Definition
High-speed design refers to systems that operate at high frequencies or data rates. Essentially, it involves circuits that utilize high-speed digital signals to transmit data between components.
In the context of PCB design and layout, high-speed design focuses on creating circuit designs with high signal integrity to avoid issues with EMI/EMC.
Fundamentally, it involves techniques to:
- Ensure reliable and efficient signal transmission,
- Minimize signal degradation,
- And maintain signal integrity.
The boundary between a high-speed design and a basic PCB with slower digital protocols can be somewhat unclear. The primary criteria used to classify a system as “high-speed” are the edge rate and rise time of the digital signals used in the system.
Modern digital designs often incorporate both high-speed (fast edge rate) and low-speed (slow edge rate) digital protocols. In the current era of embedded systems and IoT, high-speed circuit boards often feature an RF front-end for wireless communication and networking.
While all designs have their foundation in a schematic, a significant portion of high-speed PCB design focuses on connector design, PCB stackup, and routing.
Process of High-Speed design
When designing a high-speed PCB, the chosen PCB stackup plays a crucial role in determining impedance and ease of routing.
All high-speed stackups must include specific layers dedicated to high-speed signals, power, and ground planes.
Therefore, you should consider these various factors when planning the layers in your stackup.
- Layer Selection
Determine the number and types of layers needed for your high-speed design. As mentioned, high-speed PCBs often consist of multiple signal layers, supply, and ground planes.
- Order of Layers
The order of signal, power, and ground planes in your stackup is crucial. Placing signal layers between supply and ground planes improves signal integrity by minimizing crosstalk and reducing EMI. However, the order of specific layers can vary depending on the design’s requirements and limitations.
- Power and Ground Planes
Adequate distribution of power and ground planes is essential for effective noise reduction and ensuring stable power distribution. Ground planes act as a reference for the signal and carry its return currents, reducing noise and preventing interference between adjacent traces. Power planes provide a low impedance path, minimizing voltage drop in the power supply. A balanced distribution of power and ground planes throughout the stackup is advantageous.
- Impedance Control
Impedance matching is critical for high-speed design to maintain signal integrity and minimize signal reflections. Controlled impedance ensures signals propagate correctly along transmission lines and reduces the risk of signal degradation. The choice of dielectric material, trace widths, spacing, and layer thickness affects impedance values. To achieve desired impedance levels, it’s recommended to consult the PCB manufacturer’s guidelines and use preferred simulation tools. They can help establish appropriate trace widths and stackup configurations.
- Signal Integrity Considerations
High-speed signals are sensitive to signal degradation due to factors like noise, crosstalk, and impedance mismatch. Careful layer placement and overall stackup planning can help minimize these issues. Signal layers should be placed between power and ground planes to provide a solid reference plane for the signal’s return path and minimize Electromagnetic Interference (EMI).
- Return Paths
In high-speed PCB design, return paths are one of the most important factors to consider. A return path is the route that signals follow to complete their circuit loop from the signal’s path back to the source. An often overlooked challenge in high-speed routing is that the return current must closely follow the signal. When a signal is routed through a via to another layer, it’s crucial to also connect the reference layers with vias. If the return path is disrupted, it can lead to impedance mismatch and potential signal integrity issues.
- Signal Escape Routing
Escape routing involves guiding signals from high-density footprints to a location on the PCB where they can be efficiently routed as proper transmission lines. For instance, it’s used to route signals away from fine-pitched component footprints like Ball-Grid Array (BGA) components. Creating correct transmission lines near a footprint is nearly impossible, so the focus should be on making them as good as possible until there is enough space to accommodate proper transmission lines. Therefore, thorough planning for correct via placement and routing strategies is important to ensure the most efficient signal routing and to avoid potential signal integrity issues near the components.
- Thermal Considerations
It’s a well-known phenomenon that high-speed designs can generate significant heat. Therefore, adequate thermal management is crucial in the stackup planning process. To enhance heat dissipation, including thermal vias is recommended. This prevents overheating of critical components on the circuit board.
Remember to consult design guidelines as well as your preferred simulation tools. Additionally, it’s important to consider optimal design constraints to ensure the best performance.
A solid understanding and thorough handling of return paths are crucial to prevent impedance mismatch and potential signal integrity issues for your PCB.
Finally, it might be a good idea to collaborate with your available PCB production experts, who can provide valuable insights regarding stackup considerations and impedance control for high-speed design.
