Research on Precise Assembly and Calibration Technology of Flip-Chip LED in Full-Color Mini LED Displays

Posted by

On 03/20/2025

Abstract:
This paper aims to develop a flip-chip COB (Chip on Board) high-definition, small-pitch seamless LED display by studying the key technologies, including flip-chip LED transfer and packaging processes, row-column driving circuit design, high-precision assembly structures, and full-screen calibration techniques. By solving critical technical challenges, we successfully developed an advanced display with advantages such as black-screen color consistency, white-screen uniformity at all viewing angles, full-screen consistency, and high welding yield. The row-column drive circuit design incorporates column constant-current and row drive chips, achieving low power consumption, effective surface temperature control, and optimized low-gray-scale performance. The precise assembly structure ensures high flatness and rapid installation or replacement, while the calibration technology addresses brightness and color uniformity issues. The research findings provide a technical roadmap and solutions for the development of flip-chip COB high-definition seamless LED displays, further driving advancements in LED display technology and enhancing user experience.

Keywords: Small pitch, seamless splicing, LED display, transfer packaging process, row-column drive, assembly structure, calibration technology

Table of Contents

Introduction

This study focuses on the development of a high-definition, small-pitch seamless LED display using flip-chip COB technology. The research mainly explores flip-chip LED die bonding and packaging processes, row-column drive circuit scanning control technology, high-precision assembly structures for small-pitch LED displays, and calibration techniques.

To address key technical challenges, this paper focuses on several critical aspects: black-screen color consistency, white-screen uniformity across viewing angles, die bonding yield, chip rework techniques, and calibration technologies. Through process and technological optimization, we aim to achieve high-quality, stable, and reliable LED displays. The research findings have significant implications for advancing small-pitch LED display technology and promoting its widespread application.

1. Research on Assembly and Calibration Processes

1.1 Research Objectives

Through in-depth research on flip-chip LED processes, optimized drive circuit designs, improved assembly precision, and comprehensive display calibration, this study aims to develop a high-definition, small-pitch seamless LED display that meets diverse application needs and provides an enhanced visual experience for users.

1.2 Main Research Areas

1.2.1 Flip-Chip LED Transfer and Packaging Process

This study particularly focuses on the efficient transfer technology of flip-chip LEDs. To ensure welding flatness during the transfer process, a refined soldering process is employed, ensuring each LED microchip is uniformly bonded to the substrate, thereby improving yield. Additionally, we investigate the impact of chip bonding methods on display uniformity.

In the packaging stage, special attention is given to the uniformity of the black encapsulant surface. Black encapsulants effectively absorb excess light, enhancing contrast and image clarity. Simultaneously, we optimize light transmittance to ensure maximum LED brightness while minimizing reflections, thus improving the viewing experience. To guarantee the long-term stability and reliability of LED packaging, we conduct rigorous airtightness testing to protect the encapsulated LED chips from environmental influences. Moreover, precision cutting techniques enable a cutting accuracy of ±3μm, which is crucial for maintaining display integrity and aesthetics.

1.2.2 Row-Column Drive Circuit and Scanning Control Technology

LED chip brightness is influenced by current levels, but the relationship is nonlinear. Display brightness is correlated with scan count—higher scan counts improve brightness uniformity but also increase power consumption. Therefore, balancing brightness and power efficiency is critical.

Common-cathode drive: Low power consumption and lower heat generation but higher costs.
Common-anode drive: Lower cost but higher power consumption and temperature.
Integrated row-column IC: Simplifies design but offers inferior low-gray performance.
Independent row-column IC: Superior low-gray performance but complex design.

PCB layout significantly impacts low-gray display performance and electromagnetic compatibility (EMC) radiation. The scan rate and refresh frequency must be balanced to ensure optimal image quality and energy efficiency. High-density interconnect (HDI) PCB design is crucial for P0.9 and smaller pixel-pitch displays, requiring advanced HDI techniques for high-density, high-precision circuit layouts.

1.2.3 High-Precision Assembly Structure for Small-Pitch LED Displays

To achieve high-precision assembly (≤0.1mm misalignment) for small-pitch LED cabinets, several factors must be considered:

Ensuring cabinet flatness and developing an adjustable gap control mechanism.
Implementing a quick LED module installation and replacement system.
Studying the impact of cabinet design on heat dissipation.
Exploring flexible installation methods to accommodate diverse application scenarios.

1.2.4 Small-Pitch LED Screen Calibration Technology

To analyze the impact of brightness calibration on display uniformity, this study focuses on:

The effects of chromaticity calibration on LED screen consistency.
The influence of large-screen segmentation calibration on seamless splicing.
The role of multi-gray-level calibration in grayscale uniformity.
The impact of calibration techniques on spare LED module replacement.
The effects of LED chip brightness and wavelength variations on calibration results.

2. Key Technical Challenges to Be Solved

2.1 Black-Screen Color Consistency

Black-screen color uniformity is essential for minimizing color differences between LED modules and preventing reflectivity inconsistencies. Achieving uniform black-screen color requires high-quality materials, precise process control, strict quality inspections, and color calibration under various lighting conditions. Advanced technologies, such as precise pixel arrangement and color management, can further enhance performance.

2.2 White-Screen Uniformity Across Viewing Angles

Ensuring consistent white-screen brightness and color across various viewing angles is a crucial quality indicator for LED displays. The display should exhibit no dark spots, color patches, or uneven textures.

2.3 Die Bonding Yield

High die bonding yield is essential for cost control and product quality. To maintain defect rates below 60ppm, manufacturers must optimize process parameters, enhance equipment precision, and implement stringent quality control systems.

2.4 Chip Rework Technology

As defective LED chips are inevitable during production, efficient chip rework technologies must be implemented to ensure product quality and prolong the lifespan of LED modules.

2.5 Calibration Technology

LED displays can suffer from brightness inconsistency due to variations in luminous intensity, wavelength deviation, encapsulant thickness, and chip inclination. Advanced calibration techniques are required to equalize brightness and color, ensuring seamless splicing and uniform display performance.

3. Technical Roadmap and Workflow

To achieve ultra-fine pitch LED displays, the flip-chip COB (Chip on Board) technology is employed. Using 3mil × 6mil flip-chip LEDs and solder paste technology, these chips are mounted onto an HDI PCB board. The PCB substrate is designed with either a 4-layer, 1-step structure or a 6-layer, 2-step structure. The LED driver board supports both common-cathode and common-anode drive configurations, ensuring compatibility.

During the encapsulation process of the LED panel surface, a combination of molded epoxy resin and PET optical film is used. By ensuring consistency between the PCB and the optical film, the uniformity of the screen’s black-state appearance is effectively controlled.

The cabinet structure is designed with a die-cast aluminum frame, and the LED panels are installed using magnets. Each LED panel measures 150mm × 168.75mm. For screen calibration, a full-screen correction method is applied. A high-precision industrial camera samples the brightness and chromaticity of the LED display. Based on the target brightness values, brightness coefficients are calculated for each pixel, and individual LEDs are adjusted to achieve uniform white balance across the entire display.

A structural diagram of the LED cabinet and an encapsulation schematic of the LED panel are shown in Figures 1 and 2, where each cabinet consists of 4 × 2 LED panels.

Key Structural Components:

Transparent Coating Layer: Molded epoxy resin with a light transmittance greater than 80%, glossy surface, 200µm thickness, providing sealing and protection.
Film Layer: Optical PET film with approximately 40% light transmittance, 90µm thickness, enhancing black uniformity, fingerprint resistance, anti-reflection, scratch resistance, and overall protection.
Chip: Flip-chip LED with 3mil × 6mil dimensions, soldered onto the PCB using solder paste technology.
Driver IC: Row and column driver ICs positioned on the back side of the PCB.

Process Flowchart

The manufacturing process follows these steps:
Full inspection of incoming PCB → Chip solder paste printing → Die bonding → Die bonding reflow soldering → IC solder paste printing → SMT → IC reflow soldering → Testing and rework → Aging and rework → Epoxy molding → Film application → Cutting → Panel aging → Module assembly → Cabinet assembly → Full-screen aging → Calibration → Shipping → Packaging.

4. Conclusion

Through the study of flip-chip LED die bonding, packaging processes, and full-color LED drive circuit design, this research successfully developed a flip-chip COB high-definition small-pitch seamless LED display. With advantages such as black-screen color uniformity, white-screen consistency, high welding yield, reworkability, and overall screen uniformity, this display has broad application prospects, driving further advancements in LED display technology and enhancing user experience across various scenarios.