Quality control and inspection

1.Inspection points

Four checkpoints, each with a defined scope, sample basis, and disposition authority.

1.1Golden sample workflow

• Physical sample signed and dated by both buyer and supplierthe acceptable production unit.
• Production QC measures against the golden sample, not against the prototype, render, or buyer's expectations.
• Both parties keep an identical signed sampleyours in the office, theirs on the production floor.
• Photograph and documentHigh-res images, dimensional report, tag attached to the physical sample.
• Disputes resolve by reference to the golden sampleNot by argument; by inspection of the signed reference.

2.AQL sampling

ISO 2859-1 (international) and ANSI/ASQ Z1.4 (US-equivalent) define the sampling math. Same tables; different cover. Level II / Normal inspection is the default.

2.1Standard AQL levels by defect class

• **Critical defects0.0 % AQL.** Any critical defect rejects the batch.
• **Major defects2.5 % AQL.** Significantly reduces usability.
• **Minor defects4.0 % AQL.** Cosmetic, does not affect function.

Tighter AQLs (1.0 % major, 2.5 % minor) for medical, automotive, mil-spec. Looser AQLs (4.0 % major, 6.5 % minor) for promotional or one-shot goods.

2.2Sample size table (Level II, Normal inspection)

2.3Switching rules

Quality drifts over time; the inspection level adapts.

3.Defect classification

3.1Critical defects (0.0 % AQL)

• Safety hazardsSharp edges (per IEC 62368-1 test fingers), exposed conductors, battery shorts, choking risk on toys (<31 mm part per ASTM F963).
• Failure to perform primary functionDoes not power on, sensor doesn't read, transmitter doesn't transmit, app cannot connect.
• Regulatory non-complianceMissing required marks (CE, FCC ID, country of origin), wrong importer details, RoHS-restricted substances above threshold.
• Counterfeit or substituted componentsAny deviation from the approved BoM, even when functional.

3.2Major defects (2.5 % AQL)

• Reduced performanceBattery life ≤80 % of spec, sensor accuracy outside tolerance, charging slower than spec, RF range below qualified.
• Visible cosmetic flawsScratches >0.5 mm at arm's length (~50 cm), gap >0.3 mm at mating, mis-aligned parts, distorted logos.
• Wrong configurationWrong firmware version, wrong colour, missing accessory, wrong packaging insert.
• Packaging damageCrushed boxes (>10 % surface dented), missing inserts, deformed inner trays.

3.3Minor defects (4.0 % AQL)

• Subtle cosmeticMarks <0.5 mm not visible from 50 cm, minor finish irregularity, gloss inconsistency.
• DocumentationManual misprint not affecting safety, label slightly misaligned (<1 mm), language tag in wrong position.
• PackagingBarcode placement off by a few mm, slight crease in retail box.

3.4Per-product defect catalogue (template)

Each defect needs an entry. Photo each defect type at the actual scale and lighting. The catalogue is the contractual reference for accept/reject decisions.

`` Defect: Scratch on cosmetic A-surface Class: Major Limit: Visible from 50 cm under 500 lux office lighting Photo: CAT-MAJ-001.jpg (with ruler at 1:1 scale) Notes: Below 0.5 mm: minor. Above 1 mm: critical. ``

4.Process capability (Cp, Cpk)

Process capability indices quantify how well a process holds tolerance. Required for medical/automotive; useful for any tooled production.

4.1Cp — process potential

``` Cp = (USL − LSL) / (6σ)

USL = Upper Specification Limit LSL = Lower Specification Limit σ = Process standard deviation ```

• Cp ≥ 1.0Process barely fits within tolerance. Marginal.
• Cp ≥ 1.33Industry-typical target for production.
• Cp ≥ 1.67Six Sigma target. Critical features.
• Cp ≥ 2.0Ultra-stable process. Rare in consumer hardware.

4.2Cpk — process performance (centered)

`` Cpk = min[(USL − μ) / (3σ), (μ − LSL) / (3σ)] μ = Process mean ``

Cpk accounts for process centering. A process can have high Cp but low Cpk if it's not centered. Cpk ≤ Cp always.

4.3Worked example

Critical dimension: 25.00 ± 0.20 mm. Production sampled 50 units.

• Sample mean μ = 24.92 mm
• Sample std dev σ = 0.04 mm
• USL = 25.20 mm, LSL = 24.80 mm
• Cp = (25.20 − 24.80) / (6 × 0.04) = 0.40 / 0.24 = 1.67 (excellent spread)
• Cpk = min[(25.20 − 24.92) / 0.12, (24.92 − 24.80) / 0.12] = min[2.33, 1.00] = 1.00 (off-centre)
• Action: Adjust tooling to centre the mean at 25.00 mm. Cpk will rise to ~1.67.

5.Process-specific defects

Each manufacturing process has characteristic failure modes. Include the relevant set in the QC plan.

5.1Injection molding defects

5.2PCBA defects

5.3Sheet metal defects

• BurrsSharp edges from cutting; functional + safety risk. Deburr per ISO 13715 (broken edge).
• Spring-backBent angle deviates from intent. Material- and tool-dependent; compensate in tool design.
• ScratchesTooling marks or post-process handling.
• Mis-aligned holesPunch wear or stamp misalignment. Punch lifetime: typically 100k–500k strokes.

5.4Final assembly defects

• Mis-aligned screwsStripped threads (over-torque), backed-out screws (under-torque). Spec torque per fastener.
• Mis-mating connectorsForced or partial seating. Specify keying or asymmetric profiles to design out.
• Wrong accessory bundledMixed SKU at packaging station. Bar-code scan per unit at packaging eliminates.
• Surface contaminationFingerprints, dust, oil from line. Gloves and dust extraction at cosmetic stations.

6.QC plan template

The deliverable. One document, version-locked with the spec sheet.

6.1Document structure

6.2Standard inspection equipment

6.3Standard inspection lighting

• Office lighting300–500 lux. Acceptable for routine cosmetic inspection at arm's length.
• Inspection station750–1 000 lux, daylight 5500 K. Catches subtle cosmetic defects.
• Critical surface inspection2 000+ lux with directional + diffuse mix. Catches micro-scratches.