Displays of the Future: Die- Attach Challenges in MicroLED Assembly

Commercially viable Light emitting diodes or LEDs date back almost fifty years, when red LEDs were used in calculator displays. The breakthrough in LED commercialization took place when the blue LED was developed by Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura in the early 1990s, making white light possible. Over the last decade, rapid developments in LED package miniaturizations (mini LEDs) have been driven by direct emitting larger format displays and dynamic backlights for TVs and monitors. Smaller packages allow for more densely packed LEDs in the panels of direct emitting displays, hence a higher resolution. For dynamic TV and monitor backlights, a higher number of LEDs enables precise control for high-contrast images.

The next revolution will be micro LEDs, a necessary step towards direct emitting TVs and monitors. MicroLEDs are no longer used as backlights but as individual pixels emitting Red-Green-Blue (RGB) light directly without color filters and LCD light valves. There are many challenges in the industrializing of microLED consumer products. Sizes of micro LEDs can vary between 2um to 30um. Such dimensions are difficult to handle even in state-of-the-art pick and place processes.

For TV displays, a major consideration is the speed of the assembly process, as a 4K resolution display requires 25 million micro LEDs to be assembled in precise RGB pixels on the backplane with low failure tolerance.A throughput of >50 million units per hour has to be achieved for cost-effective manufacturing. Sequential pick & place processes are being replaced more and more by mass-transfer processes in order to achieve such a high throughput.

Each mass-transfer technology has its own advantages and disadvantages, but a common technological challenge is die-attach. Extremely small LED form factors require self-aligning, ultra-high resolution materials with sufficient die shear strength capabilities. For micro LEDs, die-attach materials with traditional formulations for solder pastes, fluxes, or adhesives are no longer adequate. Building on its experience in miniLED assembly, Heraeus Electronics has developed in close partnership with mass-transfer equipment and manufactures a new class of materials called WelcoTM.

“The next revolution will be micro LEDs, a necessary step towards direct emitting TVs and monitors.”

WelcoTM epoxy solders are not only self-aligning but easy to process and have excellent die-shear properties. Why is the combination of solder material with polymer so uniquely suited for microLED die-attach? Metal solder alloys take care of the self-alignment of micro LEDs on the substrate bonding pads, but the amount of interconnecting solder material between microLED electrodes and substrate pads is extremely low and would lead to fragile bonds after solder reflow. Epoxy polymers segregate in the reflow process and form a protective bond with the side walls of the micro LEDs and the substrate, which significantly increases interconnect strength. The viscosity and tackiness of WelcoTM solder pastes can be optimized to achieve maximum die placement yields for various mass transfer processes. The amount and type of polymer can be adapted as well to cater to various die shear requirements and bond pad characteristics. Clearly, all companies in the assembly value chain have to work closely together to find optimum solutions for each type of mass-transfer technology.

In summary, Heraeus Electronics has developed a “smart” solder material that self-aligns to form interconnects between microLED dice and substrate pads without confining to the limits of stencil openings. These self-aligning WelcoTM solder pastes offer best-in-class void-free and high die shear performance. It is exciting for Heraeus Electronics to play a part in revolutionizing the display industry!