Undoubtedly, most regular engineering projects make use of the standard PCB design. Also, traditional PCBs can’t work for everything. So, if you’re dealing with an advanced application, you’ll need a high-speed PCB.
However, designing high-speed PCBs can be tricky. You’ll need to pay attention to details like signal integrity, reflections, and crosstalk. If you’re unfamiliar with any of these terms, this article is for you.
In this article, you’ll learn everything about high-speed PCBs and the rules for designing one. We’ll also compare it to a high-frequency PCB.
So, let’s get started.
What is a High-Speed PCB?
High-Speed System
High-speed PCBs are devices that use high-speed signals to transfer data between components. The signals travel down the board’s copper traces until they reach their targets. Plus, the signal can be analog or digital.
However, there’s only a little difference between high-speed and standard boards. But first, you need to know this: what we use to classify a system as “high-speed” is the system’s signal edge rates. So, if it’s fast, it’s a high-speed PCB. So, the system’s signal edge rate separates high-speed from standard PCBs.
We also consider any PCB that has high signal integrity as a high-speed PCB. Other indicators that can help you differentiate a high-speed design from a regular one is that high-speed PCBs have RF front ends for wireless networking.
Also, most high-speed designs focus on interconnected designs, PCB stack-up designs, and routing. So the third area won’t be a hassle if you get things right in the first two areas.
High-Speed PCB Applications
Multiple PCBs
High-speed PCBs work in almost every application. Here are some of the applications that support using high-speed PCBs:
- You can use high-speed PCBs in network communication systems. The primary purpose here is signal integrity verification.
- High-speed PCBs can create high-speed digital test boards. These boards can test different signals, including RF signal roll-off.
- High-speed PCBs can also create powerful consumer electronics with high volumes and short time-to-market.
- High-speed PCBs can also create radio circuits with a high requirement for impedance control.
- Additionally, these boards also work in medical equipment that needs high-speed operations.
Other products that use high-speed PCBs include:
- Routers
- High-speed data channels
- Servers
- Power amplifiers
- Storage area networks
- Transceiver modules
High-Speed PCB Design Rules and Challenges
Printed Circuit Board
Like every other PCB design process, high-speed PCBs have rules that define their design method. While we look at some of the rules in detail, we’ll also examine the possible design challenges.
Impedance
You need to consider two impedance values when designing a high-PCB layout. These impedance values include single-ended impedance Zo and differential impedance Zdiff.
These are the common impedance values for both parallel and serial interfaces. Plus, sticking to the correct impedance when designing your high-speed PCBs is essential. Otherwise, your PCB tracks will have signal reflections.
That’s not all. Using the wrong impedance will also cause signal quality loss, low working frequency, and generate unbearable EMI.
Other impedance types you need to consider, even though they’re rare, include:
- Common ZCM
- Even mode Zoe
- And Odd mode zoo
Grounding
For most high-speed PCB designs, it’s impossible to trace all interfaces on one layer. Therefore, you’ll need the help of vias to transfer traces to multiple layers. However, there are specific rules that govern this process.
- Your designs must have the same GND polygon potential on all layers, especially near the signal vias. We call these GND vias stitching vias, and they need to be as close to the GND polygon as possible. Plus, sticking to this rule will ensure all high-speed traces have the same GND reference.
- Be careful when designing your vias. A poor design will cause impedance problems on your board. But, you can avoid this by ensuring all vias have the correct diameters.
- Using vias for high-speed signal routing can be tricky. So, you must place enough space between vias to avoid high current density and overheating.
Crosstalk
Crosstalk is any unwanted effect generated when transmitting signals over a communication track. These unwanted effects are usually on neighboring tracks and often cause signal change.
Additionally, crosstalk usually happens when tracks run too close to each other. In such a case, you can reduce crosstalk by leaving at least 3W space between the tracks.
For differential pairs, use the 5W rule to space your tracks properly. Also, if your board requires a differential pair transmitting periodic signals, we recommend keeping it at least 8W away from other differential pairs.
Also, there should be enough distance between tracks with asynchronous signals and high-speed signal tracks. Plus, you can minimize crosstalk on multilayered boards routing tracks perpendicular to the neighboring board’s tracks.
Components Location
PCB with Components
Component location is essential for high-speed PCB designs, before starting your design, plan—where you’ll place your components.
Ideally, you should place a separate GND polygon for analog components (if any) and keep them away from digital traces and components. That way, you can avoid EMI issues.
Also, high-speed components don’t need very long traces. So, ensure you keep enough space for length tuning so they don’t stay close to interference sources.
Additionally, you must avoid positioning high-speed components near your board’s edge. It will have adverse effects on your signal quality. Instead, you can place such components towards the center.
Track Shape
Different Track Shapes
High-speed tracks must have rounded and smooth corners. Also, you should avoid using sharp turns. But, it can take a reasonable amount of time to get the best track shape.
The best way to get the optimal track shape and avoid impedance changes is to bend them at 45-degree angles.
The Big Three Problems of High-Speed PCB Designs
High-speed PCBs have many problems that could make a circuit useless if you don’t solve them. And, of all the various issues, we’ve listed three major ones you must avoid. So let’s take a closer look at these three.
Timing
Your board has timing problems if your signals are not transmitting correctly, especially compared to other signs. A clock controls all these signals, so if your timing is terrible, the only data you’ll receive is corrupted data.
Solution: you can solve this issue by matching two coupled trace lengths. It will sync the traces with your clock rates and make them transmit at the same time.
Noise
Noise is any form of interference that affects your signals during transmission. There’s some noise on every PCB, but a high noise level can cause data corruption. Hence, making it a problem you must avoid in your designs.
Solution: Keep enough distance between your traces, making them less susceptible to noise. Proper distancing will also reduce your PCB’s noise level.
Integrity
If your signals don’t look the way they should after transmission, it has compromised integrity. Such situations are usually the result of interference that can affect a signal’s integrity during transmission.
Solution: Using a proper impedance between a receiver and transmitter can improve the integrity and quality of your signal. It will also reduce your signal’s noise sensitivity.
High-speed PCB Design Skills
PCB Design
Here are some skills you need before designing a high-speed PCB.
How to Route Traces with Impedance Control
PCB with Blue Traces
You must understand how to calculate layer stack and trace width. The results from these calculations will give you the essential impedance values. Wrong impedance values will adversely affect your signal and lead to data corruption.
Loop Area Minimization
High-speed signals are pretty susceptible to EMI/EMC problems. But, you can reduce the impact of these problems by following simple basic rules. These rules include reducing loop areas, using continuous ground planes and many stitching vias, and optimizing trace return paths.
High-Speed Routing Skills
There are many things to consider when routing high-speed traces. First, you must know how to cut ground planes and keep traces short. You must also shield your traces from interference and space digital lines to avoid crosstalk.
Use Advanced Design Software
Designing high-speed PCBs require many complex features from your CAD software. And you need the necessary skill to handle these features. That way, you can avoid designing terrible high-speed schematics.
High-Speed PCB vs. High-Frequency PCB
Printed Circuit Board
Although high-speed and high-frequency have different definitions, their board are essentially the same. They both use the sine wave as a carrier of modulated signals.
However, there are slight differences. For example, we use high-speed circuits when voltage increases and decreases in short periods. At the same time, we use high-frequency boards with short circuit cycles.
The differences aren’t that distinct to set them as separate boards. They even use the same base materials. You need to focus on keeping the signal integrity or your circuit.
Rounding Up
High-tech PCB
Many things could go wrong if you’re not careful when designing high-speed PCBs. Unlike regular PCBs, you need a lot of time and attention when developing a proper high-speed circuit schematic.
Plus, you need specific skills before making any designs, like knowing how to route your traces properly and measure impedance. You must also watch your component placement to avoid making things complicated for manufacturers.
Do you want to manufacture high-speed PCB? Then, contact us, and we’ll happily offer the best advice and services.
