Capacitive Sensing Film Sensor — Rework Cut Risk Assessment

Note: Client part numbers and product codes have been neutralized to protect confidentiality.

EXECUTIVE SUMMARY

A medical device OEM faced a design change on 2,000 finished capacitive sensing film sensors: the FPC stiffener was to be shortened from 7.00mm to 4.00mm to resolve an assembly clearance issue, and manual rework cutting of the finger area was proposed for the existing stock.

VMANX engineering evaluated the rework approach and strongly recommends Option B (new production) instead of Option A (rework cutting). Although rework appears cheaper on the surface, the cut line at 4.00mm crosses the silver trace end-face, exposing bare silver and triggering irreversible electrochemical migration (ECM) failure — a latent defect that may surface months later as intermittent shorts at the customer site.

D0 — PROBLEM DESCRIPTION

D1 — SOLUTION DEFINITION

D2 — STRUCTURE & FAILURE MECHANISM (EVIDENCE)

2.1 Actual Layer Structure (four layers, left-aligned, 0.20mm step at right)

Figure 1 shows the actual structure of the FPC finger area: Top layer — conductive carbon 6.80mm, fully covering and protecting the silver trace; Second layer — conductive silver 6.60mm, protected by carbon (0.20mm longer than silver, step at right side); Third layer — PET substrate 0.125mm, aligned with stiffener (7.00mm); Bottom layer — stiffener 7.00mm.

Figure 1 Four-layer structure: carbon 6.80mm / silver 6.60mm (left-aligned, 0.20mm step at right) / PET & stiffener 7.00mm; cut line @4.00mm crosses silver end-face

Rework cut position: the cut line at 4.00mm (from the contact end) lies inside the carbon (6.80mm) and silver (6.60mm) coverage zones. Cutting will sever both the carbon end-face and the silver layer end-face, leaving the silver end-face exposed without carbon protection.

2.2 Silver Migration (ECM) — the Core Irreversible Risk

Silver migration is a form of electrochemical migration (ECM). Under an electric field and moisture, silver ionizes at the anode, migrates, and reduces at the cathode forming dendrites:

• Anode: Ag → Ag⁺ + e⁻ (silver oxidizes to silver ions)

• Cathode: Ag⁺ + e⁻ → Ag (ions reduce into silver dendrites)

Acceleration factors — all present in this product: 1) Electric field: potential difference between adjacent 1.0mm-pitch fingers during operation; 2) Moisture: ambient humidity in consumer/industrial environments acts as electrolyte carrier; 3) Exposed conductor: the cut cross-section exposes silver with no moisture/oxygen barrier.

Figure 2 Silver migration mechanism: left — carbon coating intact (no migration); right — exposed silver at cut cross-section, Ag⁺ migration, dendrite bridging 1.0mm gap → short circuit

Failure timeline: with intact carbon coating, migration risk is minimal (over 5 years design life). After rework cutting, bare silver at the exposed cross-section, under 1.0mm pitch and electric field gradient, gradually grows dendrites over months or years. Once dendrites bridge adjacent pins, signal crosstalk, short circuit, or severe insulation resistance degradation occurs — random and intermittent (temperature/humidity dependent), hard to reproduce at the customer site yet real, drastically increasing after-sales failure rate.

D3 — FULL-DIMENSION COMPARISON MATRIX

D4 — EXPERIMENTAL DATA SUPPORT (industry reference)

Note: Data per IPC standards and industry silver-electrode migration test references; to be jointly confirmed by both parties.

D5 — RISK SUMMARY (Option A)

D6 — PRECONDITIONS IF REWORK IS MANDATORY

• Pre-cut silver end-face sealing (e.g., conformal coating or edge sealant) to restore a moisture barrier — residual risk remains, must be documented and agreed

• 100% AOI + flying-probe electrical test after rework

• Environmental screening (85°C/85%RH, 500h) on a sample lot before release

• AQL 0.65 final audit and extended warranty terms agreed in writing

• Customer to sign a risk acceptance disclaimer (rework is outside normal manufacturing process)

D7 — CONCLUSION & RECOMMENDATION

VMANX strongly recommends Option B (new production). Option A appears cheaper on the surface, but the 4.00mm cut line severs the silver end-face, triggering irreversible silver migration (ECM) with unpredictable latent failure costs far exceeding the apparent savings. Quality and reliability are the foundation of long-term partnership; this design change (7mm→4mm) is a structural optimization, while physical cutting is a destructive process contrary to normal production practice.

Customer is kindly requested to weigh the decision carefully.

D8 — CONFIRMATION