PCB Design for EMC

The design of the printed circuit board for any equipment has a major impact on its EMC performance and the amount of Electromagnetic Interference created.

EMC / EMI Design Includes:
EMC design techniques     EMC filter design     EMC PCB design     How to resolve normal & common mode EMI / noise    

EMC / EMI topics:     EMC basics     EMI interference basics     EMC standards     CISPR11     CISPR16     CISPR22     FCC 47 part 15     EMC design techniques     EMC compliance test    

One of the key areas of designing a circuit with good EMC performance is that of the PCB design.

PCB design for EMC can enable a circuit board to perform well in terms of its EMC performance, and to help there are a few basic guidelines that can be followed to provide good EMC performance.

Although it is possible to utilise multiple layers to reduce the size of the PCB, when designing a PCB for good EMC performance, this not always the optimal route to take.

The PCB design for EMC performance may require coupling to be reduced. This may require signals to be kept apart, or the distance between some components to be increased. Although small PCBs with good EMC performance can be designed, care must be taken from the outset.

Complex PCB design using digital logic and an FPGA
Typical multilayer PCB design

PCB design for EMC: some basics

When looking at optimum EMC performance a four layer board is often regarded as a good balance between board layout and EMC performance. That said, many boards with more layers can achieve good EMC performance, but require very careful design to achieve the good EMC performance.

Ground planes improve EMC performance

One technique that is particularly useful is to use one layer within the board as a ground plane.

Signal return paths are one of the most difficult issues to resolve in printed circuit boards. It can be difficult to route a ground return satisfactorily from each integrated circuit across other signal layers, etc.

The only satisfactory solution is to use a ground plane which provides a low inductance and low resistance common ground which can provides a method of providing a short lead length to ground. By having one of the layers in the PCB reserved for a ground plane, it is easy to provide a good path to ground for any signals.

For some sensitive areas it may be necessary to isolate the ground to prevent ground currents flowing cross that section of circuit. For example a sensitive section of circuit may need to have its ground isolated and have a single connection to earth especially if a higher power section close by may cause earth currents to flow across the more sensitive section.

Gridding to create ground planes

In some PCBs that may have a limited number of layers, for example one where only two layers are available a technique referred to as gridding may be used to ensure god EMC performance. This technique is a close approximation to having a ground plane in a two-layer board comes from gridding the ground to reduce EMI radiation from the signal traces.

Essentially gridding operates by creating a network of orthogonal connections between traces carrying ground. Although the ground plane is not completely contiguous, it sufficiently emulates the ground plane that is used to provide EMC improvements of a four or more layer board by providing a ground return path under each of the signal traces and lowers the impedance between the main ICs and the voltage regulation area.

Gridding is achieved by a process of expanding any ground traces and using ground-fill patterns. The aim is to create a network of connections to ground across the PCB. The gridding is achieved by expanded the ground lines to fill up as much of the empty PCB space as possible. Then, all the remaining empty space is filled with ground.

In this way as much of the available PCB space is filled with the ground grid as possible whilst still allowing connections to be made on the layer.

PCB zoning

Creating different zones on a PCB is another useful design technique to improve EMC and general noise.

PCB zoning is essentially a process of planning where the general location of components for different areas of the circuit is defined before any traces are set down.

Not only does PCB zoning places like functions on a board in the same general area, as opposed to mixing them together, but it also takes into account the speed of signals in a given area and looks at the optimum location. Thought is given to the length of lines that may radiate or pick up more noise. For example one common idea is to place high-speed logic, including microcontrollers close to the power supply. In this way the decoupling of the lines is made easer and the lengths of lines or traces that might radiate or pick up noise is reduced.

Functions on the PCB that are not so critical that have slower waveforms are located further away. Typically analogue sections of the board are located even further away as they normally carry lower frequency signals. Planning the areas of the board in this way can have a major impact on the EMC performance of the PCB.

PCB design tools

PCB design tools are becoming ever more sophisticated. Even low end ones are able to provide many functions that until recently were only found in the very high end software packages.

Some PCB design tools may assist with designing for good EMC performance. Use any facilities which may be provided to the maximum extent. Using the tools will enable the best EMC performance to be gained from the PCB design.

Other precautions for PCB EMC design

There are a few other common points to improve the PCB EMC performance.

  • Oscillators:   Care must be taken when locating and designing the layout for oscillators. Any oscillator tank loops must be located away from analogue circuits, low-speed signals, and connectors. This applies both to the board, and to the space inside the box containing the board.
  • System cable assemblies:   Another key point is to design the overall system so that cable assemblies do not pass close to an oscillator or an area that includes high speed logic, including a microcomputer after final assembly. Cable assemblies can pick up and carry noise around the overall unit and in this way degrade the EMC performance.
  • Keep high speed / noisy lines away from PCB edge:   Another good tip is to run noisy or high speed lines away from the outside edge of the board. Keeping non-noisy traces away from areas on the board were they could pick up noise, such as connectors, oscillator circuits, relays, and relay drivers also helps reduce the problem.
  • Filtering:   In some instances filtering may be required on certain lines. Ferrite beads can often provide an easy method of limiting high frequency signals, and good decoupling on the board, especially for the supply lines is necessary.
  • Filtered connectors:   On some PCBs it may be necessary to use filtered connectors to remove noise. When this is done, the earthing of the connector is important. It should be possible to earth this firmly to the PCB and the chassis.

Many of the EMC issues can be eliminated by good PCB design. In fact PCB design for EMC performance is always good practice and can prevent many time consuming investigations and costly rework. If rework is required late in the design cycle it is considerably more costly than if it is built into the design at the outset. PCB design for EMC is therefore one of the keys to a successful design.

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