mohan-bee/fitness-band-sensor-flex

manufacturing/ASSEMBLY_AND_BRINGUP.md

# Revision B — assembly and bench bring-up

## External items

- One protected 150 mAh PKCELL LP402025 pack matching the Adafruit 1317 manufacturer specification; battery is installed after reflow.
- One Semitec **103NT-4-R025H41G**, 10 kΩ / 4126 K thermistor. The common 103AT-2 is not an interchangeable substitute for this circuit.
- Eight insulated AWG32 jumpers, each ≤30 mm, and a three-wire battery/NTC harness. Provide nonconductive strain relief on both stiffened lands; no solder joint or strain-relief blob in the bend reserve.
- A keyed two-pogo bench charging fixture on 3.0 mm pitch, tip ≤0.8 mm, with an external regulated/current-limited supply. Use 5.2 V nominal, never exceed 5.25 V at the contacts; initial current limit 0.15 A. Measure at least 4.5 V at CHG_5V during charging to retain the charger's recommended headroom after fuse/diode drops. Ordinary 5 V supplies may charge more slowly under load.
- SWD probe and a 1.3 mm pitch six-contact fixture; optical gasket/window, insulating temperature-contact membrane, insulating board/cell spacer and R3 fold mandrel.

## Wiring

Both interconnect pad faces are viewed from the component side. Pad 1 is at source y +5.25; subsequent pads descend in 1.5 mm steps. Connect numbered pads **1:1** with the eight jumpers; do not overlap the boards face to face.

| Pad | Main J_FLEX → flex J_TAIL |
| --- | --- |
| 1 | GND |
| 2 | 3.3 V |
| 3 | GND |
| 4 | SCL |
| 5 | SDA |
| 6 | PPG_INT |
| 7 | 1.8 V |
| 8 | GND |

Main battery lands are rotated 180°: source coordinates **BAT (+10, −10.5), GND (+8, −10.5), NTC (+6, −10.5)**. Connect pack positive to BAT and pack negative to GND. Connect the thermistor between NTC and GND. Bond its glass head electrically insulated to the cell pouch, without puncture or compression. Keep soldering/bending ≥5 mm from the sensor head per its datasheet. Do not replace it with a fixed resistor.

Dock POS is source (−6.5, +11), GND (−3.5, +11). The external fixture must key the polarity and prevent metal bridging. Keep fixture magnets/steel outside the radio clearance zone; no magnets are fitted to the PCB itself.

SWD pads left to right at y −9.5: VTREF 3.3 V, SWDIO, GND, SWDCLK, RESET, GND. VTREF is a reference, not an uncontrolled second supply. Do not power the target from two unrelated sources simultaneously.

## Mechanical assembly

Mount the flex components over their bottom stiffeners, inspect/reflow on a flat carrier, and inspect all joints before folding. Fold at x −6 over an R3 mandrel so the optical component side faces outward toward the skin below the main assembly. Keep the flex clear of battery edges and prevent recurring motion at the fold.

Reserve battery envelope 28 × 21 × 5 mm on the reverse of the main PCB, x −4…24 and y −10.5…10.5. Add an insulating spacer; provide wire exit and swelling allowance without clamping the cell. Support the radio overhang with nonmetallic material. No battery foil, flex copper, charging magnets or skin-contact metal in its antenna zone. Physical RF/enclosure validation follows board bring-up.

Use an opaque optical barrier between LED/photodiode paths and a suitable window/gasket, with no direct LED-to-detector leakage. Do not put exposed electrical pads against skin. TMP117 requires a thin electrically insulating thermal interface, separated from charger/regulator heat. These boards alone are not a sealed wearable enclosure.

## Power and electrical acceptance

1. With no battery or dock, inspect polarity, QFN/LGA joints, paste bridges, via holes, harness numbering and thermal isolation. Check for shorts between BAT, SYS, 3.3 V, 1.8 V and GND.
2. Use a battery simulator at 3.7 V on BAT/GND, initially current-limited to approximately 20 mA. Blank MCU/no LED activity should not draw sustained high current. Stop on unexpected heating/current. Confirm approximately 3.3 V (3.2–3.4) and 1.8 V (1.75–1.85) at the flex lands before attaching the flex.
3. Verify all eight harness conductors and insulation. Attach flex; confirm both rails at U7. Program over SWD. Configure nRF52840 normal-voltage operation and the **internal LFRC** clock with calibration; no external 32.768 kHz crystal is fitted. Factory module test firmware may assume different clock hardware.
4. Start I²C at 100 kHz. Expected 7-bit addresses: BMA400 0x14, MAX17048 0x36, TMP117 0x48, MAX30102 0x57. Verify IDs, interrupt polarity and scope rise times before increasing bus rate.
5. Test the dock and charger first with a battery simulator/electronic load, not a real cell. Check input polarity protection, input headroom and 4.2 V regulation. R7 targets 50 mA; measure actual charge current. Test THERM with precision resistance substitution: a normal value near 10 kΩ permits charging; an open or short external NTC must inhibit it. Never connect a cell while a sensor-bypass test fixture is installed.
6. Fit the specified cell and attached NTC only after those checks. First charging is supervised at room temperature, with PPG LEDs disabled. Measure current, termination and timer behavior. Use controlled temperature testing to verify charging stops before cell temperature leaves 0–45°C; include NTC lag/attachment and restart hysteresis. Do not heat or cool an unprotected cell as an improvised test.
7. Enable PPG at low current first (approximately 5–12.5 mA LED setting), then measure 3.3 V transients and sensor quality before raising LED current. Verify motion, temperature trends, BLE range and current consumption across battery state and radio activity.
8. Complete cell thermal, RF-on-body/enclosure, optical contact, insulation, skin-contact materials and sweat-ingress qualification before wearing the assembled prototype. Do not charge it while worn.

Charging current/temperature limits are hardware-controlled. Firmware and mobile application development are separate work; no measurement accuracy or endurance is claimed by this PCB release.