The most comprehensive interpretation of key technologies for LED COB packaging

The LED COB (Chip On Board) package refers to an LED package technology in which an LED chip is directly fixed on a printed circuit board (PCB), and the chip and the circuit board are electrically connected by wire bonding. It can package tens or even hundreds of chips in a small area, and finally form a surface light source. Compared with the point source package, the COB surface light source package 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 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 packages, heat dissipation is a critical factor affecting long-term reliability. Increasing the junction temperature of the COB package will reduce the overall efficiency of the LED, reduce the forward voltage, resulting in red shifting of the emitted light, reducing service life and reliability.

LED thermal research mainly includes three levels: package, substrate and overall level. In solving the heat dissipation problem of high-power COB package, most researchers first propose a structural model and simulate the heat dissipation process of the entire package structure under certain conditions through software (finite element analysis software ANSYS, computational fluid dynamics software CFD, etc.). The temperature of the part is tested and the simulation results are verified. In addition, an important factor affecting the performance of high-power COB packages is the performance of the package.


1. Performance of silicone for high power LED COB package


At present, there are many kinds of silica gels that can be used in high-power LED COB packages on the market, and a large number of them are domestically produced silica gels, and the main advantage is that the price is low. Table 1 below compares the properties of some silica gels currently on the market.


As can be seen from Table 1, the refractive index of silica gel can be divided into two main grades: low refractive index (1.42) and high refractive index (1.54). The use of high refractive index silica gel during the encapsulation 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 silica gel is the light transmittance. It can be seen from the table that the transmittance of most encapsulated silica gel can reach more than 98%. Among them, Dow Corning's OE-6550 silica gel has a light transmittance of 100% and a refractive index of 1.543. The curing conditions are simple. Curing at 150 ° C for 1 h, wide temperature range (-60 ~ 200 ° C), has a great performance, but the disadvantage is expensive (price 5 700 ~ 6 800 yuan / kg).


Comparing the performance, it can be seen that the performance of many domestic silica gels is close to that of Dow Corning. Some merchants also claim that their silica gel can completely replace OE-6550 silica gel for LED packaging.


2. Research progress of high power LED COB package


The COB integrated package has better heat dissipation performance than a single discrete package, mainly because the COB package directly transfers heat to the substrate and then conducts through the substrate to the outer casing. In the high-power COB package, multiple high-power chips are closely integrated together, and the heat dissipation problem is still the first problem to be solved. In response to this, many researchers at home and abroad have studied the heat dissipation of COB package based on software simulation.


(1) Lanhai et al. used the finite element thermal simulation method to analyze the metal substrate and ceramic substrate commonly used in COB packaging process, and concluded that the thermal resistance of the ceramic substrate as the packaging material is the thermal resistance of the metal substrate. /2, and the ceramic substrate also has a larger thermal management optimization space.


(2) Ma Jian et al. used TracePro software simulation and experiment to analyze the main factors affecting the LED luminescence performance of COB package while considering the phosphor coating method and reflector structure. The research results show that the angle is 30°, cup The deep and large conical reflector is packaged, and the product has good luminescence performance. Coating the phosphor with the phosphor away from the chip can increase the luminous efficiency by about 5%.


(3) Li Weiping proposed a new COB free-form lens package structure (see Figure 1 below). The structure was simulated by TracePro. The results show that the device can achieve a specific optical distribution and the light extraction efficiency is higher than 90%.



(4) Jiang Bin et al. proposed three LED COB packaging methods, the package structure is COB-I, COB-II and COB-III, respectively. The schematic diagram is shown in Figure 2. The finite element simulation and experimental measurements show that the junction temperature of COB-III is 21.5 °C and 42.7 °C lower than COB-II and COB-I, respectively, and the thermal resistance is 25.7K/W and 58.8K/W, respectively, and COB-III. Light decay is also smaller.



(5) Hsueh-Han Wu et al. proposed five high-power COB packages with different chip spacings, of which the maximum chip spacing is 2.5mm. The simulation and experimental results of CFD software show that the larger the chip spacing, the lower the junction temperature and the luminous flux. And the higher the luminous efficiency, and the difference between the maximum and minimum junction temperatures is 3.12 °C.


(6) Jae-Kwan Sim et al. proposed LED COB package (LTC-CCOB) using low temperature infusible ceramics (the package structure model is shown in Figure 3(a) below) to improve its thermal performance. There is no LED chip and metal substrate. The insulating layer was experimentally compared with the performance parameters of the SMD-COB package (see Figure 3(b) below. The results show that the electroluminescence peak intensity of the LTCC-COB package is 1.75 times that of the SMD-COB package; LTCC- The thermal resistance between the package surface of the COB package and the SMD-COB package and air is 7.3 K/W and 7.9 K/W, respectively.



(7) In 2013, Chang Keun Lee and others also studied the heat dissipation performance of the LTCC-COB package. The LED package structure is to mount the low temperature incombustible ceramic directly on the metal-based printed circuit board (MCPCB) using a finite volume numerical simulation method ( The thermal performance of the LED module was mainly studied by using the embedded commercial software Fluent V.6.3). The results of the software simulation were consistent with the experimental results. The results show that 49%~58% of the thermal resistance of the whole substrate comes from the thermal resistance of the MCPCB; The established model overcomes the shortcomings of the traditional LTCC high power LED module with high thermal resistance.


(8) Yu Hui et al. studied a new type of LED wafer-level COB package using micro-glass caps and silicon substrates as packaging materials. Systematic study of the COB packaging process, including the following steps: 1 prepare the silicon substrate with leads; 2 use the wire bonding equipment to fix the LED on the silicon substrate; 3 the phosphor is evenly coated on the inner spherical surface of the glass bubble cap 4 fill the spherical glass bubble cap with silica gel; 5 seal the LED fixed on the silicon substrate into the spherical glass bubble cap. The experimental results show that this form of COB package is successfully implemented and the packaged chip has good thermal and luminescent properties.


(9) Feng Weifeng et al. developed a high-power ceramic COBLED module (see Figure 4 below) that can directly use 100 V AC. The module has 40 LED chips that can work directly under 110 V AC.



(10) Ming-Te Lina et al. proposed a W5II type LED vertical package structure (see Figure 5 below) that can be used in high-power optoelectronic semiconductor packages. The thermal simulation structure, manufacturing process and thermal performance were calculated. The measured values ​​characterize the thermal performance of W5II, and the photoelectric characteristics are also measured. The results show that the excellent heat dissipation performance of W5II can be used to improve the reliability and thermal fatigue resistance of high-power LED packages.



(11) Ray-Hua Horng et al. designed a double-layer heat-dissipating LED package structure (see Figure 6 below). The first layer uses a cup-shaped thin copper sheet to dissipate heat. The copper sheet and the sapphire are in direct contact to enhance the heat dissipation of the chip. The layer is made of AgSnCu alloy solder and high thermal conductivity MCPCB, and then a thin diamond layer is used to replace the traditional insulating layer. The experimental results show that the added composite solder has a good effect on reducing the LED thermal resistance, while avoiding heat. The phenomenon of agglomeration.



(12) Wang C. et al. introduced a manufacturing method of an LED module package structure, which is a silicon-based package mode in which a reflective layer and an electrode are connected to each other (as shown in FIG. 7 below), and the reflective layer is used in SU-8. The 2075 and 4620 are plated with Ni/Au/Ag to form a cathode and are formed by integrating Cu/Au and connecting electrodes, and the heat generated by the LED chip is directly emitted to the silicon substrate through the metal plating layer.



(13) Yin Luqiao et al. designed, fabricated and studied multi-chip LED modules using aluminum nitride (AlN), aluminum (Al) and alumina (Al2O3) as substrate materials, using finite element method (FEM) and electrical test. The thermal performance of the LED module was evaluated by the method. The software simulation and experimental results show that the thermal performance of the LED module with AlN as the substrate is better than that of the other two substrates, and the luminescence performance is the best.


3. Problems with high-power COB package


After a period of quietness, COB technology is gradually being applied by many manufacturers, but there are still some problems 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 package system. At present, there are two main types of substrates used for COB: aluminum substrates and ceramic substrates. The aluminum substrate is cheaper and the heat dissipation performance is poor; the ceramic substrate is expensive and the heat dissipation performance is good. In addition, these two substrates have other performance differences. There are many factors to be considered when packaging companies choose these two substrates.


(2) Selection of package adhesive: Package adhesive has a great influence on COB package and other LED package forms. At present, silica gel with better performance has gradually replaced the epoxy resin that most manufacturers will use, especially for high-power LED packaging. Silicone resin has become the first choice of many packaging manufacturers. However, there are many kinds of silica gels on the market, and the performance and price are also very different. There are many factors to consider when choosing a suitable package.


(3) Chip selection: The chip is not only related to the light efficiency of the entire LED, but also has a great relationship with heat dissipation. In the chip selection, many companies and even researchers do not fully consider the chip and the matching of phosphors, encapsulants, chip and substrate matching.


(4) Design and application of the overall heat dissipation structure: As can be seen from the foregoing, there are many heat dissipation structures that can be used in the COB package, but the design of these structures is based on the improved 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 desired effect, the process complexity of the heat dissipation structure processing, whether the manufacturing cost of the heat dissipation structure is consistent with the customer's acceptance level, etc. Wait.


4. Suggestions and prospects


In response to the problems of COB, the following suggestions are proposed:


(1) Substrate: The company selects the substrate used in the production according to the selected package chip and the positioning of the product. Different chips need to be matched with different substrates to have better heat dissipation. The best practice is to customize the substrate based on chip type and heat generation. From the current application situation and research progress of the enterprise, the performance of the ceramic substrate is better than that of the aluminum substrate. If the company is targeting high-end products, ceramic substrates can be selected. In addition, some new types of substrates, such as copper substrates, mirror aluminum substrates, and silver-plated aluminum substrates, have also appeared on the market.


(2) Encapsulation: First, we must conduct market research and fully grasp the performance and price difference of the packaging glue on the market. Furthermore, it is necessary to conduct experiments on the purchased encapsulant to better understand the properties of the adhesive, especially for some relatively low-cost adhesives. Also, different batches of glue should be tested to determine the best conditions of use.


(3) Chip: Researchers need to fully evaluate the performance of the chip based on the full compatibility of the chip with phosphor, encapsulant and substrate. Experiment with the chip to be used to compare the various types of chips to achieve the required light efficiency and heat dissipation performance in matching with phosphors, encapsulants and substrates. After the comprehensive evaluation, the final use of the chip can achieve the best results.


(4) Structure: If the company has a high level of research team, you can try to study the special heat dissipation structure and apply for a patent. This will not only prevent others from copying, but also increase the competitive advantage of their products. If you want to reduce costs, you can consider the use of traditional packaging structures, choose the right materials, and also ensure product quality.


In recent years, COB packaging, especially high-power COB packaging, has been greatly developed, and products have a certain share in the market. As long as the company continues to improve product performance in actual production, COB products will become an important part of LED packaging products.

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