The critical length is very confusing in the industry. Some people say 3 inches, some say 1 inch. I have heard many other claims, mostly because of the misunderstanding of this concept. Many people say that, if the trace is too long, it will cause signal reflection. If the trace is short, it will not reflect. This statement is very wrong, mixing several concepts like a paste. So what is the critical length, what is it, and why should we pay attention to the critical length?
The best way to understand the critical length is to analyze it from a time perspective. It takes a certain time for the signal to travel on the pcb trace. The transmission time on the ordinary FR4 board is about 6 inches per nanosecond. Of course, the surface trace and the inner trace speed are slightly different. Signal reflection occurs when there is a sudden change in impedance on the trace, which is independent of the trace length. However, if the trace is short, the reflected signal has returned to the source when the source signal has not risen to the high level, then the transmit signal is submerged in the rising edge and the signal waveform does not change much. If the trace is long, the signal at the transmitter has reached a high level, and the reflected signal reaches the source, the reflected signal is superimposed on the high level, causing interference. Then the length of the trace has a critical value. Above this value, the return signal is superimposed at a high level. Below this value, the reflected signal is flooded by the rising edge. This critical value is the critical length. Note that this definition is very inaccurate because only one reflection is considered. This is just to understand the concept needs, so for the time being.
So what is the exact definition? In practice, the reflection occurs multiple times. Although the time when the first signal is reflected back to the source is less than the rising edge time of the signal, the subsequent multiple reflections are superimposed on the high level, causing interference to the signal waveform. Then, a reasonable definition of the critical length should be: the length of the trace that can control the interference of the reflected signal within a tolerable range. The signal round trip time over this length is much smaller than the signal rise time. The empirical data found in the test is that when the signal delay on the pcb trace is higher than 20% of the rising edge of the signal, the signal will produce significant ringing. For a square wave signal with a rise time of 1 ns, when the length of the pcb trace is 0.2*6=1.2 inch or more, the signal will have a severe ringing. So the critical length is 1.2 inches, about 3cm.