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I'm creating board with basic MCU STM32F334K8T6.

STM32 have three VCC pins.

Each of them needs to be fed.

Two options:

  1. Connect them to each other.
  2. Connect all power lines at one point.

To use the second option, you must use the bottom layer. The bottom layer is used for the polygon. By adding a power line there, I will break the integrity of the polygon.

Tell me which option is better to choose?

1. enter image description here

2. enter image description here

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I'm assuming you are using the LQFP32 package and a 2-layer stackup, is that correct?

What do you mean by "polygon" on bottom layer? Are you referring to ground pour?

Is you part placement final? (I'm taking the liberty to assume "no").

Using screenshots of your layout, here what I would suggest:

  • flip LDO circuit to top layer (there is still a lot of space)
  • re-arrange circuit so that LDO's VCC output and caps are close to MCU pin 17
  • feed pin 1 and 5 using thick trace underneath MCU (or even a shape)
  • use thicker traces (= pad width) to connect to decoupling capacitors

Doing so your bottom layer is completely free for ground pours or routing traces, the board is one-sided (all components on top side) and you don't need to switch layers to feed power into the MCU.

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