4.3 What to Encode & How
Encoding is where your traceability plan turns into data that systems can trust. The codes you print are the resilient anchors of a unit's history. The key is to encode only the essential data and pair the code with the right symbology and placement for maximum readability on the factory floor.
4.3.1 The Data Dictionary: What Must Be Encoded
The content of your code (the data dictionary) should be the smallest set of fields required to meet your chosen traceability level (from Chapter 4.1). Every unnecessary character adds size and complexity.
Core Fields (Mandatory for Traceability)
These fields are non-negotiable for proving a unit's identity and configuration:
- SN (Serial Number): Unique identifier for the individual unit. If panel-level tracking is used, this is the Panel ID.
- BOM Rev / Config ID: Crucial for audits. This immediately tells the scanner and the MES system which Bill-of-Materials version was followed, allowing isolation of parts failures to a specific design release.
- Work Order / Lot / Date Code: Identifies the batch and time frame of production, essential for quick quarantine if a process fault (like a reflow oven issue) is found.
Optional Fields (Encode Only if Necessary)
- Variant / Feature Flags: Use only if one PCB assembly serves multiple final product SKUs (e.g., a flag for 'With Wifi' vs. 'Without Wifi').
- Regulatory/Customer IDs: Required if contracts dictate encoding specific customer part numbers or regulatory marks (like specific MAC addresses).
Human-Readable Lines
Always pair the machine-readable code (1D or 2D) with a clear human-readable string (usually the SN and the BOM Rev). This is essential for manual debug, repair, and auditors when a scanner fails. Ensure the human text format is consistent with what your MES expects.
4.3.2 Symbology & Durability: 1D vs. 2D
The type of barcode you choose dictates how much data you can store in a small space and, crucially, how well the code survives damage (durability).
Symbology Type | Key Feature | Ideal Placement | Durability/Damage Resistance |
1D Barcode (Code 128, etc.) | Long, single string; requires more space length-wise. | Carton labels, panel rails, packaging boxes (where space is wide). | Low. If one line is scratched, the entire code fails. |
2D Code (DataMatrix, QR) | Compact; high data density in a small square. | PCB Unit ID (where space is tight), small component labels. | High. Built-in Error Correction allows the code to be read even if 20-30% is damaged by splatter, reflow, or etching. |
Standards and Verification
- Standards: Choose a GS1/ISO standard (like GS1 DataMatrix) if your product is consumer-facing or needs global consistency. This ensures the codes are interoperable across different supplier and customer systems.
- Quiet Zone: Every code requires a quiet zone—a clear, uniform margin of empty space around the symbol. Failing to maintain this space is the most common cause of scanner failure.
- Error Correction: For the PCB unit ID (usually a 2D code), ensure the error correction level is high enough to tolerate your real-world environment (cleaning solvents, handling abrasion). Add a verify step to your route: Print/Laser $\rightarrow$ Scan to Grade – Accept/Block. WIP must be blocked if the code fails to grade correctly.
4.3.3 Deployment: Placement and Specification
The decision is not just about the data string; it's about placing the code correctly and documenting every detail.
A. Where Each Code Lives
Code Type | Placement Location | Purpose |
Panel ID | Panel Rail (Outer edge) | Used for upstream tracking (solder paste, stencil) and line-of-travel scanners. |
Unit ID | PCB (Consistent Corner) | Used for downstream tracking (AOI, Test, Rework) and customer traceability. Must be in a consistent, easily accessible corner. |
Pack-Out ID | Carton/Box | Customer-facing code, often mirroring the core fields (SN + Rev) and including shipping IDs. |
B. The Traceability Specification
The Labeling & Traceability Specification is a mandatory part of the Golden Data Pack. It removes all ambiguity and serves as the single reference for every labeling station and scanner on the floor.
This spec must detail:
- Symbologies: List every permitted symbology (e.g., "Primary unit ID is DataMatrix ECC 200").
- Field Order: Define the precise order and length of the encoded fields (e.g., "SN [12 chars], followed by Rev [3 chars]").
- Placement Map: Provide a drawing showing the exact placement coordinates, size (min/max), and minimum quiet zones for all codes.
- API Mapping: State how the scanned data fields map directly to your MES/ERP objects (e.g., "The first 12 characters map to the MES object 'Unit_SN'").