Factors that influence impedance

Why we need design PCB impedance control
With the improvement in signal transmission speed and the wide application of high frequency circuit, the requirements for printed circuit boards are higher and higher. To get complete, reliable, accurate, interference-free and noiseless signal, the printed circuit board must play a role in matching impedance, minimize signal reflection, and reduce the transmission loss, make signal completed. 

For a high frequency board, impedance matching directly affects the signal quality, perfect impedance matching can make all microwave signals along the transmission line reach the load end, there won’t signal reflect back to the source end.

Therefore, the control of characteristic impedance is very important for the PCB with high frequency signal transmission.

Factors that influence impedance
Trace width, the impedance trace width is inversely with the impedance value, the thinner the trace is, the larger the impedance value.

Space, not only the space between the impedance traces will affect the impedance value, but the ground copper around the impedance traces. Impedance traces spacing is proportional to the impedance value, the smaller the space, the smaller the impedance value.

Copper thickness, it is inversely with the impedance value, the thicker the copper, the smaller the impedance value.

Dielectric thickness, it is the thickness between the shielding layer and the impedance traces. Dielectric thickness is proportional to the impedance value, the thicker the dielectric, the larger the impedance value.

Dielectric constant, it is inversely with the impedance value, the larger the dielectric constant, the smaller the impedance value.

The thickness of the solder resist also will affect the impedance value, but it is usually ignored by the designer.

The relationship between the factors that affect the impedance 

From the above pie chart, we can intuitively see the proportion of each factor to the impedance value.
Dielectric thickness ＞ impedance trace width ＞ copper thickness ＞ Dielectric thickness 
So both the designers and factories will give priority to the greater proportion factor.