Flexible Circuit Boards and Narrow Bezel Display Technology

Learning Objectives

This learning guide aims to help you gain a deep understanding of the design, manufacturing methods, and applications of Flexible Printed Circuits (FPC) in display devices and mobile terminals. Through this study, you will be able to:

Master the main components of flexible circuit boards and their functions.
Understand the role of stress grooves in the bending sections of flexible circuit boards and their design principles.
Familiarize yourself with the advantages of implementing narrow bezel designs in display devices and mobile terminals using flexible circuit boards.
Learn about the manufacturing methods of flexible circuit boards, particularly the etching process for stress grooves.
Master the definitions of relevant technical terms.

Core Concepts and Principles

Flexible Printed Circuit (FPC): A type of circuit board with flexible and bendable characteristics, composed of a flexible substrate and conductive layers, commonly used in electronic devices that require bending or are space-constrained.
Bending Section: A section of the flexible circuit board specifically designed for bending, usually containing special structures to accommodate bending deformation.
Stress Groove: A recessed structure opened on the conductive layer to release stress generated during bending, reducing stress concentration and improving bending reliability.
First Stress Groove: A groove opened on the side of the conductive layer facing the flexible substrate.
Second Stress Groove: A groove opened on the opposite side of the conductive layer.
Interleaved Arrangement: The first and second stress grooves are arranged in an interleaved manner to avoid weakening the strength of the conductive layer and evenly distribute stress.
Conductive Layer: Typically a metal layer that carries electrical signal transmission, capable of bending along with the flexible substrate in the flexible circuit board.
Flexible Substrate: The base material of the flexible circuit board that imparts its flexible characteristics.
Adhesive Layer: Used to bond the conductive layer to the flexible substrate and can fill the stress grooves, providing elastic cushioning and support.
Narrow Bezel Design: By arranging components such as driver chips on the back of the display panel, the width of the non-display area is reduced, increasing the screen-to-body ratio.
COF (Chip On Film) Circuit Structure: A technology that packages driver chips on flexible circuit boards, commonly used for driving display panels.
Manufacturing Method: Mainly includes etching the conductive layer to form stress grooves and conductive lines, as well as bonding the conductive layer to the flexible substrate.
Reactive Ion Bombardment: An etching process used to form grooves on the surface of the conductive layer.

Key Structural Elements

Display Panel (10): Includes substrate (11), display layer (12), and touch layer (13).
COF Circuit Structure (20): Includes driver chip (21) and flexible circuit board (22).
Flexible Circuit Board (22):
Flexible Substrate (221)
Conductive Layer (222): A metal layer containing the first stress groove (223) and the second stress groove (224).
First Stress Groove (223): Located on the side of the conductive layer facing the flexible substrate.
Second Stress Groove (224): Located on the opposite side of the conductive layer.
Conductive Line (225): Multiple lines formed by dividing the conductive layer.
Main Body (226): The part of the flexible circuit board that connects to the front of the display panel.
Bending Part (227): The part of the flexible circuit board designed for bending, containing stress grooves.
Folding Part (228): The part of the flexible circuit board located on the back of the display panel after bending, usually carrying the driver chip.
Etching Mask (30): A tool used to shield and guide etching during the etching process.

Advantages and Applications

Narrow Bezel Design: By bending the flexible circuit board, the driver chip is moved to the back of the display panel, thereby reducing the width of the display device and mobile terminal’s bezel, increasing the screen-to-body ratio.
Stress Release: The first and second stress grooves effectively disperse the stress generated during bending, preventing the conductive layer from tearing or breaking, thus improving product yield and reliability.
Reduced Bending Radius: The design of the stress grooves allows the bending part to bend with a smaller curvature radius, further reducing the occupied space.
Elastic Cushioning and Support: The adhesive layer filling the stress grooves provides elastic support for the conductive lines, enhancing bending elasticity.
Application Fields: Display devices (especially OLED display panels), mobile terminals (such as smartphones and tablets), automotive electronics, and other electronic devices requiring compact and flexible connections.

Quiz

Short Answer Questions (2-3 sentences each)

What is the main function of the first and second stress grooves in the bending section of a flexible circuit board?
Why should the stress grooves in the conductive layer of a flexible circuit board extend parallel to the bending line?
How can it be ensured that the first and second stress grooves are arranged in an interleaved manner during the manufacturing process of a flexible circuit board?
Why can placing the driver chip on the folding part of the flexible circuit board achieve a narrow bezel design for mobile terminals?
Besides dispersing stress, what other role does the adhesive layer filling the stress grooves play?
What are the benefits of uniformly distributing multiple first and second stress grooves in a flexible circuit board?
In a flexible circuit board, in which direction do the conductive lines typically extend? Why is this direction important for connecting to the driver chip?
What is the COF circuit structure? What role does it play in display devices?
What advantages do OLED display panels have over traditional displays?
What is the specific process for etching the conductive layer in the manufacturing method of flexible circuit boards?

Quiz Answers

The main function of the first and second stress grooves in the bending section of a flexible circuit board is to release the stress generated during bending. This helps reduce stress concentration, preventing the conductive layer from tearing or breaking during bending, thus improving the reliability and yield of the flexible circuit board.
The stress grooves extend parallel to the bending line to effectively disperse the tensile and compressive stresses generated along the bending direction when the flexible circuit board bends. This directional design maximizes the deformation buffer space, ensuring structural integrity during the bending process.
During the manufacturing process of the flexible circuit board, the etching mask is shifted a certain distance relative to the position of the first stress groove before etching the other side of the conductive layer (to form the second stress groove), thus achieving the interleaved arrangement of the first and second stress grooves. This ensures uniformity in the thickness of the conductive layer and enhances overall strength.
Placing the driver chip on the folding part of the flexible circuit board means that the driver chip can be located on the back of the display panel. This layout avoids the driver chip occupying the non-display area on the front of the display panel, effectively reducing the width of the display device’s bezel and achieving a narrower bezel design.
In addition to dispersing stress, the adhesive layer filling the stress grooves also provides elastic cushioning and support. This elasticity helps enhance the bending elasticity of the conductive lines, further protecting them from damage during repeated bending, thus improving their durability.
The uniform distribution of multiple first stress grooves helps evenly disperse the compressive stress on the inner arc wall during bending, while the uniform distribution of multiple second stress grooves helps evenly disperse the tensile stress on the outer arc wall. This uniform distribution avoids local stress concentration, effectively preventing damage during bending.
The conductive lines in a flexible circuit board typically extend in a direction perpendicular to the bending line. This direction is important because the conductive lines need to serve as wiring between the input electrodes and the driver chip, as well as between the output electrodes and the driver chip, ensuring effective electrical signal transmission across the bending section.
The COF (Chip On Film) circuit structure is a technology that directly packages the driver chip onto the flexible circuit board. In display devices, it connects the driver chip to the display panel electrically through the flexible circuit board, thus enabling control of the display panel, making it a key component connecting the display panel and the main circuit board.
OLED display panels have advantages over traditional displays, including wider viewing angles, energy savings, faster response times, no need for backlighting, a wide color gamut, high contrast, and a lightweight overall structure. These characteristics make them increasingly popular in devices like mobile terminals.
The specific process for etching the conductive layer in the manufacturing method of flexible circuit boards is typically done through Reactive Ion Bombardment. Additionally, other etching methods such as photolithography can also be used to form the required stress grooves and conductive line structures on the surface of the conductive layer.

Paper Format Issues (No Answers Provided)

Detail the design principles of the first and second stress grooves in flexible circuit boards and their specific impact on the performance enhancement of flexible circuit boards in practical applications.
Analyze and compare the bezel design of traditional display devices with that of display devices using the patented flexible circuit board in terms of advantages and technical challenges in narrow bezel design.
Discuss the key process steps (especially etching stress grooves and conductive lines) in the manufacturing method of flexible circuit boards described in the material and their impact on the final performance and reliability of flexible circuit boards.
Discuss the role of the adhesive layer in providing elastic cushioning and support for stress grooves and conductive lines in flexible circuit boards, and consider factors that may need to be taken into account when selecting adhesive materials for different bending scenarios.
Besides mobile terminals, in which other electronic devices or fields might this flexible circuit board and its manufacturing method play an important role? Please provide examples and explain their potential advantages.

Glossary of Terms

Flexible Printed Circuit Board (FPC): A type of circuit board with bendable and foldable characteristics, composed of a flexible substrate and conductive layers, used to connect electronic components.
Main Body: The main part of the flexible circuit board that is electrically connected to the front of the display panel.
Bending Part: The part of the flexible circuit board designed for bending, containing stress grooves.
Folding Part: The part of the flexible circuit board located on the back of the display panel after bending, usually carrying the driver chip.
Flexible Substrate: The base material of the flexible circuit board that imparts flexibility.
Conductive Layer: The metal layer on the flexible substrate used for transmitting electrical signals.
First Stress Groove: A groove opened on the side of the conductive layer facing the flexible substrate, used to release bending stress.
Second Stress Groove: A groove opened on the side of the conductive layer away from the flexible substrate, used to release bending stress.
Bending Line: The axis along which the flexible circuit board bends.
Adhesive Layer: Used to bond the flexible substrate to the conductive layer and fill the stress grooves, providing elasticity and support.
Driver Chip: An integrated circuit chip used to control the display functions of the display panel.
COF Circuit Structure: A packaging technology that directly binds the driver chip to the flexible circuit board.
Display Panel: The component responsible for displaying images, such as OLED display panels.
Mobile Terminal: Refers to portable electronic devices such as smartphones and tablets.
Narrow Bezel Design: Reducing the width of the non-display area around the display screen by optimizing component layout.
Screen-to-body Ratio: The ratio of the display area to the front area of the device.
Etching: The process of selectively removing material through chemical or physical methods, commonly used in circuit board manufacturing to form wires and grooves.
Reactive Ion Bombardment: A physical etching method that uses an ion beam to bombard the surface of the material to remove it.
Conductive Line: Thin lines formed on the conductive layer for transmitting electrical signals.
Input Electrode: An interface for receiving external electrical signals.
Output Electrode: An interface for sending electrical signals.
OLED (Organic Light-Emitting Diode): A self-emitting display technology.
Low Temperature Poly-silicon (LTPS): A technology for manufacturing thin-film transistors (TFT), commonly used in OLED substrates.