While OLED technology is quite popular in the display industry, innovators are already turning their attention to the next potential disruptor– microLEDs. These tiny, individual light-emitting diodes (LEDs) offer high brightness, energy efficiency, and an incredibly wide color range. 

Recent estimates highlight its potential, marking a CAGR of 73.33% from 2024 to 2033, with the global microLED market expected to reach a size of around USD 149.97 billion.

MicroLED bonding is the process of precisely attaching these minuscule LEDs onto a substrate or backplane, transforming them into a functional microLED display. As the adoption soars, various manufacturing and research companies are stocking up on efficient microLED bonding techniques.

In this article, we explore two main microLED bonding methods and understand what makes these techniques crucial for the industry’s progress.

An Ideal MicroLED Bonding Method: Basic Features

Despite its invention in 2000, the first consumer-facing microLED demonstration didn’t occur until Sony’s 55-inch Crystal LED Display in 2012. However, this early technology was prohibitively expensive, and its manufacturing techniques lacked commercial viability. 

Still, companies continue to invest in and refine production, including crucial processes like microLED bonding. This process depends on these key factors:

1. High Throughput

A high-throughput bonding technique is necessary for the successful manufacturing of microLED displays. To maximize production, manufacturers strive to strike a compromise between speed and accuracy.

2. Bonding substance 

The bonding substance used should firmly and consistently connect the microLED chip and substrate. Solder and conductive adhesives are common materials that can be used to create both mechanical and electrical connections.

3. Temperature Control

Since MicroLEDs can produce heat, it’s critical to manage the heat throughout the bonding process to keep the LEDs safe and guarantee steady operation.

4. Cleanroom Conditions:

MicroLED bonding is usually done in a cleanroom setting to reduce the possibility of contaminants affecting the dependability and performance of the display.

5. Encapsulation

Following bonding, the microLED display might undergo encapsulation to shield the individual microLEDs from elements like oxygen and moisture that could eventually deteriorate their performance.

6. Expandable Compatibility

The microLED bonding process needs to be scalable for mass manufacturing to meet the expectations of the consumer electronics market. Manufacturers also focus on creating optimized procedures that scale up quickly without sacrificing quality.

MicroLED Bonding: Major Types

Two primary bonding techniques dominate the industry: thermocompression bonding and adhesive bonding.  Thermocompression bonding uses heat and pressure for secure attachment, while adhesive bonding relies on specialized adhesives for a strong hold. 

Thermocompression bonding is currently more widely used, but adhesive bonding is rapidly gaining popularity due to its potential advantages.

1. What Is Thermocompression Bonding?

Thermocompression bonding (TCB) utilizes pressure and heat to form a bond between microLED chips and other components. It is a widely used approach as it can provide results with a high degree of precision and is suitable for manufacturing at scale. 

MicroLED Bonding: Key Aspects of TCB

• Alignment: Precise alignment of microLEDs is essential for creating a consistent and functional display.
• Thermo-Compression: Bonding involves applying heat and pressure to microLEDs and other components, often within a controlled environment.
• Bond Formation: As materials soften under heat, they merge to form strong bonds with the substrate or other components.
• Quality Control: After bonding, the structure undergoes rigorous quality control checks to ensure it meets performance standards.

Recent Usage of TCB in MicroLED Manufacturing

This technique has several industrial use cases, including manufacturing various components, such as pressure sensors, accelerometers, gyroscopes, LEDs, laser diodes, and RF MEMS. 

Besides it is also used in:

• Assembly of high-precision devices: It is used to assemble devices that require high-precision alignments and firm bonding, especially in microelectronics and optoelectronics.
• Semiconductor packaging: TCB is crucial in the semiconductor industry when adhering microLED chips with reliability and high performance to the substrates.
  
• Wearable Technology: It will enable wearable devices, and compact and efficient bonding for microLEDs is most important for small form factors. 

MicroLED Bonding: Some Companies Leveraging TCB

• Apple: They have been engaged in microLED R&D since 2014, with an emphasis on developing wearable displays. This likely involves sophisticated bonding techniques such as thermocompression​​.
• ams-Osram: The company partnered with Apple on a microLED project, indicating the use of advanced manufacturing technologies like thermocompression bonding​​.
• Rohinni: They are focused on miniLED and microLED technologies and have expertise in precision placement and bonding processes​​.

2. What is Adhesive Bonding?

The method of firmly connecting microLEDs to a substrate or another material employing adhesion approaches is known as adhesion bonding of microLEDs.This method dates back to the 1960s. 

Adhesive bonding eventually made its way to civil aircraft, where high-performance epoxy-based resins are now often utilized. Airbus, for instance, started using adhesive bonding in 1972. There are currently around 345 bonding features in the Airbus aircraft family. 

The demand for adhesive bonding technology is expected to remain high in the future due to the constant need for improved performance, which will also lead to the creation of new applications and associated issues.

Adhesive Bonding Techniques for MicroLED Applications

1. Epoxy Bonding

Epoxy resins are frequently utilized in microLED bonding as adhesives. A tiny quantity of epoxy is dispensed onto the substrate, the microLEDs are positioned, and the epoxy is allowed to cure. Epoxy offers strong mechanical properties and exceptional adherence. 

The aerospace industry uses 2K epoxy glue solutions and one-component films made of epoxy adhesives for various applications. For example, epoxy glues are used to connect the rotor blades of helicopters because of their remarkable strength and mechanical stability.

2. Anisotropic Conductive Film (ACF) Bonding

MicroLEDs are bonded to surfaces using ACF, a substance containing conductive particles. It is appropriate for applications where electrical conductivity is critical since it offers adhesion and makes electrical connections between the microLEDs and the substrate easier. 

For example, liquid crystal display (LCD) packaging uses anisotropic conductive film (ACF) for fine-pitch bonding. 

3. Wafer-Level Bonding

This method involves fabricating many microLEDs on a wafer and bonding the entire wafer to a substrate. This procedure works well for mass manufacturing.

4. Die Attach Techniques

MicroLEDs can be attached to substrates using various die-attach techniques, including wire bonding and flip-chip bonding. These techniques involve putting the microLED in direct contact with the substrate using solder or conductive wires to form a link.

Final Note

Evidently, thermocompression bonding offers precision and scalability for microLED manufacturing, making it widely adopted. However, adhesive bonding’s potential for stronger bonds, better heat dissipation, and simpler processing is gaining traction. 

As companies like Apple delve into R&D, optimizing microLED bonding techniques will be key to unlocking microLEDs’ true potential in wearables and display technology. The method that balances performance and cost-effectiveness will emerge as the industry standard.