Differential impedance is defined as the impedance between the two lines when the line pair is driven differentially. A printed circuit board (PCB) mechanically supports and electrically connects electronic components on a circuit board. Components such as capacitors, resistors or active devices are generally soldered on the PCB. Here is an online PCB Differential Impedance Calculator to calculate the differential impedance of Printed Circuit Board.
An electrical transmission line with impedance of 25 Ohm, space between traces of 12cm, and height of trace as 18cm.
d = 2 x z x (1 - 0.48 x exp((-0.96 x (s / h))))
= 2 x 25 x (1 - 0.48 x exp((-0.96 x (12 / 18))))
= 37.345 Ohm