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Mini LED Backlight Business Is Ready to Soar, while COB Yield Rate Is the Key for Mass Production

At its spring event held in April this year, Apple Inc. launched the new iPad Pro featuring the M1 chip and, as expected, a Mini LED display for the 12.9-inch model, offering a stunning visual experience to users.

Moreover, Apple’s release of Mini LED iPad Pro will increase the penetration rate of Mini LEDs. However, shipments of this new tablet have reportedly been delayed (the original delivery time was May) due to unsatisfactory yield rates of Mini LEDs. Industry insiders maintain that COB equipment was the main reason causing low yield rates. If the COB yield rate can be enhanced, the current problem of Mini LED production can be solved.


Apple Unveils New Mini LED iPad that Brings Better Visual Experience

At the event, Apple revealed that the 12.9-inch model comes with the Liquid Retina XDR display that adopts Mini LED technology, which delivers excellent brightness and the most outstanding visual effects. Reportedly, the Liquid Retina XDR display boasts a 1 million-to-1 contrast ratio that presents true-to-life details, allowing users to view and edit HDR photos and videos or have excellent movie- and show-watching experiences. Amazingly, the 12.9-inch iPad Pro also features up to 1000 nits of full-screen brightness and 1600 nits of peak brightness, plus advanced display technologies including P3 wide color support, True Tone, and ProMotion technology.

Apple added that the new iPad Pro uses over 10,000 Mini LEDs across 2,500 local dimming zones, where each zone precisely adjusts brightness according to the content being shown on the display, thereby achieving an extraordinary 1 million-to-1 contrast ratio. When playing HDR contents with abundant details and bright highlights, such as a galaxy and explosions in action movies, the new model is able to vividly present these scenes more than ever.


Yield Rate Problems of Mini LEDs Remain Unsolved, with New iPad Pro Shipments Likely to Keep Delayed

Since Apple officially released the new iPad Pro engineered with a Mini LED design, the market has anticipated a growth in Mini LED products. However, it has been said that the yield rate of Mini LED production is low, making shipments of the 12.9-inch model slip into July.

As reported by Bloomberg, the new iPad Pro will be officially available in roughly one week, yet customers who preordered the 12.9-inch Apple iPad Pro might need to wait until July to have one in their hands. Bloomberg’s report said that the delay could be attributed to manufacturing issues of Mini LEDs. In April, Bloomberg indicated that before the release of new iPad Pro, Apple had already been facing unsatisfactory yield rates of Mini LED production, which then limited the supply. Nonetheless, sources familiar with the matter suggested that Apple’s suppliers are now trying their best to achieve mass production of such complex screens.

Sources also specified that shipments of the Mini LED iPad Pro has been delayed to July mainly because the yield rate of COB die attach remains low. At first, it was assumed that the criteria of Mini LED production must be fast bonding speed due to their high volume Mini LEDs per panel, yet the accuracy and yield rate were overlooked, resulting in a manufacturing bottleneck.

An exclusive source told LEDinside that the actual COB yield of Mini LEDs for iPad Pro is currently 95%. Assuming that each backlight backplane has 10,384 LEDs, such yield rate means that 519 of these required rework. Based on the current automatic rework rate of 30 seconds/unit on average, reworking a backplane can take about 4 hours (including defected Mini LED removal, solder refilling, and re-bond). To achieve production line balancing, a die attach system must be equipped with 74 automatic rework stations, indicating a high cost. Reportedly, the inability of real-time alignment and adjustment in that COB equipment is a major factor leading to a low yield because using ejector pin to blindly eject Mini LEDs at a high frequency is likely to cause displacement, bad connection, and short circuit.


Production of Mini LED Overcomes Its Bottleneck, Reaching a Higher COB Yield Rate

In short, Mini LED is a new display technology, of which the packaging requirements are different compared with conventional technologies, making yield rate a key to successful mass production of Mini LEDs. This leads us to an important question— Is there any COB equipment right now that can achieve a higher actual yield rate than 95%?

Industry sources said that ASM Pacific Technology (ASMPT) has already installed its COB die attach system featuring a novel bond head in major backlight module factories across Taiwan, South Korea, Japan, and China, where requirements of Mini LED mass production have been fulfilled. The yield rate of their Mini LED backlight modules is now 99.97%. In the case of iPad Pro manufacturing, reworking is required for only 3 LEDs on a backplane and can be completed in just 1.5 minutes. Unlike other models, ASMPT’s die attach system can achieve production line balancing with only 0.4 rework station when the COB capacity is fixed, which helps overcome the existing issues concerning the production of Mini LED-backlit and direct-lit RGB displays.

In addition to said COB problem, the dicing process adopted by upstream suppliers is another key to Mini LED mass production. Because the size of Mini LED chips has been greatly miniaturized compared with that of other LED chips, the number of chips cut from a same size wafer will rise, leading to increased cutting frequency. Miniaturization of Mini LED chips also requires more precise dicing process to achieve higher yield and lower cost. Sources revealed that ASMPT’s semi-surface dicing technology is able to realize dicing of thin wafers, such as Mini/Micro LEDs, more than 10 million chips per hour, which can boost Mini LED outputs and create a high yield rate that effectively improves COB die attach productivity and LED light-up rates.

In summary, to overcome the existing difficulties in Mini LED manufacturing and to achieve mass production, increasing the yield rate and using equipment with controllable cost are indispensable. With technological support from equipment suppliers, transfer, inspection, and COB processes become more efficient with reduced rework time, ultimately achieving commercialization of Mini LEDs.

By LEDinside