PCB trace routing
The traces (conducting paths) on a PCB carry signals between components in a circuit. In high-speed circuits, maintaining the proper impedance along these traces is vital to avoid signal integrity problems such as electromagnetic interference and reflections that degrade performance. Impedance matching is a process by which the designer ensures that the traces have a uniform impedance value over their length to prevent these issues. The variables on which the impedance of a trace depends include its width, copper thickness, and dielectric material. It is also important to consider how the traces are spaced and located on the PCB. Impedance matching is especially critical for differential signal pairs and single-ended signals.
In general, the higher the frequency of the circuit, the more important impedance matching becomes. As frequency increases, the power delivered to a trace by its current-carrying capacity decreases. This is because the current passes through a larger area of the trace and generates more heat, which in turn increases its resistance. As a result, the maximum allowable current density for a trace is directly proportional to its width. This is why it is important to choose the correct width of a signal line, especially for differential signals or single-ended signals at high frequencies.
Impedance matching is also performed in a pcb to avoid problems such as reflections and other distortions that can cause noise. In most cases, reflections are caused by impedance mismatches on signal lines that cause portions of the signal to be reflected back towards the source. This can result in loss of information, data jitter, and overall signal quality.
How do you perform impedance matching in PCB trace routing?
To prevent this, the designer can perform impedance matching on signals that need it by making sure that the trace widths are tightly matched based on the signal specifications and standards. In most cases, this involves designing the tracing geometry to ensure that the trace width is strictly adhered to. This can be done by calculating the needed impedance for a specific signal, determining the insulating layer stack-up and copper thickness, and using design software to optimize these variables.
There are a number of ways that a designer can verify that impedance matching has been performed on a PCB, including using simulation tools and prototype testing. Simulation programs like Eagle, Altium, and KiCad can be used to predict and analyze the impedance of a circuit board and find any areas that may require improvements or corrections. Prototype testing can be used to measure and validate the impedance of a circuit board by using instruments like time-domain reflectometry.
Impedance matching can be difficult to do correctly, but it is essential for any high-speed digital or analog PCB. As the switching speeds of electronic devices continue to increase, it will become more and more necessary to consider how impedance matching impacts signal transmission and reception. By following the guidelines and best practices for impedance matching, engineers can minimize or eliminate signal integrity issues and ensure reliable performance.