High-speed PCB design refers to any design in which signal integrity begins to be affected by the physical characteristics of the PCB such as layout, packaging, interconnects, and layer stacking, etc. And, when you start designing your board and you run into troubles like delay, crosstalk, reflections or emissions, you enter the realm of high-speed PCB design.
High-speed designs are so unique because of the focus on these issues. You may be used to designing a simple PCB, focusing primarily on the placement and routing of components. However, when working with high-speed designs, it is more important to consider factors such as their distance from the signal, how wide the signal is, where to place traces, and what kind of them are. connected. Also, with these factors in mind, it will take you to a higher level in your PCB design process.
High Speed PCB Design Tips
1. Know design software that offers advanced options
It requires a lot of complex functions for high-speed design in CAD software. Also, there probably aren't many programs aimed at hobbyists, and there are often no advanced options for web-based kits. Therefore, you need a better understanding of powerful CAD tools.
2. High-speed routing
When it comes to high-speed traces, designers need to understand the rules of basic traces, including not cutting the ground plane and keeping traces short. Therefore, protect digital lines from crosstalk at a distance and shield all interference-generating factors to avoid damage to signal integrity.
3. Traces with impedance control
For some signals around 40-120 ohms it requires impedance matching. The cue for characteristic impedance matching is the antenna and many differential pairs.
It is important that the designer understands how to calculate the trace width and stackup of the necessary impedance values. Incorrect impedance values can severely affect the signal, resulting in data corruption.
4. Length matching trace
There are many rows in the high-speed memory bus and interface bus. These lines can operate at very high frequencies, so it is critical that the signal travels from the sender to the receiver at the same time. Also, it requires a feature called length matching. Therefore, the most common standards define the tolerance value that needs to be matched to the length.
5. Minimize loop area
High-speed PCB designers need to know some tricks, high-frequency signals can cause problems such as EMI, EMC, etc. Therefore, they need to follow basic rules such as having a continuous ground plane and reducing the loop area by optimizing the current return paths of the traces, and putting in many stitched vias.
High Speed PCB Design Considerations
PCB layout is very important
Needless to say, efficient PCB fabrication in high-speed circuits is important to the end result. However, the PCB layout is not considered in the first place in this process. Therefore, it will have a significant impact on the design to achieve the necessary functions and successful PCB manufacturing, such as high-level planning and compliance with important factors. Also, you need to deal with some issues before PCB layout, such as Design for Manufacture (DFM) practices and additional considerations for high-speed PCB requirements.
Poor layout can cause performance issues when starting out for testing or for PCB manufacturing. To make matters worse, redesigns or rework are more costly and time consuming due to the need to evaluate PCB failures or performance issues and reassemble prototype layouts.
PCB Design Considerations
In reality, high-speed PCB design has a lot of constraints on designers because you need to meet various signal speeds and other design requirements. Therefore, to achieve a high-speed board design as shown below, there are a few factors to consider:
Schematic Notes: As we all know, a good schematic can provide a good foundation for PCB design. Therefore, schematics can be treated differently depending on whether you are a PCB designer or an electrical engineer. Typically, it sees a schematic as a means of communication that can be connected to a circuit board. But schematics can make a big difference in organizing and presenting your high-speed designs. Therefore, have as much information as possible available on the design schematic, such as trace lengths, necessary component placement, information on the PCB manufacturer, and more.
Trace Length Adjustment: When using a high-speed interface, you need to adjust the length of the traces to synchronize the signal transmission with the data lines. However, the interface may fail at the maximum frequency, or not work at all because it is not synchronized. Also, the higher the interface frequency, the higher the length matching requirements. So in the case of a parallel interface, you just need to adjust the length of all traces. It is important to adjust the length of these traces to obtain the desired length in a set of signals.
PCB materials and requirements for high-speed stacking: This will affect your high-speed design, such as the structure of the layer stack and the material of the PCB.
High-Speed Placement Strategies: Since changing pad size and component clearance minimizes high-speed connection lengths, it enables high-speed design using a variety of approaches to optimize component placement and improve component footprint for high speed.
Differential Pair and Trace Length Routing: It is important in high-speed designs to route differential pairs so that pairs of signals can exist simultaneously.
Crosstalk, Impedance Control, and Parallelism Considerations: In high-speed designs, there are many factors that can adversely affect your design. In addition, there are some tips to consider, such as how to minimize the impact on the design.
Understanding Stripline and Microstrip: Typically, for high-speed designs, it requires multiple approaches to routing traces. If you want to achieve high-speed routing, it is best to have a better understanding of stripline and microstrip routing technology.
Routing Topologies and Best Routing Practices: Often, if you want to achieve the circuit paths required for high-speed routing, you need to use a specific shape or topology. Also, it's great to explore various ways to keep track of trace lengths, escape characters, return paths, etc.
Simulators: For high-speed designs, simulations can be of great benefit before, during, and after layout begins. Therefore, you should have a better understanding of PCB design software to learn tips and tricks for analog design.
How to know if you need high speed PCB design?
1. Is there a high-speed interface on the board?
A quick way to find out if you need to follow the high-speed design guidelines is to check if you have a high-speed interface like DDR, PCI-e, or even a video interface like DVI, HDMI, etc.
All of these interfaces need to follow some high-speed design rules. Also, please provide exact specifications for each data in the documentation.
2. Ratio of trace length to signal wavelength
In general, if your message is the same wavelength as the trace length, your PCB will definitely require a high speed design. Because some standards (eg DDR) require trace lengths to be matched to a minimum tolerance.
A good rough number is if your trace lengths and wavelengths can be controlled within an order of magnitude of each other. Then, it's a good idea to check out the high-speed design.
3. PCB with wireless interface
As you know, every PCB has an antenna, whether through a connector or something on the board, high speed signals need to be designed. Additionally, automotive antennas require tight impedances to match the tuning length.
For boards with SMA connectors or similar, it needs to be connected to a connector with a specific impedance value.
In a nutshell, learning about high-speed PCB design relies on another piece of engineering. Although there are many factors to consider when designing for high speed. Fortunately, your CAD software for PCB design will help you with impedance calculators, trace length reporting options, differential pair routers, and other tools.
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