How to Design a High Speed PCB
There are many factors to consider when designing a high speed PCB. Whether you need a design that is optimized for fast-changing signals or not, it’s always good to know how to avoid some common problems.
First, remember to keep analog components apart from digital traces and components to prevent EMI. Also, make sure that you leave enough space for length tuning on high speed traces.
Material
The material of a high speed pcb can have an impact on its performance. It is important to select a good substrate that will support the design and its signals. Various types of PCB substrates may help you meet your needs, so work with your contract manufacturer to find the right one for your needs.
In general, a FR4 substrate is the most common choice for low to mid-frequency boards up to about 1 GHz. If your routes are going to extend into the 10″ or longer range, select a substrate that has higher material properties to minimize signal loss.
Besides selecting the correct PCB substrate, it is also critical to consider other components and routing in the design. This can help prevent problems like signal integrity issues and EMI/EMC.
1. Visualize traces as transmission lines: When designing, it is essential to consider a board’s highest operating frequency and use a digital analysis tool to determine which traces should be treated as transmission lines. This will reduce the risk of crosstalk and time delay.
2. Consider material with a low dielectric constant: The presence of a low dielectric constant will minimize the effects of crosstalk and EMI/EMC on a board’s performance. This can be achieved by placing solid ground planes between traces or using a low-dielectric-constant material in the RF layers.
3. Avoid tracing patterns that are sharply curved: Traces that are sharply curved or bent in the corners may increase reflections and crosstalk. This will interfere with the signal transfer and affect the integrity of the circuit.
4. Keep wires short and avoid loops: The length of a copper conductor can also have an effect on the high-speed signal transmission quality. The shortest length of conductor is best because it allows the signal to be transferred faster.
5. Choose the appropriate impedance: A PCB’s impedance can affect its performance by changing the radiated signal and reducing its ability to transmit it through the power supply. The correct impedance will ensure that the PCB can perform at its maximum capabilities.
The most important thing to remember in high speed pcb design is to follow the correct impedances, which can help keep the circuit functioning at its peak. This is especially true when the board has to interface with other devices or transmit data over long distances.
Layout
Layout is often the last step in the PCB design process, but high speed circuitry calls for special layout techniques to ensure accurate fabrication. While it may seem like an extra step, ensuring the correct layout is essential to a successful design and manufacturing process.
The first step in laying out a PCB is to create a schematic. This schematic is a representation of the electrical layout of your circuit, and it contains details such as signal routing and component placement.
During the layout phase, a designer will use a tool called a CAM (computer-aided manufacturing) to lay out the traces on the PCB. The tools used by a CAM can include automatic routing, which automatically aligns traces to minimize the impact of signal delays.
Assigning a single-ended or differential impedance to tracks can also help improve the quality of the circuit. However, it is important to check that the impedance values match those required by the manufacturer for your application.
Incorrect impedances can cause unwanted EMI and can affect the performance of your circuit. Fortunately, most design software systems have impedance analysis tools that make it easy to spot and correct such issues.
Another way to prevent these high speed pcb EMI issues is to ensure that you do not place vias in densely packed locations on your board. This can lead to heat problems and overheating of the conductive material within the vias.
Traces should be kept as short as possible, especially when routing high-speed signals. Long traces can deviate from trace to trace, which can cause timing errors in the circuit.
To help you align traces in a high speed design, Altium offers a new feature called Propagation Delay. This feature allows a designer to see the length of each trace and the propagation delay at the same time.
The Propagation Delay function can be very useful for high-speed design because it allows a designer to set the required propagation delay and then have Altium automatically detect mismatched tracks. It also lets you create custom rules to control the resulting propagation delay on selected tracks.
Design Rules
High speed pcbs are used for many different types of applications, from computers and servers to cell phones and Internet routers. This type of board design requires a special set of design rules to make sure the circuitry is able to operate at a speed that is adequate for the intended use.
The first rule in designing for high speed is to ensure that the signal traces are matched length-wise. If not, you might experience errors or other problems.
In addition, a designer must maintain a minimum distance between a digital line and the other traces in order to prevent crosstalk that could interfere with the signal. In addition, the designer should shield any elements that could cause interference, such as long stub traces that act like antennae.
When routing differential pairs of signals, the trace lengths should be matched to achieve an high speed pcb impedance match. This is important because it avoids the pad entries on the signals causing a difference in spacing, which can increase the impedance and possibly affect the operation of the device.
Another rule is to ensure that the traces are routed parallel to each other. This keeps the signals from being distorted as they travel along the copper.
It is also important to keep the traces short because long stub traces can create reflections that damage the signal integrity. For this reason, daisy chain routing can be a good option.
Finally, the design must be able to resist electromagnetic interference, such as from radio waves and EMI. This is usually accomplished by ensuring that the PCB stackup is designed to minimize common mode radiation.
The stackup should include a complete ground plane and a power plane, as well as a number of regulated voltage layers. This will help the designer to separate the signal lines from the ground and power planes, which can reduce impedance and common mode radiation.
The CAD tool should also provide the designer with a feature that allows them to monitor the propagation delay for all selected tracks at once. This is a great way to verify that the design rules have been met.
Impedance
Getting the right impedance for a high speed pcb is a critical part of ensuring its signal integrity. Without it, a high-speed signal may not be able to get through the board properly or at all. The trace must be able to maintain a consistent and uniform impedance throughout its length, as well as have a clear return path on a reference plane.
The impedance of the trace depends on three factors: substrate material, its width and height from the ground or power plane, and the insertion loss of the material itself. If the impedance of a trace changes from one end to the other, it can cause the reflection of a signal back to its source.
Aside from causing unnecessary reradiation of signals, uncontrolled traces can also cause signal integrity issues. If the insertion loss of a trace is too low, it will cause resonant frequencies to be absorbed by the dielectric material, which can result in distorted signals that cannot propagate.
This is known as the “skin effect.” Choosing a good PCB material is critical to controlling this. FR4 is the typical material for high-speed circuits, but some other materials can be used as well.
Generally, it is preferable to use laminates that have a low insertion loss. PTFE-based laminates, spread glass laminates, or other specialized systems are a good choice for high speed digital boards with long routes that require low insertion loss.
Other considerations include the layer count and number of interfaces on the board. Keeping all of your high-speed digital interfaces on the same layer keeps all of the signals in one region and makes it easier to maintain consistent impedance.
Another important consideration for a high-speed pcb is the power plane and ground plane. They are essential to preventing common-mode radiation and ensuring stable power delivery.
The layer count needed for your high-speed pcb can be determined by several factors, including the number of components you need to route and the size of those components. You can also get an estimate of the required layer count by calculating the number of digital signals in your design and subtracting the number of layers needed for power and ground planes.
