High-speed PCB design

In this article, Electronic Specifier takes a look into high-speed PCB design: what it is, what it means, and the advantages it offers.

What is high-speed PCB design?

High-speed PCB design refers to the practice of designing PCBs that can handle high-frequency signals, typically in the gigahertz range.

As data rates increase, signal integrity becomes a critical concern, necessitating specialised design techniques; high-speed designs ensure that the electrical performance of the PCB meets the stringent requirements of high-frequency applications.

When is a PCB design considered high speed?

Several factors such as signal frequency, data rate, rise time, and interconnect length determine whether a PCB design is considered high-speed.

High-speed PCBs deal with signals in the gigahertz range, and any signal frequency above 50MHz can start exhibiting high-speed characteristics. Systems with data rates exceeding 1Gbps typically fall into the high-speed category. Also, the speed at which a signal changes state (rise time) is crucial; shorter rise times necessitate high-speed design techniques to prevent signal integrity issues. Finally, longer interconnects can cause signal degradation, making high-speed design essential to maintain performance.

Best materials for high-speed PCB design

Selecting the right materials is important in high-speed PCB design as the materials must exhibit low dielectric loss and stable dielectric constant (Dk) across a wide frequency range.

Common materials include:

• FR-4: While standard FR-4 is ubiquitous, high-speed applications require low-loss variants with improved dielectric properties.
• Rogers materials: Rogers offers materials like RO4000 and RO3000 series, known for their low dielectric loss and stable Dk.
• PTFE (Teflon): PTFE-based laminates provide excellent high-frequency performance, making them ideal for RF and microwave applications.
• Polyimide: Polyimide materials offer superior thermal stability and mechanical properties, suitable for high-speed and high-temperature applications.

Designing for high-speed PCB

Designing a high-speed PCB involves several critical considerations. Impedance control is vital to maintain signal integrity, influenced by trace width, spacing, and the dielectric constant of the substrate.

Signal integrity techniques such as differential signalling, proper termination, and avoiding crosstalk are essential. A well-planned layer stackup minimises EMI (Electromagnetic Interference) and improves signal routing, with power and ground planes placed adjacent to signal layers to reduce noise.

Minimising the use of vias and optimising their placement reduces signal reflections and loss. Providing a clear return path for signals, especially for high-speed signals, is crucial to avoid signal integrity issues.

Designing an efficient power distribution network with adequate decoupling capacitors ensures stable power delivery and reduces noise.

Benefits of high-speed PCB design

The advantages of high-speed PCB design are manifold. High-speed designs enable faster data rates and improved overall system performance. Proper design techniques ensure that signals maintain their integrity, leading to reliable and accurate data transmission. Effective high-speed PCB design mitigates EMI, which is crucial for compliance with regulatory standards and preventing interference with other devices. High-speed techniques also allow for more compact and densely packed PCBs, essential for modern electronics. Furthermore, high-speed designs are scalable to future technologies, ensuring longevity and adaptability of electronic products.

In summary

High speed criteria:

• Signal frequency: High-speed PCBs deal with signals in the gigahertz range. Generally, any signal frequency above 50MHz can start exhibiting high-speed characteristics.
• Data rate: Systems with data rates exceeding 1 Gbps typically fall into the high-speed category.
• Rise time: Shorter rise times necessitate high-speed design techniques to prevent signal integrity issues.
• Interconnect length: Longer interconnects can cause signal degradation.

Common materials in high-speed PCB design include:

• FR-4
• Rogers materials
• PTFE (Teflon)
• Polyimide

Design

• Impedance control: Trace width, spacing, and the dielectric constant of the substrate influence impedance.
• Signal integrity: Differential signalling, proper termination, and avoiding crosstalk.
• Layer stackup: Power and ground planes should be placed adjacent to signal layers to reduce noise.
• Via management: Minimising the use of vias and optimising their placement.
• Return path: Providing a clear return path for signals is important to avoid signal integrity issues.
• Power distribution: Efficient power distribution with adequate decoupling capacitors ensures stable power delivery and reduces noise.

Advantages of high-speed PCB design:

• Enhanced performance
• Signal integrity
• Reduced EMI
• Compact designs
• Scalability

By understanding the principles, materials, and techniques involved in high-speed PCB design, engineers can design PCBs that meet the demanding requirements of high-frequency applications, making it indispensable in the swiftly evolving electronics landscape.