When walking in the mall, have not found the front of the electronic billboard seems to be missing something? That's right! The original black cut lines annoying how have gone. Thanks to the rapid decline in LED prices in recent years thanks to small dot LED display Fancy into the indoor advertising applications to replace the original LCD splicing screen, but also eliminate the black border caused by the LCD monitor (see Figure 1, Figure 2).
Figure 1 LCD splicing screen, located in Taoyuan International Airport Terminal II Mining Alliance Duty Free Shop above.
Figure 2 dot pitch LED display, located in Taoyuan International Airport, the second terminal rose Hengchang Duty Free Shop above.
In addition to commercial displays, dot-pitch LED displays rely on the advantages of high brightness, high contrast, no stitching and gradually apply force in the control room, indoor theater and other applications, instead of rear projection splicing and LCD splicing.
In July 2017, Samsung Electronics set up the first movie theater in Seoul Lotte Theater with a small dot-pitch LED display for 406-inch resolution and 4K (4,096 × 2,160) resolution. Brightness There are 146fL, high-brightness screen is conducive to display 3D images, the previous wear 3D glasses will feel the image darken, this screen will not appear this problem, and supports HDR display, also in line with DCI playback requirements. Samsung Electronics move Pixel pitch LED display application to another peak (Figure 3).
Figure 3 Samsung Cinema Screen Picture credit: Samsung
As early as June 2016, SONY demonstrated at Infocomm Las Vegas the world's first large-scale display with MicroLED as its light source (Figure 4), measuring 9.7mx2.7m in size with a resolution of 8192x2048, Is 1.26mm, SONY called CLEDIS (Crystal LED Integrated Structure). In fact, CLEDIS is not SONY for the first time to display the display with MicroLED as a light source, as early as 2012 CES, SONY had exhibited with MicroLED as a display light source 55 Inch TV, then called the Crystal LED Display (CLED Display) (Figure 5), described as the beginning of MicroLED Display, unfortunately no successor CLED Display, and CLEDIS 220-inch 8K display asking price of up to 1.03 million US dollars, it is desirable Reject, so far no installation performance.
Figure 4 SONY CLEDIS
Figure 5 SONY CLED Display
ABI Research shows that the global output value of billboards for digital signage will soar to USD4.5 billion in 2016 from USD1.3 billion in 2010. In addition, the 2015MarketsandMarkets survey also pointed out that the output value of global signage billboards will reach USD 23.76 billion in 2020 , Its annual compound growth rate will reach 8.18% (2015 ~ 2020). According to LEDinside market data, LED display output value will reach 5 billion US dollars by 2020, including small-pitch LED display output up to nearly 13.5 billion US dollars, the year The compound growth rate is as high as 32.42% (2015 ~ 2020), much higher than the average annual compound growth rate of the global digital signage bill (Table 1). I would like to make use of the technical advantages and bottlenecks of MicroLED below.
Table 1 LEDinside estimated LED display output value
Unpackaged LED display unique advantages
LED is the electrical conversion of light components to the viewer's position, the optical performance of the top priority for everything, why no package LED encapsulation than LED, the following two examples:
Eliminate moiré
However, the current dot pitch LED display solution is not perfect, the current light source is mostly surface mount three-in-one LED, because of LED encapsulants, we often encounter the use of mobile phones or cameras when the LED display, found The Moire Effect appears on the photo as shown in Figure 6. The principle is to superimpose two equal-amplitude sine waves at nearly the same frequency and the amplitude of the composite signal will vary according to the difference between the two frequencies. Pixel spatial frequency and spatial frequency of stripes in the image close to, it is easy to produce Moire.
Figure 6 The camera captures the moire of the LED display
Visual angle
Optical characteristics show that the MicroLED light field is 140 degrees (Figure 7) half the angle of the conventional SMD LED (Figure 8), and the light panel viewing angle specification will exceed the current light board size by 140 degrees. Viewing angle> 140o / 125o, also higher than the traditional SMD structure of about 140o / 60o. Side view, it will not mask or SMD package formed visually block defects, as shown in Figure 9.
Figure 7 MicroLED light field
Figure 8 SMD LED light field
Figure 9 SMD LED display side view
Therefore, an attempt has been made to use COB (Chip On Board) packageless LED to avoid the moiré and viewing angle caused by the above space interference, etc. According to the structure of the chip, the COB can be subdivided into Face-Up Chip and Flip-Flop (Flip-Chip) two forms.In the Figure 10, the formal form of COB, the electrode and the wire are in the light-emitting surface, the disadvantage is: wire (LED) will affect the LED chip light angle, electrodes and wires and other metal components Reflecting will give viewers a strong sense of specular reflection, lowering the display contrast.
The author thinks the flip-chip (Flip-Chip) of FIG. 11 is a better solution to the COB encapsulation-free LED. In terms of cost, there is less need to pay for wiring and materials; in optical theory, there is no light out of the wire Interference, a smooth surface upward. Bonding Pad down, the contrast of the display is also relatively improved. If the size of the crystal to distinguish more than 100μm × 100μm, and with a substrate, called Mini-LED, and less than this size and no The substrate is called MicroLED.
Active / passive drive pros and cons complement each other
The LED driver architecture can be divided into Passive Matrix (Figure 12) and Active Matrix (Figure 13), the advantages and disadvantages of the two driver architectures are exactly complementary. Passive Drive with Sequential Scan Drive , The scanning line is opened by columns in a set way. When the current passes, the scanning line will be lighted. The structure of the scanning line is simpler than that of the active driving and does not need to be used in the TFT process. The disadvantage is that due to the common formation of the electrodes Cross-effect, the two light-emitting pixels may have crosstalk between the phenomenon, that is, one pixel light, the other pixel may issue weak light.
Figure 12 MicroLED Passive Drive Architecture
Figure 13 MicroLED Active Drive Architecture
Each pixel of active driving is equipped with a low temperature poly-Si thin film transistor (LTP-Si TFT) with a switching function, and each pixel is equipped with a charge storage capacitor, a peripheral driving circuit, and a display array entire system Integrated on the same glass substrate.Active drive is a static drive, with a storage effect, can be 100% load drive, this drive is not limited by the number of scanning electrodes, each pixel can be independently adjusted .Active drive No duty cycle problem, the driver is not limited by the number of scanning electrodes, easy to achieve high brightness and high resolution. The disadvantage is that in addition to the high cost of LTPSTFT made in a large area glass substrate, different positions of the TFT is often at a threshold Voltage, mobility and other electrical parameters with non-uniformity, this non-uniformity will be transformed into LED display device current differences and brightness differences, and is perceived by the human eye, that Mura phenomenon Oxide TFT Although the uniformity of the process is more Well, under long-term pressurization and high temperature, the threshold voltage will drift, due to the different display screen, the various parts of the panel TFT Different threshold amount of drift, the difference will cause the display brightness, since the image relating to the previous display this difference, thus often presented as afterimage phenomenon, also known as afterimage. Therefore, the active drive TFT substrate needs to be compensated.
To LED large-scale splicing display, using PCB as the substrate, so the use of passive drive architecture, when the pixel pitch becomes dense, the number of lines to improve, then the phenomenon of crosstalk between pixels will become more serious, the common crosstalk phenomenon is organized as follows. Ghosting, ghosting most often appear in the text display, adjacent to the LED light did not originally lit, due to crosstalk caused by false light, as shown in Figure 14, this problem needs to be corrected by the driver IC into normal in Figure 15 display.
Figure 14 shows a ghosting text
Figure 15 normal display
Secondly, the first line of darkness occurs periodically, and you can notice the darkness of the first line above and in the middle of Figure 16. The first dark line is the charge stored by the parasitic capacitance of the LED light point Driven chip channel discharge caused by shunt phenomenon caused by the first line of light after the discharge of parasitic capacitance charge has been discharged, so dark phenomenon usually occurs only in the first line scan (normal display as shown in Figure 17 ).
Figure 16 Periodic first line is dim
Figure 17 shows normally
Third, the LED brightness is basically proportional to the size of the current flowing through, especially the high-efficiency LED such as MicroLED, slight differences in forward current, the human eye can feel the difference in brightness.In the passive drive line scan architecture , If the current consistency between the driver ICs is not good, the contrast of the block lightness in the unit of IC can be easily generated in comparison with the normal display of Fig. 18, as shown in Fig. 19. The second generation of Precision Drive Of driver ICs can control the current error between driver ICs within ± 1.5%, while the average drive current of MicroLEDs will be as low as 15μA or less, and the current error control target should be compressed to within ± 1.0%. The design of the driver ICs will be one Big challenge
Figure 18 shows normally
Figure 19 current uniformity is poor, resulting in uneven blocks
The fourth point is caused by the LED dead pixels bright lines, especially in the high line scan design, the cross phenomenon in addition to extending the horizontal direction of the light spot error, the vertical direction of the light spot will also increase the number of errors will be higher and extend , Once the LED dead spots appear, the cross phenomenon will be quite obvious, resulting in display flaws.Figure 20 The use of test patterns for the white monochrome oblique sweep, LED dead spots Fault situation is the blue LED fault, so the dead pixels will be yellow, At the same time there is a crossover phenomenon with a dead center as the center. Figure 21 shows the test shot after the driver IC is built-in to eliminate the cross function, and the cross phenomenon can be observed completely eliminated.
Figure 20 LED dead-end isolation caused by cross bright line
Figure 21 enables dead pixel isolation
MicroLED is still the final solution
LED has always given people the impression of saving energy and efficient, at least in the white LED to replace the traditional lighting is indeed true, but the LED display power consumption is not lower than the LCD or even higher because the LED display that each pixel Point LED is used to display rather than lighting, R / G / B light intensity of the three primary colors must be matched with each other, compromise, so it is not operating in the most efficient current, if the R / G / B RGB LEDs are operated at The most efficient current, the color emitted is very strange to the human eye.Micro-LED is regarded as a MicroLED's Sentinel before the MicroLED breakthrough technology and yield bottlenecks, the current Mini-LED chip size lags 5 × 9mil to 4x7mil, taking a 5x9mil chip size as an example. When the LED operating current is 500μA, the current density is 1.65A / cm2, and the best Internal Quantum Efficiency in LED is about 3 ~ 10A / cm2, If we can reduce the chip size to 80 × 80μm, the current density will be increased to 7.8A / cm2, taking the blue LED of Figure 22 as an example, the efficiency can be increased by 9.4%.
Figure 22 Blue LED current density and efficiency of the relationship
Another attractive feature of the MicroLED is its cost savings, which can be increased to 99.999% with the MicroLED transfer yield over time. In material cost terms, the MicroLED with 80 × 80μm grain size saves more than a 5 × 9mil Mini-LED 80% of the material costs, if enlarged to the entire system compared with the SMD LED, as shown in Figure 23, with P1.25 LED display as an example, the cost of the pixel is reduced by 82% and the cost of the entire screen is reduced by 32%.
Figure 23 P1.25 LED display cost comparison
Since the acquisition of Luxvue by Apple in 2014, MicroLED has attracted the attention of many people in the supply chain, from the upstream epitaxy, to the huge amount of transfer, driver IC and downstream system integration, many manufacturers competing to enter this market.MicroLED compared to the conventional The advantages of various display technologies are clear, but the attendant challenges are huge.Engraded crystal, huge amount of transfer, driver IC, etc. Each step has a difficult problem to be overcome, I believe, in various manufacturers efforts And by 2018, we'll see MicroLED commercialized products, whether they be large monitors, headsets or micro-projection.