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The fear of using vias

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Everywhere you read about PCB design, people always recommend against using vias for high speed tracks (I know the theory mostly). But what about when you can't avoid it? Let's say X number of pins on a chip need to be connected to X number of pins on another chip but these pins are in the wrong order (which they usually are!). Even if it's just two nets, that's two vias for one of them if you need to flip them. I assume it's the same even if you're using multilayer boards, you still need to switch layers.

Is there some magic trick I haven't heard about or is vias simply a necessary evil in most boards?

It's a case of prioritising - route the more critical circuits first, when there's plenty of room, and the less critical ones later on, where vias become more necessary.

I seem to remember reading that if you need to add a via you should put a small capacitor near it.  I don't have any practical experience of this, but of course it does all depend on what you call "High Speed". I know people who would refer to anything less than 1GHz as DC.

I would suggest going to http://www.compliance-club.com/ and looking through the back issues and articles on PCB design.


I think it is most important to choose initially a routing layer which avoids layer swaps. So it is ok to use vias near the components but worse in the middle of the trace. Next best thing to have the whole trace on same layer, is to swap to other side of the reference plane, but that is not usually possible unless you have 6 or more layers.

The trick here is to figure out where your return current is going to flow. It wants to flow as near the signal current as it can. If the return current is not able to make a neat transition near the via, it will flow anywhere it can, usually causing EMC/SI problems :)


Many high speed lines are implemented using impedance matched lines. ie LVDS are typically a 100 ohm line running around your board. You'd want to lay down 100 ohm balanced lines across a ground if at all possible for those designs. And you'd want to avoid via's for the simple reason that you wouldn't really know what was going to happen to the matched lines. Get it wrong and you may end up with alot of common mode ground noise.

But as always.. when designs are compact.. you often have little choice but to 'go for it'. Sometimes you get lucky, sometimes not. But you generally wouldn't pump fast impedance matched lines through via's if you could help it.


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