Controlled Impedance in PCBs
Controlled impedance is the process of matching the PCB’s substrate material properties with the trace dimensions and locations to create a specific electrical impedance for a signal moving along that track. This impedance is important because it can have a big impact on how well the circuit functions and on whether or not electromagnetic interference (EMI) will occur.
EMI is a significant issue in high-speed applications because it can lead to signal distortion. This is because the reflected energy from the circuit board can bleed over to nearby traces and interfere with the signals’ ability to carry energy properly. This can cause signal loss and other issues that can have a serious effect on the overall function of the device or system. Managing controlled impedance in the manufacturing process is an effective way to avoid these types of problems.
The significance of controlled impedance in pcb is that a mismatched impedance can result in a pulse of energy reflecting off the conducting surface of a circuit board and bouncing back toward the source. This can disturb the signal, causing it to slow down, lose energy, or even be interrupted completely. It can also disturb other parts of the circuit, causing unwanted electromagnetic radiation that may affect sensitive components or devices.
The Significance of Controlled Impedance in PCBs
To avoid these kinds of problems, it is crucial to design a PCB with the correct impedance levels built-in during production. This can be done through the use of controlled impedance guidelines provided by integrated circuits’ datasheets. These guidelines provide detailed information on which traces require which impedance values and how to route them correctly. These guidelines are often accompanied by a detailed impedance chart that can help you understand what each signal is expected to look like at different frequencies.
While the ideal impedance value for a PCB calculated from a schematic is fairly straightforward, the actual impedance experienced by a signal traveling on an actual circuit board can vary significantly. This is because the materials used in the substrate, the etching process, and the stack thickness can all change the impedance of a track. In addition, the layout of a circuit board can have a significant effect on impedance as well. This is because different tracing layouts can produce different signal reflection patterns.
It is also important to distinguish impedance-controlled traces from other traces on the circuit board by clearly labeling them. This will allow the fabricator to quickly identify the traces and make adjustments to achieve the correct impedance level. For example, if the traces on a circuit board need to be 5.1mils wide to reach a 50O impedance, the designer should label those as such in order to make it easier for the manufacturer to recognize them during fabrication.
Another thing to keep in mind when specifying an impedance target is that it can be difficult to hit those targets precisely the first time around. For this reason, it is crucial to communicate clear requirements to your fabricator up front so they can make the necessary changes without delay and avoid delivery delays.