PCB design of an RS-485 sensor board in KiCad with DFM checks

Halden Instruments sells climate controllers to commercial greenhouses. Each controller talks to up to 16 sensor boards over a 200 m RS-485 cable using Modbus RTU. The current sensor board is ten years old, its humidity sensor is obsolete, and field returns show two failure causes: boards killed when an installer wired 24 V backwards, and boards damaged by surges on long cable runs.

The hardware lead has written the specification for the new board: - input 24 V DC nominal, working from 18 V to 30 V, protected against reverse polarity and surges on the power and RS-485 lines; - a buck regulator to 3.3 V, an STM32G031 microcontroller, a Sensirion SHT40 temperature and humidity sensor, and a half-duplex RS-485 transceiver with a jumper for the 120 Ω termination; - a two-layer FR-4 board of 50 × 35 mm with two M3 mounting holes, pluggable terminal blocks, and all parts on the top side for machine assembly; - a bill of materials under USD 8.50 per board at 500 units; - the fab's rules: 0.15 mm minimum track and gap, 0.3 mm minimum drill, 0.15 mm minimum annular ring and 1.6 mm board with 1 oz copper.

You will turn the specification into a block diagram and power budget, draw the schematic in KiCad, choose parts and build a costed BOM, simulate the input protection and regulator start-up in LTspice, lay out the board, run design rule checks against the fab's rules, and produce Gerbers, drill files, a placement file and a DFM checklist. Everything is free software. No board will be made unless you choose to order one.

Done looks like a KiCad project that passes ERC and DRC with zero errors, a BOM within budget, an LTspice result that shows the board survives reverse polarity and starts cleanly, and a fabrication package a contract manufacturer could quote from.