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Via stubs are defined as the part of a via that is not used for signal transmission. Via stubs cause all sorts of problems, but I just don't understand why they happen in the first place.

Can't you just not make a via stub and keep only the part of the via used for signal transmission?

I can see two explanations for this:

  1. The via stub serves a purpose
  2. The via stub does not serve a purpose, but it's easier for the electronics design automation (EDA) software to just place it there, or it facilitates manufacturing in some way
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  • \$\begingroup\$ It can be done, for example through backdrilling. Costs money. \$\endgroup\$ Commented May 12, 2023 at 14:31
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    \$\begingroup\$ For a low-cost design but still with strict signal integrity requirements, one can use the strategy "always route sensitive signals on top and bottom layers, never the middle layers" to minimize via stubs without backdrilling. \$\endgroup\$ Commented May 13, 2023 at 16:12

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It is to avoid blind and/or buried vias. These are much more expensive to produce compared to a through via.

To avoid stubs you need to use either a blind or buried vias:

https://www.pcbcart.com/pcb-capability/blind-and-buried-vias.html

The Through-hole via is very cheap and easy to manufacture since it can be done after laminating. The blind and buried vias are much trickier to assemble and require more steps.

In most cases this is not a problem and through hole vias can be used. When working with very high frequencies, the stubs may become a problem and the more expensive process must be used.

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    \$\begingroup\$ As is usually the case for my questions on this site, "easy to manufacture" ended up being the answer! Thanks a lot for explaining why exactly it is easy to manufacture, much clearer now! \$\endgroup\$
    – David Cian
    Commented May 11, 2023 at 19:07
  • \$\begingroup\$ I've never done a board that didn't have blind or buried vias. That's just normal for my industry. \$\endgroup\$
    – SteveSh
    Commented May 11, 2023 at 20:43
  • \$\begingroup\$ Great answer that would be fully complete if it mentioned backdrilling :) \$\endgroup\$ Commented May 13, 2023 at 18:32
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What you're talking about is the difference between a normal via (one that goes the whole way through), a blind via (that goes from an outer layer to an internal layer), and a buried via (one that goes between internal layers.

For any applications (<GHz) the via stub doesn't really affect the signal, so it's left in. Blind and buried vias are more costly to manufacture, so they're only used when really needed.

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    \$\begingroup\$ One shortcoming of always using "normal" vias is that it takes away from routing space on layers the via passes through. This may cause you to increase the layer count, which also increases cost. \$\endgroup\$
    – SteveSh
    Commented May 11, 2023 at 20:40
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Via stubs are avoided if you have a reason to avoid them, and if you don't have a reason to avoid them then just leave them in if they don't matter.

Via is not a stub if via goes only between the layers you want. If you go from top to bottom layer, that's easy as the whole via is needed and there's no unused length of via that would be a stub.

If the via is between top and bottom layers but you only need to pass the signal to some middle layer, then the unused length of the via is a stub.

If the stub does not matter to your signal you can leave it in. If you need to get rid of the stub it is possible to drill the unused length of the via but that's expensive.

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