What is PCB Legend?

PCB legend, also known as silkscreen or overlay, is the text and symbols printed on the surface of a printed circuit board (PCB). The legend provides important information about the components, their placement, and the connections between them. It serves as a guide for assembling, testing, and troubleshooting the PCB.

The legend typically includes the following elements:
– Component designators (e.g., R1, C2, U3)
– Component values (e.g., 10K, 22μF, 74HC00)
– Polarity indicators (e.g., +, -)
– Test points
– Company logo or product name
– Revision number or date code

Importance of PCB Legend

A clear and accurate PCB legend is crucial for several reasons:
1. Assembly: It helps the assembler identify the correct components and their orientation, reducing the risk of errors.
2. Testing: Test points marked on the legend allow for easy probing and debugging of the circuit.
3. Troubleshooting: In case of a malfunction, the legend helps technicians identify components and trace the signal path to pinpoint the issue.
4. Maintenance: A well-labeled PCB makes it easier to replace components or perform upgrades in the future.

Preparing the PCB Legend

Before printing the legend on the PCB, it is essential to prepare the artwork carefully. Follow these steps to ensure a high-quality legend:

Step 1: Create the Legend Artwork

Use PCB design software, such as Altium Designer, Eagle, or KiCad, to create the legend artwork. Most software packages have dedicated layers for the silkscreen.

When designing the legend, keep the following points in mind:
– Use a clear and readable font, such as Arial or Helvetica.
– Ensure sufficient spacing between text and components to avoid overlapping.
– Keep the text size within the manufacturer’s specifications (usually 1-2mm in height).
– Avoid placing text or symbols over pads or vias.
– Use appropriate line widths for symbols (typically 0.15-0.2mm).

Step 2: Review and Verify

Once the legend artwork is complete, review it carefully to ensure accuracy and readability. Check for the following:
– Correct component designators and values
– Proper orientation of polarity indicators
– Legibility of text at the actual PCB size
– Compliance with the manufacturer’s design rules

Step 3: Generate the Gerber Files

Export the legend artwork as a separate Gerber file, usually named F.SilkS (for front side) or B.SilkS (for back side). Gerber files are the industry standard format for PCB Fabrication.

Make sure to include the legend Gerber file along with the other PCB Layers (e.g., copper, solder mask, drill) when sending the design for manufacturing.

Printing the PCB Legend

The actual printing of the legend is done by the PCB manufacturer during the fabrication process. There are two common methods for legend printing:

Silk Screening

Silk screening is the traditional and most widely used method for printing PCB legends. It involves the following steps:
1. A fine mesh screen is coated with a light-sensitive emulsion.
2. The legend artwork is transferred to the screen using UV light, hardening the exposed areas of the emulsion.
3. The unexposed areas are washed away, leaving a stencil of the legend on the screen.
4. Ink is forced through the stencil onto the PCB surface using a squeegee.
5. The ink is cured using heat or UV light.

Silk screening produces a durable and high-contrast legend, suitable for most applications. However, it has some limitations:
– Limited resolution (around 0.15mm line width)
– Ink can bleed or smudge if not applied properly
– Not suitable for very fine pitch components

Direct Legend Printing

Direct legend printing is a newer technology that offers several advantages over silk screening. It uses inkjet or laser printing to apply the legend directly onto the PCB surface.

The process works as follows:
1. The PCB is cleaned and prepared for printing.
2. The legend artwork is loaded into the printer.
3. The printer applies the ink or toner directly onto the PCB surface.
4. The ink or toner is cured using heat or UV light.

Direct legend printing offers several benefits:
– Higher resolution (down to 0.1mm line width)
– Cleaner and more precise lines
– Faster and more cost-effective for small batches
– Suitable for fine pitch components

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

Choosing the Right Legend Color

The choice of legend color depends on the PCB’s solder mask color and the desired visual contrast. The most common legend colors are white and black, but other colors like yellow, red, or blue are also available.

When selecting the legend color, consider the following factors:
– Contrast: The legend should be easily distinguishable from the solder mask. White legend on green solder mask or black legend on red solder mask provides good contrast.
– Readability: Light-colored legend on dark solder mask is generally more readable than the reverse.
– Aesthetics: The legend color can be chosen to match the product’s branding or color scheme.

Some PCB manufacturers may offer custom legend colors for an additional cost. However, stick to standard colors unless there is a compelling reason to use a custom color.

Common Pitfalls and How to Avoid Them

When designing and printing PCB legends, there are several common pitfalls to watch out for:

Overlapping Text and Components

Ensure that the legend text and symbols do not overlap with the component pads or vias. This can cause short circuits or make the text unreadable. Use the PCB design software’s design rule check (DRC) to detect and correct overlaps.

Incorrect Component Designators

Double-check the component designators and values against the schematic to avoid errors. A mislabeled component can lead to incorrect assembly or troubleshooting difficulties.

Poor Readability

Small or unclear text can make the legend difficult to read. Use a clear font and ensure that the text size is within the manufacturer’s specifications. Also, consider the contrast between the legend and solder mask colors.

Incomplete or Missing Legend

Review the legend artwork thoroughly to ensure that all necessary information is included. A missing component designator or test point can cause confusion during assembly or testing.

Incompatible Ink or Printing Method

Discuss the legend printing options with the PCB manufacturer to ensure that the chosen ink and printing method are compatible with the PCB material and finish. Some inks may not adhere well to certain surfaces or may degrade over time.

Best Practices for PCB Legend Design

To create an effective and user-friendly PCB legend, follow these best practices:

1. Use a consistent and clear naming convention for component designators (e.g., R for resistors, C for capacitors, U for ICs).
2. Place the legend text and symbols close to the corresponding components, but not too close to avoid overlapping.
3. Orient the text to be easily readable when the PCB is installed in its final position.
4. Use standard units and abbreviations (e.g., Ω for ohms, μF for microfarads).
5. Include polarity indicators for polarized components like electrolytic capacitors and diodes.
6. Mark test points and important connectors clearly.
7. Add a revision number or date code to track changes to the PCB design.
8. Follow the PCB manufacturer’s design guidelines for legend line widths, text sizes, and clearances.

By adhering to these best practices, you can create a clear and informative PCB legend that will aid in assembly, testing, and maintenance.

Frequently Asked Questions (FAQ)

1. What is the minimum text size for PCB legend?

The minimum text size for PCB legend depends on the manufacturer’s capabilities. Generally, the minimum text height is 1mm, with a line width of 0.15-0.2mm. However, some manufacturers may be able to accommodate smaller sizes. Always check with your PCB manufacturer for their specific guidelines.

2. Can I use different colors for different parts of the legend?

Yes, it is possible to use different colors for different parts of the legend, such as component designators and values. However, this will require separate silk screens or printing passes for each color, which can increase the cost and production time. It is more common to use a single color for the entire legend.

3. What is the difference between silk screening and direct legend printing?

Silk screening is the traditional method of applying the legend, using a stencil and ink. It is durable and suitable for large batches but has limitations in resolution and fine details. Direct legend printing uses inkjet or laser technology to apply the legend directly onto the PCB surface. It offers higher resolution and precision but may not be as durable as silk screening.

4. Can I change the legend after the PCB has been manufactured?

Changing the legend after the PCB has been manufactured is difficult and not recommended. If the legend is printed using silk screening, it would require creating a new screen and reprinting the legend. If direct legend printing is used, it may be possible to remove the existing legend with solvents and reprint, but this can damage the PCB surface. It is best to ensure that the legend is correct before manufacturing.

5. How do I ensure that the legend will be readable after soldering?

To ensure that the legend remains readable after soldering, choose a legend color that contrasts well with the solder mask. White legend on green solder mask or black legend on red solder mask usually provides good visibility. Also, avoid placing legend text or symbols too close to pads or areas where solder may flow during assembly.

Factor	Silk Screening	Direct Legend Printing
Resolution	Limited (0.15mm line width)	Higher (down to 0.1mm line width)
Durability	High	May require additional coatings
Cost	Cost-effective for large batches	More cost-effective for small batches
Speed	Slower setup time	Faster, no screen preparation needed
Fine Pitch Compatibility	Not suitable	Suitable

Conclusion

A well-designed and properly printed PCB legend is essential for the success of any electronic product. It provides vital information for assembly, testing, and maintenance, and can save time and reduce errors.

When creating the legend artwork, pay attention to readability, clarity, and compliance with the manufacturer’s guidelines. Choose the appropriate printing method based on the project’s requirements and budget, and work closely with the PCB manufacturer to ensure the best results.

By following the best practices and avoiding common pitfalls, you can create a professional-looking and informative PCB legend that will contribute to the overall quality and reliability of your product.