A popular rule is that no impedance matching between the trace and load is necessary if the critical length of a PCB trace (l crit) is 1/10th of the wavelength (λ). Yet is this rule of thumb correct?

You can see here that an inch of an average PCB trace can have between 0.1Ω and 15Ω of impedance depending on frequency or risetime of the signal. If we plug the impedance into Ohms law ...