As the trend towards miniaturized electronic designs continues to evolve, surface-mount devices (SMD) have become indispensable components in electronic manufacturing due to their superior performance and compact size. At the same time, flexible printed circuit boards (FPC), with their lightweight, flexible, and high-density characteristics, have emerged as critical carriers in modern electronic products. However, effectively combining these two technologies presents a significant challenge.
Compared to rigid PCB, FPC boards are softer and more fragile, posing unique technical challenges during the SMT assembly process. So, how is SMT assembly for FPC carried out? What are the critical steps and technical considerations to keep in mind? In this article, we will walk you through the process in detail.
Pre-Baking
As the demand for greater precision in electronics increases, manufacturers are required to specialize in specific fields to meet higher technical requirements. Modern PCBA manufacturing has thus been divided into two distinct processes: bare board production and component assembly, often handled by different manufacturers. This separation means circuit boards frequently undergo long-distance transportation.
For FPC, their flexibility prevents the use of vacuum packaging methods commonly used for rigid PCB during transportation, as uneven pressure from vacuum packaging could cause localized deformation or pad lifting. To mitigate this risk, anti-static packaging bags are typically used for transferring FPC.
While this method avoids deformation risks, extended transport times may cause FPC to absorb moisture from the air. During the soldering process, the high heat may vaporize this moisture, potentially causing delamination or surface bubbling issues. To address this, pre-baking is essential before FPC are assembled on production lines to remove excess internal moisture.
Recommended Pre-Baking Guidelines
- Temperature: Set between 80–100°C.
- Duration: Bake for 4–8 hours.
- Emergency Adjustments: If the project is time-sensitive, the temperature can be slightly increased, but it must not exceed 125°C to avoid damaging the FPC’s base material.
To ensure process reliability and minimize risks, it is advisable to conduct sample testing and IPQC inspections before full-scale baking. This helps confirm that the chosen temperature and time settings do not cause adverse effects such as discoloration, deformation, or warping of the FPC.
Best Practices for Pre-Baking
- Avoid Overstacking: Overstacking can lead to uneven heat distribution. Limit stack heights to 10–20 panels per group for optimal results.
- Uniform Heat Distribution: Ensure even spacing between stacks to facilitate consistent heat exposure.
Stencil and Carrier Plate Preparation
The quality of the stencil and carrier plate plays a significant role in determining the success of FPC SMT assembly, making it essential to create a dedicated stencil for FPC. This process is relatively straightforward—simply extract the positioning hole data from the PCB’s CAD file to fabricate the stencil. However, care must be taken to ensure that the diameter of the locating pins on the stencil matches precisely with the positioning holes on both the carrier plate and the FPC.
Additionally, due to specific design requirements or circuit protection considerations, the thickness of the FPC may not be uniform—some areas may be thicker, while others thinner, or even include metal reinforcement plates. To address this, specific modifications such as machining, grinding, or slotting may be necessary at the junctions between the carrier plate and the FPC to ensure that the FPC remains flat throughout subsequent processes.
The material for fabricating the carrier plate can be chosen based on the project requirements. Common options include synthetic stone, aluminum plates, silicone sheets, and specialized high-temperature-resistant magnetized steel plates. However, due to the extreme temperatures encountered during soldering, the carrier plate must exhibit low heat absorption, rapid heat dissipation, and resistance to warping or deformation after multiple thermal cycles.
Mounting FPC onto the Carrier Plate
Carrier plates can be categorized into two types: those with locating pins and those without.
For carrier plates with locating pins, the process is straightforward. Each FPC can be individually fitted onto the spring-loaded pins of the carrier plate and secured with tape. During the printing process, the spring-loaded pins are fully compressed into the carrier plate by the stencil, ensuring they do not interfere with the printing quality.
If using a carrier plate without locating pins, a locating stencil with pins is required. First, position the carrier plate onto the locating stencil, ensuring the locating pins pass through the alignment holes on the carrier plate. Then, fit each FPC onto the exposed locating pins and secure them with tape. Once the FPC is fixed, separate the carrier plate from the locating stencil before proceeding to the subsequent steps.
Regardless of the type of carrier plate used, it is essential to wear clean gloves throughout the process to prevent contamination of the board surface. Additionally, given the fragile nature of FPC, always handle them gently to avoid damage.
Securing the FPC
Due to the lightweight nature of FPC, they are prone to displacement caused by airflow during subsequent processes. To address this, tape, either single-sided or double-sided, is used to secure them in place:
- Single-sided tape: Affix the FPC to the carrier plate along all four edges—not just the corners—to ensure it remains flat and stable throughout the process.
- Double-sided tape: Apply the tape directly onto the carrier plate surface, then adhere the FPC to the tape, offering a simpler method for secure attachment.
Tape Selection Criteria
The tape must exhibit excellent adhesion under high-temperature conditions and should be easy to remove after reflow soldering without leaving any residue on the FPC surface. If your company specializes in FPC SMT assembly, it is highly recommended to invest in an automatic taping machine. This equipment can quickly cut tape into uniform lengths, significantly improving production efficiency and reducing material waste.
Maintenance and Cleaning
After repeated use of the carrier plate and tape in high-temperature environments, the adhesive properties of the tape will gradually weaken. Replace the tape immediately when its holding power becomes unreliable.
Before reusing the carrier plate, ensure it is thoroughly cleaned to maintain a smooth and clean surface. Use a lint-free cloth with a cleaning solution to wipe the surface or an anti-static adhesive roller to remove dust, solder beads, and other debris. This ensures consistent performance and prevents defects in the production process.
Solder Paste Printing
While FPC does not have specific requirements for solder paste composition, it is crucial to ensure that the solder paste has excellent thixotropic properties. This allows for easy stencil release, firm adhesion to the FPC surface, and prevents issues such as clogged stencil apertures or paste collapse after printing.
Despite being fixed to a carrier plate, slight gaps between the FPC and the carrier plate are inevitable. To address this, printing parameters should be adjusted accordingly. For instance, reducing squeegee pressure and printing speed can significantly improve printing quality, laying a solid foundation for subsequent processes.
Further Reading: Guide to Solder Paste Printing Process for SMT Assembly
SMT Placement
Since every FPC includes optical MARK alignment markers, the process of placing SMD components on FPC is similar to rigid PCB assembly. However, due to the aforementioned gaps between the carrier plate and FPC, placement machine parameters should be adjusted, such as:
- Lowering nozzle height
- Reducing blowing pressure
- Slowing down nozzle movement speed
The specific adjustments depend on the product characteristics, the number of components, and placement efficiency requirements.
Reflow Soldering
Reflow soldering is the final stage of the FPC SMT process, where components are securely soldered to the FPC surface under high temperature. It is recommended to use a forced hot air convection reflow oven to ensure uniform temperature changes, which reduces soldering defects.
Due to the differences in carrier plate materials and the variety of components on FPC, it is essential to conduct temperature profiling before handling different projects. This helps to avoid soldering issues caused by varying heat absorption.
Temperature Profiling Method:
- Place two FPC-mounted carrier plates in front and behind the test board, spaced evenly.
- Solder temperature probes to test points using high-temperature solder wire, and secure probe wires to the carrier plate with heat-resistant tape. Ensure the tape does not cover the test points.
- Select test points near the carrier plate edges, solder joints, or QFP (Quad Flat Package) pins for more accurate results.
This method ensures the profiling accurately reflects the reflow soldering conditions, minimizing quality risks.
Ending
The SMT assembly process for FPC is now complete. The final step is to remove the assembled FPC from the carrier board and perform quality inspections. It is important to note that, due to the uneven surface of FPC compared to rigid PCB, quality inspection may present some challenges. Particularly when using AOI (Automated Optical Inspection), the flexible nature of FPC can result in variations in reflection angles or focal lengths, leading to misjudgments, such as mistaking normal solder joints for defects.
Therefore, it is recommended to combine AOI and ICT (In-Circuit Testing) methods during the inspection process to leverage their respective advantages:
- AOI Inspection is suitable for verifying component placement and inspecting solder joint appearances, although it may be affected by the unevenness of the FPC.
- ICT Testing utilizes physical probes to assess electrical performance, including continuity, resistance, and capacitance, making it more reliable for evaluating the functionality of FPC, particularly for solder joints and circuit performance.
Additionally, to enhance inspection accuracy and reduce false judgments, specialized inspection fixtures can be used to secure the FPC, ensuring it remains flat and stable during the inspection process. This not only improves the effectiveness of AOI inspection but also enhances the probe contact efficiency in ICT testing.
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