What is COB? Its full name is chip-on-board, that is, chip-on-board packaging, which is a new packaging method different from SMD surface mount packaging technology. Bonding makes its electrical connection and encapsulates the chip and bond wires with glue.
This packaging method does not require packaging, but integrates upstream and downstream enterprises. From packaging to the production of LED display unit modules or display screens, it is completed in one factory, which integrates and simplifies the production process of packaging companies and display manufacturers. The production process is easier to organize and control, the point spacing of the product can be smaller, the reliability is multiplied, and the cost is closer to civilianization.
It was first used in lighting, and this application has also become a trend. It is understood that COB-packaged bulbs have occupied about 40% of the LED bulb market.
With the gradual maturity of the LED application market, users have higher and higher requirements for product stability and reliability, especially under the same conditions, requiring products to achieve better energy efficiency indicators, lower power consumption, and more competitive Powerful product prices. Based on this, compared with traditional LEDSMD chip packaging and high-power packaging, the chip-on-board (COB) integrated packaging technology directly encapsulates multiple LED chips on a metal-based printed circuit board, as a lighting module directly through the substrate. Heat dissipation can not only reduce the manufacturing process and cost of the bracket, but also has the heat dissipation advantage of reducing thermal resistance, so it has become a packaging method mainly promoted by lighting companies.
In addition to good heat dissipation performance and low cost, COB light sources can also be individually designed. However, technically, COB packaging 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 of packaging development in the future.
The application of COB in lighting has become a trend and trend. So, can this packaging technology be applied to display screens? In terms of packaging methods, some companies have made new attempts, and this attempt has also been verified and has been promoted and applied in the market. At the same time, it has also attracted widespread attention from people in the industry. So, why does the COB display get everyone's attention? There must be a reason for this.
一. Analysis of the advantages and disadvantages of COB packaging
The application of COB packaging has been used in the lighting field for many years. It has many advantages in various aspects, so it has been favored by many lighting companies. So what kind of sparks will the 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 together. It is understood that the application of COB packaging technology on the display screen has incomparable advantages of traditional packaging technology.
1. Ultra-light and thin: According to the actual needs of customers, PCB boards with a thickness of 0.4-1.2mm can be used to reduce the weight to at least 1/3 of the original traditional products, which can significantly reduce the cost of structure, transportation and engineering for customers.
2. Anti-collision and anti-compression: COB products directly encapsulate the LED chip in the concave lamp position of the PCB board, and then use epoxy resin glue to encapsulate and cure.
3. Large viewing angle: The COB package uses shallow well spherical luminescence, the viewing angle is greater than 175 degrees, close to 180 degrees, and has a better optical diffuse color and muddy light effect.
4. Bendable: Bendability is a unique feature of COB packaging. The bending of the PCB will not cause damage to the packaged LED chips. Therefore, the use of COB modules can easily make LED curved screens, circular screens, and wave-shaped screens. . It is an ideal substrate for personalized modeling screens in bars and nightclubs. It can be seamlessly spliced, the production structure is simple, and the price is far lower than the LED special-shaped screen produced by flexible circuit boards and traditional display modules.
5. Strong heat dissipation: COB products encapsulate the lamp on the PCB board, and quickly dissipate the heat of the lamp wick through the copper foil on the PCB board, and the copper foil thickness of the PCB board has strict process requirements, plus the immersion gold process, Severe light attenuation is hardly caused. So few dead lights, greatly extending the lifespan.
6. Wear-resistant and easy to clean: The surface of the lamp point is convex into a spherical surface, which is smooth and hard, and is resistant to impact and wear; if there is a dead point, it can be repaired point by point; if there is no mask, it can be cleaned with water or cloth.
7. Excellent all-weather features: It adopts triple protection treatment, and the effects of waterproof, damp, rot, dust, static electricity, oxidation and ultraviolet are outstanding; it meets all-weather working conditions, and can still be used normally in a temperature difference environment of minus 30 degrees to 80 degrees above zero.
Speaking of which, the advantages of COB display packaging are really many, especially when compared with the traditional packaging form, the contrast effect is even more obvious. Since there are many advantages, why is it not used on a large scale in the early development of LED display? Where are the shortcomings of COB packaging? "The only disadvantage of COB packaging is that the ink color of the screen is not easy to control, that is, when the light is not lit, the surface ink color is inconsistent. "The flaw of COB display packaging is that the surface consistency is not enough. It is difficult to gain customer approval. "
二. Interpretation of COB packaging process
The packaging technology of COB is classified as packaging-free or packaging-saving mode, but this packaging method does not save the packaging process, but saves the packaging process. Compared with the SMD process, the packaging process of COB needs to be saved. Going a few steps saves time and craftsmanship to a certain extent, and also saves costs to a certain extent. The production process of SMD needs to go through the steps of die bonding, wire bonding, glue dispensing, baking, stamping, color separation, taping, patching, etc. The COB process is simplified on this basis. First, the IC is pasted on the circuit board. Then solid crystal, wire bonding, testing, dispensing, baking, become the finished product.
As far as the production process is concerned, several steps are omitted. Industry insiders said that in this way, a large part of the cost can be saved. It is worth noting that the package of COB does not need to be reflowed, which has also become one of the advantages of COB.
The conventional packaging is to place the lamp beads on the PCB board for welding. When the lamps become denser, the lamp pins will become smaller and smaller, so the precision requirements for welding will be higher. How many lamps are there in a square, and a lamp has four feet, then a square will have many solder joints. At this time, the requirements for solder joints are very high, so the only solution is to reduce the solder joints. Small solder has poor stability. It may fall off if you touch it casually. This is an unavoidable problem for SMD; COB packaging eliminates the process of color separation, drying and other processes. The most critical difference is to remove the solder. Process, in the process of SMD soldering, it is extremely difficult to control the temperature. If the temperature is too high, it will cause damage to the lamp. If it is too low, the solder will not be completely melted. It is easy to cause virtual welding, false welding and other phenomena, which is a big challenge for the stability of lamp beads. And COB does not have this process, then the stability will be greatly improved.
There are many processing techniques for traditional LED displays, especially in the process of reflow soldering, the expansion coefficients of SMD lamp bead bracket and epoxy resin are different under high temperature, which can easily cause the bracket and epoxy resin encapsulation shell to fall off. gaps, and the phenomenon of dead lights gradually appears in the later use, resulting in a high defect rate. The reason why the COB display is more stable is that there is no reflow solder paste lamp in the processing technology. Even if there is a later reflow solder paste IC process, the diode chip has been cured and protected with epoxy resin glue, which avoids soldering. The problem of gaps between the lamp bead bracket and the epoxy resin caused by high temperature soldering in the machine.
三. Challenges faced by COB packaging
The emergence of a new product, new technology and new process will never be smooth sailing. It must be continuously tested and tried in the R&D and production process to discover the problem and solve the problem in real time. The emergence of every problem is the process of research and development personnel tackling key problems. In this process, it is full of hardships, but it is also accompanied by achievements and satisfaction. The development of all new things is improving little by little, and is approaching success step by step. But for now, the development of COB packaging technology cannot be called mature. After all, it will take time for new things to mature. At this stage, the packaging technology of COB still faces some challenges, and these challenges are gradually improved in the continuous efforts of enterprises.
It is understood that, at present, there are still three challenges in the packaging technology of COB.
1. The first pass rate of the packaging process
Due to its characteristics of COB packaging, COB packaging is required to be on a large board. This board has a maximum of 1024 lamps. If SMD seals a lamp, you only need to replace one. After the individual lamps are packaged, they must be tested, and all the lamps can be sealed after confirming that there is no problem. How to ensure that the 1024 lamps on the whole board are completely intact, the one-time pass rate is a very big challenge.
2. One-time pass rate of finished products
The COB product is to seal the lamp first. After the lamp is sealed, the IC drive device needs to undergo reflow soldering process. How to ensure that the lamp surface is subjected to reflow soldering treatment, and the high temperature of 240 degrees in the furnace will not cause damage to the lamp. This is another big challenge. Compared with SMD, COB saves the process of reflow soldering on the lamp surface, but the device surface needs to be reflow soldered just like SMD, that is to say, SMD needs to undergo reflow soldering twice. The thing is, when the SMD is reflowed, the temperature in the furnace will cause two kinds of damage to the lamp surface. One is the welding wire. If the temperature is too high, it will expand rapidly and rapidly, which will cause the filament to break. The second is The heat in the furnace is quickly transferred to the wick through the four pins of the bracket, and the wick may cause small fragmentation damage. This damage is fatal, and it is often difficult to detect, including aging tests. It is difficult to detect, but This tiny damage to the crystal is a tiny crack, after a period of
Use, this drawback will be highlighted, and then lead to the failure of the lamp. The COB is to ensure that when the lamp surface is reflowed, the high temperature in the furnace will not cause damage to it, and to ensure the yield, which is also a very important level.
3. Maintenance of the whole lamp
For the maintenance of COB lamps, professional repair and maintenance are required. One of the biggest problems with the maintenance of a single lamp is that after the repair, a circle will appear around the lamp. If a lamp is 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 corresponding solutions. At present, companies have come up with corresponding solutions for the problems encountered in the packaging and maintenance process of COB packaging. For example, when the lamp surface is reflowed, a certain In this way, the lamp surface is protected to reduce damage; point-by-point correction technology is used in the maintenance process to ensure the consistency between lamp beads.
Fourth, the development trend of COB packaging
One advantage of COB packaging is that it is directly packaged on the PCB board, which is not limited by lamp beads. Therefore, for COB, the point spacing is not scientific. In theory, it is very easy for COB packaging to achieve high density. . To borrow a word from industry insiders, COB packaging is tailored for small pitches.
"COB does not follow the conventional screen route, so the products made will lose their meaning. COB is mainly used in small-pitch displays. At present, small-pitch displays are widely used in the field of security, so one of the main applications of COB displays The realm is security.”
"SMD needs to solve the problem of soldering feet, one lamp bead has four soldering feet, then as the density of the display screen is higher, more lamp beads will be used per square meter, and the soldering feet will be denser and denser. If this problem is not solved, miniaturization is a very big challenge for surface mounts. COB skips this part of the bracket, and the problem of millions of solder joints is completely forgotten, so miniaturization is easier. "
"One of the characteristics of COB packaging is that it can well solve the problem of outdoor protection. First, develop small outdoor spacing. For COB, even if it reaches P3, P2.5, and P1.8, it is easy to achieve. Seize the opportunity and make breakthroughs first. Outdoor small spacing, take advantage of high reliability to detour into the field of indoor small spacing.
"We think COB has a very good development prospect, because the reliability of COB products is much higher than that of surface mount products, this is the first point; the second point, the lower the cost of COB products as the dot density is smaller, the more Close to civilianization, these are two very important characteristics. They are enough to support COB towards a better future.
"For COB, it is not limited by lamp beads. It can be easily made below 1.0, but the product will lose its meaning and value if there is no market. COB display is the hope of the future, but It will take some 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 both is similar, but The cost of COB is about 15% lower, one is a process problem, several process processes need to be omitted, and the other is that it is easier to achieve batch production.”
