MiniLED in the making: Production opportunities and challenges

MiniLED is a relative newcomer to the display category, especially for tablets, TVs and gaming monitors. This relatively new technology, used for backlighting LCD displays, offers deeper blacks, improved contrast ratios and brighter panels, compared to traditional LED-backlit LCD technology. But there’s a challenge looming as miniLED moves into mass production: miniLED backlight units require a substrate – PCB or glass – coated with a white solder mask. Why is the manufacturing process with a white solder mask a challenge? To better understand, let’s look at the benefits of miniLED, why white is the preferred solder mask, and the resulting manufacturing challenges.

According to a report by Omdia, “OLED and LCD Supply Demand & Equipment Tracker – 2Q21 Analysis”, shipments of LCD panels with miniLED backlights are expected to grow from 9.7 million units in 2021 to 37.9 million units in 2025. Specifically, TVs and tablets are the main miniLED applications, with significant growth predicted for TVs through 2025. The use of miniLEDs for laptops is also gaining momentum and expected to increase in volume over the course of 2025. the same period. MiniLED is becoming a popular choice for OEM designers because it offers better contrast, richer colors, and improved image quality, and can be a cost-effective alternative to OLED.

OLED versus miniLED

OLED is a popular technology, with self-illuminating displays and panels that do not require a backlight unit due to their light-emitting characteristics. OLED can provide better color contrast than miniLED, but it is often quite expensive and has some drawbacks in terms of lifespan and performance in outdoor environments. MiniLED, on the other hand, has excellent brightness, lower power consumption, high refresh rate and wide color gamut with less design complexity. As a result, many of the biggest manufacturers of TVs, tablets and laptops are eager to adopt miniLED technology as it offers quality and other improvements over standard LED technology and is more cost effective than standard LED technology. ‘OLED.

To provide all of these benefits, the solder mask exposure process for miniLED-based devices requires high accuracy and throughput. Manufacturers must be able to achieve higher accuracy to meet the demanding specifications of miniLED-based products as required by designers. Direct Imaging (DI) for the solder mask exposure process is an ideal solution as it enables high accuracy, while overcoming challenges such as panel topography, distortion and fine features, all of which can influence the quality and yield of the product.

MiniLED backlight unit on a large TV

Advantages and Challenges of White Solder Mask for miniLED

Designers of miniLED BLU devices (backlight units) typically require substrates manufactured by their supply chain to be coated with a white solder mask, which has a very high reflectivity rate (>90% in many cases) . This allows for high contrast and more vivid colors that deliver a sharper image on TVs, tablets, and laptops/PCs. All of these substrates, whether PCB or glass, are produced with openings to receive the miniLED chips in the next assembly step, either in package-on-board (POB), chip-on-board (COB) or chip-on-board. glass (COG). Highly accurate and uniform aperture dimensions are critical to the quality of the final miniLED product.

There are three factors in the white solder mask DI exposure process that can create challenges for manufacturers: the first is accuracy, the second is quality and yield, and the third is capability which translates also by cost.

MiniLEDs range in size from 50 to 300 microns and must be positioned very precisely, with extremely low standard deviation. For example, the uniformity of solder resist apertures (SRO) on the substrate cannot deviate more than ±5 microns for some applications. High-end devices generally require greater precision. Achieving such tight registration accuracy is even more difficult when panel targets are covered with a thick white solder mask, and special lighting or a sophisticated algorithm is required to acquire the target position with high accuracy.

Quality is the second challenge, related to the uniformity of solder resist openings and strict undercut requirements. The white solder mask used for miniLED production scatters most of the illumination wavelength, so exposure UV light is not absorbed as it would be with other mask colors resulting in a lower solder mask polymerization process. An undercut forms when the bottom portion of the solder mask layer is not fully cured during the exposure process. This can lead to the possibility of detachment. A severe undercut (see image below) is often problematic for miniLED manufacturers. A minimal undercut (see image below), which creates sturdy dams, is a requirement for high quality and yield. Creating robust dams with minimal undercuts requires the use of an optimized wide range of illumination wavelengths in the exposure process, to expose the thick, reflective white solder mask.

To address costs, manufacturers of PCBs or glass substrates for miniLED devices need to maximize their production capacity and optimize their cost per print. This can be easily achieved using a high throughput solution that also addresses the unique white solder mask exposure challenges mentioned above.

Left: White solder mask bracing with minimal undercut.  Right: white solder mask bracing with a strong undercut

Left: White solder mask bracing with minimal undercut. Right: white solder mask bracing with a strong undercut

And after?

As the demand for consumer electronics based on miniLEDs increases, new and innovative methods of exposing solder masks are needed to meet the high demands for precision, quality and capacity. Direct imaging solutions that overcome the challenges of white solder mask exposure are needed for manufacturers to produce high quality substrates at mass production levels to meet the growing demand for tablets, televisions, gaming monitors and more in the years to come.

Visit us at Touch Taiwan to learn more about KLA’s White Solder Mask Direct Imaging (DI) solution and how it can increase the production of your MiniLED backlight unit.

Since April 27-29you can find us at Touch Taiwan, Nangang World Exhibition Center, Hall 1F, Booth #M834.