In-depth Analysis of Multilayer Flexible Printed Circuit Board (FPC) Technology

This article comprehensively explores the technology of multilayer flexible printed circuit board (FPC), analyzing it one by one from aspects such as structure, advantages, applications, challenges, and trends. It aims to present in-depth technical references for practitioners in the electronics industry and help them gain insights into the essence of FPC technology.

I. Introduction

Currently, electronic devices are rapidly developing towards being smaller, lighter, multifunctional, and highly reliable. Traditional rigid PCBs are gradually showing their limitations. Multilayer flexible printed circuit boards (FPCs) have emerged at the right time. With advantages such as being bendable and having a small volume, they have shone brightly in the field of high-end electronic devices and have become a key driving force for the development of electronic technology.

II. Structural Composition

1. Flexible Substrate

The flexible substrate is the "foundation" of FPC. Polyimide (PI) film is quite common. It has high temperature resistance, good mechanical strength, and stable chemical properties, and its performance remains stable within a certain temperature range. The thickness can be selected from 12.5 to 50 μm according to requirements. For wearable devices, thinner ones can be chosen to reduce the bending radius; for automotive electronics, thicker ones are used to enhance mechanical strength.

2. Conductor Layer

The conductor layer is mostly made of copper foil, which is responsible for signal and power transmission. The thickness of copper foil ranges from 9 to 70 μm and is divided into two types: electrolytic copper foil and rolled copper foil. Electrolytic copper foil has a mature manufacturing process and low cost but is relatively rough. Rolled copper foil has high flatness and low roughness, and has advantages in high-frequency transmission and the production of fine circuits, but its cost is high. The etching process is used to carve circuit patterns, and through holes, blind holes, and buried holes build the "bridge" for electrical connection.

3. Insulation Layer

The insulation layer is placed between conductor layers and is responsible for electrical isolation. Polyimide and polyester (PET) are commonly used. The former has high temperature resistance, moisture resistance, and excellent dielectric properties, reducing signal loss and crosstalk. The latter has good flexibility, is easy to process, and has a low cost, suitable for scenarios with low requirements. The thickness ranges from 10 to 50 μm and is determined according to voltage and frequency.

4. Cover Layer

The cover layer is like a "shield", protecting the conductor and insulation layers from physical and chemical damage. The material is mostly PI film, which is attached by the lamination process. Windows are opened as needed to expose components and test points for the convenience of assembly and testing.

III. Advantages

1．Excellent Electrical Performance

(1) Good High-frequency Transmission: Low-roughness rolled copper foil combined with high-quality insulating materials makes FPC perform well in high-frequency signal transmission. When connecting the antenna and the motherboard of a 5G mobile phone, it reduces signal loss, reflection, and crosstalk, ensuring signal integrity and speeding up communication and data transmission.

(2) High Wiring Density: The flexibility enables FPC to stack more circuit layers in a limited space. Through ingenious design of layer stacking and circuit layout, it can accommodate more components and circuits than rigid PCBs, improving integration. Small sensor modules can integrate multiple functions with its help and reduce their size.

2．Good Mechanical Properties

(1) Bendable and Foldable: This is the "special skill" of FPC, which is suitable for complex installations and various device forms. Smart watches and bracelets can bend along with the devices and are durable and reliable. When folding and unfolding folding screen mobile phones, it can stably maintain electrical connections.

(2) Vibration and Shock Resistance: Compared with rigid PCBs, FPC is excellent at buffering mechanical stress. In the fields of aerospace and automotive electronics, when the equipment is subjected to vibration and shock, FPC protects circuit connections and improves the reliability of the system. FPC used for connecting sensors and controllers in automotive engine control can ensure stable signal transmission under vibration.

3．Efficient Space Utilization

FPC is flexible and thin, and can be arranged three-dimensionally with ease, skillfully avoiding components and saving space. In smartphones, it bypasses the battery and camera to connect the motherboard and sub-modules, facilitating the design of thin and light devices.

4．High Reliability

(1) Fewer Solder Joint Failures: The integrated design greatly reduces the number of solder joints of FPC. In medical and aerospace equipment, solder joint failures often lead to malfunctions. FPC reduces the number of solder joints, weakens the influence of poor soldering and thermal stress, and improves reliability.

(2) Resistance to Harsh Environments: With the support of high-performance materials, FPC remains stable in high-temperature, high-humidity, and high-salt-fog environments. It is used in marine monitoring and outdoor base station equipment to ensure stable long-term operation and a long service life.

IV. Application Areas

1．Smartphones and Wearable Devices 

These two are the main application fields of FPC. In mobile phones, FPC connects the motherboard with the display screen, camera, etc., and flexibly connects the display screen, being able to withstand frequent bending. FPC is integrated into the straps of wearable devices to connect sensors and the motherboard, promoting small-sized and multifunctional designs.

2．Aerospace

Aerospace has strict requirements for weight, volume, and reliability, and FPC fully demonstrates its advantages. It is applied in aircraft electronics, navigation, and communication systems. In satellite electronic systems, it is used to connect precision instruments to ensure stable operation in space.

3．Automotive Electronics

The intelligent and electrified development of automobiles has given rise to complex electronic systems, and the application of FPC is gradually expanding. It can be found in engine control, body electronics, and battery management systems, and can work reliably even in the harsh environment of the engine compartment.

4．Medical Electronics

Medical equipment requires reliability, small size, and biocompatibility, and FPC plays an important role. It connects sensors and circuit boards in portable monitoring equipment to reduce the size. Implantable devices can meet biocompatibility requirements and ensure long-term reliable operation in the human body environment.

V. Challenges and Development Trends

1．Challenges

(1) Cost and Supply: High-performance materials are expensive, leading to rising manufacturing costs, which restricts their application in low-cost fields. The supply of some materials is affected by resources and manufacturing processes, and there are concerns about stability.

(2) Complex Manufacturing Process: Manufacturing involves multiple processes such as multilayer lamination, etching, and drilling. The processing requirements for micro-holes are extremely high. If there are slight deviations in equipment or parameters, the quality of the holes will be affected, thus influencing electrical connections.

(3) Heat Dissipation Problem: As the power density of equipment increases, FPC has limited space for heat dissipation, and traditional methods are difficult to work effectively. It is urgent to improve its heat dissipation performance.

2． Development Trends

(1) Material Innovation: Research and develop new substrate, conductor, and insulation materials to reduce costs and improve performance. Develop high-thermal-conductivity substrates, low-cost conductors, and high-quality insulation materials to give play to their respective advantages.

(2) Process Improvement: The manufacturing process is moving towards being finer and more automated. Technologies such as laser processing and nanoimprinting are being adopted to improve precision, stabilize quality, promote automated processes, improve efficiency, and reduce costs.

(3) Technology Integration: FPC will be integrated with flexible display, wireless charging, and Internet of Things technologies to expand applications. Develop flexible screens and their driving circuits, enable wireless charging, and provide circuit solutions for Internet of Things devices to promote the intelligence and multifunction of devices.

VI. Conclusion

Multilayer flexible printed circuit board (FPC) technology plays a significant role in the development of electronic technology with its unique structure, excellent performance, and wide range of applications. Although it currently faces challenges in terms of cost, manufacturing process, and heat dissipation, with material and process innovations and technology integration in the future, it will surely empower the development of electronic devices towards being smaller, lighter, multifunctional, and highly reliable.