasitic voltages, parasitic currents, or they june be IO-related. In many cases, their root causes are not immediately recognizable in the layout.
  4. Understanding resonances as potential antennas. Almost all electrical structures can become resonant. This includes single lines and differential signals, power supply structures, cables, packages, and even vias. Fortunately, it is easy to calculate the resonance frequency for many structures using this formula: fres=1/(2π√L∙C). However, knowledge of the values for the (parasitic) inductances (L) and capacitances (C) is not quite so easy to obtain and often requires complex analysis. Also, it is not possible to completely erase resonances. Know and understand the effect, and avoid excitation where possible.
Potential noise sources on a PCB
Figure 1. Potential noise sources on a PCB.
When it comes down to it, achieving EMC compliance in PCB designs isn’t all that difficult. Understand EMC and how and why it can affect the board designs. Then learn how to manage it.
Ralf Brüning is product manager and senior consultant for high-speed design systems at the Zuken EMC Technology Center in Paderborn, Germany, responsible for product marketing and business development for the Zuken SI, PI and EMC analysis tools; ralf.bruening@zuken.com.