Introduction to USB routing

USB (Universal Serial Bus) has become the standard interface for connecting various peripherals to computers and other electronic devices. Proper USB routing is crucial for ensuring reliable data transmission and minimizing signal integrity issues. When designing a 2-layer printed circuit board (PCB), routing USB differential pairs can be challenging due to limited space and the need to maintain signal integrity.

In this article, we will explore the key concepts and best practices for USB differential pair routing on a 2-layer board. We will cover topics such as USB standards, differential signaling, impedance matching, trace geometry, and common routing techniques.

USB Standards and Data Rates

USB has evolved over the years, with different versions offering increasing data rates and improved features. The following table summarizes the main USB standards and their corresponding data rates:

USB Standard	Maximum Data Rate
USB 1.1	12 Mbps
USB 2.0	480 Mbps
USB 3.0	5 Gbps
USB 3.1	10 Gbps
USB 3.2	20 Gbps
USB 4	40 Gbps

As the data rates increase, the signal integrity requirements become more stringent, making proper USB routing even more critical.

Differential Signaling in USB

USB uses differential signaling to transmit data, which involves using two signal lines (D+ and D-) to carry the same information but with opposite polarity. This approach offers several advantages over single-ended signaling:

1. Noise immunity: Differential signaling is less susceptible to common-mode noise, as any noise induced on both lines will cancel out at the receiver.
2. Reduced electromagnetic interference (EMI): The opposite polarity of the signals helps to minimize EMI, as the electromagnetic fields generated by the two lines tend to cancel each other out.
3. Higher data rates: Differential signaling allows for higher data rates compared to single-ended signaling, as it can operate at lower voltage levels and is less affected by signal degradation.

Impedance Matching

To ensure proper signal transmission and minimize reflections, it is essential to maintain a consistent Characteristic Impedance along the USB differential pair. The USB specification defines the following impedance requirements:

• USB 1.1 and USB 2.0: 90 Ω ± 15%
• USB 3.0 and above: 90 Ω ± 7%

Achieving the desired impedance requires careful control of the trace geometry, dielectric material properties, and PCB Stack-up.

Trace Geometry and Spacing

When routing USB differential pairs on a 2-layer board, several factors must be considered to maintain signal integrity and meet the impedance requirements:

Trace Width and Spacing

The trace width and spacing between the differential pair traces directly impact the characteristic impedance. Wider traces and larger spacing generally result in lower impedance, while narrower traces and smaller spacing lead to higher impedance. The optimal trace width and spacing depend on the PCB material properties and the desired impedance.

Trace Length Matching

To minimize signal skew and ensure proper differential signaling, it is crucial to match the length of the D+ and D- traces. Any difference in length between the two traces can lead to signal integrity issues and degrade the USB performance. As a general guideline, the length difference should be kept within the following limits:

• USB 1.1 and USB 2.0: ± 150 mil (3.81 mm)
• USB 3.0 and above: ± 5 mil (0.127 mm)

Via Placement and Stitching

When transitioning between layers on a 2-layer board, vias are used to connect the USB traces. It is important to place the vias for the D+ and D- traces as close together as possible to minimize any discontinuities in the differential impedance. Additionally, using stitching vias along the length of the differential pair can help to maintain the desired impedance and reduce the risk of signal integrity issues.

Routing Techniques

Several routing techniques can be employed to optimize USB differential pair routing on a 2-layer board:

Serpentine Routing

Serpentine routing involves adding gentle bends or curves to the USB traces to match their lengths and maintain a consistent differential impedance. This technique is particularly useful when dealing with limited board space or when the traces need to be routed around other components.

Staggered Routing

Staggered routing is another approach to match the trace lengths of the USB differential pair. In this technique, one trace is intentionally lengthened by adding a series of small jogs or steps, while the other trace follows a more direct path. The staggered pattern helps to equalize the trace lengths and minimize signal skew.

Shielding and Ground Planes

Incorporating shielding and ground planes can help to improve the signal integrity of USB differential pairs. On a 2-layer board, this can be achieved by routing the USB traces between two ground planes or by placing ground traces adjacent to the differential pair. The ground planes or traces help to provide a low-impedance return path and reduce the impact of external noise sources.

Common Pitfalls and Best Practices

When routing USB differential pairs on a 2-layer board, there are several common pitfalls to avoid and best practices to follow:

1. Avoid sharp bends or corners in the USB traces, as they can cause impedance discontinuities and signal reflections. Use gentle curves or 45-degree angles instead.
2. Keep the USB traces away from other high-speed signals or noise sources to minimize crosstalk and interference.
3. Ensure that the USB traces have a solid reference plane beneath them, preferably a ground plane, to provide a stable return path and maintain the desired impedance.
4. Use appropriate via sizes and antipad diameters to minimize the impact of vias on the differential impedance.
5. Perform signal integrity simulations and analysis to validate the USB routing and identify any potential issues before fabricating the PCB.

Frequently Asked Questions (FAQ)

1. Q: Can I route USB differential pairs on a 2-layer board without a ground plane?
   A: While it is possible to route USB differential pairs on a 2-layer board without a dedicated ground plane, it is not recommended. The absence of a ground plane can lead to signal integrity issues, increased EMI, and difficulty in achieving the desired impedance. If a ground plane is not feasible, consider using ground traces adjacent to the USB traces to provide a reference path.

2. Q: What is the maximum length of a USB trace on a 2-layer board?
   A: The maximum length of a USB trace depends on the USB standard and the signal quality requirements. As a general guideline, the following trace length limits are recommended:

3. USB 1.1: 3 meters
4. USB 2.0: 5 meters

5. USB 3.0 and above: 3 meters
   However, these lengths can be affected by factors such as PCB material, trace geometry, and the presence of connectors or cables.

6. Q: How do I calculate the characteristic impedance of a USB differential pair?
   A: Calculating the characteristic impedance of a USB differential pair requires knowledge of the PCB material properties, trace geometry, and spacing. There are various online calculators and tools available that can help you determine the impedance based on these parameters. Alternatively, you can use PCB Design Software that includes impedance calculation features or consult with a PCB Manufacturer for assistance.

7. Q: What is the impact of vias on USB signal integrity?
   A: Vias can introduce discontinuities in the characteristic impedance of USB differential pairs, leading to signal reflections and degradation. To minimize the impact of vias, consider the following:

8. Place the vias for the D+ and D- traces as close together as possible.
9. Use appropriate via sizes and antipad diameters to maintain the desired impedance.
10. Minimize the number of vias along the USB traces.

11. Use stitching vias to provide additional grounding and reduce the inductance of the vias.

12. Q: Can I use autorouting for USB differential pairs on a 2-layer board?
    A: While autorouting can be a useful tool for general PCB routing, it is not recommended for critical signals like USB differential pairs. Autorouting algorithms may not always consider the specific requirements for USB routing, such as impedance matching, trace length matching, and proper spacing. It is best to manually route USB differential pairs to ensure optimal signal integrity and compliance with the USB specifications.

Conclusion

Routing USB differential pairs on a 2-layer board can be challenging, but with a solid understanding of the key concepts and best practices, it is possible to achieve reliable and high-quality USB signal transmission. By considering factors such as impedance matching, trace geometry, spacing, and routing techniques, designers can minimize signal integrity issues and ensure compliance with the USB specifications.

When embarking on your first attempt at USB differential pair routing on a 2-layer board, it is essential to take a methodical approach and pay attention to the details. Utilize the information provided in this article as a foundation, and don’t hesitate to consult additional resources, such as USB specification documents, PCB design guidelines, and expert advice.

Remember, proper USB routing is crucial for the overall performance and reliability of your electronic device. By following best practices and continuously learning from your experiences, you can master the art of USB differential pair routing on 2-layer boards and create designs that meet the highest standards of signal integrity.