During PCB wiring, it often happens that when the wiring passes through an area, due to the limited wiring space in the area, thinner lines have to be used. After passing through this area, the lines restore their original width. The change of routing width will cause impedance change, so reflection will occur and affect the signal. So under what circumstances can we ignore this impact, and under what circumstances must we consider its impact?
There are three factors related to this effect: the change of impedance, the rise time of signal and the time delay of signal on narrow line.
Firstly, the magnitude of impedance change is discussed. The design of many circuits requires the reflected noise to be less than 5% of the voltage swing (which is related to the noise budget on the signal). According to the reflection coefficient formula:
The approximate change rate of impedance can be calculated as follows:
As you may know, the typical index of impedance on the circuit board is + / – 10%, which is the root cause.
If the impedance change occurs only once, for example, after the linewidth changes from 8mil to 6mil, the 6mil width is maintained. In order to meet the noise budget requirement that the signal reflected noise at the sudden change does not exceed 5% of the voltage swing, the impedance change must be less than 10%. This is sometimes difficult to do. Take the microstrip line on FR4 plate as an example, let’s calculate it. If the line width is 8mil, the thickness between the line and the reference plane is 4mil, and the characteristic impedance is 46.5 ohms. When the linewidth changes to 6mil, the characteristic impedance becomes 54.2 ohms and the impedance change rate reaches 20%. The amplitude of the reflected signal must exceed the standard. As for the impact on the signal, it is also related to the signal rise time and the signal delay from the driving end to the reflection point. But at least this is a potential problem. Fortunately, the problem can be solved by impedance matching termination.
If the impedance changes twice, for example, after the linewidth changes from 8mil to 6mil, it changes back to 8mil after 2cm is pulled out. Then reflection will occur at both ends of the 2cm long and 6mil wide line. Once the impedance becomes larger, positive reflection will occur, and then the impedance becomes smaller, negative reflection will occur. If the interval between the two reflections is short enough, the two reflections may cancel each other, so as to reduce the impact. Assuming that the transmission signal is 1V, 0.2V is reflected in the first positive reflection, 1.2V continues to transmit forward, and -0.2 * 1.2 = 0.24v is reflected back in the second reflection. Assuming that the length of 6mil line is very short and the two reflections occur almost at the same time, the total reflection voltage is only 0.04V, which is less than the noise budget requirement of 5%. Therefore, whether this reflection affects the signal and how much it affects are related to the time delay at the impedance change and the signal rise time. Research and experiments show that as long as the time delay at the impedance change is less than 20% of the signal rise time, the reflected signal will not cause problems. If the signal rise time is 1ns, the delay at the impedance change is less than 0.2ns, corresponding to 1.2 inches, and the reflection will not cause problems. In other words, for this case, the length of 6mil wide routing line will not be a problem as long as it is less than 3cm.
When the wiring width of PCB changes, it shall be carefully analyzed according to the actual situation to see whether it has an impact. There are three parameters to be concerned about: how much the impedance changes, how long the signal rises, and how long the neck part of the linewidth changes. Roughly estimate according to the above method and leave a certain margin appropriately. If possible, minimize the length of the neck.
It should be pointed out that in the actual PCB processing, the parameters cannot be as accurate as in the theory. The theory can provide guidance for our design, but we can’t copy or dogma. After all, this is a practical science. The estimated value shall be appropriately revised according to the actual situation and then applied to the design. If you feel inexperienced, be conservative first, and then make appropriate adjustments according to the manufacturing cost.