1. Symbol grade (ISO/IEC 15415 for 2D codes)

The ISO/IEC 15415 standard measures eight parameters of a Data Matrix or QR code symbol quality and assigns each a grade from 0 (fail/poor) to 4 (excellent). The overall grade is the minimum of all eight individual grades. As covered in the QR code laser marking article, the eight parameters are: decode, symbol contrast, modulation, print growth, axial non-uniformity, grid non-uniformity, unused error correction, and fixed pattern damage.

Production minimum grades by industry:

• General industrial: Grade 1.5 minimum
• Automotive (AIAG MH10.8.2): Grade 2.0 minimum
• Medical device (FDA UDI): Grade B equivalent (Grade 2.0–2.5)
• Aerospace (AMS 2816): Grade 1.5 minimum overall

The most important single parameter to track: Unused error correction is the highest-sensitivity early warning parameter. As the process drifts (focus changes, power decreases, lens contamination increases), unused error correction degrades before the code stops scanning. A symbol that grades at Grade 2.5 overall with 75% unused error correction is robust; the same symbol grading at Grade 2.5 with 20% unused error correction is one process variation event away from failing.

2. Mark depth (for engraved marks)

Depth specification is required for marks that must survive paint application, marks subject to AMS 2816 limits, and marks on functional surfaces with defined recess tolerance. Methods:

• Contact profilometer: practical for routine production checks
• Cross-section metallography: required for qualification and audit samples

What to measure: Rt (maximum depth from surface peak to deepest valley) is the most relevant parameter for production depth verification. Measure at 3–5 locations across the mark to confirm depth uniformity.

3. Surface roughness (for annealed marks)

For medical, food equipment, and precision surfaces where annealing is specified to maintain surface continuity, measure Ra through the marked zone:

• Medical device: Ra ≤ 0.8 μm maintained through mark zone for instrument bodies; Ra ≤ 0.4 μm for implant surfaces
• NSF food equipment: verify no surface recess (profilometer confirms no depth step)

Measurement: contact profilometer with 0.8 mm cutoff wavelength (standard for surface measurement), trace across the mark center and compare to base material Ra adjacent to mark.

4. Visual/cosmetic quality

For marks that are consumer-visible (jewelry engraving, premium product identification, architectural marks), a visual inspection criterion must be established:

• Character legibility (stroke width, spacing, character height)
• Surface discoloration beyond the mark boundary (heat tinting for annealing, spatter for engraving)
• Edge definition (sharp vs. fuzzy character edges)

Define the visual criteria with a limit sample — a marked part at the acceptable quality limit, against which production samples are compared. “Looks good” is not a production criterion; a physical limit sample is.

5. Durability (for regulated and critical applications)

Durability testing exposes the mark to the service conditions it will face and verifies readability and appearance afterward. Define and test:

• For automotive: survive chemical cleaning, painting, and pressure washing
• For medical: survive 1,000 autoclave cycles at 134°C
• For aerospace: survive corrosion testing per ASTM B117 (salt spray), fuel immersion, hydraulic fluid
• For EV battery: survive thermal cycling and electrolyte exposure

Durability testing is performed during process qualification — not on every production part. Document the durability test results as part of the process qualification record.