Best High-Speed PCB Design Practices

​High-Speed PCB Design

The importance of PCB design for high-speed circuit boards is becoming more apparent as the demand for electronic devices that are faster and more complicated increases. High-speed PCBs are used for various applications, including data storage, telecommunications, and high-performance computers, to name a few.

The design of a high-speed board requires different considerations from a conventional PCB. The increased speeds and frequencies present new challenges, such as crosstalk, signal integrity, and power integrity. This article will briefly discuss some critical design considerations and techniques for designing high-speed boards.

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Power Integrity

Power integrity is the quality and reliability of the power delivery system of the PCB. The power supply can cause noise at high speeds and frequencies and interfere with the signals. Circuit boards still require power and ground planes, but managing the power delivery network is more complex than connecting an IC's power and ground pins. Each IC needs a strictly controlled power supply to comply with its power requirements.

A circuit board's power delivery network should provide stable voltage references and distribute power to all devices with acceptable noise and tolerance levels. This is known as power integrity, which helps maintain a consistent voltage level for all devices on the board.

Follow these best practices to reduce issues with power integrity:

• Decoupling capacitors should be placed close to the pins that power the component to minimize the effect of noise from the power supply on the signal. Decoupling capacitors must be routed in a wide and short manner with the vias placed appropriately.

• Keeping the traces as short as possible is recommended to achieve the best routing from power pins to bypass capacitors. When routing power supplies, it is best to use 45-degree or rounded corners and keep the traces short, wide, and direct. Wider traces are necessary to handle the increased current and temperature of power nets, reduce inductance on the line, and prevent crosstalk.

• Arrange Power and Ground planes carefully in a stack: A power plan placed next to a groundplane provides a constant voltage reference, a higher current carrying capacity, a lower voltage drop, better heat dissipation, and a shorter return path. Therefore, utilizing a low-impedance ground plane is highly advisable. To reduce EMI, using a solid plane for grounding is recommended instead of routing ground with traces. This helps with thermal management and power integrity and improves signal integrity by providing clear signal return paths for sensitive high-speed transmission lines.

Signal Integrity

Signal integrity is the quality of the signals as they travel through the PCBs and components. As signal speeds increase in electronics, they become increasingly susceptible to interference from crosstalk, impedance mismatches, ringing, and ground bounce. Signal degradation can occur at high frequencies and speeds due to reflections, crosstalk, and mismatched impedances.

These effects ultimately lead to a decline in signal fidelity or signal integrity, which can result in transmission errors and system failures. These failures may be sporadic or completely shut down the system. Additionally, signal integrity issues may not be apparent during prototyping but can cause multiple failures during the regular production of circuit boards, making the matter even more complex.

Follow these best practices to minimize problems with signal integrity:

• It is important to note that if a signal takes longer than a quarter of its rise time to travel along a trace, the trace may act as a transmission line. To avoid signal reflections caused by mismatched impedances between the source and the trace or the load and the trace, it is recommended to use controlled impedance traces that are impedance-matched to the transmission line impedance of the signal. By doing so, you can reduce reflections and ensure the integrity of your signal.

• Reduce trace lengths. When high-frequency PCB traces are of a similar length to the signal's wavelength, problems like time delays, reflections, electromagnetic interference, and crosstalk can occur. These issues can significantly impact the performance of high-frequency circuits, making the length of high-speed or high-frequency traces a critical factor to consider. Shorter traces can reduce the risk of signal degradation and parasitic effects (inductive or capacitive effects which may affect the signal).

• Proper trace routing techniques are essential. These include using vias, avoiding sharp corners, proper signal layer arrangements in a stackup, and maintaining consistent trace width. Implementing vias is a crucial aspect when routing, but it requires caution. Although vias are necessary, they can introduce added inductance and capacitance, which can cause reflections due to the alteration in characteristic impedance. Furthermore, vias increase the length of traces. Therefore, it is best to avoid using vias in differential traces.

Crosstalk

Crosstalk occurs when the unwanted electromagnetic coupling between traces is caused by interference. Any conductor with a moving charge will inevitably generate an electromagnetic field. This causes noise and possible errors. Crosstalk occurs more frequently at high frequencies and speeds, so reducing it is essential.

Follow these best practices to reduce crosstalk:

• Use a groundplane close to the high-speed layer. This shields the signal traces against crosstalk by providing a constant reference plane. It also helps control the electromagnetic fields of the aggressor signals.

• To minimize broadside coupling, it's best to configure your board layers such that adjacent signal layers have preferred routing directions that cross each other instead of running parallel. For instance, if layer two runs "north to south," layer three should run "east to west." This way, you can optimize your board's layer configuration.

• Avoid putting high-speed signals near other signals. High-speed signals can interfere with or be interfered with by other signals. It is essential to keep them as far apart as possible.

Conclusion

A high-speed PCB design requires different considerations from a conventional PCB. Understanding the power and signal integrity challenges can ensure your high-speed board is reliable.

High-speed PCB design is a constantly evolving field and very complex. Whether you are an expert or a novice in the printed circuit board design industry, staying up-to-date on high-speed trends and developments is essential. There is always something new to learn.

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