In PCB wiring, it often happens that a thinner line has to be used to pass through an area where there is limited wiring space, and then the line is restored to its original width. A change in the width of the line will cause a change in impedance, which will result in reflection and affect the signal. So when can we ignore this effect, and when must we consider its effect?

Three factors are related to this effect: the magnitude of the impedance change, the signal rise time, and the delay of the signal on a narrow line.

First, the magnitude of the impedance change is discussed. The design of many circuits requires that the reflected noise 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 △Z/Z1 ≤ 10%. As you probably know, the typical indicator of impedance on a board is +/-10%, and that’s the root cause.

If the impedance change occurs only once, such as when the line width changes from 8mil to 6mil and remains 6mil, the impedance change must be less than 10% in order to reach the noise budget requirement that the signal reflected noise at the abrupt change does not exceed 5% of the voltage swing. This is sometimes difficult to do. Take the case of microstrip lines on FR4 plates as an example. Let’s calculate. 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 line width changes to 6mil, the characteristic impedance becomes 54.2 ohm, and the impedance change rate reaches 20%. The amplitude of the reflected signal must exceed the standard. As for how much impact on the signal, but also with the signal rise time and the time delay from the driver to the reflection point signal. But it’s at least a potential problem spot. Fortunately, you can solve the problem with impedance matching terminals.

If the impedance change happens twice, for example, the line width changes from 8mil to 6mil, and then changes back to 8mil after pulling out 2cm. Then in 2cm long 6mil wide line at the two ends of the reflection, one is the impedance becomes larger, positive reflection, and then the impedance becomes smaller, negative reflection. If the time between reflections is short enough, the two reflections may cancel each other out, reducing the effect. Assuming that the transmission signal is 1V, 0.2V is reflected in the first positive reflection, 1.2V is transmitted forward, and -0.2*1.2 = 0.24V is reflected back in the second reflection. Assuming that the length of the 6mil line is extremely short and the two reflections occur almost simultaneously, the total reflected voltage is only 0.04V, less than the noise budget requirement of 5%. Therefore, whether and how much this reflection affects the signal depends on the time delay at the impedance change and the signal rise time. Studies and experiments show that as long as the delay at the impedance change is less than 20% of the signal rise time, the reflected signal will not cause a problem. If the signal rise time is 1ns, then the delay at the impedance change is less than 0.2ns corresponding to 1.2 inches, and reflection is not a problem. In other words, in this case, a 6mil wide wire length of less than 3cm should not be a problem.

When the PCB wiring width changes, it should be carefully analyzed according to the actual situation to see if there is any impact. There are three parameters to be concerned about: how much the impedance changes, how long the signal rise time, and how long the neck-like part of the line width changes. Make a rough estimate based on the above method and leave some margin as appropriate. If possible, try to reduce the neck length.

It should be pointed out that in actual PCB processing, parameters cannot be as accurate as those in theory. Theory can provide guidance for our design, but it cannot be copied or dogmatic. After all, this is a practical science. The estimated value should be revised according to the actual situation, and then applied to the design. If you feel inexperienced, be conservative and adjust to the cost of manufacturing.