LED Inside Reported on the Challenges for Micro LED display Manufacturing.
February 26, 2018
In order to precisely transfer Micro LED to a target backplane, the accuracy of equipment for Micro LED manufacturing is required to be less than ±1.5µm. However, currently, the accuracy of existing transfer equipment (Pick & Place) is ±34µm (Multi-chip per Transfer), while flip chip bonder features with accuracy of ±1.5µm (single-chip per Transfer), which failed to meet the accuracy requirements of Micro LED mass transfer. Generally, traditional LEDs like 3030 LED with 3,000µm light source can be transferred by SMT equipment, it is available to transfer chip by die bonder when the size of light source reaches 100µm, the existing equipment for pick & place faces serious challenges of accuracy when the size of light source reduces to 10µm. Chip bonding and wafer bonding cannot be applied to mass transfer due to low production capacity and high TACT, therefore, currently wafer bonding must be adopted to develop Micro LED technology and products featuring low pixel volume with the existing machines, while challenges exist in production capacity and cost, so various thin-film transfer related technologies will be needed in the future. Five thin-film transfer technologies include electrostatic adsorption, Van der Waals force-enabled transfer printing, laser ablation, phase-change transfer and fluidic assembly. The fluidic assembly is a high-speed assembly approach that achieves high UPH for various product applications, also it can reduce assembly time and cost greatly. Mass transfer is a critical technology for Micro LED manufacturing, transferring Micro LED to a target backplane quickly and accurately will be one of the topics that manufacturers need to work on the most as well as on improving UPH and yield.
Micro LED will be initially be applied to indoor displays, smart watches and smart bracelets, due to the high difficulty in transfer technology and various application products with different pixel volume, some manufacturers conduct R&D with existing wafer bonding equipment and take the application products with low pixel volume as targets to shorten the development period, also some manufacturers directly develop thin-film transfer technology, which required more resource investment and longer development period because the equipment needs to be adjusted even redesigned, moreover, there will be more manufacturing problems. However, as we have stated in the past these are very low revenue products that don’t lend themselves to differentiation from flexible OLEDs.
Currently, the highest rated volume transfer is ~250,000 LEDs/min, which calculates to a TACT 6,000 sec for 4K and 24,000 min for 8K assuming no LED redundancy was required. To get to a 2 min TACT would require 50 or 120 transfer tools respectively.