What Changes Will The PCB Design Trace Width Bring

Publish Time: January 05,2019

When PCB routing, it often happens that when the trace passes through an area, due to the limited wiring space in the area, it is necessary to use a thinner line. After passing this area, the line returns to its original width. Changes in trace width will cause impedance changes, so reflection will occur, which will affect the signal. So under what circumstances can this effect be ignored, and under what circumstances must we consider its impact?

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

Firstly lets's discuss the magnitude of the impedance change. The design of many circuits requires that the reflected noise is less than 5% of the voltage swing (this 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%. This is the basic reason why the typical index of the circuit board's resistance is +/- 10%

If the impedance change only occurs once, for example, after the line width is changed from 8mil to 6mil, the 6mil width is maintained. To meet the noise budget requirement that the signal reflection noise at the abrupt change does not exceed 5% of the voltage swing, the impedance change must be less than 10%. Sometimes it is difficult to do. Let us take the case of microstrip line on FR4 sheet as an example. If the line width is 8 mil, the thickness between the line and the reference plane is 4mil, and the characteristic impedance is 46.5Ω. After the line width changes to 6mil, the characteristic impedance becomes 54.2Ω, and the impedance change rate reaches 20%. The amplitude of the reflected signal must exceed the standard. As for how much it affects the signal, it is also related to the signal rise time and the delay of the signal 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 change occurs twice, for example, after the line width is changed from 8mil to 6mil, it will be back to 8mil after being pulled out 2cm. Then the reflection will occur at both ends of the 2cm long and 6mil wide line, one is the impedance becomes larger and positive reflection occurs, and the other is impedance becomes smaller and negative reflection occurs. If the interval between two reflections is short enough, the two reflections may cancel each other, thereby reducing the impact. Suppose the transmitted signal is 1V, the first regular reflection has 0.2V reflected, the 1.2V continues to transmit forward, and the second reflection has -0.2 * 1.2 = 0.24v reflected back. Assuming that the length of the 6mil line is extremely short and the two reflections occur almost simultaneously, then the total reflected 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 is related to the delay at the impedance change and the signal rise time. Research and experiments have shown 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 problems.

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 will not cause problems. In other words, for the case of this example, as long as the length of the 6mil wide trace is less than 3cm, there will be no problem.

When the PCB design trace line width changes, it should be carefully analyzed whether it causes an impact according to the actual situation. There are three parameters to be concerned about: how big is the change in impedance, what is the signal rise time, and how long is the neck-shaped part of the line width change. Approximately estimate according to the above method, and leave a certain margin appropriately. If possible, minimize the length of the neck part.

It should be pointed out that in actual PCB processing, the parameters cannot be as accurate as the theory. The 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 appropriately revised according to the actual situation, and then applied to the design. If you feel inexperienced, be conservative and adjust appropriately according to manufacturing costs.