Flexible Printed Circuit Assembly: Key Processes and Techniques

Introduction

Flexible Printed Circuits (FPC), also known as flex circuits, present unique challenges in the PCBA assembly and soldering process, significantly different from rigid PCBs. Due to the flexible nature of FPCs, specialized carriers are required for securing and handling during critical processes such as printing, pick-and-place, and reflow soldering. This article delves into the intricate steps involved in flexible printed circuit assembly, highlighting key procedures, potential pitfalls, and best practices for achieving optimal results.

Pre-Treatment of FPC

FPCs are highly sensitive to moisture absorption during transportation and storage since they are usually not vacuum-sealed upon delivery. Before proceeding with Surface Mount Technology (SMT) processes, pre-baking is essential to eliminate any absorbed moisture. Failure to do so can result in delamination, blistering, and other defects during the high-temperature reflow soldering process.

Pre-Baking Guidelines:  

Typical pre-baking conditions involve temperatures between 80°C and 100°C for 4 to 8 hours. In certain cases, the temperature can be increased to over 125°C, but with a corresponding reduction in baking time. It’s crucial to perform a sample test to determine if the FPC can withstand the set baking temperature or consult with the FPC manufacturer for recommended baking conditions. During baking, avoid stacking too many FPCs; a stack of 10-20 panels (PNL) is ideal. Some FPC manufacturers place isolation paper between each panel, which should be removed if it cannot endure the baking temperature. Post-baking, FPCs should be free of discoloration, warping, or other defects, and only proceed to production after passing IPQC inspection.

Fabrication of Specialized Carriers

Creating precise FPC positioning templates and specialized carriers based on CAD files is critical for accurate alignment during SMT. The diameter of the positioning pins on the template should match the holes on both the carrier and the FPC. Given that many FPCs vary in thickness due to protective layers or design features, the carrier must be machined to accommodate these variations, ensuring a flat surface during printing and placement.

Carrier Material Requirements:

Carriers should be lightweight, high-strength, and resistant to heat absorption and deformation after multiple thermal cycles. Common materials include synthetic stone, aluminum plates, silicone sheets, and high-temperature-resistant magnetized steel plates.

The Production Process

Using standard carriers as an example, the following outlines the SMT process for FPC. The use of silicone boards or magnetic fixtures simplifies the process, but the core techniques remain consistent across different carrier types.

FPC Fixation

Accurate fixation of the FPC onto the carrier is the first crucial step before SMT. The storage time between fixation and subsequent processes like printing, placement, and soldering should be minimized. Carriers come with or without positioning pins. For carriers without pins, a positioning template is used to align and secure the FPC using adhesive tape. Carriers with spring-loaded positioning pins allow for direct alignment and fixation of the FPC, simplifying the process.

Method 1 (Single-Sided Adhesive Tape Fixation):

Thin, high-temperature resistant single-sided tape is used to secure the FPC edges onto the carrier, preventing misalignment or lifting. The adhesive should be moderate, allowing easy removal post-reflow without leaving residue. Using an automatic tape dispenser can improve efficiency and reduce costs.

Method 2 (Double-Sided Adhesive Tape Fixation):

Applying high-temperature double-sided tape onto the carrier creates an effect similar to using a silicone board. Care must be taken with the adhesive’s strength; if it is too strong, it can cause FPC tearing during removal after reflow. As the tape’s adhesive properties degrade with repeated use, it must be replaced when it can no longer securely hold the FPC. This station is critical for preventing contamination, so operators should wear finger cots and clean carriers thoroughly between uses.

Solder Paste Printing

FPCs do not have specific requirements for solder paste composition, but the printing process demands high performance. The solder paste should have excellent thixotropy, allowing for easy application and release, while adhering firmly to the FPC surface without defects like collapse or clogged stencil apertures.

Printing Considerations:

Given the uneven surface of the FPC mounted on the carrier, metal squeegees are unsuitable. Instead, polyurethane squeegees with a hardness of 80-90 degrees should be used. An optical alignment system on the solder paste printer is recommended to ensure high printing quality, as any slight misalignment can significantly impact results due to the FPC’s flexible nature.

Component Placement

Both medium and high-speed placement machines can be used depending on the product characteristics, component count, and placement efficiency. The presence of optical MARK points on the FPC facilitates placement, similar to rigid PCBs. However, due to the inherent flexibility of the FPC and its carrier, parameters such as nozzle height, air pressure, and movement speed must be precisely set to avoid misalignment or component damage.

Handling Tips:

Since FPCs are typically in panel form, and due to the lower yield compared to rigid boards, it’s common for some sections to be defective. Therefore, placement machines with BAD MARK recognition capabilities are essential to maintain production efficiency.

Reflow Soldering

Forced hot air convection infrared reflow ovens are recommended for even temperature distribution across the FPC, reducing soldering defects. When using single-sided tape for fixation, the FPC’s middle section can deform under heat, causing pad tilt and solder flow issues, leading to defects like voids, bridging, and solder balls.

Temperature Profile Testing:

Due to the varying thermal properties of carriers and component types on the FPC, precise temperature profiling is critical. It is advisable to sandwich the test board with loaded FPCs on either side of the carrier during testing. Attach thermocouple probes to key points such as the edges and QFP leads using high-temperature solder and tape, ensuring the test points remain exposed for accurate readings.

Temperature Profile Settings:

For best results, use a ramp-soak-spike (RSS) temperature profile. This approach allows for easier control of each zone’s parameters and minimizes thermal shock to both FPC and components. Set the oven temperature at the lower end of the solder paste’s specification, and use the lowest airflow setting available on the reflow oven. Ensure the stability of the reflow conveyor to avoid any vibration.

Inspection, Testing, and Depaneling

Given the high heat retention of carriers, especially aluminum ones, it’s advisable to add forced cooling fans at the reflow exit. Operators should wear thermal gloves to prevent burns from the hot carriers. When removing the FPC from the carrier, apply even force to avoid tearing or creasing.

Inspection and Testing:

Post-soldering inspection under magnification (5x or greater) is necessary to identify issues such as residual adhesive, discoloration, solder contamination on gold fingers, solder balls, and IC lead defects. Due to the non-flat surface of FPCs, AOI can result in high false positives, making it less suitable. Instead, FPCs should be tested using dedicated ICT and FCT fixtures.

Depaneling:

For FPC panels, depaneling may be required before ICT or FCT testing. While manual methods like knives or scissors can be used, they are inefficient and prone to high scrap rates. For mass production of non-standard FPC shapes, it’s advisable to use dedicated punching molds for depaneling. This method greatly enhances efficiency and produces clean, aesthetically pleasing edges with minimal internal stress, reducing the risk of solder joint cracks.

Conclusion

In the flexible printed circuit assembly and soldering process for flexible electronics, precise positioning and fixation of the FPC are crucial, with success largely dependent on the quality of the carrier used. Pre-baking, printing, placement, and reflow soldering of FPCs are significantly more challenging compared to rigid PCBs. Therefore, accurately setting process parameters and implementing stringent production management are essential. Ensuring that operators strictly follow SOPs and that inline engineers and IPQC personnel conduct regular inspections is vital for maintaining defect rates within acceptable levels, typically measured in the tens of PPM.

PCBA production relies heavily on a range of machines and equipment to complete the assembly of a board. The quality of these machines directly determines a factory’s manufacturing capabilities. To sum up, if you have flexible circuit design or flexible printed circuit assembly needs, Gekunflex can provide you with services.