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Electromagnetic Interference (EMI) and Electromagnetic Compatibility (EMC) are important considerations when designing PCBs (Printed Circuit Boards). EMI is unwanted interference caused by electromagnetic radiation emitted from electronic devices, while EMC is a device's ability to function properly within its electromagnetic environment without introducing intolerable EMI.

Here are some key PCB design guidelines to reduce EMI and ensure EMC.

Proper Grounding

A good ground layout can greatly reduce EMI. Ground planes should be wide and continuous, and a multi-layer PCB design often includes a dedicated layer for ground. Ground loops should be avoided, as they can act as antennas and radiate EMI. Star grounding or ground grid techniques can be employed.

Trace Routing

High-frequency signals should be routed in shortest paths and avoided from board edges. Keep analog, digital, and power traces separate from each other and don't route high-speed signals across gaps in the ground plane.

Control of Impedance

Controlled impedance lines should be used for high-frequency signals. This can be achieved by keeping the trace width, thickness and distance to ground plane constant along the signal path.

Decoupling Capacitors

These are used to filter out high-frequency noise in power supply lines. They should be placed as close as possible to the power pins of the device they're intended to decouple.

Proper Power Distribution

Similar to grounding, the power distribution network should be designed to ensure all ICs receive clean, stable power. This usually involves careful planning and placement of power planes or power rails.

Component Placement

Place critical, high-speed, or sensitive components first and arrange them in a way that minimizes high-speed signal path lengths. Keep digital and analog components separate. Clock generators or other high-frequency components should be located centrally on the PCB to avoid radiation from the board edges.

Shielding and Filtering

For very sensitive circuits or in high noise environments, shielding can be used to prevent the ingress or egress of EMI. Filtering on I/O lines can prevent EMI from entering or leaving the board through cables.

Layer Stacking

In a multi-layer PCB, the arrangement of signal, ground, and power layers can have a significant impact on EMI. A common practice is to alternate power/ground and signal layers.

Use of Via: Vias inherently cause impedance discontinuities, which can lead to signal reflections and increased EMI. Use them sparingly on high-speed signal paths, and use ground vias to connect different ground planes.

Termination Techniques: Terminate high-speed signal lines properly to prevent signal reflections which can cause EMI. Techniques can include series termination, parallel termination, Thevenin termination, AC termination, etc.

These guidelines are a starting point, but EMI/EMC design is a complex field. For best results, consider using simulation software to predict EMI issues, and always perform EMI/EMC testing on prototypes to confirm that your design meets the required standards.