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I was watching an STM32 PCB layout video by Phil’s Lab. At 1:55:19 he routes the traces between the STM32 MCU and the two decoupling capacitors. Here is a screenshot:

enter image description here

He goes on and recommends to always connect decoupling capacitors like he did (from the MCU pin, into the two capacitors, and only then into 3V3), in order to maximize filtering. My question now is whether this indeed is a better way to wire up decoupling capacitors when compared to this layout:

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I know that the closer the decoupling capacitors are to the MCU, the better. So in the end the answer might just be, that the routing solution that results in the shortest traces between MCU pin and decoupling capacitor is optimal?

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  • \$\begingroup\$ First pic is better. Decoupling is like RC filter. Resistance of conductor works in that case despite it is small. Decoupling to prevent some current fluctuations from chip affect to the rest of circuit. \$\endgroup\$
    – user263983
    Feb 18, 2021 at 20:21
  • \$\begingroup\$ That is one way to do it. Some will say another way is better. It also depends on where the via connects to. It might be a 4-layer board with separate VCC and GND layers for low inductance, or it might be a 2-layer board with long wires and higher inductance to power supply. \$\endgroup\$
    – Justme
    Feb 18, 2021 at 20:25
  • \$\begingroup\$ Yes, his recommended way is probably better -- but what's much more important is that you have a decoupling cap for every VCC/GND pair, and that they're close to the chip. \$\endgroup\$
    – TimWescott
    Feb 18, 2021 at 20:37
  • \$\begingroup\$ The goal of decoupling layout is to have the smallest area enclosed by the loop going from the power pin to the cap and then back from the cap to the device ground pin. The second way actually has a much larger loop since it forces ground to go down through the via and then back up somewhere further away. \$\endgroup\$ Feb 18, 2021 at 23:55

1 Answer 1

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First one agrees with Recommended PCB routing guidelines for STM32F4xxxx devices AN4488 Application note. Pin -> thick track -> cap -> via.

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Power supply decoupling All power supply and ground pins must be properly connected to the power supplies. These connections, including pads, tracks and vias should have as low impedance as possible. This is typically achieved with thick track widths and, preferably, the use of dedicated power supply planes in multilayer PCBs.

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