Complete Introduction to Flexible PCB Assembly Process

The increasing demand for Flexible FPCBs in modern electronic devices is driving electronics towards lighter, thinner, and more flexible designs. With rapid advancements in IoT, wearable devices, and electric vehicles, the demand for FPCBs is expected to skyrocket. Industry forecasts predict the global FPCB market to reach $76 billion by 2027.

This article aims to provide a comprehensive and in-depth manufacturing technology guide for PCB manufacturers, electronic engineers, and researchers in related fields. It covers various aspects from basic theory to the latest applications of FPCBs. We will explore materials science, design principles, manufacturing processes, assembly technologies, and future development trends to help readers gain a comprehensive understanding of this revolutionary technology.

Fundamental Knowledge of FPCBs

Definition and Characteristics

Flexible PCBs are bendable and foldable printed circuit boards, typically composed of flexible insulation substrates (such as polyimide) and conductive layers (usually copper). Their main characteristics include:

• Lightweight: 50-70% lighter than traditional rigid PCBs
• High spatial efficiency: capable of three-dimensional bending, saving space
• Dynamic flexibility: withstands continuous bending and vibration
• Excellent heat dissipation: thin structure advantageous for heat dissipation
• Stable electrical performance: low dielectric constant, suitable for high-frequency applications

Comparison between Flexible FPCB and Rigid PCB

Applications

Flexible PCBs find widespread application in various fields including:

• Consumer electronics: smartphones, tablets, laptops
• Automotive electronics: dashboards, infotainment systems, ADAS
• Medical devices: implantable devices, portable monitoring instruments
• Aerospace: satellite communication systems, flight control equipment
• Industrial automation: robots, sensor networks
• Wearable devices: smartwatches, health monitoring devices

Material Science of FPCBs

Substrate Materials

olyimide (PI) is the most commonly used substrate material for FPCBs due to its:

• Excellent heat resistance (can withstand over 300°C)
• Outstanding chemical stability
• Good mechanical strength and dimensional stability
• Low dielectric constant (approximately 3.4)

However, PI has some limitations such as high moisture absorption and cost. Researchers are exploring alternative materials like:

• Liquid Crystal Polymer (LCP): lower dielectric constant (~3.0) and better moisture resistance
• Polyethylene Naphthalate (PEN): lower cost but slightly lower heat resistance
• Polytetrafluoroethylene (PTFE): excellent high-frequency performance, suitable for 5G and mmWave applications

Conductive Materials

Copper foil remains the most common conductive material for FPCBs, available in two main types:

• Electrodeposited copper foil: high purity, good conductivity, but less flexible
• Rolled copper foil: good flexibility but higher cost

Emerging conductive materials include:

• Graphene: ultra-high conductivity and flexibility, but challenges in mass production
• Conductive polymers like PEDOT:PSS: enable fully flexible circuits but lower conductivity
• Silver nanowires: high conductivity and transparency, suitable for transparent electronic devices

Adhesives and Coverlays

Adhesives play a crucial role in FPCBs, commonly used types include:

• Acrylic adhesives: good flexibility but lower heat resistance
• Epoxy resin: good heat resistance but less flexible
• Polyurethane: balances flexibility and heat resistance

Coverlay materials typically include:

• Photoimageable solder mask: high precision but higher cost
• Polyimide film: good heat resistance but challenging processing

Exploration of New Materials

Researchers are developing innovative materials to meet future electronic product demands such as:

• Self-healing polymers: automatically repair minor cracks, improving FPCB lifespan
• Shape memory alloys: automatically adjust shape based on temperature changes, enabling smart deformation
• Nanocomposite materials: e.g., carbon nanotube-reinforced polymers, enhancing strength and conductivity
• Biodegradable materials: suitable for temporary electronic devices, reducing electronic waste

Flexible PCBs, known for their adaptability, can be bent, folded, or twisted, making them ideal for a wide range of applications, from compact consumer electronics to critical medical devices. This article promises to enrich your understanding of flexible PCB assembly.

Advantages of flexible PCBs

• Space and Weight Reduction: One of the most significant advantages of flexible PCBs is their ability to save space and reduce weight in electronic devices. This makes them ideal for modern, compact devices such as smartphones, wearable technology, and medical devices.
• Enhanced Durability: The flexibility of these boards makes them more resistant to vibrations and movement, leading to a longer lifespan, particularly in applications where the PCB may be subject to bending or flexing.
• Heat Dissipation: Flexible PCBs generally have better heat dissipation properties compared to rigid PCBs. This is due to their ability to be positioned in a way that allows for better airflow and heat dispersion.
• Cost-Effective Assembly: While the initial cost may be higher, the overall assembly and production costs can be lower with flexible PCBs. This is because their flexibility allows for more compact and efficient designs, reducing the need for connectors and cables.
• Reliability and Performance: The reduced number of interconnects and solder joints in flexible PCBs leads to increased reliability. Their ability to conform to different shapes also means they can be designed to optimize performance in the final product.

Key Materials and Components in Flexible PCB Assembly

The assembly of flexible printed circuit boards (PCBs) involves a variety of specialized materials and components, each playing a crucial role in the functionality and reliability of the final product. Understanding these elements is key to grasping the intricacies of flexible PCB assembly.

Flexible Base Material

The foundation of a flexible PCB is the flexible base material, typically made of polyimide or polyester films. These materials provide the necessary flexibility while maintaining excellent thermal stability and electrical insulation properties. Polyimide films, for example, are favored for their high heat resistance and durability, making them suitable for a wide range of applications.

Conductive Layers

Copper is the most commonly used conductive material in flexible PCBs. It is applied to the base material in a thin layer, forming the circuit pattern. The thickness of the copper layer varies depending on the application, with higher current applications requiring thicker copper.

Adhesives and Coverlays

Adhesives are used to bond the layers of a flexible PCB together. They must provide strong adhesion while maintaining flexibility. In addition, coverlays, made of polyimide film and adhesive, are applied to protect the external circuitry of the PCB. They serve a similar function to the solder mask on rigid PCBs, offering insulation and protection against environmental factors.

Stiffeners and Reinforcements

While flexibility is a key feature of these PCBs, certain areas may require additional support. Stiffeners, typically made from materials like FR4 or polyimide, are added to specific sections to provide rigidity, supporting components like connectors that require a stable surface.

Flexible PCB Connectors

Connectors in flexible PCBs are designed to maintain flexibility while providing reliable electrical connections. These can include zero insertion force (ZIF) connectors, which allow for easy assembly and disassembly, and anisotropic conductive films (ACFs), which are used to bond flexible circuits to LCDs or other components.

The choice of materials and components in flexible PCB assembly is a critical factor that directly affects the performance, durability, and application possibilities of the final product. Innovations in materials science and component design continue to expand the capabilities and applications of flexible PCBs, making them a key element in the future of electronics design and manufacturing.

The Flexible PCB Assembly Process: Step by Step

1. Fixation of FPC

Before proceeding with SMT, the FPC first needs to be precisely fixed on the carrier board. Special attention should be paid to the shorter the storage time between the FPC being fixed on the carrier board and printing, mounting and welding, the better. The carrier board is available with or without positioning pins. The carrier board without positioning pins needs to be used with the positioning template with positioning pins. First, put the carrier board on the positioning pins of the template so that the positioning pins are exposed through the positioning holes on the carrier board. Then put the FPCs one by one on the template.

The exposed positioning pins are then fixed with tape, and then the carrier board is separated from the FPC positioning template for printing, patching and welding. The carrier plate with positioning pins has been fixed with several elastic positioning pins about 1.5mm long. You can directly put the FPC piece by piece on the elastic positioning pins of the carrier plate, and then fix it with tape. During the printing process, the spring positioning pin can be completely pressed into the carrier plate by the steel mesh without affecting the printing effect.

2. FPC solder paste printing

FPC does not have special requirements for the composition of solder paste. The size and metal content of solder ball particles are subject to whether there is a fine pitch C on FPC. However, FPC has higher requirements for the printing performance of solder paste, and solder paste should have excellent Thixotropic, the solder paste should be easy to print and demould and can be firmly attached to the FPC surface, without defects such as poor demoulding blocking the stencil leaks or collapse after printing.

The printing station is also a key station to prevent FPC contamination. It is necessary to wear finger gloves when working. At the same time, the station must be kept clean and the steel mesh must be wiped frequently to prevent solder paste from contaminating the gold fingers and gold-plated buttons of the FPC.

3.FPC Assembly

Depending on the characteristics of the product, the number of components and the placement efficiency, medium or high-speed placement machines can be used for placement. Since each FPC has an optical MARK mark for positioning, SMD placement on the -PC is not the same as on-PC placement. There is not much difference in mounting on the PCB.

It should be noted that although the FPC is fixed on the carrier board, its surface cannot be as flat as a PCB hard board. There will definitely be a local gap between the FPC and the carrier board. Therefore, the drop height of the nozzle, blowing pressure, etc. It needs to be set accurately and the moving speed of the nozzle needs to be reduced.

At the same time, most FPCs are connected boards, and the yield rate of FPC is relatively low. Therefore, it is normal for the entire PNL to contain some defective PCS. This requires the placement machine to have the BAD MARK identification function.

Otherwise, during the production of non-integrated PNL, When PNL is a good board, the production efficiency will be greatly reduced.

4.FPC reflow soldering

A forced hot air convection infrared reflow oven should be used so that the temperature on the FPC can change more evenly and reduce the occurrence of poor soldering. If single-sided tape is used, because it can only fix the four sides of the FPC, the middle part is deformed under hot air, the pad is prone to tilt, and the molten tin (liquid tin at high temperature) will flow, resulting in empty soldering, continuous soldering, and Tin beads make the process defective rate higher.

5.FPC inspection, testing and subboarding

Since the carrier plate absorbs heat in the furnace, especially the aluminum carrier plate, the temperature is higher when it comes out of the oven, so it is best to add a forced cooling fan at the outlet to help cool down quickly. At the same time, operators need to wear heat-insulating gloves to avoid being burned by the high-temperature carrier plate. When picking up the welded FPC from the carrier board, the force must be even and do not use brute force to prevent the FPC from being torn or creased.

The removed FPC is visually inspected under a magnifying glass of more than 5 times, focusing on surface glue residue, discoloration, gold finger tin staining, tin beads, C-pin open soldering, continuous soldering and other issues. Since the surface of FPC cannot be very flat, the misjudgment rate of AOI is very high, so FPC is generally not suitable for AOI inspection. However, by using special test fixtures, FPC can complete ICT and FCT tests.

The assembly process of flexible PCBs is a blend of advanced technology, precision engineering, and meticulous quality control. Each step plays a vital role in ensuring that the final product not only functions as intended but also withstands the demands of its application environment.

Conclusion

The escalating demand for Flexible Printed Circuit Boards (FPCBs) underscores their pivotal role in driving the evolution of modern electronics towards lighter, thinner, and more adaptable designs. Highleap Electronic stands at the forefront of this technological advancement with its cutting-edge Flexible PCB Assembly solutions. By offering comprehensive manufacturing capabilities and expertise, Highleap Electronic empowers industries across consumer electronics, automotive, aerospace, medical devices, and beyond to innovate with confidence.

As global forecasts predict the FPCB market to soar to $76 billion by 2027, Highleap Electronic’s commitment to delivering high-quality, reliable, and customizable FPCB solutions remains steadfast. Whether navigating complex substrate materials, integrating advanced conductive layers, or employing state-of-the-art assembly techniques, Highleap Electronic ensures that each FPCB meets stringent performance standards and application requirements.

Partnering with Highleap Electronic means embracing a future where flexible electronics not only meet but exceed expectations, fostering a new era of innovation and efficiency in electronic design and manufacturing.

FAQs for Flexible PCB Assembly

 How does the integration of smart sensors impact flexible PCB assembly?

 Smart sensors integrated into flexible PCBs require specialized assembly techniques to ensure precise alignment and connectivity without compromising flexibility or sensor functionality.

What role do environmental stress tests play in assessing the reliability of flexible PCBs?

Environmental stress tests simulate real-world conditions such as temperature fluctuations and mechanical strain to evaluate the durability and performance of flexible PCBs in diverse applications.

How does additive manufacturing (3D printing) influence the fabrication of flexible PCB components?

Additive manufacturing techniques enable the rapid prototyping of flexible PCB components with complex geometries, facilitating faster design iterations and customization.

What are the key considerations when designing flexible PCBs for high-frequency applications like 5G?

Design considerations include selecting materials with low dielectric constants and optimizing circuit layouts to minimize signal loss and interference in high-frequency environments.

How are advancements in robotics shaping the future of automated flexible PCB assembly lines?

Robotics are increasingly integrated into flexible PCB assembly lines to enhance precision, throughput, and quality control, paving the way for more efficient manufacturing processes.