What are the advantages and disadvantages of COB and SMD in different LED packaging technologies?

So, where is the advantage of COB packaging technology? How is it different from the traditional SMD package? Will it replace SMD as the mainstream of LED display in the future?

In general, whether a certain packaging technology has vitality is to always see its tail (customer application end) from the head of the industry chain ( LED chip ). Evaluate through a comprehensive analysis. Among them, the final evaluation of a certain packaging technology must be from the client application, not a link in the industry chain. This article will analyze and compare the two packages of COB and SMD to explore the best package form in the field of LED display.

In general, COB package and SMD package are on the same starting line in the selection of LED chips, and then choose different technical routes.

First, the analysis and evaluation of the packaging process

1. Different technology

In the COB package, the LED chip is directly fixed on the pad of the lamp position of the PCB board with conductive adhesive and insulating glue, and then the soldering performance of the LED chip is performed. After the test is completed, the epoxy resin is used for encapsulation.

In the SMD package, the LED chip is fixed on the pad of the lamp holder with conductive adhesive and insulating glue, and then soldered with the same conductivity as the COB package. After the performance test, it is encapsulated with epoxy resin and then split, Cutting and knitting, transporting to the screen factory and other processes.

2. Comparison of advantages and disadvantages

It is undoubted that SMD packaging plants can produce high-quality lamp beads, but the production process is too much and the cost will be relatively high. It will also increase transportation, material storage and quality control costs from the lamp bead package to the screen factory.

SMD believes that COB packaging technology is too complicated, and the first pass rate of the product has no good control of single lamp, and even an insurmountable obstacle. The failure point cannot be repaired and the yield is low.

In fact, COB packaging with current equipment technology and quality control level, 0.5K integration technology can achieve a single pass rate of about 70%, 1K integration technology can reach about 50%, 2K integration technology can make this The indicator reached about 30%. Even if there is a module that passes the pass rate detection, the whole board has 1-5 points, and there are few modules with more than 5 bad points. The test and rework before sealing can make the finished product pass rate. It reaches about 90%-95%. With the advancement of technology and the accumulation of experience, this indicator will continue to improve. At the same time, we also have a point-by-point repair technology for bad points after sealing.

3. Technical analysis and evaluation

Technical analysis evaluation form

Analysis and evaluation instructions:

The difficulty of technology implementation:

SMD package: Obviously, this single lamp bead singular packaging technology has accumulated many years of practical experience. Each family has unique skills, scale, mature technology and relatively easy to implement.

COB package: It is a new packaging technology with multiple lamp beads integration. In the process of practice, many technical experiences in production equipment, production process equipment, testing and testing methods are accumulated and verified in the continuous innovation practice. The technical threshold is high. High difficulty. The biggest difficulty currently faced is how to improve the one-pass rate of products. The COB package is faced with a technological peak, but it is not insurmountable, but it is relatively difficult to implement.

Factory failure rate control level:

Both COB and SMD packages can be controlled very well, and 0 failure rate is guaranteed when delivered to the customer.

Cost Control:

In theory, the cost control of COB in this link should be slightly better, but the current capacity is limited and has not yet formed a scale, so SMD has an advantage for the time being.

Reliability hazards:

The four-corner or hexagonal bracket used in the SMD package brings technical difficulties and reliability hazards to subsequent production steps. For example, the lamp bead surface reflow soldering process needs to solve the problem of a large number of bracket pin soldering yields. If the SMD is to be applied outdoors, it is also necessary to solve the problem of outdoor protection of the bracket pins.

The COB technology is precisely because this bracket is omitted, there will be almost no technical difficulties and reliability problems in the subsequent production links. There are only two technical hills: one is how to ensure that the IC driver chip has no failure point on the bead surface when reflow soldering, and the other is how to solve the problem of module ink color consistency.

Second, the analysis and evaluation of reflow soldering

Lamp bead surface reflow soldering process

Technical difference

The COB package does not have this process because there is no bracket.

The SMD package is purchased by the display manufacturer for the lamp beads of the SMD package factory, and then the chip is processed onto the PCB.

2. Some questions

A. Competition issues

This link is different from the packaging process. The packaging link is in the middle of the industry chain. There are only a handful of companies, and there will be a certain profit margin. The screen factory link is in the downstream of the SMD industry chain. There are many enterprises, the technology threshold is relatively low, the competition is fierce, the profit is thin, and the survival pressure is high. Among the many screen manufacturers, companies that have been listed through financing have a dominant position by virtue of their financial strength, brand advantage and channel advantages in foreign markets. Other SMEs can only be differentiated if they do not have differentiated products. The uneven development level of enterprises will inevitably lead to an increase in the degree of differentiation of product quality. How will the company do in such an environment, and how?

If you want to do a good job in this link and try to reduce the problem of product reliability degradation, the technology used seems simple, but it is not. The following three factors deserve everyone's attention?

The quality of the PCB board design quality, material, manufacturing process, process and storage quality control.

The accuracy of SMT equipment and the quality control level of SMT production process.

Faced with market competition, managers' cost awareness and quality awareness.

We believe that competition to reduce raw material quality and sacrifice product quality will have disastrous consequences for the industry chain.

B. Product cost issues

There is no such process in COB packaging, and the cost at this stage is always zero.

As the dot density of the SMD package increases, the patch technology becomes more difficult and the cost of the product increases. Moreover, the higher the dot density, the more the cost increases, and the nonlinear growth accelerates the relationship.

C. Reliability issues

There is no such process in the COB package, and there is no reliability reduction problem in this link.

SMD package Because there are generally 4 solder fillets in each bracket, there must be reliability reduction in this section. This is determined by the reliability theory. According to the reliability principle, the less control a product has, the higher the reliability. Please see the table below:

Comparison of quality control of COB package and SMD package for module lamp bead reflow process

From the table we can clearly see the fact that as the pitch of the COB package becomes smaller and the dot density increases, the control section of the bead surface is always 0, and the SMD package becomes smaller as the dot pitch becomes smaller. And the increase in dot density, its control per square meter will increase by a factor of four with the corresponding increase in density per square meter.

D. Lamp bead failure caused by reflow oven temperature

SMD lamp bead devices typically require a temperature of about 240 degrees during reflow soldering, and lead-free soldering can even reach furnace temperatures of 260 degrees to 280 degrees. There are two failure possibilities for the bead device when passing through the reflow soldering machine.

A failure is a hidden danger of cracking and shattering of LED chips under high temperature: LED chips, like humans, also have normal and abnormal individual differences. When weak and disabled individuals pass the high temperature, they will not stand the test. Kind of problem. If it fails immediately after the furnace is over, the problem is not big enough. It can be replaced. The fatal thing is that this kind of hidden danger can still be lit after the furnace is over, and it can also pass the aging test, after the transportation vibration, or after the user has used it for a period of time.

Another type of failure is the breakage of the LED chip solder wire.

E. Application process friendliness

The COB package lamp bead is sealed on the PCB by epoxy resin. The epoxy resin and PCB board have strong affinity and have the following physical properties:

Compressive strength: 8.4kg/mm2

Shear strength: 4.2kg/mm2

Impact strength: 6.8kg*cm/cm2

Hardness: Shore D 84

Therefore, it is not afraid of static electricity, not afraid of bumping, not afraid of impact, bendable deformation, wear resistance and easy cleaning. Therefore, the contact with people is friendly, not delicate and durable.

The SMD package lamp beads are soldered to the PCB board through the pins of the bracket, and the physical strength test performance is not high. It is afraid of touching, afraid of static failure caused by touch, and the contact with people is not strong.

3. Analysis and evaluation

At this stage, the advantages of COB packaging have emerged, there is no vicious competitive environment, and there are no problems with cost and reliability. So COB is easy to achieve small spacing not only refers to the concept of physical space, but also the advantages of cost and reliability.

The SMD package faces a breakthrough in how to improve the solder pin yield technology at this stage, which requires a lot of manpower, material resources and financial resources. And as the dot density becomes denser, the cost and reliability issues become more and more prominent.

Third, the analysis and evaluation of the ink color processing

1. Technical difference

COB packaging has long been plagued by ink color consistency issues, and it is now technically possible to solve this problem.

The SMD package uses a mask process to solve the ink problem.

2. Analysis and evaluation

Both packages are roughly equivalent in cost to solve the problem of ink color, and there is no impact of reduced product reliability.

Fourth, the analysis and evaluation of outdoor protection treatment links

1. Technical difference

The LED wick of the COB package has been encapsulated with epoxy resin, and there is no exposed soldering foot. The problem to be solved by the outdoor protection is only the protective treatment and waterproof structure design of the PCB board, and the technology is very simple.

A. PCB board protection

There are two levels of protection.

General outdoor application grades (such as rental and solid): three anti-paint treatment process.

High and low temperature, humidity, salt spray environment application level: nano coating + anti-UV + three anti-paint treatment process.

B. Waterproof structure design

After the COB module is treated with double-sided outdoor protection, the front and back sides are not afraid of water. In order to make the power supply, driver card and driver IC in the cabinet work in a good environment and work longer, the module structure design adopts the outdoor protection full-sealing design, and the COB module driver IC has The silicone sealing ring is sealed, and there is a heat-dissipating aluminum plate under the sealing ring. The aluminum plate is embedded and fixed in the plastic parts kit. There is also a layer of silicone sealing ring between the kit and the box to ensure that no water enters the module and the box.

The SMD package needs to be filled with the SMD device's holder chip pins, especially to the density above P4, which is quite difficult.

2. Facing problems

A. Competition issues

COB packaging has not yet formed large-scale production capacity. At present, it has not formed a cost advantage at the P8-P10 level. At present, it is only in the stadium and rental market, which needs to be not afraid of collision, outdoor display and high and low temperature, humidity, salt spray application environment and other subdivisions. The special application market has application advantages. The cost at the P5-P6 level is comparable to SMD. Cost will be an absolute advantage in pure outdoor applications at P4-P3 and even denser levels. Once the capacity is formed in the future, COB packaging will have a price advantage at all point density levels, and there is no competition between COB counterparts.

SMD faces competition from peers as mentioned above, so the quality of the bracket is crucial. To reduce the height of the bracket for cost saving, it will increase the difficulty of filling technology and reduce the yield of glue. It will not save cost but will make reliability. Reduced and cost of glue processing.

B. Product cost issues

The cost of the COB packaging process is basically calculated based on the outdoor processing cost per square meter, and has little to do with the increase in dot density.

The cost of SMD package outdoor protection treatment increases with the increase of product dot density and the difficulty of glue filling technology. Moreover, the denser the dot density, the more the cost increases, and the nonlinear growth accelerates the relationship.

C. Reliability issues

The COB packaged lamp surface has been encapsulated with epoxy resin. In this process, there are no exposed chips and pins to be processed. It is only necessary to handle the outdoor protection of the PCB board, and there is basically no problem of reliability reduction.

There are 4 soldering legs in each bracket of SMD package lamp surface, which needs glue filling and encapsulation treatment, so there is a glue encapsulation yield index of one bracket pin. As with the reflow soldering process, there is a problem of reliability degradation caused by the bracket pins in this section. According to the reliability principle, the less control a product has, the higher the reliability. Please see the table below:

Comparison of COB package and SMD package quality control link for module lamp bead surface outdoor protection technology

From the table we can clearly see the fact that as the pitch of the COB package becomes smaller and the dot density increases, the control of the bead surface is still 0, and the SMD package becomes smaller as the dot pitch becomes smaller. As the dot density increases, its control will continue to increase by a factor of four with the corresponding dot density increase. The cumulative control link refers to the accumulation of SMD reflow soldering process control and potting process control. As can be seen from the data in the table, this value increases by an eight-fold increase with the increase in dot density. In an overly competitive environment, the height of the outdoor SMD lamp bead pins is continuously reduced, which increases the difficulty of implementing this technology. How to fill the glue in such a small height space, cover all the pins, and control the level of the module is indeed a super-difficult level technology.

D. Outdoor protection problem

The surface of the lamp bead in the COB package is a hemispherical curve. All the devices on the lamp surface are encapsulated by epoxy resin. No device pins are exposed outside, and the curve is excessively rounded and has no edges and corners. If the spraying technology is adopted, no matter whether it is the principle of electrostatic adsorption or the principle of vacuum vapor deposition, it can be processed without dead angle. Therefore, no matter whether it is applied indoors, outdoor environment or harsh high temperature, humidity, salt spray environment, there is no worry about lamp bead failure caused by oxidation of device pins. The COB package PCB board must be an immersion gold process, which ensures that the device is not easily oxidized during the entire process, thus ensuring no oxidation hazard of the device pins during the front and back plating of the PCB.

SMD packaged lamp beads have many raised squares with sharp edges and corners. The bracket pins exposed on the bead surface need to be protected by outdoor protection. If the indoor lamp bead is to be modified for semi-outdoor use, especially in humid and salt spray environments, it must be sprayed. If the PCB board adopts the tin-spray manufacturing process, the device pins are prone to oxidation hazard during the process, and then the sputtering is performed, and the effect is not satisfactory. There is a spray dead angle in the shaded area below the abdomen of the lamp bead.

3. Analysis and evaluation

At this stage, the advantages of COB packaging are once again apparent, and there is no problem of reliability reduction. At present, outdoor technology has broken through to the P3.0 level.

SMD package continues to face the technical problem of how to improve the outdoor protection of the bracket pin. For small outdoor spacing, the technical difficulty is greater, and it takes a lot of manpower, material and financial resources to break through this technology. And as the dot density becomes higher, cost and reliability issues become prominent.

Fifth, the end customer's reliability experience link

The two packages have come to the end customers through different technical routes, and their experience is the most powerful. Theoretical analysis needs to be evaluated through their practical application.

With the increasing number of actual cases of COB packaged displays and the expansion of special applications, the market will certainly analyze and select these two packaging technologies. The power of this choice comes from the market, from the very end of the industry chain.

Here we only give the reliability data of COB outdoor products after one year of use in the terminal client (except for the COB products of the cottage version):

Full color screen point failure rate: less than five hundred thousand

Single and double color screen point failure rate: less than 8 parts per million

Conclusion:

In summary, a detailed comparative analysis of each link, using the following two figures to summarize the two technical styles in the LED display industry industry chain.

COB packaging technology:

From the beginning of packaging, to the completion of display manufacturing, COB packaging technology integrates the middle and lower reaches of the LED display industry chain, all production is done in one factory. This kind of production organization is simple, the process is compact, the production efficiency is higher, and it is more conducive to the fully automated production layout. This form of organization is also more conducive to the quality control of the entire process of the product. This kind of organizational form is still an organic whole. In the product development stage, it is necessary to consider the problems that may be encountered in each production process, and comprehensively evaluate and formulate technical implementation plans. This form of organization can also better assume quality responsibility for end customers.

From the perspective of the difficulty of technology implementation and the impact on product reliability in the whole production process, it can be seen from the following figure that the white abscissa represents the main process links in the industrial chain, and the process in the white abscissa The white column line on the node indicates technical difficulty. The red line above the figure shows the reliability curve of the product from the package to the client application. The blue line segment represents the technical difficulty curve on different links of the industry chain.

As can be seen from the figure, the COB package is only facing a technical peak on the road of multi-lamp integrated packaging technology in the field of LED display, which appears in the packaging of the lamp bead. Moreover, this technology is not insurmountable, but it is not a way for anyone to climb. It is a manifestation of a comprehensive technology that requires countless failures and lessons learned. It requires years of technical accumulation and precipitation, and it needs to be firm and down-to-earth. A craftsman who is not afraid of difficulties and innovation. Once the peak of this technology is crossed, like the squid jumping the dragon gate, the road behind the mountain will be a horse, and there is no technical difficulty in the whole production process. It can be seen from the red product reliability curve that once the lamp bead is sealed, the subsequent production links have little effect on the reliability of the product. After one year of application, the reliability index and the package are almost the same.

SMD packaging technology:

The packaging companies and display companies in the SMD display industry chain are two types of independent enterprises, and the industrial profits are shared by these two types of enterprises. Although the cake is large, there are many companies, fierce competition and low profits. This kind of production organization is complicated, it will waste part of the industry's profit and efficiency, and the quality control of products is relatively difficult. Since the packaging link and the display plant are independent of each other, it is difficult to effectively cooperate with the technical difficulties in the production process and coordinate the research. Once the end-users use the product, if there are quality problems, there are many links involved, and it is difficult to blame.

From the perspective of the difficulty of technology implementation and the impact on product reliability in the whole production process, the color and meaning of the curve in the figure below are the same as the previous figure. In the figure, it can be seen that there is a double hump-type technology peak in the SMD display packaging industry chain. These two technical difficulties are appearing in the screen factory, and the packaging process is relatively small due to the mature and stable technology. Therefore, the technical difficulty of SMD display will definitely exceed the difficulty of COB packaging technology.

The deterioration of the competitive environment and the imbalance of profit distribution affect the trend of product reliability red lines starting from the packaging factory. Although the red lines of each factory are high and low, their downward trend when reaching the client application is graceful. The question is determined by the reliability theory. This is the real reason why the client application feels inconsistent with the reliability of the product and the factory standard of the packaging factory. Therefore, the reliability of SMD products is mainly determined by the screen factory.

In the future, in the field of display, which packaging method is more vital, I believe that the end user will make the right choice. Providing customers with cost-effective display products is the direction of COB packaging efforts. COB packaging will make an important contribution to the development of the industry in terms of product reliability and price.