What is the problem with uneven PCB board thickness

The solder mask of the PCB board surface will reduce the Z0 value of the signal line by 1 to 3 Ω. In theory, the thickness of the solder mask should not be too thick, and the effect is not very large. The surface of the copper wire is in contact with air (εr = 1), so the Z0 value is measured to be high. However, after the solder mask, the Z0 value will drop by 1 to 3 Ω, because the εr of the solder resist is 4.0, which is much higher than the air.

     The inner layer must find the wire gap and the notch. For the 2GHZ high-speed signal, even the 0.05mm gap must be scrapped; controlling the inner line width and defects is the key.