Characteristic Impedance Multi-layer 8 Layers Pcb

What is PCB trace impedance?

In custom pcb fabrication, High Quality Prototype PCB Fabrication
The wires and traces are usually made of copper because it is the element with the least resistance other than silver. Put the ohmmeter on the trace, the DC resistance is almost negligible.
PCBA AC impedance cannot be said to be the same. Unlike resistance, impedance is based on frequency. All wires and traces will have at least some impedance to the current from any driver. Generally, for signals, a fairly fast rise time is usually necessary to make the trace impedance a problem, but it is important to realize that the trace impedance exists and may become a problem.
But why do we need to worry about trace impedance? Why does it exist and can be controlled? If you have heard the term "controlled impedance", you probably already know that you can control impedance. But to understand how and why, it is important to understand the nature of the trace impedance as a result.