PCB Silkscreen: Adding Value and Precision in Fabrication
Rush PCB Ltd adds the silkscreen layer on the PCBs we make as the last step in the fabrication process. The silkscreen layer serves to indicate the position of components on the board, their test points, warning symbols, company logos, miscellaneous marks, and so on. Manufacturers typically use the silkscreen on the component side of the PCB. Sometimes, it is necessary to use a silkscreen on the solder side also. The basic aim of silkscreen on the PCB is to help designers and assemblers identify components quickly.
Manufacturers use a highly formulated non-conductive epoxy ink for the silkscreen process. Although it is possible to use different colors, the PCB manufacturing industry preferably uses three common colors—white, black, and yellow. Depending on the space available on the PCB, the PCB designer can select a standard font from the PCB software.
PCB Silkscreen—Basic Function
With modern advancements in technology, a PCB silkscreen, apart from providing information on onboard components, also provides beautification images in the artwork design. It offers text-based information on various circuit points, components, parts, and additional circuitry as required. This information helps in identifying:
- The Manufacturer—Company logos, name, etc.
- PCB Version Number
- Project Identification
- Component Numbers and Orientation
- Test Points
- Alignment Marks
- Warning Signs—for alignment, high voltage, etc.
- Processing Information
The printed circuit board offers mechanical support and electrical interconnections to the on-board components. The PCB is a non-conductive substrate with a laminated copper sheet. During fabrication, the manufacturer removes unwanted copper, leaving only the necessary signal traces as conductive pathways.
Without proper identification, inspecting and troubleshooting a PCB assembly is a tough task. To avoid this situation, the designer adds customized information on the board for the assistance of the manufacturer, inspectors, and the end-users.
In general, the PCB silkscreen plays the following roles:
- Identifying Circuit Points—parts, testing points, etc.
- Troubleshooting—locating defective components, undesirable outputs, etc.
- Identifying Components—for replacement, design evaluation, etc.
- Safety Precautions—showing warning signs, grounding, earthing, high voltage, etc.
- Branding and Marketing—for manufacturing information.
- Safeguarding Intellectual Property—against illegal duplication.
- Product Evaluation—Rating verification, etc.
The silkscreen information on the PCB depends on the board size. For a small board, the designer must prioritize the critical information that must appear on the board. Overall, the PCB silkscreen affects the PCB quality, grading, and reliability.
Primarily, there are three methods for application of silkscreen information on a PCB. Manufacturers select the most optimum method depending on the number of boards, complexity, and cost.
Manual Screen Printing
The manual screen-printing process is the oldest and most affordable of all the PCB silkscreen printing methods. Manufacturers use a screen with a negative image of the pattern they want to print on the board. After aligning the screen with the PCB, they push epoxy ink through the screen onto the board using a squeegee. This transfers the image on to the board for drying.
The manual screen-printing method is a manageable process with a simple, low-cost setup. However, this process is suitable for a large board with average printing resolution.
Liquid Photo Imaging
For liquid photo imaging, the operator first coats the copper surface of the PCB with an epoxy solution with photo imaging properties. Then they transfer the silkscreen image on to the board by exposing it to ultraviolet light.
Requiring less space than the manual screen-printing method, liquid photo imaging produces better resolution and precision. However, this method consumes more ink, requires a greater amount of setup time, and is more expensive.
Direct Legend Printing
Direct legend printing uses a high-end ink jet printer to transfer ink onto the board. The biggest advantage is this method can transfer the CAD information directly to the PCB.
This is a modern printing method offering the highest efficiency, accuracy, resolution, and precision. However, it is also the most expensive method.
According to Rush PCB Ltd, the silkscreen process is a delicate process directly affecting the overall quality and reliability of the board, while adding value through refined artwork. Manufacturers select the printing process based on the cost factor and the resolution necessary.