Breakout Board Design Guidelines

jcajiao

jcajiao

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.

flux

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
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C4
Capacitance
22uF
VBAT
BAT1
L1
Not Recommended for New Designs
Inductance
2.2uH
IC2

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