To support product traceability, many PCBs include a white soldermask area for subsequent laser engraving of a QR code. In other cases, the QR code is laser marked directly on the soldermask. These two approaches differ significantly in process structure, adhesion, durability, and readability. The following sections compare the two methods and summarize the key design and manufacturing considerations.
Fundamental Process Differences
Approach A: Print the White Area First, Then Laser Engrave
A white ink block is screen-printed onto the PCB surface as a base layer. A laser then ablates selected areas of the white ink, exposing the underlying soldermask or substrate. The laser-processed regions appear dark, according to the color of the soldermask or exposed substrate, while the unprocessed areas remain white. The resulting contrast forms the QR code pattern.
Approach B: Laser Mark the White Area Directly on the Soldermask
In this approach, the white area itself is formed directly by the laser-marking process on the soldermask. Because it does not have a full screen-printed ink layer serving as a mechanical foundation, the resulting white marking has relatively low adhesion to the PCB surface.
Both approaches have advantages and limitations. However, for reliable volume production, Approach A—printing the white area first and then laser engraving it—is generally more robust. The continuous screen-printed ink layer provides a stronger foundation and better adhesion. Approach B is more vulnerable to peeling during the high-temperature shock of SMT processing or during subsequent cleaning operations.
Potential Process and Reliability Problems
Material and Adhesion Problems
White Area Peeling
Insufficient surface tension: When the surface tension of the soldermask is below 36 dyn/cm, ink adhesion can drop sharply. In one reported case, a 5 °C deviation in soldermask pre-baking temperature caused the dyne value to fall from 38 to 32, while the peeling rate in a character-adhesion test increased by 300%.
Incomplete surface preparation: Inadequate cleaning of the substrate, residual silicone oil, or other contaminants can prevent the white ink from bonding properly to the surface.
Material compatibility: The white ink must be compatible with the underlying soldermask system. Incompatible ink chemistry can reduce interlayer adhesion and increase the risk of peeling after thermal or chemical exposure.
The white area formed by direct laser marking in Approach B lacks the continuous screen-printed ink layer used in Approach A. Its adhesion is therefore weaker, making it more susceptible to detachment.
Potential consequence: If the white area peels away, the QR-code information may be partially or completely lost, causing the product traceability system to fail.
White Area Discoloration and Yellowing
Residues from electroless nickel immersion gold processing: Residual chemicals from the immersion-gold process may remain on the white ink surface. During SMT exposure to high temperatures, chemical residues can react with the ink and cause discoloration. Trace nickel ions remaining after the process may also reduce ink adhesion.
High-temperature aging: Reflow soldering, with a peak temperature of approximately 260 °C, can oxidize the epoxy resin system. White pigments such as titanium dioxide may also degrade under prolonged or repeated thermal exposure.
UV aging: Long-term exposure to ultraviolet radiation can cause photo-oxidation and gradual yellowing of the white ink.
Once the white area turns yellow, its optical reflectance changes. The contrast between the light QR-code modules and the dark laser-ablated regions is reduced, making the code more difficult for scanning equipment to recognize and increasing the risk of misclassification.
Insufficient White-Ink Thickness and Substrate Damage
If the white ink layer is too thin, the laser may penetrate through it and directly damage the copper foil or underlying substrate. In applications where exposed copper is unacceptable, this may expose the copper surface and increase the risk of subsequent oxidation.
Conversely, an excessively thick white layer can produce uneven ablation and increase burrs or roughness along the QR-code edges.
Potential consequences: Electrical performance may be affected, the exposed copper may oxidize, and the PCB may fail visual-quality requirements.
Appearance and Processing Defects
Charring and Carbonization
If the laser focus is too deep or the laser power is excessive, the material may carbonize. The marked area can become severely scorched, producing a dark, blurred appearance rather than a clean QR-code pattern.
Potential consequence: Module boundaries become indistinct, contrast becomes nonuniform, and decoding performance deteriorates.
Blurred Marking and Edge Burrs
Incorrect focusing or an excessively high scanning speed can cause broken lines, ghost images, or incomplete modules. Uneven white-ink thickness can also cause some areas to be fully ablated while others remain partially covered. In addition, the reflective properties of the ink affect how uniformly the laser energy is absorbed.
Potential consequence: The QR code may receive a low grading result or fail to decode entirely.
Surface Spatter and Contamination
When laser energy acts on the white ink, part of the material may vaporize or melt and eject onto adjacent areas. This can contaminate the region surrounding the QR code.
Potential consequences: Deposited material may interfere with component placement, contribute to soldering or contact problems, and reduce the cleanliness and optical consistency required for reliable QR-code recognition.
Readability and Machine-Recognition Problems
Insufficient Contrast and Low Recognition Rate
Improper laser parameters: Laser power, pulse frequency, scanning speed, and fill density directly affect the marking result. Insufficient power produces shallow ablation and weak contrast, while excessive power can cause charring and carbonization.
Insufficient color difference between the white area and the underlying soldermask: If the soldermask is itself a light color, such as light green or light blue, laser ablation may not produce enough contrast for reliable scanning.
Uneven PCB surface: Height differences at the boundary between copper foil and substrate can affect screen printing. For example, an outer-layer copper thickness of 2 oz, approximately 70 μm, and an inner-layer copper thickness of 1 oz, approximately 35 μm, can create a step difference of approximately 0.05–0.1 mm. This step causes uneven squeegee pressure during printing. One communications-equipment manufacturing case reported a character-line-width deviation of up to 30% in untreated step areas, accompanied by a 50% reduction in QR-code recognition rate.
Nonuniform module dimensions or insufficient spacing: If the gaps between dark and light QR-code modules are too small, blurred screen printing may cause adjacent boundaries to merge. This reduces the distinction between modules and can prevent reliable decoding.
Potential consequences: Scan failures increase, the traceability system becomes ineffective, and the production cycle may be disrupted.
Under ISO/IEC 15415, QR-code quality is graded from A to F. Grades A, B, and C are generally considered acceptable, D is marginal, and F is unacceptable. Contrast is one of the key grading parameters. The stated criteria are a contrast of at least 80% for Grade A, 70%–79% for Grade B, and 60%–69% for Grade C, while contrast below 40% is directly classified as Grade F.
Approach A is more likely to produce a high-contrast QR code because the thickness of the white ink can be controlled and the printed surface is more uniform.
Design and Manufacturing Control Plan
1. White-Area and QR-Code Dimensions
- Design the white area on an independent PCB layer rather than sharing the same block with other silkscreen characters.
- Reserve a blank boundary of at least 0.5 mm around the QR code to prevent obstruction by components, traces, or vias.
- Do not make the QR code excessively small. A recommended overall size is at least 3 mm × 3 mm.
2. Character and Silkscreen Design Rules
- Maintain a minimum character stroke width of 5 mil. The required stroke width should increase as copper thickness increases.
- Maintain a minimum character height of 28 mil. Character height should also be increased when thicker copper creates larger surface steps.
- Keep silkscreen characters at least 6 mil away from soldermask openings and pads. This prevents ink from printing onto the pad and affecting solderability.
3. Avoid High-Stress and Step Areas
- Keep characters at least 0.3 mm away from regions where copper thickness changes abruptly, and avoid printing across such steps.
- As copper thickness increases, provide additional design margin because the requirements for screen-printing accuracy become more demanding.
4. Soldermask Color Selection
- High-contrast combinations include a white ink area on a black soldermask or on another dark soldermask color.
- Laser-engraved QR codes on white soldermask naturally have insufficient contrast and should be avoided whenever possible.
5. White-Ink Thickness
The target white-ink thickness should generally be controlled within 15–25 μm. This requires coordinated control of screen-printing parameters, including mesh count, printing speed, and squeegee pressure.
Conclusion
Laser engraving a QR code on a PCB white area is not simply a matter of applying ink and marking it. It is a combined engineering process involving material science, thermal behavior, optical recognition, and precision processing.
For volume production, printing the white area first and then laser engraving it generally provides better recognition performance, durability, and traceability than direct laser marking. During the initial design stage, engineers should evaluate the required QR-code recognition rate, PCB surface complexity, production environment, and reliability requirements. The design and manufacturing teams should also align on ink selection, laser-marking capability, process windows, and inspection criteria. These controls help ensure that the QR code remains readable and traceable throughout the product life cycle.
