The traces in the PCBs are used to transfer maximum signal power from the source to the load. If the source’s impedance value does not match the impedance value of the load, some signals will be reflected back to the source. Then only part of the signals travels to the destination. As the demand for high-density and high-speed PCBs has increased dramatically, the PCB designs with controlled impedance have become very popular.
We may be no need to worry about the impedance control if the traces are short or low-frequency. But the reactance and the impedance are vital factors for high-frequency circuits. The higher the frequency, or the longer the traces, the more necessary it is to control the traces’ impedance.