Principles of PCB Design: 16 Essential Rules You Must Know

16 Essential Principles for PCB Design

1. When designing PCB component placement, fully follow the principle of placing components in a straight line along the signal flow direction, and avoid back-and-forth routing as much as possible.

   Reason: Prevent direct signal coupling that impairs signal quality.
2. On a PCB, filtering, protection and isolation devices for interface circuits should be placed close to the interfaces.

   Reason: Effectively achieve the functions of protection, filtering and isolation.
3. If an interface is equipped with both filtering and protection circuits, follow the principle of protection first, then filtering.

   Reason: Protection circuits are used to suppress external overvoltage and overcurrent. Placing protection circuits after filtering circuits will result in damage to the filtering circuits due to overvoltage and overcurrent.
4. For PCBs where the clock frequency exceeds 5MHz or the signal rise time is less than 5ns, a multilayer board design is generally required.

   Reason: Using a multilayer board design allows excellent control over the signal loop area.
5. For multilayer boards, critical routing layers (the layers where clock lines, buses, interface signal lines, RF lines, reset signal lines, chip select signal lines and various control signal lines are located) should be adjacent to a complete ground plane, preferably between two ground planes.

   Reason: Critical signal lines are usually strong radiation or highly sensitive signal lines. Routing close to a ground plane can reduce the signal loop area, thereby lowering radiation intensity or improving anti-interference ability.
6. In multilayer boards, the power plane should be recessed by 5H-20H relative to its adjacent ground plane (H is the distance between the power and ground planes).

   Reason: Recessing the power plane relative to its return ground plane can effectively suppress edge radiation issues.
7. In layer stack design, avoid arranging routing layers adjacent to each other as much as possible. If it is unavoidable, appropriately increase the layer spacing between the two routing layers and reduce the layer spacing between the routing layer and its signal loop.

   Reason: Parallel signal traces on adjacent routing layers will cause signal crosstalk.
8. Strong radiation devices such as crystals, oscillators, relays and switching power supplies should be placed at least 1000mil away from the board's interface connectors.

   Reason: Such devices will radiate interference outward directly or couple currents onto external cables for outward radiation.
9. Sensitive circuits or devices (such as reset circuits, WATCHDOG circuits, etc.) should be placed at least 1000mil away from all edges of the board, especially the edge on the interface side of the board.

   Reason: Areas such as board interfaces are the most susceptible to coupling of external interference (e.g., electrostatic discharge), and sensitive circuits like reset circuits and watchdog circuits can easily cause misoperation of the system.
10. PCB traces must not have right-angle or acute-angle bends.

    Reason: Right-angle traces lead to impedance discontinuity, which causes signal reflection, resulting in ringing or overshoot and forming strong EMI radiation.
11. Avoid arranging routing layers adjacent to each other as much as possible; if unavoidable, make the traces on the two routing layers perpendicular to each other or ensure the length of parallel traces is less than 1000mil.

    Reason: Reduce crosstalk between parallel traces.
12. Critical signal traces such as clock lines, buses and RF lines should comply with the 3W principle when running parallel to other traces on the same layer.

    Reason: Prevent crosstalk between signals.
13. Critical signal traces must not cross split planes (including reference plane gaps caused by vias and pads).

    Reason: Crossing split planes will lead to an increase in the signal loop area.
14. When a signal line (especially a critical signal line) changes layers, a ground via should be designed near the via for layer change.

    Reason: Reduce the signal loop area.
15. The distance between a critical signal trace and the edge of the reference plane should be ≥3H (H is the height of the trace from the reference plane).

    Reason: Suppress the edge radiation effect.
16. For components with metal housings connected to ground, a ground copper pour should be laid on the top layer in the projection area of the metal housings.

    Reason: Suppress outward radiation and improve anti-interference capability through the parasitic capacitance between the metal housing and the ground copper pour.