mohan-bee/fitness-band-sensor-flex

manufacturing/FABRICATION.md

# Revision B — fabrication and assembly instructions

Fabricate **two independent PCBs**, not an integrated rigid-flex. All dimensions below are millimetres. Source coordinates use board centre (0,0), +Y up; native KiCad coordinates are X = 100 + source X, Y = 100 − source Y. Do not recenter individual layers or drill files.

Read MANUFACTURING_STATUS.md before placing an order. File checks pass; fabrication/assembly acceptance is pending, particularly the flex via process, custom coverlay and panel support.

## Rigid main board

| Item | Specification |
| --- | --- |
| Outline | 50 × 24; use main-Edge_Cuts.gm1, not module/courtyard outline |
| Material | FR4, Tg ≥150°C, nominal 0.8 mm; standard four-layer stack, no impedance-controlled nets |
| Layer order | F.Cu / In1.Cu GND / In2.Cu / B.Cu |
| Copper | 35 µm outer; manufacturer standard inner copper ≥18 µm |
| Finish | ENIG, lead-free assembly; black or green solder mask |
| Minimum width/space | 0.10 / 0.10; ordinary routes 0.15, fine-pitch neckdowns approximately 0.112 |
| Vias | Through, finished hole 0.20, land 0.45, minimum annulus 0.125; tent both faces where not inside an exposed component aperture |
| Other holes | Two NPTH Ø1.6; use separate NPTH drill file |
| Stencil | 0.10–0.12 mm stainless; use F.Paste. U2 EP has four 1.1 mm windows; do not replace with a full EP aperture |
| Components | 32 SMT parts, top side; no battery during reflow |

The radio module extends 3.2 mm beyond the left board edge. Do not add copper, ground pour, rails, metal fasteners or panel break tabs under its antenna. Remove manufacturing rails before installing/testing the module. Host copper begins beyond its all-layer antenna reserve; preserve the separate top-only notch. Support the module mechanically in the prototype fixture.

## Sensor flex

| Item | Specification |
| --- | --- |
| Outline | 36 × 14; laser profile from flex-Edge_Cuts.gm1 |
| Material | Two-layer adhesiveless polyimide core, 25 µm dielectric |
| Copper | 18 µm each side, rolled-annealed preferred for the static fold |
| Coverlay | Yellow PI, both sides; 12.5 µm PI + approximately 15 µm adhesive per face |
| Finished thickness | Approximately 0.12 mm excluding stiffeners; use manufacturer's 0.12 mm build and published tolerances |
| Finish | ENIG; no hard gold/contact mating connector requirement |
| Minimum trace/pad clearance | Trace width 0.15; exposed-pad to unrelated trace 0.15; via-to-trace 0.10; copper-to-outline ≥0.30 |
| Vias | **Advanced 0.20 mm drilling / 0.45 mm lands**. Do not substitute the regular 0.30/0.55 mm process |
| Components | Eight SMT parts, top side, using a rigid reflow carrier/support panel |
| Stencil | Approximately 0.10 mm; optical sensor apertures per F.Paste; no paste on tail or test contacts |

### Layer assignment — flex

| File | Actual manufacturing role |
| --- | --- |
| flex-F_Cu.gtl, flex-B_Cu.gbl | Top/bottom copper |
| TOP_COVERLAY_OPENINGS.gbr | **Positive laser openings** in top coverlay, including solder contacts. This is not adhesive paste. Native CAD storage layer is F.Adhesive |
| BOTTOM_COVERLAY_NO_OPENINGS.gbr | Bottom coverlay, entirely closed; an empty openings layer is intentional |
| BOTTOM_FR4_STIFFENER_0p4mm.gbr | Two bottom FR4 support regions; not copper or paste. Native storage is B.Adhesive |
| BOTTOM_PI_STIFFENER_0p15mm.gbr | Bottom PI tail support; not copper. Native storage is Eco1.User |
| flex-F_Paste.gtp | Component stencil only |
| flex-B_Paste.gbp | Empty: no bottom components |
| flex-Edge_Cuts.gm1 | Finished flex outline |
| PTH drill / drill map | Plated vias; no components or plated holes in the bend |

The ordinary native F.Mask is only a per-pad CAD reference and is **not the laser-coverlay definition**. It is deliberately omitted from the flex fabrication package. Grouped laser windows expose multiple fine-pitch pads; solder deposition remains controlled by the separate stencil. Some top vias/traces inside SMT windows are exposed intentionally; apply no paste to vias. Bottom vias remain covered. Inspect those regions and apply an appropriate insulating coating after assembly, keeping optical apertures and temperature contact surfaces clear.

All top laser windows have ≥0.5 mm web separation after merging. See validation/flex-coverlay-windows.json. Preserve these merged openings rather than attempting narrow PI dams between QFN/LGA pads. Copper/connectivity DRC and laser-coverlay review are separate checks; KiCad does not model the complete flex lamination process.

### Stiffeners and fold

All stiffeners are on the **bottom**. Adhesive nominal 0.025 mm; allow manufacturer thickness tolerances and include adhesive in the stack report.

| Region, source coordinates | Material |
| --- | --- |
| x −18…−12, y −7…7 | PI 0.15 mm |
| x 0…10, y −7…7 | FR4 0.4 mm |
| x 11…18, y −7…7 | FR4 0.4 mm |

No stiffener, adhesive squeeze-out, hole, via, component or extra copper may enter x −11…−1. The bend axis is x −6; a 180° static fold at neutral radius ≥3 mm occupies approximately 9.43 mm developed length. Fold over a mandrel once after assembly; this is not a cyclic flex hinge. Eight top traces cross perpendicular to the bend; keep bottom copper absent there.

The 1 mm gap between the two FR4 supports is intentional for temperature isolation. Request panelization and reflow support from the fabricator/assembler: 5 mm handling rails and fiducials outside the finished flex, suitable break tabs outside the bend/sensor areas. These single-circuit files define the finished boards; fabrication tooling must not change their geometry.

## Assembly handoff

Use exact MPNs in each BOM; supplier stock and consigned-part arrangements must be checked when placing the order. No automatic substitute is approved. Position CSV is in KiCad native convention (X right, Y up from absolute origin, rotation in degrees). The assembly-orientation CSV uses absolute KiCad X/Y-down for component centres and pad 1; use it and the native board to map the assembler's library rotation. Verify first-article polarity and pin 1 before reflow, especially U1, U2, U6, U7 and D1.

Mechanical contacts, SWD pads, test pads and AUX mechanical items are excluded from the BOM/placement file. Mount battery and NTC by the separate harness instructions after board inspection and electrical tests. Native project constraints contain no electrical DRC exclusions. Read the full reports; a zero copper/connectivity report does not qualify optics, RF, cell thermal tracking or enclosure materials.