Marking and identification: where traceability becomes physical

Three layers, often confused

“Marking” on a harness drawing can mean three different things with different governing documents, and a single line saying “mark per drawing” usually leaves at least two of them unspecified.

The third layer is worth dwelling on, because it is the one buyers assume the marking covers. A part number on a label tells you what something is supposed to be. It does not tell you which lot of connectors went into it, or which reel of wire, which is the question a counterfeit or nonconformance investigation actually asks. That link lives in the build record, and it only exists if it was required.

The requirement that can damage the part

Wire identification has to survive the life of the harness without compromising the insulation carrying it. Those two goals are in tension, and the method chosen decides which one wins.

SAE's guidance on the hot stamp process is direct about the limit: hot stamp marking of wire identification is prohibited on aerospace vehicle wiring required to conform to AS50881. The reason is physical — the stamping dies can fracture the insulation wall and reach the conductor, and that risk rises as insulation gets thinner. Methods that do not deform the insulation — ink jet, dot matrix, or UV laser — are encouraged instead, particularly on insulation walls below roughly 0.010 inch.

This is the sort of requirement that survives on a legacy drawing long after the wire specified on it changed. A drawing that says “hot stamp per...”, carried forward to a build using modern thin-wall wire, is specifying a process against a material it was never written for. Check the marking note whenever the wire changes — not only when the marking changes.

Marking has to outlive the assembly

Whatever method is used, a mark that becomes unreadable has failed at exactly the point it was needed. Worth stating on the drawing:

• Legibility after processing. Marks pass through potting, overmolding, heat-shrink recovery, cleaning and solvents. Which of those will this mark see?
• Contrast and durability requirements, and how they will be verified.
• Placement and repeat interval along a wire, so an identifier is visible after the harness is installed and a breakout is buried.
• What happens at a repair — a spliced or replaced length needs the same identification, and that is where marking discipline usually lapses.

What to put on the drawing

1. Which of the three layers is required — item, conductor, or both.
2. The governing standard for each, with its revision and current designation. Custody of these documents moves; see shield termination for how a superseded number becomes ambiguous.
3. The permitted marking methods, and any that are excluded for the insulation in use.
4. The identifier content and format — what actually gets marked, character by character.
5. Placement, orientation and repeat interval.
6. Legibility and durability acceptance, and when it is inspected. Workmanship criteria for marking sit in IPC/WHMA-A-620 alongside crimps and shield terminations.
7. Whether lot traceability is required, and to what depth — that is a records requirement, and it belongs in words rather than implied by a label.

Points three and four are where quotations diverge most. Marking method drives equipment, setup and inspection time, and a supplier reading “mark per drawing” will price the method it already owns. See the RFQ checklist for the rest of what a quotation needs, and harness drawing requirements for the surrounding package.

This is a buyer's orientation, not a substitute for the standards. Confirm current designations, revisions and any platform-specific requirements against the published documents and the contract.