High Speed PCB Design
High speed pcb is when signal integrity begins to become impacted by physical characteristics such as board size, interconnection, layer stackup and routing. Getting the basics right will help to avoid problems such as crosstalk, reflections and ground bounce.
It is often not possible to trace all high speed interfaces on one layer so vias (electroplated holes) are used to move traces between layers. GND polygons on differential layers should be placed near signal vias.
EMI/EMC
High speed signals require special attention to maintain their signal integrity and to minimize electromagnetic interference (EMI) and crosstalk. This requires good component placement and routing techniques. In addition, careful consideration of power distribution and clock distribution is also needed. It’s best to have these aspects of the design considered from the start of the PCB layout process.
For example, when two parallel traces are touching, they can create EMI by inductively and capacitively coupling. This can affect the signal on one trace and cause it to deviate. This is called “cross-talk,” and can make the circuit unreliable. To minimize this, traces should be separated by at least twice the width of the trace.
Another way to minimize EMI is by ensuring that all signals have proper terminations. This can be accomplished by using low-inductance capacitors and by reducing the number of layers in a signal path. It’s also important to minimize the distance between different layers in a PCB. This can be done by placing components close together and by using short, wide traces.
Having the right PCB manufacturer is also crucial. They should be familiar with the challenges associated with high-speed design, and they should have the required tools and materials in stock. In addition, they should be able to offer advice on the high speed pcb correct material stackup and board materials. This includes choosing a dielectric with a stable frequency response, selecting copper with a low profile and minimizing the thickness of solder masking.
Trace Routing
The signal lines of a high speed PCB work at very fast frequencies and have to be tuned to the proper lengths to ensure they can carry data with the highest efficiency. This requires the designer to follow certain rules such as not cutting ground planes, keeping traces short, shielding traces from interference and spaceing digital lines to prevent crosstalk. They also need to ensure that the distance between differential pairs is equal in length and use length equalization on certain signals. This feature is important in order to synchronize signal propagation through these types of copper tracks and make them work at the highest possible frequency.
Another aspect of high speed traces that needs to be taken into account is the impedance they have. This is especially true for differential pairs and characteristic impedance matched traces that are used in many high speed interfaces. The designer must be able to calculate the proper impedance for these signals by using a layer stack calculator and determining the correct trace width and spacing. Wrong impedance values can cause significant data corruption on a signal and must be avoided.
Lastly, the high speed designer must be able to recognize different routing topologies and know how to choose the right one for each circuit path. This is important because it can minimize EMI/EMC issues and allow for the most efficient data transfer on the board.
Trace Lengths
Trace lengths can be a crucial factor in high speed PCB design. If the High-Speed PCB Supplier traces don’t match the lengths specified in the signaling standard, they may not function correctly. This can result in a number of problems such as signal loss and phase distortion.
This can be caused by several factors including component specifications, PCB laminate losses, and routing path lengths. These can cause skew and phase distortion which can lead to signal quality problems such as poor performance or even failure.
The other major issue can be trace inductance which is caused by the magnetic field generated by the current flowing on the traces. This is usually caused by long traces and not having enough ground planes. In order to minimize this effect, it is necessary to use the correct lengths and avoid putting unnecessary components on top of each other.
Another important rule is to ensure that all of your GND connections are a continuous and a single unified potential. This is called the stitching via rule and should be followed to prevent any issues with signal return paths and EMI coupling. It is also important to stick to the right impedances when designing your circuit board. Adhering to these rules will help you create a high-speed design that is free of any unwanted effects.
Layout Optimization
In high speed PCB design, layout optimization is one of the most important aspects of the circuit board. It encompasses a variety of factors, including power and ground plane placement, impedance matching, and component footprint enhancements. The right ECAD software can help you create a circuit board layout that is optimized for high speed.
For example, you can use backdrilling to get rid of stubs on high-speed signals; look at the datasheet to see if on-die termination is available to avoid signal reflection; and minimize skew by using tuning structures. Having the right distance between nets can also help improve signal integrity and reduce noise sensitivity. In addition, ensuring that differential pairs are routed separately can help to prevent interference.
It is also crucial to place the ground planes near the signal traces, as this can help to minimize EMI issues. However, you should be careful to avoid any gaps or splits in the ground planes. These can cause impedance discontinuity, which may lead to EMI problems.
The best way to optimize your PCB layout for high-speed is to start by creating a clear and concise schematic. This will allow you to visualize the layout and understand what is happening in the design. It will also make it easier to work with your fabricator and ensure that you are taking all the necessary steps to prepare your board for high-speed manufacturing.
