Introduction to USB routing

USB (Universal Serial Bus) is a widely used interface for connecting various peripheral devices to computers. Proper USB routing is crucial for ensuring reliable data transmission and minimizing signal integrity issues. When designing a PCB (Printed Circuit Board) with USB functionality, it is essential to follow best practices and guidelines to achieve optimal performance.

In this article, we will focus on the challenges and considerations involved in routing USB differential pairs on a 2-layer PCB. We will discuss the importance of maintaining signal integrity, the constraints imposed by a 2-layer design, and provide practical tips for successful USB routing.

Understanding USB differential pairs

What are differential pairs?

Differential pairs are two conductors that carry complementary signals, typically denoted as D+ and D-. The signals transmitted over these conductors are equal in magnitude but opposite in polarity. This differential signaling technique helps to reduce electromagnetic interference (EMI) and improve noise immunity.

Why are differential pairs used in USB?

USB uses differential pairs to transmit data because of their ability to cancel out common-mode noise. By sending signals with opposite polarities, any noise induced on both lines will be effectively cancelled out at the receiver end. This results in a cleaner and more reliable signal, enabling faster data transmission rates.

Characteristics of USB differential pairs

USB differential pairs have specific characteristics that must be considered during PCB design and routing. These include:

• Impedance matching: USB differential pairs should have a characteristic impedance of 90Ω ±10%. This ensures proper signal transmission and minimizes reflections.
• Length matching: The length of the D+ and D- traces should be matched within a certain tolerance to maintain signal integrity. The maximum allowed length difference depends on the USB version and speed.
• Coupling: The D+ and D- traces should be tightly coupled to each other to maintain their differential properties. This is typically achieved by routing them close together and parallel to each other.

Challenges of routing USB on a 2-layer board

Limited routing space

One of the main challenges of routing USB differential pairs on a 2-layer board is the limited routing space available. With only two layers, the top layer is usually dedicated to component placement and signal routing, while the bottom layer is used for ground and power planes. This leaves little room for routing the USB traces, especially when considering the required impedance and length matching.

Signal integrity concerns

Maintaining signal integrity is crucial for USB functionality. However, on a 2-layer board, the USB traces are more susceptible to crosstalk and electromagnetic interference from other signals. The close proximity of the traces to the ground plane can also introduce unwanted capacitance, which can degrade the signal quality.

Impedance control

Achieving the required 90Ω ±10% characteristic impedance for USB differential pairs can be challenging on a 2-layer board. The impedance is influenced by factors such as trace width, trace spacing, dielectric thickness, and the proximity to the ground plane. Careful calculations and simulations are necessary to determine the optimal trace geometry and spacing.

Best practices for USB routing on a 2-layer board

Keep USB traces short

To minimize signal integrity issues, it is recommended to keep the USB traces as short as possible. Place the USB connector close to the USB controller or hub to reduce the overall trace length. Shorter traces have lower resistance, inductance, and capacitance, which helps to maintain signal quality.

Use a ground plane

Dedicating one layer of the 2-layer board to a solid ground plane is essential for USB routing. The ground plane provides a low-impedance return path for the USB signals and helps to reduce EMI. It also acts as a reference plane for controlling the impedance of the USB traces.

Route USB traces symmetrically

When routing USB differential pairs, it is important to maintain symmetry between the D+ and D- traces. This means routing them parallel to each other with a consistent spacing along their entire length. Any bends or turns should be symmetric as well, typically using a 45-degree angle or an arc of the same radius.

Avoid splitting the ground plane

If possible, avoid splitting the ground plane around the USB traces. A solid, uninterrupted ground plane provides better signal integrity and minimizes potential noise issues. If splitting the ground plane is unavoidable due to space constraints, use appropriate stitching techniques to maintain a continuous ground reference.

Minimize crosstalk

To minimize crosstalk between the USB traces and other signals on the board, follow these guidelines:
– Keep the USB traces away from other high-speed signals or noisy components.
– Use ground shields or guard traces to isolate the USB traces from adjacent signals.
– Avoid routing other signals parallel to the USB traces for long distances.

Consider using thicker dielectric

If the 2-layer board allows for it, consider using a thicker dielectric (insulating layer) between the top and bottom layers. A thicker dielectric helps to reduce the capacitance between the USB traces and the ground plane, which can improve impedance control and signal quality. However, this may increase the overall board thickness and manufacturing cost.

Simulation and verification

Before finalizing the USB routing on a 2-layer board, it is highly recommended to perform simulations and verifications to ensure the design meets the required specifications. Some important steps include:

1. Impedance calculations: Use Impedance Calculators or simulation tools to determine the appropriate trace width and spacing for achieving the target characteristic impedance of 90Ω ±10%.

2. Signal integrity simulations: Perform signal integrity simulations to analyze the USB signals for potential issues such as reflections, crosstalk, and noise. Adjust the routing and termination strategies based on the simulation results.

3. Design rule checks (DRC): Run DRC on the PCB layout to verify that the USB routing complies with the manufacturing and design rules specified by the PCB fabricator. This includes checking for minimum trace widths, spacings, and clearances.

4. Prototype and testing: Once the design is finalized and verified through simulations, it is essential to fabricate a prototype board and perform thorough testing. Test the USB functionality, signal quality, and compliance with the relevant USB standards to ensure reliable operation.

FAQ

1. Can I route USB traces on both layers of a 2-layer board?

While it is technically possible to route USB traces on both layers of a 2-layer board, it is generally not recommended. Routing USB on a single layer with a solid ground plane on the opposite layer provides better signal integrity and impedance control. Splitting the USB traces across layers can introduce discontinuities and degrade performance.

2. What is the maximum length for USB traces on a 2-layer board?

The maximum length for USB traces depends on the USB version and speed. For USB 2.0 Full Speed (12 Mbps), the maximum trace length is typically around 5 meters. For USB 2.0 High Speed (480 Mbps), the maximum trace length is reduced to about 5 inches (12.7 cm) without additional signal conditioning. It is recommended to keep the traces as short as possible within these limits to ensure reliable operation.

3. How do I control the impedance of USB traces on a 2-layer board?

Controlling the impedance of USB traces on a 2-layer board involves several factors:
– Trace width: Wider traces have lower impedance, while narrower traces have higher impedance.
– Trace spacing: The spacing between the D+ and D- traces affects the differential impedance. Closer spacing results in higher coupling and lower impedance.
– Dielectric thickness: The thickness of the insulating layer between the traces and the ground plane influences the impedance. Thicker dielectrics lead to higher impedance.

Use impedance calculators or simulation tools to determine the optimal trace geometry and spacing for achieving the desired impedance of 90Ω ±10%.

4. Can I use vias in the USB differential pair routing?

While it is possible to use vias in USB differential pair routing, it is generally discouraged, especially on a 2-layer board. Vias introduce discontinuities and can cause impedance mismatches, leading to signal reflections and degradation. If vias are absolutely necessary, ensure that both D+ and D- traces transition through vias at the same point to maintain signal symmetry.

5. How can I test the USB signal integrity on my 2-layer board?

Testing the USB signal integrity on a 2-layer board can be done through various methods:
– Oscilloscope measurements: Use a high-bandwidth oscilloscope to capture and analyze the USB signals. Look for any signs of signal distortion, ringing, or reflections.
– USB compliance testing: Perform USB compliance tests using specialized equipment and software to verify that the USB signals meet the requirements specified by the USB standards.
– Functional testing: Connect USB devices to the board and test the functionality and reliability of the USB communication. Verify that data transfers occur without errors or interruptions.

It is recommended to perform a combination of these testing methods to ensure the USB signal integrity and overall system performance.

Conclusion

Routing USB differential pairs on a 2-layer board presents unique challenges due to the limited routing space and signal integrity concerns. However, by following best practices and guidelines, it is possible to achieve reliable USB functionality on a 2-layer design.

Key considerations include keeping the USB traces short, using a solid ground plane, maintaining trace symmetry, and minimizing crosstalk. Impedance control is critical for USB routing, and careful calculations and simulations should be performed to determine the optimal trace geometry and spacing.

Before finalizing the USB routing, it is essential to conduct thorough simulations and verifications to ensure the design meets the required specifications. Prototyping and testing the manufactured board are also crucial steps in validating the USB signal integrity and overall system performance.

By understanding the challenges and applying the best practices outlined in this article, designers can successfully route USB differential pairs on a 2-layer board while maintaining signal quality and reliability.