4.2 Marking Methods & Materials

Marking methods transform bare boards into traceable products,products, bridging the physical and digital worldsworld of manufacturing. Whether etched with lasers, printed with high-temp labels, or deposited with inkjet,Choosing the durabilityright method and contrastmaterial of these identifiers decidedetermines if genealogy survives ovens, cleaners, and coatings. Placement is equally critical—marks on mask or rails endure, while those on pads or finishes invite failure. When tied cleanly into MES scans, a simple code evolves into a permanent passport, proving where a unit came from and what it has been through.

4.2.1 What a “mark” must survive (and prove)

Everythe unit ID orsurvives 2Dharsh codeprocesses has to (1)like stayreflow, readable after reflow,aggressive cleaning, and handling;conformal (2)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 scanpermanent fastenough onto outlive the line;product's andwarranty (3) match what MES expects so genealogy stays intact. That means picking the right methodperiod and wherecontrasted enough itto lands,be plusread definingreliably theby data it encodes in your label/traceability specscanners and MESAOI route.cameras.

4.2.2 Methods compared (pick the right tool)

Method	What it isProcess	Durability through(Survival reflow/cleanRate)	Contrast & readability	Finish/coating compatibilityCost/Complexity	Typical usesUse Case
Laser Marking (direct part marking, DPM)	Direct-to-Board. Ablates/etches the solder mask or FR-4copper to createreveal text/2Da codes.high-contrast mark.	ExcellentExcellent. (nothingNothing to melt);melt or peel; survives solvents.solvents and high heat.	BestHigh oninitial matte,cost high-contrastfor mask;the keeplaser a clean mask window.system.	NoPermanent impact on finishes—mark on mask, not on pads. Plays well under/over coat when kept in clear zones.	Unit SN/2D on PCB,codes, panel marks,IDs, post-assembly branding.
High-tempTemp labelsLabels	Applied Sticker. Pre-printed label (polyimidePolyimide) +with thermal transfer ink)	Printed,ink, pressure-sensitiveapplied labelsby rated for reflow.machine.	Very goodGood. ifIf spec’drated for lead-free temps; choose adhesives rated for your profiletemperatures and cleaners.required cleaning solvents.	ExcellentLow withinitial darkcost; textongoing onmaterial lightcost; label;requires scanner-friendlyclean ifapplication quiet zone preserved.	Place on mask or rails; avoid solderable finishes/pads. Confirm your clean/coat sequence; some labels are coat-through, others need a mask window.surface.	Unit SN, MAC/IMEI, panelcustomer-facing ID;IDs, shipping/packwhere labels.data changes frequently.
Inkjet on(Legend board (legend printer)Printer)	Direct-to-Board.Direct Prints ink onto the mask (like silkscreensilkscreen, but printed)printed post-fab).	GoodGood. whenNeeds proper surface preppreparation isfor right;adhesion; protectless fromresilient to aggressive cleaners.cleaning than laser.	NeedsIntegrated clean,into mattethe maskline; forfast crispand edges; avoid pads/metal.	Keep off solderable finishes; plan coating so text stays legible.flexible.	Supplemental text, small 1D/2D codes where laser/labelslaser aren’isn't preferred.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:Golden rule: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 put marks on solderable copper pads or finishestest points.

B. Inkjet & Legend

• Inkjet:—keep legend/ink/labelsUses offindustrial solderableprinter 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 andlayer VIPPOor caps;defined useby dedicatedthe solder mask windowsopening. orThis rails.mark is created during fabrication and cannot be changed during assembly.

4.2.3 ContrastApplied & placementLabeling (soThe scannersSticker don’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 hunt)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.

• ContrastForbidden first:Zones: matteNever maskplace boostsa edgelabel contraston fora bothsolderable 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 laserwhen and ink/legendwhere; chooseto mask/legend colors that read under your AOI/test lighting as well as hand scanners.

Useprevent the rails:mark reservefrom spacebeing onruined by subsequent processes.panel

rails for the big, line-of-travel barcodes/2D codes; keep board-level unit codes in a consistent corner.

Keepouts: don’t straddle V-score/tab lines; leave quiet zones around codes so verifiers grade cleanly (tie to panel drawing).

Off the copper: marks and labels live on mask/FR-4, not on solderable areas or test pads. (Your DFM notes should already say this.)

4.2.4

A. Finish & coatingCoating interactions (avoid surprises)

Interactions

• Finishes:Surface Finish: ENIG/ImmAg/ImmSnMarks are fine under/nearor labels must if you don’tnot cover OSP (Organic Solderability Preservative) coated copper, as the pad; OSP areaswill oxidize—keep marksoxidize and labelsmay off active copper, period. (OSP still okay elsewhere onaffect the board.)adhesive bond.
• CleaningConformal & coat:Coat: planIf surfacethe prepproduct souses ink/labelsconformal stick,coating, andyou must define whether labelsthe aremark goes under or over conformalthe coat.coat:
  • Under IfCoat: over,The speclabel/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 materialsis thatapplied toleratelater. yourThis chemistry;requires if under, addreserving a small mask window so(a clear area without coating) where the coat edge doesn’t creep under the label.
    

  4.2.5 When to mark (process timing)

  • Pre-reflow labels: common for unit SN on top side—only if label is applied to ensure adhesion to the PCB mask, not the slick coating.

B. Placement & Timing

1. Placement: Define a dedicated, consistent zone for the primary unit ID mark (e.g., bottom-left corner on the bottom mask). reflow-ratedUse the panel rails; verifyfor readabilitylarge, afterline-of-travel AOIcodes andneeded cleaning.for automation.
2. Post-Timing: Specify when the marking operation occurs (e.g., "Laser mark post-reflow laserand pre-test" or "Apply pre-reflow polyimide label at Station 1").
3. Verification:: ideal for permanent IDs without adhesive risk; addIntegrate a short scan gate ininto the MES rightroute immediately after marking.
3. Pack-out labels: shipping/customer labels printed at the last station—tie their data to the unit/panel scans so genealogy stays intact. (Encode BOM rev if your customer requires it.)
   

4.2.6 Data & verification (what the system expects)

• Put the data fields and formatsmarking (e.g., whichLaser 1D/2D– symbologies,Verify whatScan goes– intoMES each)Log). inThe line must be blocked if the mark fails verification.

ME/Designer Checklist

• LabelingMethod & Traceability specChosen: inLaser, yourlabel, Goldenor Datainkjet Pack.
• Add scan points to the MES route (print→verify→apply; laser→verify), and block WIP if a code fails decode. That’s how you avoid “shadow spreadsheets.”
• Keep sample images of good/poor marks in the pack so line leads train operators quickly.
  

4.2.7 Designer/ME checklist (print this with your fab pack)

• Chosen marking methodspecified per location: laser vs high-temp label vs inkjet legend.location.
• ArtworkDurability Confirmed: showsLabel dedicatedmaterial zonesrated onfor reflow temperature and cleaning solvents.
• panelNo-Go railsZones Honored: and on the PCB; noNo marks on pads/finishes.
• Contrastpads, ensured:test mattepoints, maskor wherecopper marks live; readable legend fonts.finish.
• Coating planPlan: clear:Clearance label(window) under/overdefined coatif withthe propermark windows/materials.goes under the conformal coat.
• Label/traceabilityMES specSync: listsThe data fields, symbologies, BOM-rev policy,fields and scan points inare locked into the MES route.MES.
  

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Conclusion: Selecting the right marking method, placing it for durability, and integrating it into MES ensures every identifier remains both scannable and trustworthy. The payoff is fast genealogy retrieval, resilient IDs through process stress, and reliable evidence for audits and customer returns.