Breakout Board Design Guidelines
Design a high-reliability I2C environmental sensor breakout board named “Sentinels Hub Air Hex”.
Primary sensor:
- Use Bosch BME690 as the main environmental sensor.
- Keep the footprint compatible with BME688 if possible.
- Use I2C mode only.
- Tie CSB directly to VDDIO for I2C.
- Use SDO for address selection: default 0x76, optional 0x77.
- SDO must never float.
Board outline:
- Unique point-top hexagonal PCB.
- Recommended size: 26.0 mm horizontal flat-to-flat, approximately 30.0 mm vertical point-to-point.
- White soldermask.
- 1.6 mm PCB thickness.
- Origin at PCB center.
Mounting holes:
- Two M3 mounting holes on the left and right flat sides, not on vertices.
- Hole type: NPTH.
- Drill diameter: 3.20 mm.
- Hole centers:
- Left: X = -8.00 mm, Y = 0.00 mm
- Right: X = +8.00 mm, Y = 0.00 mm
- Center-to-center distance must be exactly 16.00 mm.
- Add at least 6.5 mm circular mechanical keepout around each hole.
- Do not route critical traces or place components under screw heads.
Sensor island / cape:
- Place the BME690 on a small thermal and airflow peninsula.
- Create a routed C-shaped or U-shaped slot around the sensor, leaving only a narrow neck for traces.
- Slot width: 1.0 to 1.2 mm.
- Use rounded slot ends, no sharp internal corners.
- Sensor peninsula size around 8 mm x 9 mm.
- Neck width around 2.0 to 2.5 mm.
- Do not place copper pours or ground planes on the sensor island except required pads and short traces.
- Keep heat sources away from the sensor island.
- Place no LEDs, no regulators, no TVS diodes, and no connectors near the sensor.
- Add silkscreen text: “NO COAT”, “NO TOUCH”, “SENSOR AREA”.
Connectors:
- Add two Qwiic / STEMMA QT compatible JST-SH 1.0 mm 4-pin connectors for daisy chaining.
- Pinout: GND, 3V3, SDA, SCL.
- Add one 2.54 mm through-hole or large solder-pad header with the same pinout: GND, 3V3, SDA, SCL.
- Place connectors on the main PCB body, away from the sensor island and airflow path.
Power:
- Qwiic power is 3.3 V only.
- Use VDD = VDDIO = 3V3 for the BME690.
- Add clear silkscreen: “3V3 ONLY”.
- Add local 100 nF decoupling capacitors close to the BME690 supply pins.
- Add 1 uF to 4.7 uF bulk capacitance on the main board.
I2C pullups:
- Add 4.7 kΩ pullups from SDA and SCL to 3V3.
- Make pullups disconnectable with a cuttable/solder jumper labelled “PU_EN”.
- Default pullups enabled.
Address jumper:
- Add a robust 3-pad solder jumper:
- GND pad
- SDO center pad
- VDDIO pad
- Default address 0x76 by connecting SDO to GND with a small cuttable trace or 0-ohm link.
- Allow 0x77 by cutting the default bridge and soldering SDO to VDDIO.
- Add a weak 1 MΩ pulldown on SDO so the address pin never floats.
- Silkscreen: “ADDR 0x76 DEFAULT / 0x77”.
Protection:
- Add low-capacitance ESD protection for SDA and SCL near the connectors.
- Add ESD/TVS protection for 3V3 near the connectors.
- Add optional 22–47 ohm series resistors on SDA and SCL.
- Add reverse-polarity protection for the through-hole header power input if feasible.
- Avoid placing protection components near the sensor island.
Optional Pro feature:
- Add optional DNI footprint for Sensirion SHT45-AD1F on a separate small airflow-exposed area or mini-peninsula.
- Keep it away from BME690 heater and all heat sources.
- Label as “OPTIONAL T/RH PRO SENSOR”.
- Do not populate in base variant unless requested.
Manufacturing and reliability:
- Design for IPC Class 3 manufacturing.
- Bare PCB target: IPC-6012 Class 3.
- Assembly target: J-STD-001 Class 3.
- Inspection target: IPC-A-610 Class 3.
- Use ENIG or ENEPIG finish.
- Use high-Tg FR-4, preferably Tg >= 170 C.
- Add global fiducials.
- Add test pads for 3V3, GND, SDA, SCL, ADDR/SDO.
- Add board revision, serial/QR area, and Sentinels Hub logo on silkscreen.
- Do not apply conformal coating to the BME690, optional SHT45, connectors, or routed airflow slot.
Thought for 1 min 11 sec
I've made a plan, and started work on it.
Add SHT45 DNI
Use 4-layer stackup
Run ERC/DRC
Document Class 3
Add export targets
