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  • How to deal with cross-segmentation of signal lines in PCB design ?
     
    Date :2017-8-27

     

    Disclaimer: The following method is not applicable to all designs, the face of different designs, the need for engineers to choose the appropriate method for processing.

     

     

    In the PCB design process often encounter high multi-layer, high-density design, then in this case will inevitably cross-segmentation of the situation, as shown below:

     

     

    This is a HDI project, very dense, many points will appear cross-segmentation, at the beginning of the design, for DDR3, after simulation, that the problem is not, so the design engineers and I communicate, told him no problem, the results Take a look at the time, dumbfounded Which is cross-segmentation, is simply split it Back and forth in the gap on the gap or differential line has a single line directly on the gap, in all likelihood will cause problems, not the SI problem, it may cause PI problems, or EMI problems.


    Because the cross-split itself will cause the impedance of the discontinuity, the impedance will cause a discontinuity reflex, reflection is too large may cause radiation or interference, and then SSN, crosstalk, EMI, and so the problem is likely to follow.

     

    So what if the signal must be split across? In fact, this is not a very good way, can only be said to be "bad in the election". For example: the shortest distance across the split, do not wrap on the gap:

    In addition, the patch capacitor is also a commonly used means, that is, cross-segmented signal to provide a shorter return path. Usually in the signal across the division placed a 0201 or 0402 size ceramic capacitors, the capacitance of the value of 0.01uF or 0.1 uF, this parameter is not static, need to change according to the frequency of the signal changes.

     

     

    Of course, in the low-speed, in the EMI treatment method, you can also use the cross-way, that is, in the signal line through the gap area plus a 0 ohm resistance.

     

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