How are PCB silk screens made?

1. What is PCB Silkscreen?

PCB Silkscreen is the layer containing text and symbolic markings on a printed circuit board, applied using screen printing technology with epoxy resin ink onto the PCB surface. These markings include crucial information such as component identifiers, polarity indicators, and version information, providing essential references for Montagem de PCB, testing, and repair.

2. Core Value of PCB Silkscreen

• Improves Assembly Efficiency: Clear component designators and polarity markings reduce soldering errors.
• Facilitates Debugging and Repair: Clear identifiers speed up the troubleshooting process.
• Enhances Product Traceability: Version numbers, date codes, and other information improve the quality management system.
• Brand Image Display: Company logos and certification marks enhance product professionalism.

3. Key Information Contained in PCB Silkscreen

3.1 Essential Markings

1. Component Reference Designators: Such as R1, C5, U3, etc.
2. Polarity Markings: Anode/Cathode indicators for components like diodes and electrolytic capacitors.
3. Pin 1 Marker: Indicates the starting pin for integrated circuits.
4. Component Outline: Bounding box of complex components
5. Test Point Identifiers: Mark key signal test locations.

3.2 Informational Markings

1. Company Logo and Copyright Information
2. PCB Version Number and Date Code
3. Manufacturer Information and Serial Number
4. Safety Certification Marks (UL, CE, RoHS, etc.)
5. Warning Symbols and Special Operation Notes

4. PCB Silkscreen Production Process Flow

4.1 Design Stage

EDA Tool Setup Specifications:

• Set the silkscreen layer separately as the Top/Bottom Silkscreen layer.
• Font Selection: Use standard Sans-serif fonts.
• Standard Sizes: Height 25-35mil, Line Width 5mil.
• Color Configuration: White is primary; yellow, black, or red for special needs.

Design Key Points:

• Avoid silkscreen overlapping with pads or vias.
• Maintain uniform character orientation (left-to-right, bottom-to-top).
• Use leader lines for annotations in high-density areas.
• Ensure polarity component markings are clear and prominent.

4.2 Comparison of Three Main Production Processes

4.2.1 Traditional Screen Printing

Fluxo do processo:
Frame Preparation → Stretching Mesh → Coating with Photosensitive Emulsion → Exposure → Development → Printing → Curing

Vantagens: Low cost, high efficiency, suitable for large-volume production.
Limitações: Limited precision (minimum line width > 0.15mm), not suitable for high-density boards.

4.2.2 Liquid Photo Imaging (LPI)

Fluxo do processo:
Coating Liquid Photoimageable Solder Mask → UV Exposure → Development → Curing

Vantagens: High resolution (up to 0.1mm), good flatness.
Aplicações: PCB boards with medium precision requirements.

4.2.3 Direct Legend Printing (DLP)

Fluxo do processo:
Direct drive from CAD data → Inkjet Printing → UV Curing

Vantagens: Highest precision (up to 0.05mm), digital workflow, suitable for prototypes and small batches.
Limitações: Higher cost, strict requirements on surface finish.

5. Detailed PCB Silkscreen Design Specifications

5.1 Layout Design Principles

1. Consistency Principle: Maintain uniform character orientation on the same side.
2. Clearance Principle: Keep silkscreen at least 3mil away from pads and vias.
3. Clarity Principle: Ensure sufficient contrast between characters and the background.
4. Association Principle: Place identifiers at an appropriate distance from their corresponding components.

5.2 Silkscreen Handling for Special Components

BGA/QFN Components: Silkscreen size must exactly match the actual chip size.
Conectores: Clearly mark PINs and interface orientation.
Polarized Components: Use “+/-” symbols or cathode band indicators.
High-Voltage Areas: Add warning symbols and safe distance indications.

5.3 Design for Manufacturability (DFM) Considerations

• Font Selection: Avoid overly complex fonts.
• Size Control: Minimum character height should not be less than 20mil.
• Position Optimization: Avoid placing on curved or uneven areas.
• Compatibilidade de processos: Consider the characteristics of different silkscreen processes.

6. Quality Control and Common Issue Resolution

6.1 Silkscreen Quality Inspection Standards

1. Completude: All necessary markings are complete without omission.
2. Clarity: Character edges are sharp, without burrs.
3. Adesão: Passes cross-cut test, no peeling.
4. Registration Accuracy: Deviation from pad position is < 0.1mm.

6.2 Common Problems and Solutions

Blurry Characters: Adjust ink viscosity or exposure parameters.
Desvio de posição: Optimize the screen alignment process.
Poor Adhesion: Improve substrate pre-cleaning.
Insufficient Resolution: Choose a more suitable production process.

7. Trends in Advanced Silkscreen Technology

7.1 Digital Silkscreen Technology

• 3D Silkscreen Printing: Adapts to printing on irregular surfaces.
• Intelligent Color Management: Achieves precise control for multi-color silkscreening.
• Variable Data Printing: Meets personalized customization needs.

7.2 Application of Eco-Friendly Materials

• Tintas à base de água: Reduce VOC emissions.
• UV Curing Technology: Reduces energy consumption.
• Biodegradable Materials: Comply with green manufacturing requirements.

8. Practical Design Recommendations

1. Early Involvement: Consider silkscreen design during the PCB layout stage.
2. Process Consultation: Confirm specific process capabilities and limitations with the manufacturer.
3. Prototype Verification: First articles must confirm silkscreen effectiveness.
4. Documentation Standards: Provide complete Gerber files for the silkscreen layer.
5. Controlo de versões: Ensure silkscreen information matches the BOM list.

By adhering to the above specifications and process requirements, you can ensure that the PCB silkscreen provides necessary information without affecting the performance and reliability of the circuit board, offering strong support for subsequent assembly, testing, and maintenance work.