Printed circuit board

PCB design alignment width changes generated by the reflection

In the PCB wiring, it often happens that the alignment through a certain area, due to the limited wiring space in the area, had to use a thinner line, through this area, the line and then restore the original width. The change in the width of the alignment causes a change in impedance, so reflections occur and have an impact on the signal. So when can this effect be ignored, and in what cases must we consider its impact?

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

First, discuss the size of the impedance change. Many circuits are designed to require reflected noise less than 5% of the voltage swing (which is related to the noise budget on the signal), according to the reflection coefficient formula can be calculated from the impedance of the approximate rate of change required: △ Z / Z1 ≤ 10%. As you may know, the typical indicator of impedance on the board is + / – 10%, the fundamental reason for this.

If the impedance change occurs only once, for example, the line width from 8 mil to 6 mil after the change, has been maintained 6 mil width of this case, to achieve a sudden change in the signal reflection noise does not exceed 5% of the voltage swing this noise budget requirements, impedance change must be less than 10%. This is sometimes difficult to do, in the case of microstrip lines on FR4 sheets, for example, let’s calculate. If the line width is 8 mil and the thickness between the line and the reference plane is 4 mil, the characteristic impedance is 46.5 ohms. After the line width changes to 6 mil the characteristic impedance becomes 54.2 ohms, and the impedance change rate reaches 20%. The amplitude of the reflected signal must exceed the limit. As for how much impact on the signal, but also with the signal rise time and the drive end to the signal delay at the reflection point. But at least this is a potential problem point. Fortunately this can be solved by impedance matching termination.

If the impedance changes twice, for example, after the line width changes from 8mil to 6mil, and then pulls out 2cm and changes back to 8mil, then reflections will occur at both endpoints of the 2cm long 6mil wide line, once with a higher impedance and a positive reflection, then with a lower impedance and a negative reflection. If the interval between the two reflections is short enough, it is possible for the two reflections to cancel each other out, thus reducing the impact. Suppose the transmission signal is 1V, the first positive reflection has 0.2V reflected, 1.2V continues to be transmitted forward, and the second reflection has -0.2*1.2 = 0.24V reflected back. Assuming again that the 6mil line length is extremely short and the two reflections occur almost simultaneously, the total reflected voltage is only 0.04V, which is less than 5% of this noise budget requirement. Therefore, whether and how much this reflection affects the signal is related to the time delay at the impedance change and the signal rise time. Studies 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, then the time delay at the impedance change is less than 0.2ns corresponding to 1.2 inches, the reflection will not cause problems. In other words, for this case, the length of the 6 mil wide alignment will not be a problem as long as it is less than 3 cm.

When the PCB design alignment line width changes, the actual situation should be carefully analyzed, whether it causes the impact. Need to focus on the parameters by three: how much impedance change, how much signal rise time, how long the neck-shaped part of the line width change. According to the above method to roughly estimate the amount of appropriate allowances. If possible, try to reduce the length of the neck section.

It should be noted that the actual PCB processing, the parameters can not be as accurate as in theory, the theory can provide guidance to our design, but can not be copied, not dogmatic, after all, this is a practical science. Estimated values to do appropriate revisions according to the actual situation, and then applied to the design. If you feel inexperienced, then be conservative first, and then in the appropriate adjustment according to the manufacturing cost.


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