How is the signal impedance on the PCB board calculated?

How is the signal impedance on the PCB board calculated?

If you use Powerpcb, set the stacking mode, layer thickness, copper thickness, dielectric constant, and then click on a certain line to display the impedance in the status bar.

PCB impedance, the signal is a function of the transmission time, transmission distance variable, so the signal may change in every part of the connection. Therefore, the AC impedance of the connection, that is, the ratio of the change of the voltage to the change of the current is determined as the characteristic impedance of the transmission line: the characteristic impedance of the transmission line is only related to the characteristics of the connection itself. In the actual circuit, the resistance value of the wire itself is smaller than the distributed impedance of the system. Especially in the high-frequency circuit, the characteristic impedance mainly depends on the distributed capacitance of the unit distributed capacitance and the unit distributed inductance.