What is COB? Its full name is chip-on-board, which is a chip-on-board package. It is a new type of package that is different from SMD surface-mount package technology. Specifically, the LED bare chip is adhered to the PCB with conductive or non-conductive glue, and then Wire bonding enables electrical connection and encapsulation of the chip and bond wires with glue.
This kind of packaging is not a package, but it integrates upstream and downstream enterprises. The production from LED chip package to LED display unit module or display screen is completed in one factory, which integrates and simplifies the production of packaging companies and display manufacturers. Process, production process is easier to organize and control, product point spacing can be smaller, reliability is multiplied, and cost is closer to civilians.
COB packaging was first used in Lighting, and this application has also become a trend. It is understood that COB-packaged bulbs already occupy about 40% of LED bulbs.
With the gradual maturity of the LED application market, users are increasingly demanding product stability and reliability, especially under the same conditions, requiring products to achieve better energy efficiency indicators, lower power consumption, and more competition. Product price. Based on this, compared with the traditional LEDSMD chip package and high power package, the on-chip (COB) integrated package technology directly encapsulates multiple LED chips on a metal-based printed circuit board as a lighting module directly through the substrate. Heat dissipation not only reduces the manufacturing process and cost of the bracket, but also has the advantage of reducing the thermal resistance of the thermal resistance, thus becoming a kind of packaging method promoted by the lighting enterprise.
In addition to good heat dissipation and low cost, COB light source can be personalized. However, in terms of technology, COB package still has shortcomings such as light decay, short life, and poor reliability. If it can be solved, it will be one of the leading directions for future package development.
The application of COB in lighting has become a trend and trend. Can this packaging technology be applied to the display? In terms of packaging methods, some companies have made new attempts, and such attempts have been verified and promoted and applied in the market. At the same time, it has also attracted widespread attention from people in the industry. So, why is the COB display getting everyone's attention? There must be a reason in it.
I. Analysis of advantages and disadvantages of COB packaging
The application of COB package has been applied in the field of lighting for many years. It has many advantages in various aspects, so it has been favored by many lighting companies. So what kind of spark will COB packaging technology be applied to the display screen? Will there be some levels of acclimatization? Let's analyze the advantages and disadvantages of COB packaging. It is understood that COB packaging technology is applied to the display screen, which has the incomparable advantages of traditional packaging technology.
1. Ultra-thin: According to the actual needs of customers, PCB board with thickness from 0.4-1.2mm can be used to reduce the weight to 1/3 of the original traditional products, which can significantly reduce the structural, transportation and engineering costs for customers.
2. Anti-collision and compression: The COB product directly encapsulates the LED chip in the concave lamp position of the PCB board, and then is cured by epoxy resin. The surface of the lamp point is convex into a spherical surface, which is smooth and hard, and is resistant to collision and wear.
3. Large viewing angle: The COB package uses shallow well spherical illumination, the viewing angle is greater than 175 degrees, close to 180 degrees, and has a better optical diffuse color dimming effect.
4. Bendable: Bendability is a unique feature of COB packages. PCB bending does not cause damage to packaged LED chips. Therefore, LED arc screens, circular screens, and wavy screens can be easily fabricated using COB modules. . It is an ideal substrate for personalized screens in bars and nightclubs. It can be seamlessly spliced, the production structure is simple, and the price is far lower than the LED shaped screen made by the flexible circuit board and the traditional display module.
5. Strong heat dissipation: COB products are packaged on the PCB board, and the heat of the wick is quickly transmitted through the copper foil on the PCB board. The thickness of the copper foil of the PCB board has strict technical requirements, plus the immersion gold process. Hardly causing severe light attenuation. Therefore, there are very few dead lights, which greatly extend the life of the LED display .
6, wear-resistant, easy to clean: the surface of the lamp point is convex into a spherical surface, smooth and hard, collision-resistant and wear-resistant; there are dead spots, you can repair point by point; no mask, dust can be cleaned with water or cloth.
7, all-weather excellent characteristics: the use of triple protection treatment, waterproof, moisture, corrosion, dust, static electricity, oxidation, UV effect is outstanding; to meet all-weather working conditions, the temperature difference of minus 30 degrees to zero 80 degrees can still be used normally.
To put it bluntly, the advantages of the COB display package are really quite a lot, especially when compared with the traditional package form, the contrast effect is more obvious. Since there are many advantages, why not get a large-scale application in the early development of LED display? What are the inadequacies of COB packaging? Hu Zhijun, deputy general manager of Shenzhen Weiqiaoshun Optoelectronics Co., Ltd. said: "The only shortcoming of COB packaging is that the screen ink color is not well controlled, that is, when the lamp is not lit, the surface ink color is inconsistent." Shenzhen Aolida Technology Co., Ltd. Yang Rui, the company's marketing director, said frankly: "The scarcity of the COB display package is that the surface consistency is not enough. If this problem is not solved, it will be difficult to get the customer's approval."
Second, the interpretation of COB packaging process
The COB packaging technology is categorized as a package-free or provincial-saving package. However, this package does not eliminate the packaging process, but saves the packaging process. Compared with the SMT process, the COB packaging process is saved. Going to a few steps saves time and process to a certain extent, and also saves costs to a certain extent. SMD's production process needs to undergo solid crystal, wire bonding, dispensing, baking, stamping, splitting, color separation, tape, and patch. The COB process is simplified on this basis. First, the IC is attached to the circuit board. Then solid crystal, wire bonding, testing, dispensing, baking, and become a finished product.
In terms of the production process alone, several steps have been omitted, and industry insiders say that this can save a large part of the cost. It is worth noting that the COB package does not require reflow soldering, which is one of the advantages of COB.
Array's marketing director Yang Rui said that the conventional package is to place the lamp bead on the PCB for soldering. When the lamp is getting denser and denser, the lamp pin will become smaller and smaller, so the precision of soldering will be higher. How many lamps in a square, one lamp has four feet, then there will be many solder joints in one square. At this time, the requirements for solder joints are very high, then the only solution is to reduce the solder joints. Very small solder stability is very poor, it may be accidentally touched, it may fall off, this is a problem that SMD can not avoid; COB package eliminates the process of splitting, color separation, drying, etc. The most critical difference is to remove the solder. Process, SMD in the process of soldering, the temperature control is extremely difficult to master, the temperature is too high, will cause damage to the lamp, too low, the solder does not completely melt. It is easy to cause the phenomenon of virtual welding, false welding, etc., which is a great challenge to the stability of the lamp bead. And COB does not have this process, then the stability will be greatly improved.
The processing technology of the traditional LED display screen is quite numerous, especially in the process of reflow soldering. Under the high temperature state, the expansion coefficient of the SMD lamp bead holder and the epoxy resin is different, which easily causes the bracket and the epoxy resin encapsulation to fall off. In the gap, the dead light phenomenon gradually appears in the later use, resulting in a high defect rate. The reason why the COB display is more stable is because there is no reflow soldering lamp in the processing technology. Even if there is a post-reflow soldering IC process, the diode chip has been cured and cured with epoxy resin, thus avoiding soldering. There is a problem of gaps between the lamp holder and the epoxy resin caused by high-temperature soldering in the machine.
Third, the challenges facing COB packaging
The emergence of a new product and new technology and new technology will never go smoothly. It is necessary to continuously test in the R&D and production process, and constantly try to find the problem, in order to solve the problem in real time. The emergence of each problem is the process of research and development personnel, in this process, full of hardships, but also accompanied by achievements and satisfaction. The development of all new things is perfected little by little, and it is also close to success step by step. But for now, the development of COB packaging technology can not be called mature, after all, the development of new things still takes time. At this stage, COB packaging technology still faces some challenges, and these challenges are gradually improved in the continuous efforts of enterprises.
It is understood that at present, COB packaging technology currently has three challenges.
1. One pass rate of the packaging process
COB package method Because of its characteristics, the COB package is on a large board. This board has a maximum of 1024 lights. If the SMD seals a lamp, you only need to change one, but the 1024 of the COB package. After the lamp package is completed, the test is performed, and all the lamps are confirmed to have no problem before the seal can be applied. How to ensure that the entire board of 1024 lights is completely intact, one pass rate is a very big challenge.
2, the finished product pass rate
The COB product is first sealed. After the lamp is sealed, the IC driver device is subjected to a reflow process. How to ensure that the lamp surface is over-reflowed, the high temperature of 240 degrees in the furnace does not damage the lamp. This is another big challenge. Compared with SMD, COB saves the surface reflow soldering process, but the device surface needs to be reflow soldered like SMD, which means that SMD has to be reflowed twice. When SMD is reflowed, the temperature in the furnace will cause two kinds of damage to the lamp surface. One is the wire. If the temperature is too high, it will rapidly expand rapidly, which will cause the filament to break. The second one is The heat in the furnace is quickly transmitted to the wick through the four pins of the bracket. The wick may cause small fragmentation damage. This damage is fatal, and the detection is often difficult to detect, including aging tests are difficult to detect, but This tiny damage of the crystal is finely cracked after a period of time
Use, this drawback will be highlighted, which will lead to the failure of the lamp. The COB is to ensure that when the lamp surface is over-reflowed, the high temperature in the furnace does not cause damage to it, and the yield is guaranteed. This is also a very important aspect.
3, the whole lamp repair
For the maintenance of COB lamps, professional maintenance and maintenance are required. One of the biggest problems with single-lamp maintenance is that after repairing, a circle will appear around the lamp, and a lamp will be repaired. The surrounding circle will be smoked by the welding torch, and the maintenance difficulty is relatively high.
If there are challenges, it is necessary to find a corresponding solution. At present, the problems encountered in the packaging and maintenance process of the COB package are solved by the enterprise. For example, when the lamp surface is over-reflowed, a certain solution is adopted. The way to protect the lamp surface to reduce damage; the point-by-point correction technology is used in the maintenance process to ensure the consistency between the lamp beads.
Fourth, the development trend of COB packaging
One advantage of the COB package is that it is packaged directly on the PCB, which is not limited by the lamp bead. Therefore, the point spacing for COB is not scientific. In theory, it is very easy for the COB package to achieve high density. . In the words of the industry, the COB package is tailored for small spaces.
Already Marketing Director Yang Rui said that "COB does not follow the conventional screen route, so the products made will lose meaning. The COB is mainly used in small-pitch display. At present, the small-pitch display has more applications in the security field, so One of the main areas of application for COB displays is security."
Hu Zhijun, deputy general manager of Wei Qiaoshun Optoelectronics Co., Ltd. also expressed the same view, "SMD needs to solve the problem of soldering feet. One lamp has four solder fillets, so the density of the display is higher, used in unit square meters. The lamp bead will be more, the solder fillet will be more and more dense, this problem is not solved, miniaturization is a very big challenge for the surface mount, COB skips this part of the bracket, millions of solder joints The puzzles are all left behind, so miniaturization is easier."
"One of the characteristics of COB packaging is that it can solve the problem of outdoor protection very well. Wei Qiaoshun adopted a strategy of 'rural encircling the city', first developing outdoor small spacing, for COB, even to P3, P2.5, P1.8, it is easy to achieve, so Wei Qiaoshun intends to seize the time, seize the opportunity, first break through the outdoor small spacing, take advantage of high reliability and return to the small penetration of the indoor area." Hu Zhijun describes the future development blueprint.
“We believe that COB has a very good development prospect, because the reliability of COB products is much higher than that of surface-mount products, which is the first point; the second point is that the lower the density of COB products, the lower the cost, the more Close to civilians, these are two very important features that are sufficient to support the COB towards a better future.
"For the COB, it is not limited by the lamp bead. It can be easily made below 1.0, but the product will lose its meaning and value without the market. The COB display is the hope of the future, but It takes a certain amount of time for this road to go smoothly. Because more efforts are needed to solve the consistency problem. COB is a very good development trend because the price of the two is similar, but The cost of COB is about 15% lower. One is a process problem, it is necessary to save several process flows, and the other is to make batchization easier."
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