COB technology: a new generation standard for small-pitch LEDs

In 2016, the small-pitch LED industry once again achieved a bumper harvest. Judging from the growth of the entire LED display industry, small-pitch LED TVs have achieved almost 90% growth. While the small-pitch LED display products are growing rapidly, the small-pitch LED display technology is also making continuous progress. This year, COB packaging products have become the new darling, and there is a tendency to become the future king.

What is COB (Chip On Board) small-pitch display technology? That is, the technical method of directly packaging LED light-emitting wafers on PCB circuit boards and combining CELL units into displays. At present, industry giants such as Weichuang and Sony have given strong support to this technology.

Weichuang, a leading domestic high-end large-screen display brand, believes that the development of small-pitch LED displays can be divided into two stages: the first stage is to solve the problem of being usable and usable, and the core technology breakthrough is reflected in the reduction of the pixel pitch to less than 2mm. , P1.5 and P1.2 products are mass-produced; the second stage of the development of small-pitch LED displays is mainly to provide higher product reliability and visual experience, among which COB packaging is one of the most critical technical directions .

In the spring of 2016, Vtron officially launched P1.5 standard COB packaging products. This product won the “2016 China Audio and Video Industry Technology Innovation Award” from AVF, China Electronic Video Industry Association, and has been strongly recognized by customers in the industry. Industry experts believe that with the initial achievements of COB packaged small-pitch LED TV products in 2016, this type of products may gradually enter the “supply-side explosive growth period” in 2017.

Simplify the complexity, why is the small-pitch LED in COB package so magical

For the application of small-pitch LED screens, dead lights are the biggest problem. Taking P1.6 products as an example, there are more than 390,000 lamp beads and nearly 1.6 million leads per square meter. These components constitute a very complex process system problem: that is, such a complex system must use a process called “reflow soldering” to realize the connection. The reflow soldering process itself means “artificial high temperature” (240 degrees, far exceeding the normal operating temperature of the led display).

During high-temperature operation, due to the different materials in the SMD package of LED lamp beads, such as copper brackets, epoxy resin materials, and crystals with different thermal expansion coefficients, the lamp beads themselves will inevitably undergo thermal stress changes. This has become the core “culprit” of broken lights and dead lights in small-pitch LED screens.

With COB packaging technology, in the packaging process after chip splitting, the LED crystal becomes the smallest CELL display unit at one time, and there is no need for secondary “surface mount” welding in the later stage. This engineering process, by reducing a high-precision and high-temperature environment operation, maximizes the stability of the electrical and semiconductor structures of the LED crystal, and can reduce the failure rate of the display screen by more than an order of magnitude.

In other words, the characteristic of COB packaging is that after LED packaging, the traditional “surface mount” process is no longer required. Owing to the omission of a high-temperature and high-precision process, the reliability of the entire engineering system is enhanced. This is the core reason why COB packaging is favored by the small-pitch LED industry.

Overall packaging, COB technology benefits a lot

From the perspective of broken lights and stability of small-pitch LED screens, in addition to the “high temperature” damage process of reflow soldering, there are the following aspects that need to be highly valued:

First, the collision process of the display unit. The lamp beads of SMD surface mount products are not seamlessly connected with the PCB board, which makes it easy to cause stress concentration on a single lamp bead during the collision process. However, the transportation and installation of the large-screen system will inevitably have vibrations and collisions. This has resulted in an “engineering” increase in the failure rate of small-pitch LED displays. COB packaging technology, through the highly integrated bonding molding of epoxy resin, chip, and PCB board, can effectively protect the stability of the chip and chip electrical connection parts.

Second, the temperature uniformity in the working process of the system. The smaller the spacing of the SMD packaging products, the more it is necessary to use high-power small-particle LED crystals. At the same time, the gap between the lamp bead and the display panel will hinder the thermal conductivity of the wafer during operation. Due to the adoption of a more integrated overall process for COB packaging, chips with lower power area density and larger crystal particles can be selected in crystal selection, thereby reducing the working intensity of the core luminous point. At the same time, the COB package realizes the all-solid seamless heat dissipation under the full epoxy resin package, which reduces the heat concentration of the LED crystal in the working state, which is beneficial to prolong the product life and improve the system stability.

Third, the overall packaging process, COB achieves “sealed and five defenses”. That is to say, COB can do a good job of “waterproof, moisture-proof, dust-proof, anti-static, and anti-oxidation” for crystals and crystal electrical connection parts. Compared with SMD packaging, in the process, especially after vibration and collision, the long-term chemical and electrical stability damage of electrical connections often occurs-this is one of the main culprits of continuous failure of lamps in long-term applications.

Therefore, on the whole, COB packaging is a process that is more capable of providing “high reliability” than SMD products.

Starting from the visual effect, the change of COB is not a little

What are the benefits of LED display systems? Bright and colorful. However, in a system based on a single lamp bead, pixel granulation, viewing angle, and picture softness are also defects in its display effect.

The traditional LED display system is mostly used for outdoor display and long-distance viewing display, and its requirements for viewing angle and visual comfort are not obvious. However, with the popularization of small-pitch LED applications and the emergence of a large number of indoor display systems and close-up viewing systems, how to improve the viewing comfort of LED displays has become a major industry problem.

Compared with SMD packaging, small-pitch LED products under COB technology are naturally “non-granular display”, and the uniformity of image quality, color curve, and viewing angle effects have been improved to varying degrees. After adopting the built-in point-by-point tuning technology of the PCB cell board, COB has also made great progress in the uniformity of crystal and unit effects. The combination of these changes makes COB packaging the best technical route to realize the “visual comfort” and “experience effect improvement” of small-pitch led screen solutions TVs.

Vtron’s description of this is that COB “realizes the conversion of LED display units from ‘point’ light source to ‘surface’ light source, with a more uniform picture and no spot, combined with advanced surface coating technology, the image display of COB small-pitch LED products It is softer, effectively reduces light intensity radiation, eliminates moiré and glare, and reduces damage to the viewer’s retina, which is conducive to close-up and long-term viewing.”

Dingding the new standard of next-generation small-pitch LED TV, COB is on the way

Although COB display technology has become a rising star in the small-pitch LED industry, this does not mean that COB technology is perfect.

In fact, because COB technology is a one-time CELL packaging technology, the surface mount process with reflow soldering as the core is completely eliminated, which itself means that the complexity of the “packaging process” increases. Including the number of crystals in a unit cell, from three red, green and blue crystals of SMD to thousands or even more. for complex.

At the same time, COB packaging technology itself started relatively late, and has long been used in high-end high-brightness light source and customized light source markets. In the field of small-pitch LED screen display, the accumulation of process technology and material technology is not as good as that of SMD technology. As a result, COB is still inferior to traditional SMD technology in terms of extremely small pitch products and overall cost. According to the forecast, the key to COB technology in 2017 is to make more technological breakthroughs and cost breakthroughs in P1.5, P1.2 and P1.0 products.

However, relying on the display performance advantages of COB technology itself, COB products have achieved very good market response. For example, in the field of radio and television studios, due to the anti-moiré characteristics of COB technology itself, camera system personnel are very fond of this new technology. In the command and dispatch center market, COB’s high stability, convenient maintenance and viewing comfort have also been highly recognized by customers. In the leasing industry, COB is more able to prevent collision and vibration damage, which makes the leasing market very optimistic about the promotion of this product. P1.9 COB products have become the new darling of the leasing industry.

In short, technological innovation is always on the way. After the small-pitch LED display solves the problem of usability, how to “use it” has become the focus of industry competition-this will be the biggest opportunity for COB.