Multi-Sensor Fitness & Health Wristband — PCB Design Case Study
Ultra-compact 4-layer wearable PCB integrating optical heart rate, skin temperature, galvanic skin response, and 6-axis IMU into a 30 mm × 22 mm board. Designed for high-volume consumer production with aggressive BOM cost optimization and UL/CE certification path.
Problem Statement
A consumer electronics brand needed a wearable biosensor PCB to undercut competitors on price while adding galvanic skin response (GSR) that no rival wristband offered at the sub-$80 retail price point. The board had to last 7 days on a 100 mAh battery, pass FCC Part 15 and CE RED, and be assembly-ready for a 10,000-unit pilot at a Southeast Asia EMS partner.
Design Constraints & Specs
- 4-layer board, 30 mm × 22 mm, 0.6 mm total thickness — rigid-flex interface connector at one edge
- BLE 5.3 chip antenna with FCC/CE pre-certified module option for regulatory shortcut
- 7-day runtime from 100 mAh LiPo — target average current < 595 µA
- GSR electrodes on board edge — stainless steel pads with 2.54 mm pitch, 100 nA leakage max
- All 0201 passives; LGA and QFN only — no BGAs to reduce assembly cost at target EMS
- Unit BOM cost target: $6.40 at 10,000 units
Design Challenges Solved
- Achieved 7-day runtime by mapping all sensors to time-division sampling windows with the MCU in deep sleep between readings — measured average draw 541 µA with BLE notifications enabled
- GSR measurement accuracy required 100 nA leakage from electrode pads; solved via guard ring traces biased at the same potential as the measurement node, reducing leakage to 18 nA
- BLE antenna performance degraded when wrist-worn due to body loading; tuned matching network using smith chart analysis to shift resonance back to 2.44 GHz, recovering 3.1 dBi effective gain
- Hit $6.38 BOM target by redesigning voltage reference from precision IC to a calibrated resistor divider with factory one-point trim, saving $0.22 per unit
Deliverables
- Schematic optimized for BOM cost with approved alternates identified for all sole-sourced components
- 4-layer PCB layout with EMS design-for-assembly review completed
- BOM with pricing at 1k/10k/50k unit breaks from three distributors
- FCC/CE pre-compliance antenna test report
- Pick-and-place and solder paste stencil aperture files for high-speed SMT line
- DFM report signed off by the target EMS partner
Outcome
First 50-unit pilot assembled at EMS partner with 99.2% first-pass yield. FCC Part 15 and CE RED certification completed in 6 weeks. Product launched on schedule; wristband achieved #3 ranking in category on launch platform within 30 days.
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