In high-speed electronic design, the PCB (Printed Circuit Board) is no longer just a carrier for components and connecting lines. It directly affects the signal quality and system performance. Signal interference is a common problem in high-speed PCB design, which can lead to signal distortion, increased bit error rates, and even system instability. Therefore, optimizing PCB layout and routing to reduce signal interference is crucial for improving signal integrity.

I. Signal Integrity (SI) Basics

Before delving into layout and routing optimization, we need to understand the concept of signal integrity. Signal integrity refers to the ability of a signal to maintain its original shape and quality during transmission. The main factors affecting signal integrity include:

Reflection: Occurs when a signal encounters an impedance mismatch, causing a portion of the signal to bounce back along the transmission line.

Crosstalk: Signal energy on one signal line couples to an adjacent signal line, causing interference.

Attenuation: The gradual loss of signal strength during transmission.

Power Noise: Noise on the power and ground planes that can affect signal quality.

II. PCB Layout Optimization

Proper PCB layout is the first step in reducing signal interference. It can effectively reduce reflections, crosstalk, and power noise.

Component Placement Principles:

Place Critical Components Close Together: Place critical components (such as high-speed chips, connectors, etc.) on high-speed signal paths as close as possible to shorten signal transmission distance, reduce signal attenuation and reflection.

Consistent Signal Flow: Arrange components according to the direction of signal flow, avoiding signal line crossings and detours.

Separate Analog and Digital Circuits: Separate analog and digital circuits to avoid noise from digital circuits interfering with analog circuits.

Separate High-Frequency and Low-Frequency Circuits: Separate high-frequency and low-frequency circuits to reduce interference from high-frequency signals to low-frequency signals.

Distribute Heat-Generating Components: Distribute heat-generating components to facilitate heat dissipation and avoid localized high temperatures affecting component performance.

Place Decoupling Capacitors Close: Decoupling capacitors should be placed as close as possible to power pins to provide a fast current path and reduce power noise.

Power and Ground Plane Design:

Complete Power and Ground Planes: Use complete power and ground planes to provide a stable reference plane, reduce signal impedance, and reduce signal reflection and crosstalk.

Multilayer Board Design: It is recommended to use multilayer board design to separate power and ground plane layers to provide better power integrity and signal integrity.

Ground Plane Partitioning: If it is necessary to partition the ground plane, avoid high-speed signal lines crossing the partition gaps, otherwise it will cause signal impedance discontinuity and produce reflections.

Power Plane Filtering: Add filtering circuits on the power plane to suppress power noise.

Connector Placement:

Select the Appropriate Connector Type: Select the appropriate connector based on the type and rate of the signal. For example, high-speed signals should choose connectors with good signal integrity.

Connector Placement Position: Connectors should be placed on the edge of the PCB for easy signal input and output.

Signal Line Assignment: Reasonably assign signal lines on the connector and evenly distribute ground lines between signal lines to reduce crosstalk.

III. PCB Routing Optimization

Reasonable PCB routing is the key to reducing signal interference. It can effectively control signal impedance, delay, and crosstalk.

Signal Line Routing Principles:

Shortest Path: Signal lines should use the shortest path to reduce signal transmission distance and reduce signal attenuation and reflection.

Avoid Right Angles: Avoid 90-degree right-angle traces, as right angles create impedance discontinuities that can cause signal reflections. It is recommended to use 45-degree bevels or rounded transitions.

Reduce Vias: Vias increase signal impedance and should be minimized.

Maintain Consistent Line Width: Signal lines should maintain consistent line width to avoid impedance changes.

Ground Line Accompanying: High-speed signal lines should be accompanied by ground lines to provide a good reference plane and reduce signal impedance and crosstalk.

Differential Routing: For differential signals, use equal-length, equal-spaced differential routing to maintain consistent differential impedance.

Meandering: When signal line lengths are inconsistent, meandering can be used for compensation, but pay attention to controlling the spacing and curvature of the serpentine lines to avoid introducing excessive inductance and capacitance.

Impedance Control:

Calculate Impedance: Calculate the appropriate impedance value based on the signal rate and transmission distance.

Control Line Width and Layer Stackup: Achieve impedance control by adjusting the signal line width, distance from the reference plane, and layer stackup.

Impedance Testing: Use tools such as TDR (Time Domain Reflectometry) to perform impedance testing to verify that the impedance meets design requirements.

Crosstalk Control:

Increase Signal Line Spacing: Increasing the spacing between signal lines can reduce crosstalk.

Ground Line Isolation: Adding ground lines between signal lines can effectively isolate signals and reduce crosstalk.

Shielded Lines: Use shielded lines to protect particularly sensitive signal lines.

Avoid Parallel Long-Distance Routing: Avoid parallel long-distance routing of signal lines, especially high-speed signal lines.

Power and Ground Line Routing:

Thicken Power and Ground Lines: Thicken power and ground lines to reduce resistance and inductance, and improve power supply capability.

Star Routing: Use star routing to radiate power and ground lines from a central point to reduce common-mode noise.

Decoupling Capacitors: Place decoupling capacitors between power and ground lines to provide a fast current path and reduce power noise.

IV. Common Signal Integrity Simulation Tools

Mentor Graphics HyperLynx: Powerful signal integrity, power integrity, and electromagnetic compatibility analysis tool.

Cadence Allegro Sigrity: Integrated signal integrity analysis solution that seamlessly integrates with the Cadence Allegro PCB design platform.

Ansys HFSS: Electromagnetic field simulation software based on the finite element method, which can perform three-dimensional electromagnetic field analysis and is suitable for high-speed signal simulation.

V. Conclusion

Optimizing PCB layout and routing to reduce signal interference is a complex and detailed process. Comprehensive consideration is required from component layout, power and ground plane design, signal line routing, and other aspects. By following the above principles and techniques and combining professional signal integrity simulation tools, we can effectively improve the signal integrity of the PCB and improve the performance and reliability of the system.

VI. Some Additional Experience Sharing:

Plan Early, Simulate Early: The earlier signal integrity problems are discovered, the lower the cost of solving them.

Practice More, Summarize More: Accumulate experience and continuously improve your PCB design skills.

Communicate with PCB Manufacturers: Understand the process capabilities of PCB manufacturers to ensure that the design can be successfully produced.