Compared with other display technologies, LED electronic screens have the advantages of self-illumination, excellent color reproduction, high refresh rate, power saving, and easy maintenance. The two characteristics of high brightness and super-large size through splicing are the decisive factors for the rapid growth of LED electronic screens in the past two decades. In the field of large-screen outdoor display, so far no other technology can compete with LED electronic screen technology. What challenges will small-pitch LED electronic screens bring to the chip side?

However, in the past, LED electronic screens also had their shortcomings. For example, the distance between the packaged lamp beads was large, resulting in low resolution, which was not suitable for indoor and close-up viewing. In order to improve the resolution, it is necessary to reduce the spacing between the lamp beads, but the size of the lamp beads is reduced, although the resolution of the whole screen can be improved, the cost will rise rapidly, and the high cost will affect the large-scale commercial application of small-pitch LED displays .

In recent years, thanks to the efforts of chip manufacturing and packaging manufacturers, IC circuit manufacturers and screen manufacturers, the cost of single-package devices has become lower and lower, LED package devices have become smaller and smaller, and the pixel pitch of display screens has become smaller and smaller. The resolution is getting higher and higher, making the advantages of small-pitch LED electronic screens in indoor large-screen display more and more obvious.

At present, small-pitch LEDs are mainly used in advertising media, stadiums, stage backgrounds, municipal engineering and other fields, and continue to develop markets in transportation, broadcasting, military and other fields. It is estimated that by 2018, the market size will be close to 10 billion. It can be predicted that in the next few years, small-pitch LED electronic screens will continue to expand market share and squeeze the market space of DLP rear projection. According to the forecast of Everbright Securities Research Institute, by 2020, the replacement rate of small-pitch LED electronic screens for DLP rear projection will reach 70% to 80%.

The author is engaged in the blue-green LED chip manufacturing industry and has been engaged in product development for many years. From the perspective of product design and process technology, the requirements for blue-green LED chips from the development of small-pitch LED electronic screens are discussed below, as well as possible countermeasures on the chip side.

2. The demand for LED chips from small-pitch LED displays

As the core of the LED electronic screen, the LED chip plays a crucial role in the development of small-pitch LEDs. The current achievements and future development of small-pitch LED electronic screens depend on the unremitting efforts of the chip side.

On the one hand, the dot pitch of indoor LED electronic screens has gradually decreased from P4 in the early stage to P1.5, P1.0, and P0.8 in development. Correspondingly, the size of lamp beads has been reduced from 3535 and 2121 to 1010. Some manufacturers have developed 0808 and 0606 sizes, and some manufacturers are even developing 0404 sizes.

As we all know, the reduction of the size of the packaged lamp beads will inevitably require the reduction of the chip size. At present, the surface area of blue-green chips commonly used in small-pitch displays in the market is about 30mil2, and some chip factories are already mass-producing 25mil2 or even 20mil2 chips.

On the other hand, the reduction of the surface area of the chip, the decrease of the brightness of a single core, and a series of problems affecting the display quality have also become prominent.

The first is the requirement for grayscale. Unlike outdoor screens, the difficulty of indoor screens is not brightness but grayscale. At present, the brightness requirement of indoor large-pitch screens is about 1500 cd/m2 -2000 cd/m2, and the brightness of small-pitch LED displays is generally around 600 cd/m2 -800 cd/m2, and the display screen suitable for long-term attention is the most The best brightness is around 100 cd/m2 -300cd/m2.

One of the current problems of small-pitch LED electronic screens is "low brightness and low gray". That is, the grayscale is not enough at low brightness. To achieve "low brightness and high gray", the current solution used on the package side is a black bracket. Since the reflection of the black bracket on the chip is weak, the chip is required to have sufficient brightness.

The second is the problem of display uniformity. Compared with conventional screens, the smaller the pitch, there will be problems such as afterglow, darker first sweep, low brightness and reddishness, and uneven low gray. At present, in response to the afterglow, the first scan is dark and the low gray is red, the packaging end and the IC control end have made efforts to effectively alleviate these problems, and the brightness uniformity problem under low grayscale is also corrected point by point. Technology has eased. However, as one of the root causes of the problem, the chip side requires more effort. Specifically, the brightness uniformity under small current is better, and the consistency of parasitic capacitance is better.

The third is reliability. The current industry standard is that the allowable value of the LED dead light rate is 1/10,000, which is obviously not suitable for small-pitch LED electronic screens. Due to the high pixel density of the small-pitch screen and the short viewing distance, if there are 1 dead light in 10,000, the effect is unacceptable. In the future, the dead light rate needs to be controlled at one in 100,000 or even one in a million to meet the needs of long-term use.

In general, the development of small-pitch LEDs requires the chip segment to be reduced in size, improved in relative brightness, good in brightness consistency at low current, good in parasitic capacitance consistency, and high in reliability.