Impedance Matching for Short-Low Frequency Traces
Forget about clock or signal frequency. Think about edge rise time instead. A perfect square wave of, say, 320 Hz actually contains much higher frequency components:



You can see frequency components going right up to 2000 Hz (and they go beyond too). But if you slow the rise time of the square wave, then you actually remove these high frequency components.



As we add higher and higher frequency components, we can see the rise time of the wave getting shorter and shorter.

To decide what frequencies your signal contains, look at the rise time on an oscilloscope.



The rise time of a signal is usually considered to be the time taken to go from 10% to 90% of the amplitude. Once you have taken this measurement, the maximum frequency you should worry about is about:

freq = 0.5 / rise time

A 100MHz clock will be a big problem if the rise time is 10ps!