Micro-LED Conquered, Again?
Micro-LEDs are a numbers game, with 24.9m very small LEDs needing to be transferred from a wafer to a display substrate in a cost effective manner. As we have noted previously there are many ways in which the display industry has been approaching this problem, a major stumbling block to the commercialization of micro-LED displays. They range from sophisticated pick and place tools, fluidic self-assembly, where micro-LED die ‘float’ into place on the substrate in a liquid, stamps that stick to the die, to laser based tools. Each of these processes has its pluses and minuses, with tradeoffs usually being speed, accuracy, and pre and post steps. While some of the transfer processes we have seen can move millions of micro-LEDs per hour, many involve pre-processing before the transfer takes place or a bonding step as or after the transfer is made. While these are in most cases separate from the transfer process itself, they have to be considered part of the transfer and included in the total transfer time, which can then become radically different from the stated transfer rate of the tool.
While this seems nitpicking, it has a significant effect on the overall cost of producing micro-LED displays, and as the feature size of micro-LEDs continues to be reduced toward 5um or less, the cost increases. In order to solve this problem, and the problem of repairing those micro-LEDs that were damaged during the transfer (even at five 9’s there would still be at least 249 LEDs that would need to be replaced), the Electronics & Telecommunications Research Institute (ETRI) in South Korea has come up with a solution called SITRAB, which stands for Simultaneous Transferring and Bonding, or simply a way to transfer and stick the die to the display substrate in one step. By using a laser (aka Laser Assisted Bonding or LAB) to heat this material (known as an anisotropic solder paste or ASP) the die can be picked up and bonded as they are placed, but instead of a very precise laser that steps through a bond for each die, a wider beam laser can be used to bond a large number of die at once.
According to ETRI this will reduce the time it takes to transfer an bond I reduced to 1/10th of normal, and since the ASP is easily applied and bonded, repair time can be reduced to 1/100th of normal. But there is a catch, although the researchers say the equipment investment cost is low and domestic equipment could be used, but so far ETRI has been able to produce a 100 mm2 display that contains 1,225 micro-LEDs, which would be the equivalent of just over 76 pixels/inch for a mono-chrome display and 25 PPI for a full color RGB display, which is far below commercial pixel densities. Taking it one step further, ETRI says the process could be adapted to related products within two years, which seems quite optimistic in our view.
As noted this is one of many potential ‘solutions’ to the issues facing micro-LED commercialization, which we believe will see viable product in 2025 at the earliest, with new materials and processes being developed almost daily. The good news is that considerable sums are being spent on the development of micro-LEDs under the expectation that it will eventually be, in some form, a replacement for LCD technology, and those with key IP and process technology will have an edge over rank and file display producers. That said, success is certainly not guaranteed, and what eventually becomes a high volume micro-LED product could look completely different from what we have seen thus far.
While this seems nitpicking, it has a significant effect on the overall cost of producing micro-LED displays, and as the feature size of micro-LEDs continues to be reduced toward 5um or less, the cost increases. In order to solve this problem, and the problem of repairing those micro-LEDs that were damaged during the transfer (even at five 9’s there would still be at least 249 LEDs that would need to be replaced), the Electronics & Telecommunications Research Institute (ETRI) in South Korea has come up with a solution called SITRAB, which stands for Simultaneous Transferring and Bonding, or simply a way to transfer and stick the die to the display substrate in one step. By using a laser (aka Laser Assisted Bonding or LAB) to heat this material (known as an anisotropic solder paste or ASP) the die can be picked up and bonded as they are placed, but instead of a very precise laser that steps through a bond for each die, a wider beam laser can be used to bond a large number of die at once.
According to ETRI this will reduce the time it takes to transfer an bond I reduced to 1/10th of normal, and since the ASP is easily applied and bonded, repair time can be reduced to 1/100th of normal. But there is a catch, although the researchers say the equipment investment cost is low and domestic equipment could be used, but so far ETRI has been able to produce a 100 mm2 display that contains 1,225 micro-LEDs, which would be the equivalent of just over 76 pixels/inch for a mono-chrome display and 25 PPI for a full color RGB display, which is far below commercial pixel densities. Taking it one step further, ETRI says the process could be adapted to related products within two years, which seems quite optimistic in our view.
As noted this is one of many potential ‘solutions’ to the issues facing micro-LED commercialization, which we believe will see viable product in 2025 at the earliest, with new materials and processes being developed almost daily. The good news is that considerable sums are being spent on the development of micro-LEDs under the expectation that it will eventually be, in some form, a replacement for LCD technology, and those with key IP and process technology will have an edge over rank and file display producers. That said, success is certainly not guaranteed, and what eventually becomes a high volume micro-LED product could look completely different from what we have seen thus far.
ETRI STB Process tool - Source: Newsis.com
Conceptual Diagram of ETRI Simultaneous Transfer & Bonding Process includingloading, transfer, inspection, rework and unloading steps - Source: Newsis.com
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