Disclaimer: The following methods are not applicable to all designs. Facing different designs, engineers need to choose the appropriate methods to deal with them.
In the process of PCB design, high-multilayer, high-density designs are often encountered, so cross-segmentation is inevitable in this case, as shown in the following figure:
This is an HDI project, which is very dense, and there will be cross-segmentation at many points. At the beginning of the design, for DDR3, after simulation, I felt that there was no big problem, so when the design engineer communicated with me, I told him that there was no problem. As soon as I brought it for review, I was dumbfounded. This is not a cross-division, it is simply a division. There is a single wire directly laid on the gap when winding or differential wires back and forth on the gap. Nine times out of ten, it will cause problems. If it is not the problem of SI, it may cause the problem of PI or EMI.
Because cross-segmentation itself will cause impedance discontinuity, impedance discontinuity will cause reflection, and excessive reflection may cause radiation or interference, and then SSN, crosstalk, EMI, and so on may follow.
So, what if the signal has to cross-divide? In fact, there is no very good way to do this. It can only be said that “the best of the bad”. For example: the shortest distance across the division, don’t wire on the gap:
In addition, the gap-mending capacitor is also a common method, that is, it provides a short return path for the signal across the division. Usually, a ceramic capacitor with the size of 0201 or 0402 is placed at the signal cross-division. The capacitance of the capacitor is 0.01uF or 0.1uf. This parameter is not static and needs to be changed according to the frequency of the signal.
Of course, at low speed, in the EMI treatment method, you can also use a jumper, that is, add a 0ohm resistor in the area where the signal line passes through the gap.
