Flexible Printed Circuits (FPCs), also known as flexible circuit boards, differ significantly from rigid printed circuit boards in their PCBA assembly and soldering processes. Due to their soft and pliable nature, FPCs cannot be directly handled or transported like rigid boards. Without the use of a specialized carrier board, it is difficult to fix and transport them during the SMT (Surface Mount Technology) process. Standard SMT steps such as printing, placement, and reflow soldering become challenging without proper support.
One key step in the FPC assembly process is **pre-baking**. FPCs are often not vacuum-packed at the factory, making them prone to moisture absorption during storage and transportation. To prevent issues like delamination or blistering during reflow soldering, they must be pre-baked at 80–100°C for 4–8 hours. In some cases, higher temperatures (up to 125°C) may be used, but with shorter baking times. Before baking, it's essential to test the FPC to ensure it can withstand the heat. After baking, the FPC should show no signs of discoloration, warping, or deformation. It must pass an IPQC (In-Process Quality Control) inspection before proceeding to the next stage.
Another critical aspect is the **production of a special carrier board**. This board is designed based on the FPC’s CAD data and ensures precise alignment through positioning pins and holes. Since FPCs often have varying thicknesses due to design requirements, the carrier must be carefully processed to maintain flatness during printing and placement. Common materials for carrier boards include synthetic stone, aluminum, silicone, and high-temperature resistant magnetized steel. These materials offer durability, thermal stability, and minimal warpage.
During the **SMT process**, the FPC is fixed onto the carrier using either single-sided or double-sided tape. The choice depends on the application and the need for easy removal after reflow. For example, single-sided tape allows for easier post-reflow handling, while double-sided tape offers more secure fixation. Careful attention is needed to avoid excessive pressure that could damage the FPC. The printing stage requires a polyurethane scraper (hardness 80–90) to ensure even solder paste distribution, as metal scrapers might cause uneven application or damage the FPC.
The **placement** of components on FPCs is similar to that on rigid PCBs, but the flexibility of the FPC means that the nozzle height and suction pressure must be precisely adjusted. Additionally, since FPCs tend to have lower yields, the placement machine should have a BAD MARK identification feature to detect and isolate defective units quickly.
For **reflow soldering**, a forced hot air infrared oven is recommended to ensure even heating. If single-sided tape is used, the middle portion of the FPC may sag, leading to misalignment and poor solder joints. Therefore, the temperature profile must be carefully set. Testing the temperature curve by placing two FPC-loaded carriers on a test board helps ensure uniform heating across all components.
After reflow, the FPC is removed from the carrier, and a cooling fan is often used to reduce the temperature quickly. Operators must handle the FPC gently to avoid tearing or creasing. Visual inspection under a magnifying glass (at least 5x) is crucial for identifying issues like residual glue, discoloration, tin beads, or incomplete solder joints. While AOI (Automated Optical Inspection) is commonly used for PCBs, FPCs are often better suited for ICT (In-Circuit Test) or FCT (Functional Circuit Test) using dedicated fixtures.
Finally, for mass production, a custom stamping die is recommended to separate the FPCs cleanly and efficiently. This reduces stress on the board and minimizes the risk of solder joint cracking.
Overall, the SMT process for FPCs is more complex than for rigid PCBs, requiring careful planning, precise equipment settings, and strict quality control. Each step, from pre-baking to final testing, plays a vital role in ensuring the reliability and performance of the final product.
In addition to FPC-specific considerations, the overall **PCBA manufacturing process** relies heavily on advanced machinery and equipment. Key tools include solder paste printers, component placement machines, reflow ovens, AOI systems, wave soldering machines, and cleaning systems. Each piece of equipment contributes to the efficiency and quality of the final assembly.
When outsourcing PCBA production, it is essential to follow strict guidelines, including accurate BOM (Bill of Materials) compliance, anti-static measures, proper component orientation, and adherence to soldering standards. Components must be handled with care to prevent static damage, and packaging must protect the boards during transport.
By following these best practices, manufacturers can ensure high-quality, reliable PCBA assemblies that meet the demands of modern electronics.
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