Apple jumpstarts miniLED backlights

After more than three years of hype, excitement, and unfulfilled promises, miniLED backlights are finally ready for prime time. While not the first adopter, Apple will once again be the catalyst that drives the technology from anecdote to mainstream. But Samsung, TCL, BOE and others will also contribute substantially to the rise of miniLEDs in 2021. Supply chains in Taiwan and China have been preparing and are ready to fight for supremacy.

LCDS get a boost with local dimming backlights

MiniLEDs are often presented as a stepping stone to microLEDs. There is indeed some level of continuity in direct view LED display applications, such as those large LED videowalls often seen in shopping malls, stadium, entertainment districts and airports. The size of the LED chips in those applications is typically in excess of 5 mil x 9 mil, or 125 µm x 225 µm. But as manufacturers keep shrinking die, they can eventually qualify as microLEDs, such as Sony’s Crystal LED (CLED) displays built with 20 µm-long LED die.

In liquid crystal display (LCD) backlight applications one point of confusion disappears, although another emerges. LCD display makers have long been using full array local dimming (FALD) in order to improve the contrast of their panels. In LCDs, depolarization effects in the TFT transistor substrate, liquid crystal cell, and color filters result in light leakage. Therefore, even when a pixel is turned off, there is always a residual amount of light leaking from the backlight unit (BLU), resulting in poor contrast. To enhance LCD display performance, global and local backlight dimming are used. Local dimming segments the display into multiple zones where the backlight can be controlled individually and completely shut down in dark areas of the image, as shown in figure 1.

FALD has been used mostly in TVs, rarely in desktop IT monitors, but not in mobile displays such as laptops, tablets or mobile phones. One reason is that placing the LEDs on the back of the display and facing the viewer increases the thickness of the device. Not only is the LED package rather thick, enough distance is required for the light from individual chips to spread enough that the display can be evenly illuminated without hot spots.

Here comes miniLEDs

MiniLEDs solve the issue by multiplying the number of LED chips, thereby reducing the spacing between each light source and the thickness of the backlight, as shown in figure 2. More interestingly, they also can significantly increase the number of zones, which in turn reduces blooming, the halo that appears when bright objects or highlights much smaller than the individual dimming zones must be displayed against a very dark backdrop.

Ultimately, well-designed miniLED backlights with a high number of zones allow LCD displays to reach contrast performance close to that of OLEDs while maintaining the high brightness characteristics of LCDs. They can also reduce power consumption. Since in dark images, most zones will be off or dimmed at very brightness.

MiniLED performance and cost: one or the other?

Why don’t we see miniLED backlights everywhere? The answer is cost. More LEDs means higher chip cost and higher number of zones means more driver ICs. So why not make the LED chip smaller? After all, their cost more or less scales with surface area.

Well, first of all, producing very small LEDs below 100 µm is complicated. Defects are more likely to kill a device. The width of the dicing streets between chips on the wafer becomes disproportionally large. Then, as chip size decreases, so do the bonding pads and the gap between them. This makes attaching the die onto the printed circuit board (PCB) more difficult. Higher placement accuracy is required. Demands on PCB flatness and lithography accuracy become more stringent and cost increases further, as shown in figure 3. At this stage, using glass rather than PCB becomes a credible option. And if using glass, then why not use TFT glass, the same backplane used for the LCD cell itself? This also simplifies driving, rids the board of many drivers and enables high performance, low power consumption active matrix driving. The jury is still out. PCB makers are developing improved materials and traditional, passive matrix BLU driver suppliers such as Macroblock in Taiwan are developing convincing and cost-effective solutions based on multi-channel, multiplexing drivers that significantly reduce the IC count. For now, die sizes are often still above 5 mil x 9 mil. Roadmaps move to 3 mil x 5 mil in the short term. For some applications, the end game would be 2 mil x 4 mil.

Another significant cost contributor is the miniLED assembly. Traditional LED chip bonders have throughput in the 15,000 to 25,000 unit per hour range. So it would take a piece of equipment an hour or more to assemble a single TV with 25,000 chips. This number would more than quadruple for a TV using 100,000 chips. Not only is that four times more chips, but those would likely be smaller, which implies higher positioning accuracy and reduces the equipment throughput.

There is therefore a need for miniLED-dedicated assembly technologies and equipment to enable broader adoption. Rohinni was definitely the pioneer in the field. In collaboration with equipment maker K&S, the company introduced the first miniLED equipment back in late 2018. The tool can assemble 180,000 chips per hour with an accuracy of ± 10 µm. Rohinni is planning to improve this further to 360,000 chips per hour or higher, see our interview here. To no-one’s surprise, other equipment makers have taken notice and ASM, with its AD 420 tool, has also started to introduce assembly tools aimed at miniLEDs.

Just like there is confusion between microLED and miniLED, there is also confusion between traditional FALD and miniLED backlights. There is no industry-wide accepted definition and, once again, many advertise miniLED displays using traditional SMD LED package and a number of zones not much different from previous generations of FALD. While there is a continuum between traditional FALD and miniLED, Figure 4 shows our attempt to more clearly separate the two for TV applications, and the same could be done for other applications but with different thresholds. The goal is to really identify disruptive technologies.

MiniLED TVs are coming in 2021

With that definition in mind, TCL is expected to commercialize what we consider as the first, glass-based, active matrix driven miniLED TV later in 2020. The 75” TV, showed at the US Consumer Electronics Show (CES) 2020 but first presented at a company event back in August 2019, will have 5,184 zones powered by more than 20,000 miniLEDs. At CES, BOE, leveraging its Pixey joint venture with Rohinni, also showed a 75” prototype with more than 10,000 zones and 100,000 LEDs on glass backplanes. The company hopes to have commercial products by the end of 2020. Samsung is also expected to adopt miniLEDs on its flagship QLEDs in 2021, although most probably using PCB and passive matrix driving. Samsung alone is hoping to sell 2 to 3 million of those miniLED TVs in 2021. It wouldn’t be a surprise to see LG join the ranks, although more quietly so as not confuse the company’s message that OLED is the technology of the future for high-end TVs. The company showed some prototypes at CES 2020.

In the TV and smartphone markets, miniLED LCDs compete directly with OLEDs. MiniLED performance in term of contrast and blooming will at best only approach that of OLEDs. The cost and price gap must therefore be in favor of LCDs to provide enough incentive for the buyer to choose miniLED. LCD makers must therefore optimize their design and engineering to carefully adjust performance and cost of their miniLED solutions to occupy the sweet spot between existing high end LCDs and OLEDs and try to expend their position from there. 

Mid-size displays are a sweet spot for miniLEDs

In smaller displays, there is a sweet spot for miniLEDs. The Red/Green/Blue (RGB) OLED used in smartphones are still quite expensive for larger sizes such as laptops or tablets. For larger panels, LG’s white OLED (WOLED) technology is also quite expensive for desktop monitors. And, although some gamers are likely to purchase LG’s 48” or even 55” G-Sync compatible TVs as monitors, the company lacks the flexibility and capacity in its WOLED fabs to address small and intermediate sizes in the more mainstream 21” to 32” ranges. For IT, users might be staring at productivity applications like word processing and spreadsheets all day long. With OLED, this means serious risks of burn-in and premature aging. Power consumption also becomes a concern. In all those product segments, the space is therefore virtually wide open for miniLEDs as soon as it gets cost under control.

ASUS first introduced a 27” miniLED monitor with 2,304 LEDs defining 576 zones back in 2019. The product was followed by a 32” version with 1,152 zones. Acer followed suit at CES 2020, showing its X32 miniLED monitor, a 32” display, also with 1,152 zones. Lenovo released its P27, a 27” monitor with 1,152 zones and 10,368 miniLEDs. Apple also has its Pro Display XDR with 576 zones. All those products are geared toward high-end, professional users for creative and editing work. The Asus 32”, for example, retails for US$4,500 while the Apple XDR fetches US$5,000. On the laptop front MSI introduced the Creator 17, a laptop with 240 zones.  Most of those products wouldn’t qualify as real miniLED by our standards but they do constitute a significant evolutionary step by bringing FALD into their respective product categories.

Apple raises awarness and triggers supply chain shake up

The big catalyst in smaller displays is likely to be Apple, with the 2021 iteration of its 12.9” iPad pro to feature miniLEDs. The choice of the Pro as the first iPad adopting miniLED is not a surprise. First, its higher end product positioning is more suited to absorb the extra cost. Second, customers are more likely to use productivity apps on this device. A late 2020 launch seems increasingly unlikely though. Early 2021 is more probable. The supply chain however is already ramping up. The LCD panels will be supplied by LG Display which will incorporate the full BLU to be assembled by local partner Heesung Electronics. Upstream, the 10,384 8 mil x 8 mil miniLED chips will most likely be provided by Taiwan-based Epistar and assembled by TSMT using K&S’ Pixalux, the miniLED assembly equipment featuring Rohinni’s technology.

From the LED perspective, Taiwan is the big winner for now. The island’s mature and strong LED and display ecosystems have been extremely reactive and started working collaboratively early on to enable miniLED adoption. While Epistar is the big winner for Apple’s iPad, AUO’s LED affiliate Lextar supplies its parent companies with the miniLED backlight used in Asus and Acer monitors and MSI’s notebooks.

Interestingly, the miniLED opportunity is even reshaping the local industry. Earlier this year, Epistar announced a US$200 million investment, US$160 million of it for miniLED capacity. To put this in perspective, as of writing of this article, Epistar’s paid-in capital is US$371 million. The company plans to dedicate up to 95% of its blue LED capacity to miniLEDs, albeit in the short term most of it is for direct view LED displays. In June 2020, Epistar and Lextar announced the creation of a common holding company, “Ennostar”, effectively merging the two entities, with the former to focus on chip development and manufacturing, and the latter on packaging, applications and modules. The merger makes Taiwan-based LCD panel maker AUO the single largest shareholder of the new holding with a bit less than 10% of the share, enough to gain a seat on the board. Beside the need for the two players to reach a critical mass to fend off the rise of Chinese LED makers such as San’an and HC Semitek, there is no doubt that miniLED, and in the longer term, microLED opportunities were instrumental in the decision to merge the companies. With Apple also very active in the island on microLED developments, it is likely that the consumer electronic giant might have weighed in the decision.

Chinese competitors are not standing still though. In April 2019, Sanan announced it was raising US$1.8 billion for miniLED and microLED development and production. Local rival HC Semitek is raising US$217 million with a similar goal. The company has signed a collaboration agreement with leading LCD panel maker BOE and is positioning itself to serve Samsung’s 2021 miniLED TV needs. 2021 will therefore likely be the year miniLED reaches prime time. To capitalize on this initial success, the supply chain and manufacturers must however keep refining their designs, improve the technologies and, above all, reduce costs. Designs choices have far from converged yet, as the industry is still experimenting with many options in term of die size, quantities, number of dimming zone, driving type and PCB choices. For large display TVs, with more than 5,000 dimming zones, active matrix on TFT glass appears to be an efficient choice. The TFT could be used in conjunction with discrete silicon-based minidriver integrated circuits (ICs) assembled on the backplane with the same tools used for the miniLED chips. For smaller displays, passive matrix and PCB remain competitive for the midterm. The industry is far from converging, as can be seen in figure 6. In all cases, die size reduction will be a major contributor to cost reduction.

While OLEDs were once thought to be the sure winner in high-end TVs, miniLEDs provide LCD makers a credible opportunity to reduce the performance gap and leverage on their massive installed LCD capacity. For midsize panels such as tablets and laptops, it brings an opportunity to establish a strong foothold in high end product segments before OLED can get a chance to establish itself. But as often in the display and LED industry, cost reduction will be key in enabling significant adoption.

About the author

Eric Virey, PhD. serves as a Principal Display Market and Technologies Analyst within the Photonics, Sensing & Display division at Yole Développement (Yole).
Eric is a daily contributor to the development of the Display activity at Yole, with a large collection of market and technology reports on display technologies, Quantum Dots, MicroLEDs, TFT backplanes as well as multiple custom consulting projects: business strategy, identification of investments or acquisition targets, due diligences (buy/sell side), market and technology analysis, cost modelling, technology scouting, etc.
Eric has spoken in more than 50 industry conferences worldwide over the last 10 years. He has been interviewed and quoted by leading media over the world including: The Wall Street Journal, CNN, Fox News, CNBC, Bloomberg, Financial Review, Forbes, Technology Review, etc. He is also a regular contributor to various display industry media and organizations.
Previously Eric has held various R&D, engineering, manufacturing and business development positions with Fortune 500 Company Saint-Gobain in France and the United States.
Eric Virey holds a PhD in Optoelectronics from the National Polytechnic Institute of Grenoble. He is currently based in Portland, OR.

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