Generally speaking,3 levels of service are available for an Impedance Control PCB.
1.No impedance control: The impedance tolerance is enough wide. Only impedance PCB design with standard specifications and without extra precautions, the manufacturer will manufacture the correct impedance.This is the fastest and cheapest option because it has no extra burden for the impedance control PCB manufacturer.
2.Impedance watching: Before manufacturing impedance control PCB, the PCB manufacturers may need to adjust the width of the trace and height of the dielectric and gets approval on the proposed specifications.A TDR (Time Domain Reflectometry) test will be performed to confirm the impedance for an additional cost.
3.Impedance control. Firstly, the impedance control PCB manufacturer makes the circuit board, getting as close to the target impedance as possible. Next, do a TDR test to determine if the impedance is within specification and if necessary, manufacturer maybe made some adjustments.
In the case of a microstrip, the impedance will depend on 4 parameters:
1.H is the height of the dielectric. It can be changed in steps. In this example +/- 1 mil results in +/- 2 ohms.
2.Er is the dielectric of the material. It is fixed once the material is chosen. Having a good idea of the Er is necessary since +/- 0.1 results in +/- 0.5 Ohms.
3.T is the trace thickness. An outer trace is plated, providing a 20% uncertainty in exterior traces. This results in a small uncertainty of +/-0.2 ohms.
4.W is the trace width. Typical trace width uncertainty is +/-2 mil which results in an uncertainty of +/- 2 ohms.
As the example provided, if the target impedance is 50 ohm, a 26 mil trace width is required.Because there is a tolerance on the input parameters, it translates into a tolerance on the trace width. Achieving the calculated trace size should produce required impedance.
Last but not least, a typical tolerance on final impedance is +/- 10%.
In order to understand the point well, you should get a good understanding of Er values and experience about how dielectric laminates behave.your PCB manufacturer has the knowledge and capabilities to meet your requirements. Specifying impedance control ensures you will need to work closely with your circuit board provider but the results are worth it.