PCB Silk-screen and Cure

Posted by

What is PCB Silkscreen?

PCB silkscreen, also known as legend or nomenclature, is a layer of text and symbols printed on the surface of a printed circuit board (PCB) to provide essential information for assembly, testing, and maintenance. The silkscreen layer is typically applied using a screen-printing process with white or yellow ink, although other colors may be used for specific applications.

Silkscreen serves several important functions on a PCB:

  1. Component identification: Silkscreen labels components with their reference designators (e.g., R1, C2, U3) to help identify their locations during assembly and troubleshooting.

  2. Orientation markers: Silkscreen may include orientation markers, such as pin 1 indicators or polarity symbols, to ensure correct component placement.

  3. Test points: Test points for debugging or quality control may be labeled on the silkscreen layer.

  4. Branding and logos: Company logos, product names, and version numbers can be printed on the silkscreen for branding and identification purposes.

  5. Warning messages: Safety warnings or handling instructions may be included on the silkscreen to prevent damage to the PCB or components.

Silkscreen Design Considerations

When designing the silkscreen layer for a PCB, several factors should be considered to ensure readability and manufacturability:

  1. Font size and style: Choose a font that is clear and easy to read at small sizes. Sans-serif fonts, such as Arial or Helvetica, are commonly used. Minimum font size should be 0.8mm (0.032″) for readability.

  2. Spacing: Maintain sufficient spacing between silkscreen elements and components to avoid obscuring important features or interfering with soldering.

  3. Alignment: Ensure that silkscreen labels are aligned with the corresponding components and do not overlap with pads or traces.

  4. Contrast: Select a silkscreen color that contrasts well with the PCB substrate color for optimal readability.

  5. Manufacturing limitations: Be aware of the minimum line width and spacing that your PCB manufacturer can achieve with their silkscreen process.

Silkscreen Application Process

The silkscreen layer is typically applied to the PCB using a screen-printing process. This involves the following steps:

  1. Screen preparation: A fine mesh screen is coated with a light-sensitive emulsion and exposed to a film positive of the silkscreen 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 onto the PCB surface.

  3. Curing: The printed ink is then cured using heat or ultraviolet (UV) light to ensure adhesion and durability.

Silkscreen Ink Types

There are two main types of ink used for PCB silkscreen printing:

  1. Epoxy-based ink: This type of ink is heat-cured and offers excellent durability and resistance to solvents and abrasion. Epoxy-based inks are commonly used for high-reliability applications.

  2. UV-curable ink: UV-curable inks are cured using UV light and provide faster curing times and good durability. They are suitable for most standard PCB applications.

The choice of ink type depends on the specific requirements of the PCB, such as the desired durability, curing time, and compatibility with the substrate material.

Silkscreen Curing Methods

After the silkscreen ink is applied to the PCB, it must be cured to ensure proper adhesion and durability. There are two primary methods for curing silkscreen ink:

Heat Curing

Epoxy-based inks are typically heat-cured using a conveyor oven. The PCB is passed through the oven at a controlled temperature and duration to allow the ink to crosslink and harden. The specific curing temperature and time depend on the ink formulation and PCB substrate material but typically range from 120°C to 150°C for 30 to 60 minutes.

Advantages of heat curing:
– Excellent durability and resistance to solvents and abrasion
– Good adhesion to a wide range of substrate materials

Disadvantages of heat curing:
– Longer curing times compared to UV curing
– Higher energy consumption due to the need for elevated temperatures

UV Curing

UV-curable inks are exposed to high-intensity UV light to initiate a photochemical reaction that causes the ink to crosslink and harden. UV curing is a fast process, typically requiring only a few seconds of exposure. The specific wavelength and intensity of the UV light depend on the ink formulation.

Advantages of UV curing:
– Fast curing times, allowing for higher production throughput
– Lower energy consumption compared to heat curing
– Reduced risk of thermal damage to heat-sensitive components

Disadvantages of UV curing:
– Limited penetration depth of UV light, which may affect curing of thick ink deposits
– Potential for shadowing effects, where tall components block the UV light and prevent proper curing

The choice of curing method depends on the specific ink type, PCB substrate material, and production requirements. In some cases, a combination of heat and UV curing may be used to achieve the desired results.

Silkscreen Inspection and Quality Control

After the silkscreen layer is applied and cured, it is important to inspect the PCB to ensure that the silkscreen meets the required quality standards. Inspection criteria may include:

  1. Readability: Check that all silkscreen elements are clear, legible, and free from defects such as smudging, bleeding, or incomplete printing.

  2. Registration: Verify that the silkscreen elements are correctly aligned with the corresponding components and features on the PCB.

  3. Adhesion: Perform a tape test to ensure that the silkscreen ink is properly adhered to the PCB surface and does not flake or peel off.

  4. Dimensional accuracy: Measure the size and position of critical silkscreen elements to ensure they meet the specified tolerances.

Automated optical inspection (AOI) systems can be used to quickly and accurately inspect silkscreen quality, while manual inspection may be necessary for more complex or critical PCBs.

Troubleshooting Common Silkscreen Issues

Several issues can arise during the silkscreen process that may affect the quality and appearance of the final PCB. Some common problems and their potential solutions include:

  1. Incomplete or missing silkscreen: This may be caused by a clogged screen, insufficient ink, or incorrect exposure of the screen emulsion. Ensure that the screen is properly cleaned and exposed, and that the ink is applied evenly.

  2. Smudging or bleeding: This can occur if the ink is too thin or the screen mesh is too coarse. Use a higher viscosity ink and a finer mesh screen to improve print quality.

  3. Poor adhesion: This may be due to contamination on the PCB surface, incompatible ink, or incorrect curing parameters. Clean the PCB surface thoroughly, select a compatible ink, and ensure that the curing process follows the manufacturer’s recommendations.

  4. Misalignment: This can happen if the screen is not properly aligned with the PCB or if the PCB shifts during printing. Use alignment marks and fixtures to ensure accurate registration between the screen and the PCB.

By understanding the causes of these issues and implementing appropriate solutions, manufacturers can ensure high-quality silkscreen printing on their PCBs.

Frequently Asked Questions (FAQ)

  1. What is the difference between silkscreen and legend?
  2. Silkscreen and legend are two terms that refer to the same thing: the layer of text and symbols printed on the surface of a PCB to provide information for assembly, testing, and maintenance.

  3. Can silkscreen be applied to both sides of a PCB?

  4. Yes, silkscreen can be applied to both the top and bottom sides of a PCB, depending on the design requirements. However, it is more common to have silkscreen on the top side only, as this is the side where most components are placed.

  5. Is silkscreen necessary for all PCBs?

  6. While silkscreen is not strictly necessary for the functional operation of a PCB, it is highly recommended for most applications. Silkscreen provides valuable information that aids in assembly, testing, and troubleshooting, which can save time and reduce errors.

  7. Can silkscreen be applied over soldermask?

  8. Yes, silkscreen is typically applied over the soldermask layer on a PCB. The soldermask provides electrical insulation and protection for the copper traces, while the silkscreen is applied on top of the soldermask for labeling and identification purposes.

  9. How does the choice of PCB substrate material affect silkscreen adhesion?

  10. The choice of PCB substrate material can impact silkscreen adhesion, as some materials have different surface properties that may affect ink bonding. For example, polyimide substrates may require special ink formulations or surface treatments to ensure proper adhesion. It is important to select a silkscreen ink that is compatible with the specific substrate material being used.