Electronic – Decoupling cap routing on a 4 layer PCB


I would like to place some decoupling caps for an IC. The IC is on the top while the caps will be on the bottom, opposite side. How should I route them:

  • A single VIA that connects the IC pin, power plane and cap

  • Two VIAs, one that connect cap with power plane and another that connects cap with IC pin

Please excuse me if this has been asked previously, but I read a load of resourses and could not find an answer to this. Thanks in advance.

Edit: More detailed picture of the 2 scenarios

(But first, I want to thank everyone for the great answers and comments!)

I have decoupling cap. One pin of the cap is connected to the ground plane by a VIA. That is for both scenarios. The other pin of the cap will be connect a) to power, b) to IC input. Thus, three things have to be connected together ultimately: IC pin, power, cap pin.

  • Scenario 1: A VIA connects the 3 of them, that is a trace from IC pin goes to the VIA, the VIA is goes through power plane and actually connects with power plane, a trace connects this very same VIA with the cap pin.

  • Scenario 2: A VIA is connected to power plane and a trace connects cap pin with this VIA. Then another VIA connects the IC pin with the that trace, but the second VIA although it goes through the power plane does not connects with it. So you have two VIAs: one that connects the cap with the power plane and another that connects the cap with the IC pin.

Best Answer

It actually does not matter too much, according to Henry Ott's book on EMC. Proximity, trace length, and loop size matter more.

If the IC and cap are on different sides of the PCB, placing both vias along the edge of the capacitor pad side (rather than the ends) so they are closer also reduces inductance since magnetic fields of opposite currents in the via cancels out. Multiple vias also helps. Note this also applies to connecting IC power pins to the power and ground planes. Those are part of the loop too. enter image description here

From Electromagnetic Compatibility, Henry Ott 2009

Similarly, placing two caps on the same side on opposite sides of the IC does something similar since the loop currents flow inside each loop in opposite directions and the magnetic overlap a fair amount if the loops are close to each other. enter image description here From Electromagnetic Compatibility, Henry Ott 2009

So you can see that the first requires a pair vias for both the cap and pins, but the vias can be tightly coupled, while the second requires longer traces/loops but no vias. It's twelve of one or a dozen of the other.