Ni Zhang (Jufei Optoelectronics Co., Ltd., Shenzhen, Guangdong 518100, China)
Abstract: This study aims to develop a high-definition, fine-pitch, seamless COB (Chip-on-Board) Mini LED display by researching and integrating key technologies such as flip-chip LED transfer packaging, row-column driving circuit design, high-precision assembly structures, and full-screen calibration techniques. By overcoming critical technical challenges, the display system has been successfully developed. The resulting LED display offers advantages including uniform black-level appearance, consistent white-screen performance across viewing angles, excellent screen-wide uniformity, and a high soldering yield rate. The row-column driving architecture utilizes a combination of column constant-current driver ICs and row scanning driver ICs, enabling low power consumption, controlled surface temperature, and minimized grayscale distortion. The precision assembly structure design ensures excellent flatness and enables quick installation and maintenance. The calibration process effectively addresses brightness and color uniformity issues. This research provides a comprehensive technical route and practical solution for the development of flip-chip COB fine-pitch seamless LED displays, which will further drive technological advancements in the LED display industry and enhance the end-user experience.
Keywords: fine-pitch; seamless splicing; LED display; transfer packaging process; row-column driving; assembly structure; calibration technology
This paper focuses on the development of a high-definition, fine-pitch, seamless flip-chip COB LED display through comprehensive research on flip-chip LED die bonding and packaging processes, row-column driving circuit design for full-color LED modules, high-precision assembly structures, and full-screen calibration techniques. The study emphasizes key aspects such as flip-chip LED transfer and packaging, scanning control technology based on a combination of column constant-current driver ICs and row driver ICs, precise structural design for fine-pitch LED panel alignment, and calibration methodologies.
To address critical technical challenges, the research targets several core issues, including black screen uniformity (ink consistency), white screen consistency across various viewing angles, die bonding yield rates, chip reworkability, and effective calibration processes. Through process and technical optimization, the LED display aims to achieve high quality, enhanced stability, and superior reliability.
The outcomes of this research are expected to significantly contribute to the advancement of fine-pitch LED display technology, promoting innovation and wider adoption across the LED display industry.
1. Research on Assembly and Calibration Processes
1.1 Research Objectives
The primary objective of this study is to develop a high-definition, fine-pitch, seamless flip-chip COB LED display by optimizing flip-chip LED processing, refining driver circuit design, improving assembly precision, and implementing comprehensive screen calibration. The goal is to meet the demands of diverse application scenarios and deliver an enriched visual experience for end users.
1.2 Key Research Areas
1.2.1 Flip-Chip LED Transfer and Packaging Process
In the domain of LED chip technology, special emphasis has been placed on the efficient transfer process of flip-chip LEDs. To ensure the flatness and uniformity of LED chip soldering during transfer, a refined bonding process was adopted. This process ensures that each miniature LED chip is evenly soldered onto the substrate, thus enhancing overall yield.
The study also explores how different die-bonding techniques impact the display’s uniformity.
During the packaging phase, we focused particularly on the uniformity of the black encapsulation surface. Black encapsulants can effectively absorb stray light, thereby improving both contrast and image clarity. At the same time, we optimized light transmittance to ensure maximum luminous output through the encapsulant, thereby achieving the desired brightness level.
Uniform surface reflectivity was another critical area of focus, as reducing unwanted reflection directly enhances the overall visual experience.
To ensure long-term stability and reliability, airtightness of the encapsulation material was rigorously tested to protect the internal LED chips from environmental interference. Furthermore, we paid close attention to the precision cutting of the packaged LED modules. With advanced cutting technology, we achieved a cutting accuracy of ±3 μm, which is essential for maintaining the visual sharpness and aesthetic integrity of fine-pitch LED displays.
1.2.2 Scan Driving Control Using Column Constant-Current + Row Driver ICs
The brightness of LED chips increases with current but does not follow a strictly linear relationship. Display brightness is also affected by the scan ratio: higher scan ratios improve brightness uniformity but also increase power consumption, necessitating a balance between luminance and energy efficiency.
Common-cathode driving offers low power consumption and thermal output but comes at a higher cost. In contrast, common-anode designs are more cost-effective but result in higher power usage and heat generation.
While integrated row-column driver ICs simplify the circuit design, they typically deliver poorer low-gray performance. On the other hand, separate row and column ICs offer superior grayscale performance but complicate the design process.
PCB layout plays a critical role in both low-gray level performance and electromagnetic compatibility (EMC) and must be optimized accordingly. It is also essential to balance the scan rate with the refresh frequency to maintain image quality while reducing power draw.
For designs with sub-P0.9 pixel pitch, advanced HDI (High-Density Interconnect) technology is required to support high-density and high-precision circuit design. A deep understanding of these aspects will guide the effective design and production of fine-pitch LED displays【2】.
1.2.3 High-Precision Splicing Structure Design for Fine-Pitch LED Displays
To achieve high-precision splicing (flatness, gap width, and misalignment < 0.1 mm) in fine-pitch LED display cabinets, several factors must be considered:
Optimization of cabinet flatness and adjustability of panel gaps.
Development of mechanisms for quick installation and module replacement.
Analysis of cabinet structure for effective thermal dissipation.
Exploration of flexible installation strategies to accommodate different on-site mounting environments.
1.2.4 Calibration Technology for Fine-Pitch LED Displays
To assess the impact of brightness calibration on display uniformity, the following areas were investigated:
The influence of chromaticity calibration on color consistency across the screen.
The effect of zonal calibration on the visual seamlessness of large display assemblies.
The role of multi-grayscale calibration in achieving stepwise grayscale uniformity.
The compatibility of calibrated screens with spare module replacements.
The impact of different brightness and wavelength ranges of LED chips on calibration effectiveness.
3. Technical Approach and Workflow
To achieve ultra-fine pixel pitch in LED displays, the flip-chip COB (Chip-on-Board) technology was employed. Specifically, 3 mil × 6 mil flip-chip LEDs were used and soldered onto HDI (High-Density Interconnect) PCBs using solder paste technology. The PCB substrates were designed in either a 4-layer single-step or 6-layer two-step architecture to meet high-density requirements.
The LED driver boards were designed to support both common-cathode and common-anode driving schemes, ensuring broad compatibility across different system architectures.
For the surface encapsulation of the LED modules, a combination of molded epoxy resin and PET optical film was applied. By ensuring consistent alignment between the PCB and the optical film, we were able to control the ink-color uniformity of the display under black screen conditions.
The cabinet structure was designed using die-cast aluminum frames. LED modules were mounted onto the cabinet via magnetic attachment for convenient assembly and maintenance. Each LED module measures 150 mm × 168.75 mm, and each cabinet accommodates 4 × 2 modules (i.e., 8 modules per cabinet).
Full-screen calibration was employed to ensure display uniformity. A high-precision industrial camera was used to sample the brightness and chromaticity of the entire LED screen. Based on the target luminance values, individual brightness coefficients were calculated for each pixel, enabling fine-tuned calibration of every single LED. This process ensures consistent white balance and visual uniformity across the entire screen.
Illustrative diagrams of the LED cabinet structure and module packaging are shown in Figure 1 and Figure 2, respectively.
Fig1 Schematic diagram of the structure of the LED cabinet
Fig.2 Schematic diagram of LED light board package
(1) Transparent Coating Layer:
A molded epoxy adhesive layer with a thickness of 200 μm and light transmittance greater than 80%. The surface is glossy and serves as a sealing and protective barrier.
(2) Optical Film Layer:
An optical PET film with approximately 40% light transmittance and a thickness of 90 μm. This layer improves black-level consistency (ink-color uniformity), offers anti-fingerprint and anti-reflection properties, enhances scratch resistance, and adds overall surface protection.
(3) LED Chips:
Flip-chip LEDs with dimensions of 3 mil × 6 mil, soldered onto the PCB using solder paste bonding to ensure firm attachment and electrical connectivity.
(4) Driver ICs:
Row and column driver ICs are mounted on the back side of the PCB for display control.
Process Flow Diagram
The full process route for manufacturing the LED display modules is as follows:
Incoming PCB Inspection → LED Die Solder Paste Printing → Die Bonding → Reflow Soldering for Die Bonding → IC Solder Paste Printing → SMT Placement → IC Reflow Soldering → Testing & Rework → Aging & Rework → Epoxy Molding → Optical Film Lamination → Precision Cutting → LED Board Aging → Module Assembly → Cabinet Assembly → Full-Screen Aging → Calibration → Final Inspection → Packaging
4. Conclusion
Through in-depth research into flip-chip LED die bonding, packaging processes, and the design of row-column driving circuits for full-color LED panels, this study successfully developed a high-definition, fine-pitch, seamless flip-chip COB LED display. The resulting display offers several significant advantages, including consistent black-level appearance, uniform white screen performance across viewing angles, high soldering yield, reworkability, and excellent full-screen uniformity. These features position the display for broad application prospects and are expected to strongly promote the advancement of LED display technologies, improve user experience, and meet the needs of various application scenarios.
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[5] Omitted.
