4.2 Marking Methods & Materials

Marking methods transform bare boards into traceable products, bridging the physical and digital world of manufacturing. Choosing the right method and material determines if the unit ID survives harsh processes like reflow, aggressive cleaning, and conformal coating. If the mark fails, traceability stops, risking high costs during a failure analysis.

4.2.1 The Marking Decision: Permanence vs. Cost

The marking decision is a cost-to-risk trade-off. You must select a method that is permanent enough to outlive the product's warranty period and contrasted enough to be read reliably by scanners and AOI cameras.

Method	Process	Durability (Survival Rate)	Cost/Complexity	Typical Use Case
Laser Marking (DPM)	Direct-to-Board. Ablates/etches the solder mask or copper to reveal a high-contrast mark.	Excellent. Nothing to melt or peel; survives solvents and high heat.	High initial cost for the laser system.	Permanent Unit SN/2D codes, panel IDs, post-assembly branding.
High-Temp Labels	Applied Sticker. Pre-printed label (Polyimide) with thermal transfer ink, applied by machine.	Very Good. If rated for lead-free temperatures and required cleaning solvents.	Low initial cost; ongoing material cost; requires clean application surface.	Unit SN, MAC/IMEI, customer-facing IDs, where data changes frequently.
Inkjet (Legend Printer)	Direct-to-Board. Prints ink onto the mask (like silkscreen, but printed post-fab).	Good. Needs proper surface preparation for adhesion; less resilient to aggressive cleaning than laser.	Integrated into the line; fast and flexible.	Supplemental text, small 1D/2D codes where laser isn't available.
PCB Features	Fabrication. Using copper, mask openings, or silkscreen dots during board manufacture.	Permanent. Integrated into the PCB structure.	Free (part of fab cost).	Static codes, date codes, version IDs (cannot change during assembly).

4.2.2 Direct-to-Board Methods (The Permanent Mark)

These methods create a mark that is fundamentally tied to the PCB itself.

A. Laser Marking (DPM)

Laser marking is the most common choice for high-volume, high-reliability traceability codes because of its durability.

Mechanism: The laser creates contrast by ablating (vaporizing) the top layer (solder mask) to expose the contrasting color of the layer beneath (often the white fiberglass or black/white ink layer below the mask).
Contrast is King: Laser marks require a clean, matte surface and a high-contrast mask color combination (e.g., green mask on white FR-4) for reliable reading by automated scanners.
Placement: The mark must be on the solder mask or exposed FR-4, never on solderable copper pads or test points.

B. Inkjet & Legend

Inkjet: Uses industrial printer heads to apply epoxy-based ink directly to the mask. This is often used to add traceability codes to bare boards during the front-end fabrication process.
Copper/Mask/Silk: Simple IDs (like a single revision letter or a small 2D code box) can be integrated into the copper layer or defined by the solder mask opening. This mark is created during fabrication and cannot be changed during assembly.

4.2.3 Applied Labeling (The Sticker Mark)

Labels are preferred when the ID needs to be human-readable, large, or contain data that is generated late in the process (e.g., IMEI number).

A. Material Durability

The material and adhesive must be specifically rated to survive the manufacturing environment. Polyimide (Kapton) is the industry standard for high-temp labels. Polyimide labels use high-performance adhesive that is chemically resistant and rated for the high heat of the lead-free reflow profile (240˚C to 260˚C).

* Adhesive Failure: Labels fail when heat or chemicals (solvents during cleaning) break down the adhesive, causing the label to peel or fall off—eliminating traceability. Always spec labels that match your process profile.

B. Label Placement Rules

Labels must be placed on a clean, flat, non-solderable surface to ensure long-term adhesion.

Forbidden Zones: Never place a label on a solderable pad, test point, gold finger, or any copper area.
Clearance: Ensure the label has adequate clearance from panel scores/tabs, component bodies, and any required rework zones.

4.2.4 Process Integration & Final Checklist

The marking specification must define the when and where to prevent the mark from being ruined by subsequent processes.

A. Finish & Coating Interactions

Surface Finish: Marks or labels must not cover OSP (Organic Solderability Preservative) coated copper, as the OSP will oxidize and may affect the adhesive bond.
Conformal Coat: If the product uses conformal coating, you must define whether the mark goes under or over the coat:
- Under Coat: The label/mark is applied, then the board is coated. You must ensure the coating doesn't affect the code's readability.
- Over Coat: The board is coated, and the label is applied later. This requires reserving a small mask window (a clear area without coating) where the label is applied to ensure adhesion to the PCB mask, not the slick coating.

B. Placement & Timing

Placement: Define a dedicated, consistent zone for the primary unit ID mark (e.g., bottom-left corner on the bottom mask). Use the panel rails for large, line-of-travel codes needed for automation.
Timing: Specify when the marking operation occurs (e.g., "Laser mark post-reflow and pre-test" or "Apply pre-reflow polyimide label at Station 1").
Verification: Integrate a scan gate into the MES route immediately after marking (e.g., Laser – Verify Scan – MES Log). The line must be blocked if the mark fails verification.

ME/Designer Checklist

Method Chosen: Laser, label, or inkjet specified per location.
Durability Confirmed: Label material rated for reflow temperature and cleaning solvents.
No-Go Zones Honored: No marks on pads, test points, or copper finish.
Coating Plan: Clearance (window) defined if the mark goes under the conformal coat.
MES Sync: The data fields and scan points are locked into the MES route.