NHK Scientists Develop Tiled Display Concept
The SID has published an article in its Journal that describes a method of making display tiles that have invisible seams when joined together. Usually there are seams visible on tiled displays. The article is available for purchase or free to SID members.
Why is this interesting for 8K?
One of the challenges of making large 8K displays is the problem of yields. If you are making small displays on a single substrate and you find a defect or two, you can afford to throw away defective panels. If the defect is on a big display, you have to lose the whole thing. As an example, In a typical Gen 8.5 production line (close to the maximum size for current OLEDs, for example), a 2200 x 2500 panel (overall) can make 18 32″ panels or three 65″ panels. If there are two defects in different parts of the substrate, you might lose two 32″ panels (just 11% of the total) but if you lose two 60″ panels, you would discard 66% of the value. In practice you would probably also make some 32″ panels alongside the 65″, but the overall issue is still the same, you lose a lot of the value. (There is a detailed look at the way substrates are cut here)
Because of the increased complexity of the 99 million sub-pixels (each with several transistors in the case of an OLED or microLED) in an 8K RGB LCD or OLED, there is more chance of defects so yield issues can be challenging.
I you were able to make the displays as tiles, you would then have the advantage that if there were defects, you would only lose the tiles, not the whole display. The writer looked at this in some detail in an article back in 2021.
Drivers
A challenge for most flat panel technologies (LCD and OLED especially) to getting to seamless tiling is the need to have driver chips connected to the ends of each row and column on the front of the display. Although the driver chips can be mounted on flexible circuit boards and bent out of the way, or integrated into the edge of the panel itself, they still take up some space. One way around this is to have conductive pads on the edges of each tile that can receive a signal from a driver through other tiles. Because the human eye is so sensitive, it is very hard to do this seamlessly, especially in displays with smaller pitch.
(In larger direct LED displays the different driving scheme for large LEDs allows each tile to be driven from the back. The whole architecture is designed to be modular. For smaller LEDs and microLED designs, tiling remains a key topic.)
NHK’s Idea
NHK’s idea is to use through-plastic-vias to connect the back of the panel to the front. This means that an ultra-narrow bezel could be created around the display of just 8 μm. The group tested the use of many vias (which you might need to go through multiple displays) and found that TFTs could be correctly operated even with the current passing through 1,000 Vias.
Unlike the kind of designs used in LED walls, the NHK group made a module with a 3 cm x 3 cm size and 160 x 160 pixels, giving a fine pixel pitch of 133ppi. This would enable an 8K display to be made with an array of 48 x 27 modules, allowing the creation of an 8K display with just a 50″ diagonal.
NHK believes that this approach could allow the development of a wide range of different shapes, sizes and configurations and getting away from the fixed formats needed for most flat panel technologies.
Of course, this is an R&D project, so it’s still some way from production, but the attractiveness of the idea is clear.
