In the electronic industry, high speed circuit boards are frequently used. It is crucial that PCBs used in modern electronic devices meet specific requirements. Therefore, the demand for high speed PCB design has continued to rise overtime. Also, hardware engineering professionals now focus more on high speed PCB design.
A high speed circuit board is crucial for developing high-tech devices. As high tech devices keep getting smaller, data transmission keeps getting faster. Also, these devices are more handy and functional. So, it is crucial for PCB designers to understand the basics of high speed board design. In this article, we will be providing important information about high speed design. Also, we will delve in high speed PCB design guidelines.
What is a High Speed Board Design in PCB?
A high speed circuit board design offers signal transfer at a very high speed. This means that a device featuring a high speed circuit board can transmit data at a very high rate. High speed circuit design offers a lot of advantages to manufacturers and engineers. This PCB design helps to develop the most advanced electronic solutions.
However, there are certain difficulties at the development stage related to signal integrity. The right choice of PCB material can help to reduce integrity problems. However, you can still face problems like crosstalk, electromagnetic interference (EMI), and signal degradation. Therefore, engineers pay more attention to these problems.
Designing a simple PCB might not require much of your time. However, it is important to consider some decisions when designing a high speed board. Here, you need to consider where to place your traces and the type of width you need to use. Also, you will need to consider the closeness of the traces to other signals.
The integrity of signals impacts the physical characteristics of this design. These characteristics include interconnection, stackup, and layout. A high speed design is a system that uses high speed digital signals to transmit data between components. The edge rate of digital signals in the system denotes the term “high speed.”
Routing, interconnect design, and PCB stackup design are the major focus of the high speed PCB design. If you achieve success in these areas while designing your board, you will have a functional high speed PCB.
What is Considered High Speed Digital Design?
Not all PCBs are high speed. There are ways to distinguish a high speed circuit board from other boards. Also, certain characteristics help you to identify a high speed PCB design. Therefore, a design is high speed if:
- Digital signal frequency is 50MHz and more
- Circuit features several sub-circuits linked to each other via high speed interfaces like CSI and LVDS
- PCB size is very small and locating the components is more challenging
- It utilizes Ethernet, UB, and HDMI for fast data transmission
- Signal propagation time over the track is at least one-third of the signal rise time
A high speed PCB design is usually applied to high frequency PCB that use high speed interfaces. Therefore, the amount of speed and data transferred is of great importance.
Factors to Consider When Designing High Speed PCB
There are rules and challenges when designing a high speed layout and schematic. Also, there are certain factors to consider as some challenges may arise.
Impedance
When designing a high speed board layout, evaluate the single-ended impedance Zo. Also, you should consider a differential impedance Zdiff. The impedance of single tracks not united in differential pairs refers to single-ended impedance. Also, differential impedance refers to impedance between coupled tracks.
Other types of impedance include common Zcm, even mode Zoe, and odd mode Zoo. Also, these impedances aren’t common. However, they are important. It is crucial to adhere to the right characteristics impedance when designing a high speed circuit.
Components location
The placement of components is a crucial point in high speed circuit layout. Therefore, you need to figure out the location of the components on your PCB. For instance, sort out the components based on their functionality. Also, locate components close to each other if they perform similar functions. Analog components should feature GND polygons.
Also, place analog components separately from digital traces and components. This helps to avoid electromagnetic interference. Also, create enough space for the length tuning. You don’t need to place high speed components close to interference sources. Also, you shouldn’t position components associated with a high speed interface too close to the board’s edge.
This placement can impact the signal quality negatively. Therefore, it is more ideal to move these components to the board center. Components location is more challenging when a circuit board is very small in size.
Crosstalk
This phenomenon happens when a transferred signal over a communication track causes an unwanted effect in other tracks. This effect indicates a change in signal. Also, the lengths of the section where there are parallel tracks determine crosstalk.
You can reduce crosstalk by putting some things in place. Therefore, the distance between the tracks should be at least three times longer than the width of the track. Also, differential pairs should have distance between them. This distance should be at least five times longer than the width of the track. Also, this helps to reduce crosstalk between differential pairs.
Trace length tuning
It is important to tune the length of traces for signal propagation synchronization via data lines. If there is synchronization, the interface may not work at all or malfunction at the maximum frequency. Therefore, trace length tuning is an important aspect in high speed design.
Higher interface frequency results in higher requirements of length matching. You only tune the traces’ length for a parallel interface. However, there are several traces without space for length tuning. You can join signals in differential pairs to achieve a serial interface.
It is crucial to remember the interconnecting delays in FPGAs and CPUs for very high speed interfaces. The rules for matching of length for differential pairs are more complex. All traces must have same length.
Grounding
It is impossible to trace high speed interfaces on just one layer. Therefore, you have to use vias to move traces to other layers. Also, vias are some electroplated holes that help traces connect with one another. GND polygons on differential layers should be near signal vias. Therefore, place such GND vias close to signal vias. Also, GND vias are stitching vias. With this, you can maintain the same GND reference throughout the high speed trace.
You need to be careful with how you use vias for high speed signal routing. Over populated vias can result in high current density. Also, this can result in consequently overheating. Ensure you create enough space between vias. Also, ground and power planes are important considerations for this design.
Signal Integrity, Routing, and Power Integrity
Signal integrity
This often begins with designing to a certain impedance value in a PCB. Also, it involves maintaining the specific impedance value throughout routing and layout. There are other strategies that help to achieve signal integrity. These include:
- Removing stubs in extremely high speed lines with backdrilling
- Reducing routing through vias
- Consulting with the manufacturer on the process and materials that can avoid fiber weave effects
- Keeping a list of nets and buses that need length matching. This will help you to apply tuning structure to remove skew
- Focusing on shorter routes between components to create high speed signals
- Using a rough crosstalk simulation or calculation to identify the right spacing between nets in your PCB layout
High speed routing
Following the design rules for your high speed project will help to meet spacing and impedance requirements. Also, you can enforce vital rules in differential pair routing. You can encode trace geometry limits with the best routing tools. It is very crucial to place ground planes close to your traces in high speed design.
Construct the layer stack to have ground planes in adjacent layers to impedance controlled signals. This helps to maintain consistent impedance and define return path in the high speed PCB layout.
Power integrity
It is important to maintain stable delivery of power to high speed components. This is because power integrity problems are often disguised as signal integrity issues. Also, they cause unnecessary radiation from buses and interconnect as transients develop strong oscillations. Therefore, utilize decoupling capacitor groups to ensure power delivery.
Also, use power and ground planes pair on adjacent layers. This helps to offer more capacitance to maintain low PDN impedance.
Signal Integrity in High Speed Board Design
There are going to be signals in your copper traces. These are analog and digital signals. A digital signal is a clock or square wave signal. Also, this signal is a simpler representation of values. It comprises low point and high point. An analog signal features a set range of positive and negative values. This type of high speed signal can feature a degree of results according to their signal frequency and strength.
The problem with these two signals is that they are vulnerable to interference. When an environment affects a signal, signal problems arise. To ensure signal integrity, you have to put certain things in place. signal integrity is crucial in high speed PCB layout.
For instance, if there is a net transferring some signal containing data from point A to point B on a PCB layout. Point A is the transmitter while point B is the receiver. There are certain things that disrupt the integrity between point A and B. Some of these things are:
- Signal ringing
This happens when there is an unwanted shifting in the current on your trace. Therefore, this results in extra current flow. This causes delay for your high signals.
- Signal reflection
It occurs when there is a signal in transfer along a copper trace. Then the signal reflects back to its starting point instead of getting to its destination.
- Signal time
Signal timing happens when high speed signals sent to their final destination don’t get there on time. Therefore, you might interpret your signal as 1 when this occurs.
- Signal noise
Signal noise occurs when there is a random fluctuation from another signal on a PCB. This can impact other high speed signals close to it.
Advanced Tools for High Speed Design and Layout
There are PCB design softwares for achieving high speed circuit design. These software provide enhanced functionality when used for this design. Also, Designers have to carry out a lot of work to ensure electromagnetic compatibility and power integrity. However, the appropriate tools can help to achieve great results.
There are more developed PCB design software that helps to achieve industry-standard results. Some simulation programs are specifically designed to evaluate signal and power integrity. Also, simulations are important in this PCB design. This is because they figure out certain PI, SI, or EMI issues before manufacturing a design.
Advanced software like EAGLE, Altium Designer, and more can help you achieve great results. There are new features included in Altium Designer 20. These features help to enhance a high speed layout. For instance, the propagation Delay is a great feature. This feature helps you view the length of the signal propagation delay and signal traces simultaneously.
Circuit designers trust the tools provided by Altium Designer for high speed layout and design. Ensure you work with the best tools when designing a high speed PCB. Also, this helps to make your design process easier and faster.
Conclusion
Here comes the end of our article. High speed PCBs play a crucial role in the electronics industry. The PCB design process comes with a lot of challenges. Also, engineers have tried to look for ways to tackle these challenges. Therefore, a high speed PCB design should be handled by an expert.
For your high speed design project, RayMing has got you covered. We offer quality high speed PCBs and also adhere to high speed design guidelines. Also, we provide assistance and professional review of your existing design.
