8D Corrective Action Report — Membrane Switch Key Failure

Decision Summary (Conclusion First)

The metal snap domes in the returned samples have inverted (snapped through): the pre-formed convex disc has buckled permanently into the concave direction and now rests against the printed contact pad. This single physical condition produces all three reported symptoms simultaneously — loss of tactile feedback, reduced travel, and a permanently closed (stuck-on) circuit.

Assessment conclusion: dome inversion is an irreversible mechanical over-stress event, not a wear-out or ageing mode. Lot 24-09-03 passed a 100% tactile and electrical continuity test before shipment, which an inverted dome cannot pass by definition; the over-stress was therefore introduced after outgoing inspection — during transport, storage, handling or assembly. VMANX accepts the systemic gap identified in Why 4: the handling requirements of the snap dome were not sufficiently communicated with the product. Sections D5 and D7 close that gap from both directions.

D0 Preparation and Emergency Response

D1 Cross-Functional Team

D2 Problem Description (5W2H)

Returned sample, lot 24-09-03 / P/N Vmanx.1916 — both metal snap domes inverted; red arrows mark the affected domes.

The red arrows and dashed circles mark the two metal snap domes on the circuit layer of the returned sample. Both show the inverted condition: the dome centre is buckled into the concave direction and sits permanently against the printed contact pad, which is why the key reads as continuously closed. The lot marking 24-09-03 / P/N Vmanx.1916 at the lower right confirms sample traceability to the reported batch.

D3 Interim Containment Actions

D4 Structure and Failure Mechanism Evidence

The product is a six-layer membrane switch. The metal snap dome is retained in a pocket formed by the double-sided spacer (layer 4) and acts against the printed contacts of the circuit layer (layer 5). The backing plate supplied by the customer forms the load-bearing surface beneath layer 6 and is the element that physically limits dome travel.

Metal snap dome — normal (open), pressed (closed) and inverted (failed), with the six-layer construction.

Normal operation. The metal snap dome is a pre-formed convex disc. Actuation causes a controlled elastic buckling event that generates the tactile "click" and bridges the two printed contacts. On release, the stored elastic energy returns the dome to its convex state and the circuit opens. The transition ring at the dome apex is the geometric datum from which the dome radius is developed; any damage to this ring degrades or destroys the return behaviour.

Inverted dome (snap-through). When the applied force drives the dome beyond its designed maximum travel, the arch buckles past its elastic limit into a second stable position. The dome becomes bi-stable: it no longer returns to the convex state but remains concave, with its centre held against the circuit layer. The consequence is simultaneous loss of tactile feedback, reduced travel and a permanently closed contact — precisely the symptom set reported by the customer.

Critical characteristic: dome inversion is a mechanical over-stress event, not a gradual wear-out or ageing mode. It occurs instantaneously under a single over-travel actuation, impact, bending action or sustained compressive load, and it is irreversible. An inverted dome cannot be recovered by any field action and the affected switch must be replaced.

D4.1 Five-Why Analysis

D4.2 Cause and Effect Analysis (4M1E)

D4.3 Verification of the VMANX Manufacturing Process

Traceability conclusion: the failure did not originate in the VMANX manufacturing process. Every unit of lot 24-09-03 passed a 100% tactile and continuity test before shipment. An already-inverted dome forms a permanently closed circuit, which the outgoing continuity test detects and rejects by definition — such a unit cannot pass and cannot be shipped. The inversion therefore occurred after outgoing inspection.

Positioning statement: the purpose of this section is factual boundary-setting, not attribution of blame. Irrespective of where the mechanical stress was applied, VMANX accepts responsibility for the systemic gap identified in Why 4 and is committed to supporting the customer in eliminating the field conditions that allow over-stress to occur.

D5 Permanent Corrective Actions

D5.1 Handling Standard — Correct versus Prohibited Practice

Membrane switch handling guide for metal snap dome products — correct practice versus prohibited practice.

D5.2 Assembly, Packaging and Storage Standard

Assembly, packaging and storage work standard for metal snap dome membrane switches.

D5.3 Risk Notice — Consequences of Non-Compliance

D6 Implementation and Verification

D6.1 Preconditions for Continued Use of the Affected Lot

Should the customer wish to release any quarantined material rather than replace it, the following engineering preconditions must be satisfied in full:

① Engineering change review and written customer confirmation of the screening method, acceptance criteria and responsibility boundary.

② First-article verification: 100% tactile check on a rigid reference fixture, confirming full dome return and no reduced travel.

③ Electrical verification: continuity test in the released state, confirming no permanently closed contact on any key.

④ 100% inspection before release, with a bilateral retained sample confirmed by both parties.

Residual risk statement: a dome that has been partially over-deflected but not yet fully inverted cannot be reliably detected by tactile or continuity testing. Screened material therefore carries a residual field-failure risk that cannot be fully eliminated, and its acceptance requires written confirmation by both parties.

Ongoing verification plan: the next three consecutive production lots will undergo 100% tactile and electrical continuity inspection with a target defect rate of zero. The inspection record will be issued to the customer with each shipment.

D7 Prevention of Recurrence

The essence of this issue is a mismatch between an intrinsic product characteristic — the high mechanical sensitivity of the metal snap dome — and the actual field condition, namely the absence of a defined rigid support. The corrective strategy therefore does not seek to allocate blame. It closes the mismatch from two directions simultaneously: improved information transfer through the warning card, the illustrated specification and the training video, and physical protection through upgraded packaging and an optional rigid cover design. Both directions are required; either one alone leaves residual risk.

D8 Conclusion, Recognition and Bilateral Confirmation

Recommended action: replace the affected units rather than screen and release them. Dome inversion is irreversible and partially over-deflected domes are not reliably detectable, so replacement is the only route that restores full field reliability. The corrective actions in D5 eliminate the recurrence path for all future lots.

One-sentence conclusion: an inverted snap dome is a permanently closed switch inside the customer product — reliability is not restored by inspection, it is restored by removing the over-stress condition and replacing the affected parts. Quality and reliability are the foundation of a long-term partnership.

Information Requested from the Customer

This report is issued by the VMANX Engineering Department / Quality Assessment Center, which retains the right of interpretation. It constitutes a technical assessment and does not by itself approve or release any disposition of the affected material.