Something You Ought to Know About PCB Silkscreen Printing
6 min
PCB silkscreen (also called legend or nomenclature) is the layer of printed text, symbols, marks, and images on a PCB surface. It is applied on top of the solder mask layer using special epoxy ink, providing critical information for component placement, assembly instructions, testing, and identification.
JLCPCB applies silkscreen on the top and/or bottom side (component side) after the solder mask and copper layers are completed. Common ink colors are white (default), black, or yellow for optimal contrast against the solder mask background.
Benefits of PCB Silkscreen
There are several key advantages to adding silkscreen:
1. Component Identification
2. Assembly Guidance
3. Circuit Understanding
4. Branding and Logo Placement
5. Aesthetics
6. Compliance and Certification
7. Documentation and Reference
For example, clear component labels (e.g., "R1", "C1") speed up assembly and troubleshooting. On JLCPCB, silkscreen can also include version numbers, dates, or logos. Note: JLCPCB automatically removes silkscreen over pads, vias, or traces to avoid assembly issues.
Silkscreen Techniques in PCB Manufacturing
JLCPCB primarily uses Liquid Photo Imaging (LPI) for its balance of precision, resolution, and cost-effectiveness in volume production. Below are the three common methods, with their processes and key advantages/disadvantages.
Manual screen printing
This traditional method applies epoxy ink through a nylon mesh stencil by hand. It is suitable when line widths exceed 7 mil (0.178 mm) and registration tolerance is ±5 mil. After printing, the board is cured in an oven.
Pros: Extremely low setup cost for very small runs or prototypes—no photomask or digital files needed; highly flexible for last-minute design changes (just modify the stencil); works well on uneven or curved surfaces (though rare for PCBs); ink thickness can be controlled manually for bolder legends. Ideal for hobbyists or low-volume custom boards.
Cons: Highly labor-intensive and operator-dependent—variations in squeegee pressure or angle can cause uneven ink distribution, smudging, or incomplete coverage; poor repeatability across batches; limited resolution (struggles with fine text <1 mm or thin lines); risk of misalignment between top/bottom sides; not scalable for medium/high volume due to time per board.
Liquid photo imaging (LPI)
LPI is the industry-standard method for modern mass production, including JLCPCB's primary choice. The detailed process:
1. The completed PCB (copper + solder mask) is cleaned and coated with liquid photo-sensitive epoxy ink (usually via curtain coating or spray for uniform thickness ~20–30 μm).
2. The ink is pre-baked to remove solvents.
3. A photographic film (photomask) with the legend artwork is aligned and placed in contact with the board.
4. UV light exposes the ink through the photomask—exposed areas polymerize/harden.
5. Unexposed ink is washed away in a developer solution (alkaline or solvent-based).
6. Final high-temperature curing ensures durability.
Double-sided boards require separate processing for each side with precise alignment pins.
Pros: Excellent resolution and registration accuracy (±0.05 mm typical); supports very fine features (down to 0.15 mm lines/0.8 mm text); highly consistent and automated once photomask is created; thick, durable ink layer resistant to chemicals and reflow soldering; perfect for complex, dense legends with logos, barcodes, or small text; scalable for high-volume production with minimal per-board variation.
Cons: Requires expensive photomask fabrication for each design revision; multi-step process (coating → pre-bake → exposure → develop → cure) increases setup time and facility requirements (cleanroom, UV exposure units, chemical handling); higher waste from developer chemicals; not ideal for ultra-small runs due to photomask cost.
Direct legend printing (DLP)
This fully digital, non-contact method uses specialized inkjet printers adapted for PCBs. The process:
1. The PCB is loaded into the printer after solder mask curing.
2. Piezoelectric or thermal inkjet heads deposit tiny droplets of UV-curable epoxy ink directly onto the surface, following digital Gerber/artwork files.
3. Each printed line/pass is immediately UV-cured for instant hardening.
4. Multiple passes build desired ink thickness; no physical contact with the board.
5. Final inspection and optional additional cure.
No photomask or stencil required—purely digital workflow.
Pros: Highest flexibility—design changes are instant (just update the file); fastest turnaround for prototypes and small-to-medium batches; excellent precision for intricate patterns, gradients, or multi-color legends (if equipment supports); minimal waste (ink only where needed); easy double-sided printing with automated flipping; supports variable data (e.g., serial numbers per board).
Cons: Equipment is very expensive (industrial inkjet systems cost significantly more than LPI setups); ink formulation is specialized and costly; potential issues with ink adhesion on certain solder mask types; slightly thinner ink layer compared to LPI (may affect long-term durability); throughput slower than LPI for very high volumes; limited availability at many fabs (JLCPCB offers on request but not standard).
Comparison Table (JLCPCB Specifications)
| Method | Description | Min Line Width/Text Height | Pros | Cons | JLCPCB Usage |
| Manual Screen Printing | Ink applied through stencil by hand | ~0.178 mm / 1 mm | Low cost for prototypes | Labor-intensive, lower resolution | Rarely used |
| Liquid Photo Imaging (LPI) | Photo-sensitive epoxy ink, UV exposed | 0.15 mm / 0.8 mm | High resolution, precise, automated | More setup time | Primary method |
| Direct Legend Printing (Inkjet) | Digital inkjet directly onto board | 0.15 mm / 0.8 mm | Fast for small batches, flexible | Higher equipment cost | Available on request |
Note: JLCPCB default silkscreen color is white (black/yellow optional). Silkscreen is automatically clipped from pads and vias.
Conclusion
PCB silkscreen is essential for usability, assembly, and branding. When designing for JLCPCB, use clear fonts, avoid placing silkscreen over pads/vias, and adhere to minimum size rules (0.15 mm line, 0.8 mm text) for best results. This ensures high-quality production and easy assembly.
Frequently Asked Questions (FAQ)
1. What is PCB silkscreen?
Silkscreen (legend) is the printed layer of text, symbols, and marks on the PCB surface, applied over the solder mask using epoxy ink for component identification, assembly guidance, and branding.
2. What are the key benefits of adding silkscreen?
Improves component identification, assembly/troubleshooting speed, circuit understanding, branding, aesthetics, compliance markings, and long-term documentation.
3. What silkscreen specifications does JLCPCB support?
Default white ink (black/yellow optional); min line width 0.15 mm (6 mil); min text height 0.8 mm (31 mil). Silkscreen is automatically clipped from pads and vias.
4. Which silkscreen method does JLCPCB primarily use?
Liquid Photo Imaging (LPI) for high resolution and reliability in production; manual rarely used; direct inkjet available on request.
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