What is the Silkscreen on a PCB?
A printed circuit board consists of multiple parts, each playing a crucial role in the construction and functionality of the PCB. Among them, there is the substrate that provides mechanical support, the components that carry out the actual circuit functions, the conductors that transmit current and connect the components, and the vias that establish connections between different layers, and more. These parts are all closely scrutinized by designers as they directly impact the PCB’s performance and functionality.
However, despite the significance of these aspects, there is one detail that is easily overlooked: Silkscreen. In design or manufacturing, the role of the silkscreen layer is often not thoroughly discussed. In this article, FS Technology will provide a comprehensive discussion on the PCB silkscreen layer, introducing its concept, and emphasizing its importance.
Significance of Silkscreen in PCB
Although the silkscreen is just a layer of ink traces on a circuit board primarily used for identifying components, test points, symbols, and other details, its role in projects is often underestimated. The purpose of this ink layer is for labeling and does not directly affect the circuit’s performance. However, for companies like FS Technology specializing in PCBA, silkscreen holds significant importance. These markings assist engineers and manufacturers in tracking all components on the board and provide guidance for them.
During the silk screen manufacturing process, we use specially formulated ink, typically opting for a non-conductive epoxy resin ink. The ink’s color can be customized according to customer requirements, with black, yellow, and white being common choices. In most cases, we choose white ink as PCB board typically has a green color, making white silkscreen markings more visibly clear. Additionally, font styles can also be customized per customer requests, and these design elements are included when outputting Gerber files.
Traditional silkscreen printing typically involves equipment such as stretched polyester screens on aluminum frames, laser etching machines, spray developer solutions, and curing ovens. Silkscreen is usually applied on the assembly side of components for observation and identification. However, in some projects, silkscreen may also be applied on the soldering side, but this can lead to higher production costs and therefore needs careful consideration.
Necessity of Silk Screen Layer
Different industries have distinct requirements for PCB. In high-demand sectors such as aviation, automotive, and medical devices, safety and quality parameters are exceptionally high. These industries impose strict specifications on the silkscreen layer of PCB.
It helps manufacturers use reference indicators such as polarity markings and component values to identify integrated circuits, resistors, capacitors, and other components. This provides engineers with clear guidance that minimizes the possibility of component misconfiguration, thereby avoiding costly rework.
Furthermore, the silkscreen layer provides valuable data about the PCB’s version, manufacturer’s identification, manufacturing date, and other relevant details. It helps track the history and origin of the PCB throughout its working life, providing necessary background information for maintenance and repairs.
Assembly heavily relies on the use of components, and the silkscreen layer plays a crucial role in guiding and preventing common errors throughout the process. The silkscreen layer provides various useful markings, such as polarity indicators (positive and negative polarity markings), Pin-1 markings, and orientation indicators. On integrated circuits, the Pin-1 marking defines the orientation of the chip. Particularly in the prototyping and development stages, the silkscreen layer offers great flexibility, allowing engineers to quickly update component labels, indicators, or guidelines without the need for extensive redesign, thus enabling rapid PCB prototyping.
Furthermore, silkscreen markings play a crucial role in testing. During electrical testing, measurement, and troubleshooting processes, these markings provide test points and debugging data, allowing easy access to specific nodes for diagnostics without the need for additional documentation. For manufacturing, silkscreen markings enable technicians to employ visual inspection, speeding up the inspection process. By visually identifying silkscreen markings, they can easily spot missing or misaligned components, soldering issues, or other errors, thereby improving assembly speed and increasing the yield of good products.
Customization and Branding
For some major companies, they require consistency in their products. Even for circuit boards used as internal components, they insist on adding their company logo, serial number, or identifier to enhance their brand image. This improves the overall user experience and adds an aesthetic appeal to the devices. Well-executed and thoughtful silk-screen printing can make a project more user-friendly and visually appealing, which is attractive to users.
Key Design Considerations for Silk Screen Layer
The size of text and symbols in the silk screen layer must be sufficiently large for easy reading by engineers and designers during testing and assembly. Standard components use a font size of at least one millimeter. For small-sized components like 0402 resistors or capacitors, fonts of around 0.5 mm are typically used. Component outlines or indicators need to be visible without leaking into adjacent features, so they require adequate width. Typically, line widths of 0.15 mm to 0.2 mm are used. Maintaining proper spacing between different silk screen elements avoids overcrowding and confusion.
To maintain neatness, align silkscreen indicators and markings with components. Place indicators near the component pins or other key reference points, avoiding positioning component silkscreen that might obstruct test points, connectors, or other vital functions. This helps engineers accurately position components during assembly and establishes clear pathways for testing and troubleshooting.
Ensure consistent silkscreen colors for products within the same batch. When selecting colors, make sure the silkscreen color contrasts sharply with the board background. Typically, use white silkscreen printing on green boards or black silkscreen printing on white boards. For specific modules, consider using different colors for easy identification. For example, different colors can highlight connections, LEDs, or power-related components.
PCB Silk Screen Printing Techniques
Liquid Photo Imaging
Commonly referred to as LPI, this PCB silkscreen printing technique utilizes liquid ink and is favored for its durability and precision. The process unfolds as follows:
- Begin by applying photosensitive emulsion to a screen. The role of the photosensitive emulsion is to create the desired pattern based on the design from the film or artwork when exposed to ultraviolet (UV) light.
- Place the film or artwork onto the layer of photosensitive emulsion and expose it to UV light. UV exposure causes the emulsion in the unexposed areas to wash away, leaving only the parts exposed to light on the silk screen, forming the desired pattern.
- Apply liquid ink to the template, and then use a squeegee to force the ink through the mesh and onto the surface of the PCB. During the development process, areas on the circuit board where the emulsion has been removed will have ink adhering to them.
- Use a lamp to cure or dry the ink to ensure it adheres to the PCB’s surface and remains stable.
Direct Legend Printing
When using DLP technology for silkscreen printing, there’s no need for traditional templates or screens. Instead, it involves directly printing silk screen using a specialized printer. This modern technology offers several advantages, especially for prototypes and small-batch projects. Here is the process of DLP silk screen printing:
- Begin by designing the graphics and labels in digital form. In computer-aided design (CAD) software, users can customize the size, font, and placement of graphics on the PCB, allowing for flexible creation of the desired design.
- DLP printers are equipped with special print heads designed to dispense ink or epoxy resin directly onto the surface of the PCB.
- Utilizing the principles of digital light processing, the design and graphics are formed by projecting a digital image or design from a light source onto the PCB.
- After printing, the graphics need to be cured to ensure adhesion and durability. Curing is typically achieved using ultraviolet (UV) light or heat.
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