I have seen the answer for when to consider PCB trace as a transmission line in many places.
Typically, if the signal pulse rise time is ‘small’ compared to the time it takes for the signal pulse to propagate (e.g. ½ of the total time the signal takes to travel along the trace) then you need to consider your PCB as a high-speed circuit
(Source: EMA Design Automation)
No one explains how the signal integrity problems arise when the rise time is smaller than two times the propagation delay.
- What happens to a signal when its rise time is less than the propagation delay?
- How will this condition cause signal integrity issues?