How to do PCB Legend Print

What is PCB Legend Print?

PCB (Printed Circuit Board) legend print, also known as silkscreen printing, is the process of applying text, symbols, and logos onto the surface of a PCB. The purpose of the legend print is to provide essential information about the board’s components, their orientation, and any necessary warnings or instructions. This information helps in the assembly, testing, and troubleshooting of the PCB.

Types of PCB Legend Print

There are two main types of PCB legend print:

Ink Legend Print: This method involves applying ink directly onto the PCB surface using a silkscreen printing process. The ink used is typically white, but other colors like black, yellow, or red can also be used depending on the PCB’s solder mask color.
Etched Legend Print: In this method, the legend is etched into the solder mask layer of the PCB. This process involves removing the solder mask in the areas where the legend needs to be printed, exposing the underlying copper layer.

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Why is PCB Legend Print Important?

PCB legend print serves several important functions:

Component Identification: The legend print helps identify the various components on the PCB, such as resistors, capacitors, ICs, and connectors. This information is crucial for assembly and troubleshooting purposes.
Orientation Guidance: The legend print provides information about the correct orientation of components, such as the polarity of capacitors and the pin numbering of ICs. This helps ensure that components are placed correctly during assembly.
Warning and Instructions: The legend print can include warnings and instructions, such as “Do Not Touch” or “High Voltage,” to ensure the safety of users and prevent damage to the PCB.
Branding and Aesthetics: The legend print can include company logos, product names, and other branding elements to enhance the visual appeal of the PCB and promote brand recognition.

How to Prepare Artwork for PCB Legend Print

To ensure high-quality PCB legend print, it is essential to prepare the artwork correctly. Here are the steps to follow:

Create the Artwork: Use a PCB design software or a vector graphics editor to create the artwork for the legend print. Ensure that the artwork is clear, legible, and follows the PCB manufacturer’s guidelines.
Choose the Right Font: Select a font that is easy to read and has a sufficient stroke width to ensure clarity after printing. Sans-serif fonts like Arial or Helvetica are commonly used for PCB legend print.
Set the Appropriate Size: The size of the legend print should be large enough to be easily readable but not so large that it interferes with other components on the PCB. A minimum text height of 1.5mm is recommended.
Consider the Color: If using ink legend print, choose a color that contrasts well with the solder mask color to ensure good visibility. White ink on a green solder mask is a common combination.
Provide the Artwork in the Right Format: Supply the artwork to the PCB manufacturer in a vector format, such as EPS or AI, to ensure the best print quality. If using a raster format like TIFF or PNG, ensure that the resolution is at least 600 DPI.

PCB Legend Print Techniques

There are several techniques used for applying PCB legend print, each with its own advantages and limitations.

Silkscreen Printing

Silkscreen printing is the most common method for applying ink legend print on PCBs. The process involves the following steps:

A fine mesh screen is coated with a light-sensitive emulsion and exposed to UV light through a film positive of the legend artwork.
The unexposed areas of the emulsion are washed away, leaving a stencil of the legend on the screen.
The screen is placed over the PCB, and ink is forced through the mesh onto the PCB surface using a squeegee.
The ink is then cured using heat or UV light to ensure durability.

Silkscreen printing is suitable for high-volume production and can produce sharp, durable prints. However, it has limitations in terms of minimum font size and stroke width.

Direct Legend Printing

Direct legend printing is a digital printing method that uses inkjet or laser technology to apply the legend directly onto the PCB surface. This method offers several advantages over silkscreen printing:

It allows for finer detail and smaller font sizes, as there are no physical limitations like mesh size.
It enables quick turnaround times and low-volume production, as there is no need for screen preparation.
It supports full-color printing, allowing for more visually appealing designs.

However, direct legend printing may not be as durable as silkscreen printing and may require additional protective coatings for harsh environments.

Etched Legend Print

Etched legend print involves removing the solder mask in the areas where the legend needs to be printed, exposing the underlying copper layer. The process is as follows:

The legend artwork is used to create a photomask or a film negative.
The PCB is coated with a photosensitive solder mask and exposed to UV light through the photomask.
The exposed areas of the solder mask are developed and removed, leaving the copper exposed in the legend areas.
The exposed copper is then plated with a contrasting metal, such as tin or nickel, to create the legend.

Etched legend print offers excellent durability and can produce very fine details. However, it is more expensive than ink-based methods and may not be suitable for all PCB designs.

PCB Legend Print Design Considerations

When designing PCB legend print, there are several factors to consider to ensure readability, clarity, and manufacturability.

Font Selection

Choose a font that is easy to read and has a sufficient stroke width to ensure clarity after printing. Sans-serif fonts like Arial, Helvetica, or Verdana are commonly used for PCB legend print. Avoid using fancy or decorative fonts that may be difficult to read at small sizes.

Text Size and Spacing

The size of the legend print should be large enough to be easily readable but not so large that it interferes with other components on the PCB. A minimum text height of 1.5mm is recommended. Ensure adequate spacing between lines of text and between the text and other components to avoid confusion.

Color Contrast

If using ink legend print, choose a color that contrasts well with the solder mask color to ensure good visibility. White ink on a green solder mask is a common combination. If using etched legend print, ensure that the exposed copper color contrasts well with the solder mask.

Placement and Orientation

Position the legend print in a way that is logical and easy to understand. Place component identifiers near the corresponding components, and ensure that the orientation of the text matches the orientation of the components. Avoid placing legend print too close to the edge of the PCB or in areas where it may be obscured by other components.

Language and Symbols

If the PCB will be used in different countries or by people who speak different languages, consider using internationally recognized symbols or providing legends in multiple languages. Ensure that any symbols used are clear and unambiguous.

PCB Legend Print Quality Control

To ensure the quality and consistency of PCB legend print, it is essential to implement proper quality control measures.

Visual Inspection

Visually inspect the legend print for clarity, legibility, and proper placement. Check for any smudging, bleeding, or inconsistencies in the print. Use a magnifying glass or microscope to examine fine details.

Dimensional Verification

Verify that the legend print meets the specified dimensions and tolerances. Use a measuring tool, such as a caliper or a vision system, to check the text height, stroke width, and spacing.

Adhesion Testing

Test the adhesion of the legend print to the PCB surface to ensure that it will not flake off or degrade over time. Use a tape test or a scratch test to assess the print’s durability.

Compatibility Testing

Ensure that the legend print is compatible with the PCB’s intended use environment. Test the print’s resistance to solvents, moisture, and extreme temperatures, as applicable.

Process Control

Implement process control measures to ensure consistency in the legend print process. Monitor and control variables such as ink viscosity, screen tension, squeegee pressure, and curing temperature and time.

Common PCB Legend Print Defects and Solutions

Despite careful design and process control, PCB legend print defects can still occur. Here are some common defects and their solutions:

Defect	Description	Possible Causes	Solutions
Incomplete or missing print	Parts of the legend are missing or not printed completely	– Clogged screen mesh – Low ink viscosity – Insufficient squeegee pressure	– Clean or replace the screen – Adjust ink viscosity – Increase squeegee pressure
Smudging or bleeding	The edges of the legend are blurred or the ink bleeds into adjacent areas	– High ink viscosity – Excessive squeegee pressure – Incompatible ink and solder mask	– Adjust ink viscosity – Reduce squeegee pressure – Use compatible ink and solder mask
Poor adhesion	The legend print flakes off or detaches from the PCB surface	– Contaminated PCB surface – Incompatible ink and solder mask – Insufficient curing	– Clean the PCB surface before printing – Use compatible ink and solder mask – Ensure proper curing conditions
Misalignment	The legend print is not positioned correctly relative to the components or the PCB edges	– Incorrect screen alignment – PCB shrinkage or expansion – Inconsistent PCB placement	– Use alignment marks and fixtures – Compensate for PCB dimensional changes – Use consistent PCB placement methods

By identifying and addressing these defects, manufacturers can improve the quality and reliability of their PCB legend print.

Frequently Asked Questions (FAQ)

What is the minimum text size for PCB legend print?
The minimum recommended text height for PCB legend print is 1.5mm. However, smaller sizes may be achievable depending on the printing method and the PCB manufacturer’s capabilities.
Can PCB legend print be done in colors other than white?
Yes, PCB legend print can be done in various colors, including black, yellow, and red. The choice of color depends on the solder mask color and the desired visual effect.
Is it possible to have legend print on both sides of the PCB?
Yes, legend print can be applied to both sides of the PCB. This is commonly done for double-sided PCBs that have components on both sides.
How durable is PCB legend print?
The durability of PCB legend print depends on the printing method and the ink or material used. Silkscreen printing and etched legend print are generally more durable than direct legend printing. Proper curing and protective coatings can enhance the durability of the print.
Can PCB legend print be removed or changed after production?
Removing or changing PCB legend print after production is difficult and not recommended. It is essential to ensure that the legend artwork is correct and complete before proceeding with the printing process.

Conclusion

PCB legend print is a crucial aspect of PCB design and manufacturing. It provides essential information about the board’s components, orientation, and instructions, aiding in assembly, testing, and troubleshooting. By understanding the different printing methods, design considerations, and quality control measures, manufacturers can produce high-quality, reliable PCB legend prints.

When preparing artwork for PCB legend print, it is essential to choose the right font, size, and color, and to provide the artwork in the appropriate format. Manufacturers should also consider the intended use environment and implement proper process control measures to ensure consistency and minimize defects.

By following best practices and continuously improving their processes, PCB manufacturers can deliver products with clear, durable, and visually appealing legend prints that meet the needs of their customers.