Things like vias, non-homogeneous dielectric, thickness variation and other component paracitics all contribute to impedance mismatch. In any printed circuit board design, it is almost impossible to maintain a constant impedance of the transmission path from transmitter to receiver. Any change in the characteristic impedance results in reflections which manifests itself into noise on the signal. To ensure good signal integrity, it is important to maintain a constant impedance at every point along the way.
A uniform transmission line terminated in its characteristic impedance will have a constant ratio of voltage to current at a given frequency at every point on the line. Likewise, the current passing through them produces a corresponding magnetic field. As the leading edge of a signal propagates down a transmission line, the electric field strength between two oppositely charged conductors creates a voltage between them. One conductor is the signal path and the other is its return path. A transmission line is any two conductors with some length separated by a dielectric material.
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