What is PCB Legend Print?

PCB (Printed Circuit Board) legend print, also known as silkscreen printing, is the process of adding text, symbols, and other identifying marks on the surface of a PCB. The legend print serves several purposes, including:

• Providing component identification and orientation
• Indicating test points, connectors, and other important features
• Displaying company logos, part numbers, and revision information
• Enhancing the overall appearance and professionalism of the PCB

The legend print is typically applied to the component side of the PCB, but it can also be added to the solder side if needed.

Types of PCB Legend Print

There are two main types of PCB legend print:

1. Silk Screen Printing: This is the most common method for applying legend print. It involves using a fine mesh screen and a squeegee to transfer ink onto the PCB surface. Silk screen printing is cost-effective and suitable for most PCB Applications.

2. Direct Legend Printing: This method uses a specialized inkjet printer to directly apply the legend print onto the PCB surface. Direct legend printing offers higher resolution and more flexibility in terms of design compared to silk screen printing. However, it is more expensive and typically used for high-end or low-volume PCBs.

Preparing PCB Legend Print Artwork

Design Considerations

When designing the legend print artwork, consider the following factors:

• Readability: Ensure that the text and symbols are clear and easy to read. Use a font size of at least 0.8mm (0.032″) for standard text and 1.5mm (0.06″) for critical information.
• Placement: Position the legend print in a way that does not interfere with component placement or soldering. Avoid placing text or symbols too close to pads, vias, or other features.
• Color: Choose a legend print color that contrasts well with the PCB Surface Finish. White is the most common color for legend print, but other colors like black, yellow, or red can be used for specific applications.
• Consistency: Maintain consistent font styles, sizes, and formatting throughout the legend print artwork. Use a grid or template to ensure proper alignment and spacing.

Creating the Artwork

To create the legend print artwork, follow these steps:

1. Use a PCB design software or vector graphics editor to create the artwork.
2. Import the PCB layout file into the software and align the legend print artwork with the component placement.
3. Add text, symbols, and other elements to the artwork, following the design considerations mentioned above.
4. Export the artwork as a high-resolution vector file (e.g., EPS, PDF, or SVG) or a Gerber file, depending on your PCB manufacturer’s requirements.

Applying PCB Legend Print

Silk Screen Printing Process

The silk screen printing process involves the following steps:

1. Screen Preparation: A fine mesh screen is coated with a light-sensitive emulsion and exposed to UV light through a film positive of the legend print artwork. The exposed areas harden, while the unexposed areas remain soluble.
2. Ink Application: The screen is placed over the PCB, and ink is applied to the screen using a squeegee. The ink passes through the open areas of the screen and onto the PCB surface.
3. Curing: The printed PCB is then dried and cured using heat or UV light to ensure the ink adheres properly to the surface.

Direct Legend Printing Process

The direct legend printing process follows these steps:

1. Printer Setup: The inkjet printer is loaded with the appropriate ink cartridges and configured for the specific PCB material and surface finish.
2. Printing: The PCB is placed in the printer, and the legend print artwork is sent to the printer. The printer directly applies the ink onto the PCB surface.
3. Curing: The printed PCB undergoes a curing process, similar to silk screen printing, to ensure the ink’s durability and adhesion.

Quality Control and Inspection

After applying the legend print, it is essential to conduct quality control and inspection to ensure the print meets the required standards. Some key aspects to check include:

• Legibility: Verify that all text and symbols are clearly readable and free from defects like smudging, bleeding, or inconsistent thickness.
• Registration: Check that the legend print is properly aligned with the component placement and other features on the PCB.
• Adhesion: Perform a tape test to ensure the ink is well-adhered to the PCB surface and does not peel or scratch easily.
• Continuity: Test the PCB for any short circuits or open connections that may have been caused by the legend print process.

Common Challenges and Solutions

Ink Bleeding or Smudging

Problem: The legend print ink bleeds or smudges, resulting in unclear or distorted text and symbols.

Solutions:
– Adjust the ink viscosity and screen mesh count to achieve the desired print quality.
– Ensure proper curing time and temperature to allow the ink to set correctly.
– Clean the PCB surface thoroughly before printing to remove any contaminants that may affect ink adhesion.

Misalignment or Poor Registration

Problem: The legend print is misaligned or poorly registered with the component placement or other features on the PCB.

Solutions:
– Use fiducial marks or alignment pins to ensure proper registration between the legend print artwork and the PCB layout.
– Verify that the artwork and PCB Files are correctly aligned and scaled before printing.
– Implement a vision system or manual inspection to check the registration during the printing process.

Insufficient Ink Adhesion

Problem: The legend print ink does not adhere well to the PCB surface, leading to peeling, scratching, or fading.

Solutions:
– Select an ink that is compatible with the PCB surface finish and has good adhesion properties.
– Ensure the PCB surface is clean and free from oils, dust, or other contaminants before printing.
– Optimize the curing process to achieve the required ink adhesion and durability.

Best Practices for PCB Legend Print

To achieve the best results for PCB legend print, consider the following best practices:

• Collaborate closely with your PCB manufacturer to ensure the legend print artwork meets their specific requirements and guidelines.
• Use vector-based artwork files for the highest print quality and scalability.
• Maintain a minimum clearance of 0.25mm (0.01″) between the legend print and other features on the PCB.
• Apply a clear conformal coating over the legend print to enhance its durability and protection against environmental factors.
• Regularly maintain and calibrate the printing equipment to ensure consistent print quality and accuracy.

Frequently Asked Questions (FAQ)

1. What is the minimum font size for PCB legend print?

2. The minimum recommended font size for standard text is 0.8mm (0.032″), while critical information should have a minimum font size of 1.5mm (0.06″).

3. Can PCB legend print be applied to both sides of the PCB?

4. Yes, legend print can be applied to both the component side and the solder side of the PCB, depending on the specific requirements of the design.

5. What is the difference between silk screen printing and direct legend printing?

6. Silk screen printing uses a fine mesh screen and squeegee to transfer ink onto the PCB surface, while direct legend printing uses a specialized inkjet printer to directly apply the ink. Direct legend printing offers higher resolution and design flexibility but is more expensive compared to silk screen printing.

7. How can I ensure proper alignment between the legend print and the PCB layout?

8. Use fiducial marks or alignment pins to align the legend print artwork with the PCB layout. Additionally, verify that the artwork and PCB files are correctly scaled and aligned before printing.

9. What should I do if the legend print ink is not adhering well to the PCB surface?

10. Ensure that the ink is compatible with the PCB surface finish and has good adhesion properties. Clean the PCB surface thoroughly before printing and optimize the curing process to achieve the required ink adhesion and durability. If the problem persists, consult with your PCB manufacturer for further guidance.

Characteristic	Silk Screen Printing	Direct Legend Printing
Print Resolution	Lower	Higher
Design Flexibility	Limited	High
Cost	Lower	Higher
Suitable for	Most PCB applications	High-end or low-volume PCBs
Ink Compatibility	Wide range of inks	Specialized inks
Equipment Requirements	Screen, squeegee, and curing system	Specialized inkjet printer and curing system

PCB legend print is a critical aspect of PCB design and manufacturing, providing essential information and enhancing the overall appearance of the board. By understanding the different types of legend print, designing appropriate artwork, and following best practices for application and quality control, you can ensure that your PCBs have clear, durable, and professional-looking legend prints.