In the field of electronics, flexible circuits are typically found in two forms: rigid-flex PCB used to connect different rigid parts, or flex PCBA that provide functionality directly for electronic devices. With the continued trend towards miniaturization in electronics, the importance of miniaturization technology has increased significantly, and flexible PCBA has distinct advantages over rigid PCBA in this regard.
To assemble a PCB, components must be mounted on the bare board. This article TechSparks will provide a comprehensive guide to flexible PCB assembly, aimed at assisting readers in the successful completion of miniature electronic manufacturing projects.
- Challenges in applying SMT assembly: With SMT accounting for up to 70% of the assembly process, direct application of flexible circuit in SMT assembly is challenging due to their thinness, fragility, and susceptibility to deformation. To achieve high-quality assembly, specialized jigs are required to hold the flexible PCB in place on the rigid board.
- Low assembly density: The assembly density of flexible board is typically lower than that of rigid board, with the number of components on a flex PCB usually less than 50. This is due to the limitations of line width, spacing, and soldering techniques. Advanced design and assembly processes are required to ensure stability and reliability.
- Larger panel sizes: To facilitate manufacturing, individual PCBs are typically combined into larger panels. Flexible printed circuit board is more commonly used in consumer electronics products, which are characterized by high demand, leading to the production of larger panels. To improve assembly efficiency, manufacturers assemble the panels and then separate them by punching holes.
- High precision assembly requirements: Depending on the application, flexible PCB is classified as either dynamic or static. Dynamic applications involve repeated folding and bending, requiring greater precision, anti-static measures, and solderability testing.
- High assembly costs: Due to the challenges in the production process, the use of flexible PCBs is associated with higher costs, both in bare board manufacturing and assembly.
Components on Flex PCB
PCB assembly involves the process of soldering electronic components onto the surface of a circuit board. To ensure smooth execution of this process, it is essential to prepare a Bill of Materials (BOM) file during the design phase, which lists all the materials required to build a PCBA. A well-crafted BOM file is vital to the success of the project in several ways:
- Helps computers identify different materials required for project planning.
- Serves as the foundation for SMT robotic picking and placement.
- Provides sources for project traceability, serving as the basis for outsourcing projects.
To create a BOM file, designers must use reliable flexible PCB design software with a built-in component library. The component library contains the necessary information about each component, which is integrated into the circuit design.
Now that BOM is mentioned, we have to mention component procurement. There are two options to consider:
- Self-sourcing: Care must be taken when sourcing components from online suppliers. Choosing a reputable and authentic supplier can result in increased costs, but it mitigates the risk of using counterfeit electronic components.
- Project Outsourcing: Opting for turnkey PCBA manufacturing can save project managers from unnecessary trouble. Professional manufacturers can carry out the entire project, from procurement to manufacturing and assembly. TechSparks provides a list of flex PCB manufacturer for project outsourcing.
Flex PCB Assembly Process
- Bare Board Baking: In flexible PCB manufacturing, a baking process is required before assembly to remove moisture and volatile organic compounds from the board surface. This involves placing the board in an oven where hot air flows through the vents to the board surface, with temperature and time settings based on the temperature tolerance of the components used in the project.
- FPC Fixation: During assembly, soft boards may become bent or deformed, and therefore require fixation prior to assembly. Two methods for fixation are commonly used: using glue or double-sided tape to attach the board to cardboard, or using a specialized clamp.
- Solder Paste Handling: Solder paste is a necessary compound used in assembly. Before use, it is important to check its preservation state and viscosity, and thoroughly stir and mix it.
- Solder Paste Printing: When applying solder paste, a PCB stencil is commonly used to ensure precision and uniformity. This process can be performed manually or using a solder paste printing machine, with manual printing requiring careful attention to ensure uniform force and machine printing requiring parameter settings.
- SMT Assembly: In this step, feeders are used to install surface mount device (SMD) components. These components are taken from a component library and placed on the correct position on the solder paste using suction cups, with high precision devices utilized.
- Through-Hole Assembly: For larger components, a through-hole process may be used, which involves drilling during manufacturing.
- Cardboard Removal: After the flexible PCB assembly process is complete, the cardboard is removed and residue on the board is cleaned. Careful attention should be paid during cardboard removal, as the solder paste is in a paste state at this time and could cause component flow.
- Reflow Soldering: This is the final step in securing electronic components to a circuit board. The board is heated in a reflow oven to melt the solder paste and connect the electronic components.
- Testing: After reflow soldering, board testing is required to ensure all electronic components are properly connected and the board is functioning as intended.
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