High-Speed PCB: A Comprehensive Guide

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, and they 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 with high signal integrity to be a high-speed PCB.

Another indicator 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, if you get things right in the first two areas, the third area won’t be a hassle.

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 examine some rules in detail, we’ll also examine possible design challenges.

Impedance

When designing a high-PCB layout, you need to consider two impedance values: single-ended impedance Zo and differential impedance Zdiff.

These are the common impedance values for both parallel and serial interfaces.

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, tracing all interfaces on one layer is impossible. Therefore, you’ll need to transfer traces to multiple layers using Vias. However, specific rules 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. 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 usually affect neighboring tracks and often cause signal changes.

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 requires many complex features from your CAD software. And you need the necessary skills 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.

Best Material In High-Speed PCBs

You will need to figure out the purpose of your high-speed PCB and its usage before choosing a material.

In such cases, the material can affect the signal’s performance and the board’s durability.

It may also affect the final price. Here are a few examples you may want to look into:

Thermoset hydrocarbon laminate: High-speed PCBs will benefit from this material since it has mechanical stability.
PTFE: This one is perfect for high-frequency RF — it doesn’t cause much dielectric loss. Also, the signal performs better with PTFE, which provides greater impedance control.
Polyimide: Polyamide is a very strong material with high thermal/mechanical properties. This makes polyamide-based high-speed PCBs useful in moisture-heavy areas.
Enhanced Epoxy: If you choose the enhanced epoxy material, you’ll see an uptick in performance. After all, it offers enhanced electric properties that work best for multi-layered PCBs.

High-Speed PCB Design Factors To Think About

Some important considerations for your high-speed PCB include:

PCB Schematics: You should produce an easy-to-read schematic. Doing so helps the manufacturer understand how the circuit’s design. Try to provide information like where each component goes, trace lengths, etc.

Stray capacitance: Also called parasitics, this affects high-speed circuitry by causing instability. So, figuring out where it comes from can go a long way to prevent future issues.

Traces: Signal performance might get worse due to the application of stripline or microstrip layouts. For example, the material’s trace thickness/width and dielectric constant will affect trace capacitance, resistance, and inductance. As a result, the relevant signal path won’t work as it should.

Routing: High-speed PCBs can run into problems when signals disturb each other. So, circuit designers should ensure signal paths don’t have that issue. They can do this by making sure each signal trace isn’t close to the other. Also, avoiding long traces will help to keep this issue at bay.

When Should You Use A High-Speed PCB?

At this point, you can easily tell when it’s time to rely on a high-speed PCB for your project. All of these factors will give you that perfect idea:

The circuit board has digital radiation, skin effect, and ionization loss in it
Linkage length is higher than λ/12
Signal speed changes impedance and how the components function
Digital signal delay is more than the rising edge time by 20%

Software For Your High-Speed PCB Design Needs

So many tools out there will get the right design down to a tee. However, before starting it off, those working on it must confirm the PCB is electromagnetic-supported and handles power integrity.

Certain programs get the job done without failing.

Even then, simulators are another practical tool—they find any setbacks in a circuit, such as EMI, SI, or PI-related challenges.

Designers love the Altium Designer 20 software. That’s because it loads with extraordinary features, like propagation delay, to ensure a flawless circuit board.

In this case, they can peek at both the signal trace and signal propagation delay.

What’s better, other programs, including EAGLE, will make everything nice and easy to work with. Now you’re pretty much guaranteed a promising result in the end!