PCB Technical - The Importance and Future Prospects of High Speed PCB Design

What is high speed pcb design? High-speed design refers specifically to systems that use high-speed digital signals to transfer data between components. The dividing line between high-speed digital designs and simple boards that use slower digital protocols is blurred. The general metric used to characterize a particular system as "high speed" is the edge rate (or rise time) of the digital signals used in the system. Most digital designs use both high speed (fast edge rate) and low speed (slow edge rate) digital protocols. In today's era of embedded computing and the Internet of Things, most high-speed boards have an RF front end for wireless communications and networking.

Planning Your High-Speed PCB Stacks and Impedances

The PCB stack you create for your high-speed board will determine the impedance and how easy it is to route. All PCB stacks include a set of layers dedicated to high-speed signal, power, and ground planes. Here are some points to consider when assigning layers in a stack:

Board size and number of nets: How big is the board and how many nets do you need to route in the PCB layout. Boards that are physically larger may have enough space to allow you to route nets throughout the PCB layout without using multiple signal layers.

Routing Density: With a high number of nets and a board size limited to a small area, there may not be much room for routing around the surface layers. As a result, you will need more internal signal layers when alignments are closer together. Using a smaller board size may force higher wiring density.

Number of Interfaces: Sometimes it is a good strategy to wire only one or two interfaces per layer, depending on the bus width (series vs. parallel) and board size. Keeping all signals in the same layer for high-speed digital interfaces ensures that all signals see consistent impedance and skew.

Low-speed and RF signals: Will there be any low-speed digital or RF signals in your digital design? If so, these may take up space on the surface layer that could be used for high-speed buses or components, and may require additional internal layers.

Power Integrity: One of the cornerstones of power integrity is the use of large power planes and ground layers for each of the voltage levels required for large integrated circuits. These should be placed on adjacent layers to help ensure that there is high planar capacitance to support the stabilized power from the decoupling capacitors.

Routing Design Guidelines for High-Speed PCBs

1. Cabling Basics and Strategies

Routing Methods: These include single-sided, double-sided, and multilayer routing, as well as automatic and interactive routing. Interactive wiring is suitable for strict requirements of the line, automatic wiring can be dealt with before the key parts to avoid parallel lines caused by reflection interference.

Optimization of wiring: The efficiency of automatic wiring depends on good layout and preset rules, such as the number of wire bends, the number of vias, etc.. Firstly, carry out fast and short connecting line wiring, and then optimize the path globally.

2. Power and ground processing

Decoupling capacitor: add between power and ground to reduce noise.

Line width design: ground line > power line > signal line, signal line width is recommended to be 0.2-0.3mm, power line 1.2-2.5mm.

Large copper layer: utilize the unused space as ground line, or separate the power and ground layers in the multilayer board.

3. Common ground treatment for digital and analog circuits

Isolation: Separate the internal ground of digital and analog circuits, only a little shorted at the interface outside the board to reduce interference.

Layout: High-frequency signals away from sensitive analog components to ensure the shortest signal path, and separate input and output lines.

4. Signal lines and layer selection

Power (ground) layer wiring: When the signal layer resources are tight, consider wiring in the power or ground layer, giving priority to retaining the integrity of the ground layer.

5. Large area conductor connection

Thermal isolation: component legs and large-area copper surface connection using cruciform pads to reduce the risk of soldering false soldering.

6. Network system and grid design

Grid density: based on 0.1 inches or its integer multiples of the settings, balancing the amount of data and wiring path, to avoid ineffective path.

7. Design Rule Check (DRC)

Comprehensive review: Ensure wire spacing, power/ground width, critical signal processing, independent ground, avoiding short circuits, etc. meet requirements.

8.Special Considerations

High-speed nodes: clock wiring needs to consider delays and use termination devices to optimize SI.

Technology selection: driving technology should match the task requirements, consider rise time and noise tolerance.

Pre-wiring simulation: Determine the parameter range, and the simulation results guide the wiring constraints.

Post-wiring simulation: Check SI and timing issues, make timely adjustments.

Model Selection and Future Trends: Emphasize model accuracy, pursue automated tools to simplify SI design, and focus on new technologies.

High speed PCB wiring design requires careful planning, from power and ground processing to digital and analog circuit isolation, to the choice of wiring strategy, each step needs to follow strict rules and CAD tools to assist.

Through continuous simulation verification and rule checking to ensure that the final design not only meets the electrical performance requirements, but also has a good manufacturability.

Alignment impedance is an important element of high-speed PCB design, with the increasing complexity of electronic systems, impedance hazards are also increasing, engineers need to do a good job in order to ensure the competitiveness of the product impedance design, and one of the sources of impedance is the alignment, talk about how to do a good job in the alignment impedance design, reduce its impact.

In general,the common alignment impedance can be divided into single-ended 50ohm and differential 100ohm,of course,more and more impedance types also appear, such as 27ohm, 33ohm,75ohm,120ohm (diff), etc., but at present to 50ohm and 100ohm mainly.

Multiple single-wire impedances and differential impedances may exist in the same design at the same time, which requires the use of different alignment widths.

High speed PCB design plays a critical role in modern electronic devices,with the core objective of optimizing signal transmission and ensuring signal integrity. As technology continues to advance, the increasing complexity and integration of electronic systems makes impedance design even more important.Engineers must focus on alignment impedance control to minimize signal delay, distortion and interference to ensure product performance and competitiveness.

The electrical performance of high speed pcb can be effectively improved through accurate PCB stacking, power and ground handling, signal line layout, and strict design rule checking. In addition, simulation verification and model selection will play a key role in the design process to ensure that the final design meets manufacturing requirements. Looking ahead, with the development of automation tools and the emergence of new technologies, high-speed PCB design will move toward greater precision and efficiency, paving the way for intelligent and high-performance electronic products.