PCB Blog - EMC PCB board design skills

EMC is closely related to the generation, transmission, and reception of electromagnetic energy, and EMC is not expected in PCB board design. Electromagnetic energy comes from multiple sources mixed together, so special care must be taken to ensure that when different circuits, traces, vias, and PCB board materials work together, the various signals are compatible and do not interfere with each other. EMI, on the other hand, is a destructive effect produced by EMC or unwanted electromagnetic energy. In this electromagnetic environment, the PCB board designer must ensure that the generation of electromagnetic energy is reduced, causing interference.

7 Tips to Avoid Electromagnetic Problems in PCB Board Design

1. PCB board grounding

An important way to reduce EMI is to design the ground plane of the PCB board. The first step is to make the ground area as large as possible within the total area of the PCB board, which reduces emissions, crosstalk, and noise. Special care must be taken when connecting each component to a ground point or plane, otherwise, the neutralizing effect of a reliable ground plane cannot be fully utilized.

A particularly complex PCB board design has several stable voltages. Ideally, each reference voltage has its own corresponding ground plane. However, if there are too many ground planes, it will increase the manufacturing cost of the PCB board and make the price too high. The compromise is to use ground planes in three to five different locations, each of which can contain multiple ground sections. This not only controls the manufacturing cost of the circuit board but also reduces EMI and EMC. A low impedance grounding system is important if EMC is to be achieved. In a multi-layer PCB, there is a solid ground plane, not a copper thieving or scattered ground plane, because it has low impedance, provides a current path, and is a reverse signal source. The length of time the signal takes to return to the ground is also very important. The time for the signal to and from the source must be comparable, otherwise, an antenna-like phenomenon occurs where the radiated energy becomes part of the EMI. Also, traces that carry current to/from the signal source should be as short as possible, if the source and return paths are not of equal length, there will be a ground bounce, which will also generate EMI. If the time of the signal entering and leaving the source is not synchronized, an antenna-like phenomenon occurs, which radiates energy and causes EMI

2. Distinguish EMI

Since EMI is different, a good EMC design rule is to separate analog and digital circuits. Analog circuits with higher amperage or current should be kept away from high-speed traces or switching signals. If possible, they should be protected with a grounded signal. On a multi-layer PCB, analog traces should be routed on one ground plane, while switch or high-speed traces should be on the other. Therefore, signals of different characteristics are separated. A low-pass filter can sometimes be used to remove high-frequency noise coupled with surrounding traces. The filter suppresses noise and returns a stable current. It is important to separate the ground planes for analog and digital signals. Since analog and digital circuits have their own unique characteristics, it is important to separate them. Digital signals should have a digital ground, and analog signals should terminate at the analog ground. In digital circuit design, experienced PCB board layout and design engineers pay special attention to high-speed signals and clocks. At high speeds, the signals and clocks should be as short as possible and adjacent to the ground plane because, as mentioned earlier, the ground plane keeps crosstalk, noise, and radiation under control.

Digital signals should also be kept away from power planes. If the distance is close, noise or induction can be created, which can weaken the signal.

3. Crosstalk traces are the key

Traces are especially important to ensure proper current flow. If the current is coming from an oscillator or other similar device, it is especially important to keep the current separated from the ground plane, or not to run the current in parallel with another trace. Two parallel high-speed signals generate EMC and EMI, especially crosstalk. The resistance path must be kept short and the return current path as short as possible. The return path trace should be the same length as the transmit trace. For EMI, one is called the "aggressor trace" and the other is the "victim trace". Inductive and capacitive coupling can affect "victim" traces due to the presence of electromagnetic fields, causing forward and reverse currents on the "victim traces". In this way, ripples are generated in a stable environment where the transmit and receive lengths of the signal is nearly equal. In a well-balanced environment with stable traces, the induced currents should cancel each other out to eliminate crosstalk. However, we live in an imperfect world where such a thing does not happen. Therefore, the goal must be to keep the crosstalk level for all traces. If the width between parallel traces is twice the trace width, the effect of crosstalk can be reduced. For example, if the trace width is 5 mils, the distance between two parallel traces should be 10 mils or more. As new materials and new components continue to emerge, PCB board designers must also continue to deal with electromagnetic compatibility and interference issues.

4. Decoupling Capacitors

Decoupling capacitors reduce the unwanted effects of crosstalk and should be placed between the power and ground pins of the device to ensure low AC impedance, reducing noise and crosstalk. To achieve low impedance over a wide frequency range, multiple decoupling capacitors should be used. An important rule of thumb for placing decoupling capacitors is to place the value capacitors as close to the device as possible to reduce inductive effects on the traces. This particular capacitor is placed as close as possible to the device's power pins or power traces and connects the capacitor's pads directly to vias or ground planes. If the traces are long, use multiple vias to make ground impedance.

5. Avoid 90° angles

To reduce EMI, avoid traces, vias, and other components that form 90° angles, as right angles will generate radiation. At this corner, the capacitance will increase and the characteristic impedance will change, causing reflections, which in turn cause EMI. To avoid 90° angles, traces should be routed to the corners with at least two 45° angles.

6. Use vias sparingly

In almost all PCB board layouts, vias must be used to provide conductive connections between different layers. PCB layout engineers need to be especially careful because vias create inductance and capacitance. In some cases, they also produce reflections because the characteristic impedance changes when vias are made in the trace. Also keep in mind that vias increase trace length and need to be matched. In the case of differential traces, vias should be avoided as much as possible. If unavoidable, vias should be used in both traces to compensate for delays in the signal and return paths.

7. Cable/Physical Shielding

Cables carrying digital circuits and analog currents create parasitic capacitance and inductance that cause many EMC-related problems. If a twisted pair cable is used, the coupling level is kept low, eliminating the resulting magnetic field. For high-frequency signals, shielded cables must be used, with both the front and background to eliminate EMI interference. Physical shielding is the encapsulation of the whole or part of the system with a metal package to prevent EMI from entering the circuit on the PCB board. This shield acts like an enclosed grounded conductive container, reducing antenna loop size and absorbing EMI.