What is silkscreen in PCB and how to make it better?

As users, we can see several parts of the printed circuit board. The first visible part is the electronic components mounted on the board, which serve the actual functional purpose. Next, we can see the bare board itself, which includes the green solder mask that provides protection, the metal traces that carry current, and various numbers and characters created with silkscreen printing. Among these parts, the PCB silkscreen serves a specific purpose. While it may be possible to remove it, it plays an important role in design and manufacturing.

In this article, TechSparks aims to provide a comprehensive guide to silkscreening. It will help you understand what is a silkscreen and learn how to design and manufacture PCB to achieve a more precise and flawless end product.

silkscreen

Table of Contents

What is Silkscreen Layer in PCB

The screen printing layer on a PCB consists of ink traces, text, or graphics applied directly to the surface by the manufacturer. Its purpose is to provide essential information for assembly, testing, and other processes. During the PCB manufacturing process, epoxy resin ink is commonly used for screen printing.

When designing the silkscreen, consideration is given to the color that contrasts with the solder mask to ensure visibility. For example, a white silkscreen is used on a green solder mask, while a black silkscreen is used on a red solder mask. This helps make the screen print more noticeable without conflicting with the solder mask color.

While silk screen printing may add to the manufacturing costs, it is considered necessary to aid manufacturers and engineers in locating and identifying components. The silkscreen ink is non-conductive, ensuring it does not affect the electrical performance of the circuit when applied. It should also be resistant to corrosion and high temperatures to withstand harsh environments and avoid issues during component repair and replacement. Traditional silkscreen printing requires a polyester screen stretched over an aluminum frame, a laser photographic plotter, a spray developer, and a curing oven.

    Information Included in PCB Silkscreen and Its Significance

    Silk screen cleaning visible PCBA
    • Polarity reference marks: These marks assist in indicating the orientation or polarity of components like diodes or electrolytic capacitors to ensure correct mounting.
    Polarity reference marks
    Polarity Reference Marks
     
    • Test points: They designate specific locations on the PCB/PCBA where electrical testing or probe testing is necessary during the manufacturing process.
    • Component reference indicators: These indicators, which may include numbers or letters, mark the location of each component for easy assembly and potential rework.
    Component designation reference designation
    Component Designation Reference Designation
    • Component outlines: They define the spatial boundaries between components, preventing overlap or misalignment.
    • Pin markers: These markers identify the pin numbers or assignments of the components, ensuring alignment with their corresponding pads.
    Component Pin Marking
    Component Pin Marking
    • Unique identification number: This number provides traceability throughout production, testing, and maintenance stages.
    • Company logo: Some brand owners require the manufacturer to print their exclusive logos or slogans on the finished PCBs to prevent misuse.
    company logo
    Company Logo
    • Version number: This indicates a revision or iteration of the PCB design, providing information about design changes.
    • Date code: The date code indicates the production date or production batch, serving quality control, tracking, and troubleshooting purposes.
    date code
    Date Code
    • Manufacturer’s marks: These marks are specific to the PCB manufacturer, such as their logo, serial number, or lot number. They aid in traceability and quality control.
    • Warning symbols: These symbols, often exclamation points, highlight potential hazards or safety precautions during handling, installation, or operation.
    • Regulatory symbols: Regulatory symbols like CE, FCC, or RoHS indicate compliance with specific industry standards or regulations.
    Regulatory marking
    Regulatory Marking
     

    How to Create PCB Silk Screen Printing

    Traditional Silkscreen Printing:

    Traditional screen printing drawings
    Traditional screen printing drawings

    The most commonly employed method in silkscreen PCB is the transfer of ink to the circuit board surface through a fine mesh screen. This method is cost-effective as it primarily requires the preparation of a silkscreen stencil. The process typically involves the following steps:

    1. Create the stencil: Prepare the stencil required for printing based on the design file.
    2. Align and apply ink: Position the stencil accurately onto the board and apply the ink onto the screen.
    3. Squeegee application: Use a squeegee to push the printing ink through the screen, allowing it to flow onto the board’s surface.
    4. Repeat and dry/cure: Repeat the printing process as necessary, ensuring adequate drying or curing time between layers.

    Liquid Photo Imaging (LPI):

    Liquid Photo Imaging Screen Printing
    Liquid Photo Imaging Screen Printing

    LPI is an advanced PCB manufacturing technique that utilizes a photosensitive emulsion to achieve precise and intricate printed patterns. This technique enables the creation of complex designs with small text and fine lines. LPI ink materials offer improved resistance to temperature, humidity, and chemicals, and their use facilitates automated manufacturing, saving time and increasing efficiency. The process of manufacturing LPI silkscreen is similar to applying a solder mask and involves the following steps:

    1. Application of liquid photosensitive ink: Apply the photosensitive ink onto the PCB surface, ensuring even coverage.
    2. Placement of the design film: Position the screen-printed design film onto the layer of ink.
    3. UV exposure: Expose the board to UV light, which accelerates the hardening or curing process of the ink.
    4. Development: Wash off the unexposed ink using a developer solution, revealing the desired screen-printed design.
    5. Curing or drying: Cure or dry the remaining ink to enhance its durability and adhesion to the PCB surface.
    6. Quality control: Inspect the finished screen printing for accuracy and perform quality control checks.

    Direct Legend Printing (DLP):

    Load the PCB into the screen printing machine
    Load the PCB into the screen printing machine

    DLP is considered the most precise method among the three PCB silk layer manufacturing techniques described by TechSparks. However, it comes with a higher cost. In this method, an inkjet projector is utilized to directly print the screen-printed design onto the PCB surface. This eliminates the need for a stencil, as acrylic ink is applied directly from the CAD data onto the original PCB. The DLP approach offers speed and accuracy, although it is not recommended for silkscreen thicknesses below 0.02 inches, as it may result in illegible prints. The following steps outline the DLP process:

    1. Prepare a digital file: Create a digital file containing the desired screen-printed design.
    2. Set up a dedicated printer: Configure a specialized printer capable of directly printing the legend, ensuring proper alignment.
    3. Load the PCB: Insert the PCB into the printer and adjust the printing parameters accordingly.
    4. Initiate the printing process: Start the printing process, during which the printer applies ink directly onto the PCB surface based on the digital design.
    5. Allow for drying or curing: Let the ink dry or undergo curing to ensure proper adhesion and durability.
    6. Perform quality control: Verify the printed screen for accuracy, clarity, and perform necessary quality control checks.

    PCB Silk Screen Layer Design

    Font Size and Line Weight

    To ensure readability, the font size used for silkscreening should be carefully selected in relation to the size of the PCB and the available space. It is important to communicate with the manufacturer beforehand to understand the materials they can provide for the silkscreening process. Additionally, it is recommended to proactively provide the manufacturer with information such as the desired size, height, and color for the silkscreen.

    Typical sizes for silkscreening include a height of 35 mils with a line width of 5 mils, as well as a height of 25 mils with a line width of 5 mils.

    Screen size and line width

    Mark the Pins on the Silkscreen

    During the design process, it is essential to include silk screen markings for component pins. In the provided image, the P3 connector is clearly labeled with four pin numbers, enabling easy commissioning and installation. It is highly recommended to mark areas with densely packed pins, such as chips, FPC sockets, and other components, to ensure efficient troubleshooting and maintenance.

    Mark the pins on the silkscreen

    Screen Printing Direction

    To ensure optimal readability, it is advisable to minimize the number of orientation changes for reference designators on the silk screen. This can be achieved by arranging the layout in a left-to-right or bottom-to-top manner. The provided illustration demonstrates the alignment of the silk screen with the chip’s orientation, which helps reduce the likelihood of soldering in the wrong direction. However, it is important to note that for certain components like electrolytic capacitors, different considerations come into play, such as indicating positive and negative polarity. Therefore, the orientation advice may not apply uniformly and should be adjusted accordingly.

    Component Placement Directions up to 2

    It is advisable to align the component reference marks with the reading direction of the silkscreen. For horizontally placed components, the characters should be positioned on the left side, while for vertically placed components, the characters should be located at the bottom.

    When designing the silkscreen for components like resistors, capacitors, and diodes, it is recommended to avoid using multiple orientations. Using multiple orientations can lead to increased difficulty for operators during tasks such as debugging, maintenance, and soldering.

    In cases where the PCB assembly density is high and there is limited space for placing silkscreen characters, it is recommended to position them in nearby blank spaces. Additionally, marking them with arrows and picture frames can help provide additional clarity and guidance.

    Component placement direction

    Dense Silkscreen Symbols

    Key Positions Avoid Silk Screen Marks

    It is not recommended to place silkscreen characters directly on top of component pads as it can have negative effects on the solderability of the component. Additionally, it may obstruct the visibility of the printed text in the area where the component is mounted. This can lead to confusion, as illustrated in the figure below, where the via hole covers the silk screen marking for components R48 and R49, making it difficult to distinguish between them.

    Avoid silkscreen markings at critical locations

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