LED screen: the difference between DIP, SMD, COB three packaging modes

- Jul 14, 2020-

  1. What is COB packaging?

  COB package is an abbreviation of Chip On Board in English. Literal translation means that the chip is placed on the board. It is a new packaging method different from DIP and SMD packaging technology.

  In the field of LED display technology, the COB packaging process is to fix the LED bare chip on the lamp pad of the PCB with conductive glue or insulating glue, and then use ultrasonic welding technology to conduct conductive functional wire bonding of the LED chip, and finally use epoxy Resin glue encapsulates the lamp position to protect the LED light-emitting chip.

  Second, the difference between COB packaging technology and DIP and SMD packaging technology

  DIP package is an abbreviation of dual inline-pin package, commonly known as plug-in display. It was first developed among the three packaging modes. The lamp beads are produced by LED lamp bead encapsulation manufacturers, and then inserted by LED module and display manufacturer into the LED PCB lamp board, and the DIP semi-outdoor module and outdoor waterproof module are made by wave soldering.

  SMD package is an abbreviation of Surface Mounted Devices, meaning: surface mount device, it is one of SMT (Surface Mount Technology Chinese: surface adhesion technology) components. Three-in-one is a kind of SMD technology of LED display, which means that SMT lamps with three different colors of RGB LED chips are packaged in the same gel at a certain interval.

 The DIP and SMD packaging processes are no different from the COB packaging in terms of solid bonding wires. The biggest difference is that the red part of the bracket is used. As everyone knows, the bracket generally has four solder legs, which need to be soldered to the PCB board through SMT. Therefore, the biggest difference between the COB packaging process and the DIP and SMD packaging process is that a single lamp saves a bracket, so it also saves the surface mount welding process of the lamp bead surface reflow soldering machine.

  3. Advantages of COB packaging process

 1. High reliability, the important indicator for evaluating reliability is the dead light rate: the current national standard used in the LED display industry is: three ten thousandths, and the COB packaging process can currently achieve this index: full color screen: less than one hundred thousand Five-in-one, single- and dual-color screens: less than eight in one million.

  Why COB package display has such high reliability, we analyze through the following five aspects:

  A: The control link of single lamp production process is reduced.

 Everyone knows that a full-color lamp bead requires five welding lines. From the perspective of the production of the lamp bead surface, the COB packaging process only needs to control the quality of these five welding lines during the production process, and the SMD packaging process except these five welding lines The quality of the wire should be well controlled, and the welding quality of the four solder legs of the bracket during the reflow soldering process of the lamp bead surface must be well controlled. According to the principle of reliability, the fewer the control links of a system, the higher the reliability. The control links of COB and SMD are 5 and 9, respectively, so the reliability of COB is at least nearly double that of SMD in this respect.

  Taking one square meter of LED display as a unit, if producing 1 square meter of P10, the COB packaging process will save 40,000 control points. If the dot density is doubled, that is, the dot density reaches the P5 level, the COB packaging process will save 160,000 solder joints per square meter. If the dot density is further reduced to the P2.0 level, the COB packaging process will save 1 million solder joints per square meter. If the dot density reaches P1.0 level, the COB packaging process will save 4 million solder joints. On the contrary, how to ensure that these millions of solder joints do not appear false soldering, continuous soldering, and virtual soldering in SMD packaging is a very headache problem.

  The COB packaging process has created a revolutionary step, eliminating the link surface mount soldering. This is also the factor that guarantees the highest weight among the high reliability factors of COB packaging.

  B. COB packaging eliminates the over-reflow soldering process of the lamp bead surface, and no longer causes the high temperature of the traditional packaging process reflow soldering furnace to invalidate the LED chip and the bonding wire.

 As we all know, a high temperature of 240° is generally found in a reflow oven. If the TG point of the epoxy resin adhesive is too low, or moisture is absorbed during the packaging process, the high temperature will cause the non-linear expansion of the colloid, which will lead to the failure of the LED chip wire breakage. In addition, the high temperature will quickly conduct heat to the chip through the support pins, which will increase the possibility of chip body cracking and failure. This kind of problem is the most terrible. Generally, there will be no problem during the factory aging test. The problem will gradually be exposed after being transported to the client and used for a period of time.

  C. Good heat dissipation. The COB packaging process directly fixes the bare LED chip to the pad, so the heat dissipation area is larger than the traditional packaging process, the material's comprehensive thermal conductivity is also high, and the heat dissipation is good.

 The traditional package is to fix the bare LED chip on the pad in the bracket, and the pad needs to transfer the heat to the PCB board indirectly through the metal pins of the bracket.

  D. Shenjin PCB board process. The COB packaging process circuit board adopts the immersion gold process, and does not adopt the PCB board tin spraying process commonly used in traditional packaging. Therefore, under outdoor application conditions, living in humid heat and salt fog environment application conditions, the PCB board circuit has high oxidation resistance.

  E. Outdoor protective treatment process has no dead ends. In the COB packaging process, the curve of the lamp surface is smooth and hemispherical. All the devices on the lamp surface are encapsulated by epoxy resin, and no device pins are exposed. Therefore, no matter whether it is used indoors, outdoors, or in a harsh, humid, salt-spray environment, there will be no fear of lamp bead failure caused by device pin oxidation.

 The surface of the SMD package does not have a smooth and excessive curve, there will be many convex squares, and there are obvious corners on the square. The exposed pins of the lamp beads need to be protected by outdoor protection.

  COB packaging actually only requires nano-coating, anti-ultraviolet coating and three-proof paint spraying outdoor protection treatment on the PCB of the lamp bead and the PCB and device of the driver IC. The treatment area does not have any shadow areas and no dead corners. The SMD package is used to deal with the outdoor protection of the four-leg or six-leg pads of surface-mounted lamp beads. It will face another huge challenge in how to deal with the oxidation resistance of these millions of solder joints.

  2. Cost saving, compared with the traditional packaging process, COB packaging process saves costs, mainly from the following four aspects:

  A. Saving raw material costs, COB packaging no longer uses metal raw materials such as brackets and braids.

  B. Save the processing cost, COB packaging saves the cutting, splitting, taping and reflow soldering process of the lamp bead circuit board.

  C. Save the transportation cost, COB packaging no longer uses the bracket, saving the weight of the bracket. For example, one square meter of SMD P3 full-color screen will use 111111 brackets.

 The COB package uses a point-by-point precise dispensing process to protect the bare LED chips, so the amount of glue used is very small. Taking P3 full-color as an example, the amount of glue used for a 1024-lamp module is less than 3 grams. Therefore, the weight of the module is also saved. Saving weight saves logistics costs.

  D. Simplify the production organization process and make it easier to control. The COB packaging process integrates the production process of the mid-stream and downstream companies of the LED display industry chain. The production process from LED lamp bead packaging to LED display can be completed within one enterprise. , Saving the cost of production organization, packaging and logistics costs of intermediate links, quality control costs, etc.

  The SMD packaging process is done by the lamp bead packaging factory, and the packaging is shipped to the LED display manufacturer.

  3. It is easy to achieve a small dot pitch. From the perspective of physical space size, COB package is no longer limited by the size of the bracket when designing the lamp bead diameter. The current technology can design the lamp bead diameter to 1.2mm, lamp beads and lamp beads The safety distance between them can reach 0.5mm. In theory, the small dot pitch can achieve the P1.7 level. In the future, with the advancement of LED chip technology, the size will be further reduced, or there will be flip-chip LED chips. It is not far from breaking through the P1.0 level.

  4. Lightweight, 180° wide viewing angle, easy to bend

  A. Light and thin: The weight of the COB package module will be 1/2 lighter than the weight of the SMD package module. Compared with outdoor modules of the same point density, COB modules are about 5-10kg lighter per square meter than SMD modules.

 B. 180° large viewing angle, because the COB package uses a hemispherical lens to emit light, and there is no cover to cover, so the theoretical emission angle can reach 180°, and the SMD is generally at 125°, and the maximum can reach about 160°.

  C. Flexible, because the COB package has no bracket welding, the LED chip is sealed in the lamp position by epoxy resin, so it can be bent arbitrarily, the bending ability is determined by the size of the module and the thickness of the PCB board, and SMD The module cannot be bent.

  5. Compression resistance, impact resistance, wear resistance, easy to clean

 A. Compression resistance, impact resistance, and wear resistance. The lamp position of the COB module is encapsulated with epoxy resin. The high TG point glue has good physical properties as follows:

  Compressive strength: 8.4kg/mm²

  Shear strength: 4.2kg/mm²

  Impact resistance: 6.8kg*cm/cm²

  Hardness: Shore D 84

  Take P4 lamp beads as an example: the diameter of the lamp beads is D=2.8mm

  The lamp bead package area is: S=πr²=3.14X1.4²=6.15mm²

  The pressure of a single lamp bead is: 6.15X8.4=51.66kg

  The lateral shearing force of a single lamp bead is: 6.15X4.2=25.83kg

  B. It is easy to clean. No mask is used on the surface of the COB module light board. After the screen is used outdoors, it can be rinsed directly with water.

  4. Conclusion

  COB packaging, because it integrates and simplifies the production process of packaging companies and display manufacturing companies, the production process is more reasonable, easier to organize and control, the dot pitch of products can be smaller, the reliability is doubled, and the cost is closer to civilians. The higher the density, the more obvious the cost advantage. In the future, the application of small pitch to civilian applications, COB packaging will play an important role.

  The performance of COB packaging products in terms of high reliability will also be impressive. Especially for outdoor small pitch applications, once the production capacity is formed, it will occupy an absolute advantage in technology and price.

 In the future, SMD will face huge challenges in terms of reliability, achieving smaller pitches, and costs. The main problem facing SMD in the future is not in the packaging link, but its biggest problem will be in the screen factory link. Although the factory that encapsulates the lamp beads can make the quality of the lamp beads very good, the overall level of the screen factory is uneven. PCB material, PCB manufacturing process, quality of driver IC, precision of SMT equipment, production level of SMT, process and method of outdoor protection treatment, management's concept of quality control, users' desire for uncontrollable low prices, market The feedback information, user confidence and other factors do not seem to have a plausible answer in an environment of excessive competition.

  On the contrary, due to the revolutionary breakthrough of COB packaging, the big burden of the bracket is thrown away, and the package will move forward lightly, and the future becomes bright. The future development of the industry is summarized in one sentence as a concluding remark: "Six feet can't run but four feet, four feet can't run without feet."