Product
LED COB (Chip On Board) packaging refers to the LED packaging technology that directly fixes the LED chip on the printed circuit board (PCB) and electrically connects the chip and the circuit board through wire bonding. It can package dozens or even hundreds of chips in a very small area, and finally form a surface light source. Compared with point light source packaging, COB surface light source packaging technology has the advantages of low price (only about 1/3 of the same chip), space saving, easy heat dissipation, improved luminous efficiency, and mature packaging process technology.
Due to its superior heat dissipation performance and low manufacturing cost, COB packaged LED light sources are popular among many packaging companies. For high-power COB packaging, heat dissipation is a crucial factor affecting its long-term reliability. The increase in the junction temperature of COB packaged products will reduce the overall efficiency of the LED, reduce the forward voltage, cause the emitted light to red shift, and reduce the service life and reliability.
The heat dissipation research of LED mainly includes three levels: packaging, substrate and overall level. When solving the heat dissipation problem of high-power COB packaging, most researchers first propose a structural model, and use software (finite element analysis software ANSYS, computational fluid dynamics software CFD, etc.) to simulate the heat dissipation process of the entire packaging structure and the temperature of each part under certain conditions, and then conduct experiments to verify the simulation results. In addition, an important factor affecting the performance of high-power COB packaging is the performance of the packaging glue.
1. Performance of silicone for high-power LED COB packaging
There are many types of silicone available for high-power LED COB packaging on the market, among which the largest number is domestic silicone, and its main advantage is low price. Table 1 below compares the properties of some silicones currently on the market.
As can be seen from Table 1 above, the refractive index of silicone can be divided into two main grades: low refractive index (1.42) and high refractive index (1.54). Using high refractive index silicone in the packaging process can effectively reduce the loss of photons at the interface, thereby increasing the luminous flux of the light source.
Another important parameter affecting the performance of silicone is light transmittance. As can be seen from the table, the transmittance of most encapsulated silicones can reach more than 98%, among which Dow Corning's OE-6550 silicone has a transmittance of 100% and a refractive index of 1.543. The curing conditions are simple, and it only needs to be cured at 150℃ for 1h. It has a wide temperature resistance range (-60~200℃) and has great advantages in performance, but the disadvantage is that it is expensive (priced at 5,700~6,800 yuan/kg).
Comparative performance also shows that the performance of many domestic silicones is close to that of Dow Corning's product. Some merchants also claim that their silicone can completely replace OE-6550 silicone for LED packaging
2. Research progress of high-power LED COB packaging
COB integrated packaging has better heat dissipation performance than single discrete packaging, mainly because COB packaging is a chip that directly conducts heat to the substrate and then conducts it to the shell through the substrate. In high-power COB packaging, multiple high-power chips are integrated together in close proximity, and the heat dissipation problem is still the first problem to be solved. In view of this, many researchers at home and abroad have studied the heat dissipation of COB packaging based on software simulation.
(1) Lan Hai et al. used the finite element thermal simulation method to analyze the metal substrate and ceramic substrate commonly used in the COB packaging process. The conclusion is that the thermal resistance of the ceramic substrate as the packaging material is 1/2 of the thermal resistance of the metal substrate, and the ceramic substrate also has a larger space for thermal management optimization.
(2) Ma Jianshe et al. used TracePro software simulation and experiments to analyze the main factors affecting the luminous performance of COB packaged LEDs under the conditions of considering the phosphor coating method and the reflector cup structure. The research results show that the product has better luminous performance when encapsulated with a conical reflector cup with an angle of 30° and a slightly larger cup depth. The luminous efficiency can be increased by about 5% by coating the phosphor away from the chip.
(3) Li Weiping et al. proposed a new COB free-form surface lens packaging structure and simulated the structure using TracePro. The results show that the device can achieve a specific optical distribution and the light output efficiency is higher than 90%.
(4) Jiang Bin et al. proposed three LED COB packaging methods, with packaging structures of COB-Ⅰ, COB-Ⅱ and COB-Ⅲ. The results of finite element simulation and experimental measurement show that the chip junction temperature of COB-Ⅲ is 21.5℃ and 42.7℃ lower than that of COB-Ⅱ and COB-Ⅰ, respectively, and the thermal resistance is 25.7K/W and 58.8K/W lower, respectively. In addition, the light decay of COB-Ⅲ is also smaller.
(5) Hsueh-HanWu et al. proposed five high-power COB packaging forms with different chip spacings, among which the maximum chip spacing is 2.5mm. The results of CFD software simulation and experimental measurement show that the larger the chip spacing, the lower the junction temperature, the higher the luminous flux and luminous efficiency, and the difference between the maximum and minimum junction temperatures is 3.12℃.
(6) Jae-KwanSim et al. proposed the use of low-temperature eutectic ceramics for LED COB packaging (LTC-CCOB) to improve its thermal performance. There is no insulating layer between the LED chip and the metal substrate. The performance parameters of the LTCC-COB package and the SMD-COB package were compared and analyzed experimentally. The results showed that the peak electroluminescence intensity of the LTCC-COB package was 1.75 times that of the SMD-COB package; the thermal resistance between the package surface and the air of the LTCC-COB package and the SMD-COB package was 7.3K/W and 7.9K/W, respectively.
3. Problems with high-power COB packaging
After a period of silence, COB technology is now gradually being used by many manufacturers, but there are still some problems that need to be solved.
(1) Selection of substrate: The heat dissipation performance of the substrate plays a key role in the heat dissipation of the entire packaging system. There are currently two main types of substrates used for COB: aluminum substrates and ceramic substrates. Aluminum substrates are cheaper and have poor heat dissipation performance; ceramic substrates are more expensive and have better heat dissipation performance. In addition, these two substrates have some other performance differences. Packaging companies need to consider many factors when choosing these two substrates.
(2) Selection of encapsulation glue: Encapsulation glue has a great impact on COB packaging and other LED packaging forms. At present, silicone with better performance has gradually replaced the epoxy resin glue used by most manufacturers, especially for high-power LED packaging. Silicone resin has become the first choice of many packaging manufacturers. However, there are many types of silicone on the market, and the performance and price are also very different. There are many factors to consider when choosing an encapsulation glue that suits you.
(3) Selection of chip: The chip is not only related to the light efficiency of the entire LED, but also has a great relationship with heat dissipation. In terms of chip selection, many companies and even researchers have not fully considered the matching of chips and phosphors, packaging glue, and matching of chips and substrates.
(4) Design and application of overall heat dissipation structure: As mentioned above, there are many heat dissipation structures that can be used for COB packaging, but the designs of these structures are improved based on the heat dissipation structure originally proposed. There are still many problems to be solved in the actual production of these heat dissipation structures, such as whether the heat dissipation performance of the structure can achieve the expected effect, the complexity of the heat dissipation structure processing, whether the manufacturing cost of the heat dissipation structure is consistent with the customer's acceptance, etc.
In recent years, COB packaging, especially high-power COB packaging, has made great progress and the products have occupied a certain share in the market. As long as enterprises continue to improve product performance in actual production, COB products will become an important part of LED packaging products.