Who knows via in PCB design?

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Joyce Taylor
Posts: 6
Joined: 12 Jul 2020, 22:22

Who knows via in PCB design?

#1 Post by Joyce Taylor » 24 Jul 2020, 05:29

Is it recommended to put the vias in the middle of the pad? I am making a 4 layer PCB. I want to connect the SMD(package 1210) components from the 1st layer(signal layer) to 2nd layer (ground). So Can I place the via in the mid of the component pad or should I place it near to the component and connect it through a track?

Thanking in advance.

Alex
Technical Support
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Joined: 14 Jun 2010, 06:43

Re: Who knows via in PCB design?

#2 Post by Alex » 11 Nov 2020, 04:09

Via in pad are not recommended to use because solder leakage through the hole. The exceptions are small hole vias where risk of leakage is very small , blind vias, tented vias, etc.

mpaalanen
Posts: 18
Joined: 21 Jul 2010, 17:05

Re: Who knows via in PCB design?

#3 Post by mpaalanen » 12 Nov 2020, 16:22

Normally you would want to avoid this for the reason given above.
However, when designing a high frequency circuit the parasitic couplings change the situation. At hundreds of MHz, and definitely in the GHz, the shortest extra traces can introduce significant parasitic capacitances and/or inductances. For example a bypass cap can be rendered completely ineffective if it resonates with the inductive trace stub connecting it to a via.
So, if you are designing a RF board then yes, often via-in-pad is a necessary/beneficial feature. For a low frequency board, not so much. Though remember, that signal frequencies are not the only defining feature, edge rates count equally. A modern logic circuit can snap the signal between states real quick and the transition represents frequencies (remember your Fourier series) possibly in the GHz. So demands to bypassing can still be rigorous.
Anyway, don't put vias in pads as a rule. If the PCB lacks space to route the stubs, use smaller components. 1210 is huge, 0805 is no harder and 0603 is easy after a little practice.
One way to mitigate parasitics for decoupling and bypass is to put the vias on left and right of the pads, not the end.

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