When to remove via stubs and non-functional pads?

When to remove via stubs and non-functional pads?

For SERDES channel designs, board/system designers are told to remove stubs and non-functional pads (NFP) of vias on channels to ensure signal quality at the receiver ends. What confuses them the most is when to consider these effects: does one have to remove all NFPs and stubs in any channel carrying signals with data rates over 1Gbps? Can a design still work even if via stubs are kept?

The actual consideration here should be the channel budget. An interface specification usually requires the insertion loss (in dB) of an entire channel within a pre-defined margin at or below certain frequency points. This is the allowed noise budget of the channel. (Note that the important concept here is not data rates in time domain, but frequencies in frequency domain.) As an interconnect component of a channel, via response generates noise, therefore, consumes the budget. If an existing channel carrying the signals with certain data rates has the overall loss within the required margin, the design will work and it is likely that a designer does not need to modify via designs, such as removing NFPs and stubs of vias. This is an important aspect for designers to understand: vias on a channel may have bad behavior but the effects may be out of the range that channel budget specifies; then no action is needed to make changes to vias.

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Read this article to see the examples where the effects of NFPs and stubs may happen within the interested frequency range, and only then designers need to consider the possible signal degradation from vias.

http://pcdandf.com/cms/component/content/article/171-current-issue/8975-integrated-3d-analysis

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