PCB Manufacturers suggest specifying Impedance

PCB Manufacturers suggest specifying Impedance
This paper will from different way to discuss specifying Impedance.
1, The reasson Why should Specifying Impedance in your PCB Design?
2, How to Specifying Impedance in your PCB Design

Why should Specifying Impedance in your PCB Design?

If you are designing a product where the impedance is critical then you will need to clearly specify the impedance value required on the specific traces.
These may be designs with tight tolerances or ones that cannot be designed using common Microstrip configurations.

How to Specifying Impedance in your PCB Design?
It is important to note that when you define a trace as having an impedance control requirement, it is the impedance of the trace that is critical and not the size of the trace.
By defining a trace as impedance controlled you are giving permission to the PCB manufacturer to vary parameters such as the size of the trace or thickness of the laminate in order to achieve the desired impedance.
The manufacturer will probably go with the recommended parameters from the field solver first, then test the PCBs impedance and if necessary make appropriate changes and produce the PCB again until the required result is achieved.