1.21 Surface Prep & Cleanliness

A traceability code is only useful if it's readable a year later. Surface preparation is the quiet step that ensures your unit ID doesn't lift, smear, or fade during harsh processes like reflow, cleaning, or coating. A mark that sits on flux, oil, or glossy residue is destined to fail.

1.21.1 Process Timing: When to Mark (Clean, Then Mark)

The rule is simple: The surface must be clean and dry immediately before the mark is applied. You must time the marking step to avoid the harshest processes and residue deposits.

Quick Matrix: Best Timing by Method

Planning Around Coating

Conformal coating is the final hurdle. The traceability mark must be accounted for in the coating process:

• Under Coat: If the code lives under the coating, you need a high-quality mark that won't smear during application.
• Over Coat: If the label goes over the coating, you must design a mask window—a small zone on the PCB where the coating is deliberately skipped—so the label adheres to the PCB mask/FR-4, not the slick coating itself.

1.21.2 Adhesion Killers & Cleanliness Recipes

Most mark failures come from contamination. You must identify and eliminate the adhesion killers unique to your line.

Adhesion Killers (What Ruins the Mark)

• Flux Residues: The biggest killer. No-clean flux films under a label or ink guarantee weak adhesion and smearing during cleaning.
• Oily Contamination: Silicones, tapes, or oily residue from gloves/fixtures cause labels to lift at the corners and ink to bead.
• Glossy Mask: A glossy solder mask reduces the surface energy required for adhesive bonding and scatters laser light, degrading contrast. Use a matte mask in the marking zone for better laser contrast and adhesion.
• Moisture: Trapped water vapor under labels after washing causes bubbles and eventual lifting. Boards must be dry before labeling.

Cleanliness Recipes (Simple Prep)

Match the preparation step to the method to avoid both under-cleaning (failure) and over-cleaning (wasting time/materials):

• Laser Marking: Minimal prep required. A simple blow-off or vacuum to remove dust from the mask is usually sufficient.
• Label Application: IPA-wipe (Isopropyl Alcohol) or approved mild prep on the mask/FR-4 area immediately before application. Apply firm pressure to the label to activate the adhesive.
• Ink/Legend: Wipe, print, and then cure immediately. Do not print near running saw blades or depanel machines where fiber dust can ruin the print edge.

1.21.3 Verification and Specification

Your quality assurance (QA) process must treat the traceability mark as a critical process output, verifying its quality before the unit moves on.

The Verification Gate

Do not trust your eyes. Every code must be verified by a scanner.

• Add a mandatory verification gate in your MES route immediately after the code is printed or applied.
• Set a minimum pass grade for each symbology (e.g., DataMatrix must grade 'B' or better).
• If the code fails the grade, the system must block WIP and trigger a defined reprint or rework flow. This prevents bad codes from entering the finished goods inventory.
• Keep a golden sample board/label with photo references in the work instruction for operator training and fast dispute resolution.

The Marking Specification

Your Labeling & Traceability Spec in the Golden Data Pack must lock down these preparation requirements:

• Process Sequence: Define the when (e.g., "Post-wash, pre-conformal coat").
• Surface Prep: List the exact cleaner and method (e.g., "IPA wipe and air dry for 30 seconds before label application").
• Verification: Mandate the scanner grade threshold and the API write-back to MES/ERP for the verification step.
• Design Constraints: Reference the PCB drawing, confirming that the mark is off pads, off breakaway lines, and in a dedicated mask window (if needed for over-coat policy).