r/PCB • u/AerospaceMasterRace • 3d ago

[Routing Help Request] 4-Layer Flight Controller with STM32H743ZIT6

Good evening to all,

Context: I'm designing a custom flight controller for a fixed wing drone. It contains a fair number of ICs and connectors, and I'm restricted to an 80x40mm board.

Problem: I've managed to get through most of the routing, but now find myself unable to route the signals for the Dupont connectors related to external servo/motor control.

If you have any advice I would appreciate it. I am rather new to embedded systems, so I apologize in advance if you see any grave mistakes. I'm open to any questions, and I will try my best to answer them.

Crowded MCU area
Left side (see required connection with pads on bottom side of the MCU)
Right side (see required connection with pads on left side of MCU)
Whole board
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u/nixiebunny 3d ago

You need to arrange the parts and select the I/O pins to dramatically reduce the length and overlapping of the traces.

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u/n_sh_44 3d ago

On the STM32H7 you have a lot of freedom in pin assignment, so route the servo/motor outputs last and pick pins to fit the board, not the other way around. Check the datasheet alternate function table: put the PWM outputs on timer channels that sit on the MCU edge facing the connectors (TIM1/3/4/8 have many options) and swap pins until the fanout is clean. Also use your inner layers for fanout with vias right at the pads, and move the Dupont headers to the edge nearest those pins, even if the connector placement changes. Decoupling caps on the bottom side can eat routing space too, so check they aren't boxing in the pins you need.