Detailed explanation of the specific methods for creating LED high-efficiency COB packaging products

With the continuous innovation of LED packaging technology and the implementation of energy-saving and emission reduction policies at home and abroad, the proportion of LED light sources used in the lighting field is increasing, and new packaging forms are constantly being introduced. Ouyang Minghua, a senior engineer at Yuanlei, said: "LED's performance in heat dissipation, light efficiency, reliability, and cost performance is still a focus. If these cannot be broken through, or if new products other than LED can achieve breakthroughs in the future, then the choice in the lighting field may not be LED." COB (Chip on Board) is an LED packaging product launched by the industry under this background. Compared with traditional discrete LED packaging products, it has better primary heat dissipation capabilities and high-density luminous flux output. In addition to explaining some of the characteristics of COB, this article focuses on how to improve the light efficiency of COB from the basic principles and seeks methods to meet the core value points of lighting.

COB has good heat dissipation capability

When designing the LED packaging structure, the chip junction temperature should be reduced as much as possible. The heat dissipation path of the COB packaged chip is the shortest, which can quickly transfer the heat of the working chip to the metal substrate, and then to the heat sink. Therefore, COB has better heat dissipation capacity than traditional discrete component assembly. The current material choices for COB metal substrates include copper, aluminum, aluminum oxide, aluminum nitride, etc. In terms of comprehensive cost, heat dissipation capacity, and corrosion resistance, aluminum is mainly selected as the metal substrate to make COB. The figure below is the structure diagram of Yuanlei's COB product.

COB can achieve high-density luminous flux output

When we use LEDs in modular design, we usually don’t have much space for the light source, but we want to have a sufficiently high brightness output in a very small size. This solution is almost impossible to find on discrete component LEDs. Of course, some may choose 3535 ceramics or other small-sized products with relatively high luminous flux, but none of them can match the high-density luminous flux output of COB or MCOB. Therefore, the advantages of COB in product modularization are reflected: it helps modular design while maintaining a high luminous flux.

The drive design of COB is very flexible. The application end can select COB based on the existing drive conditions to meet a variety of solutions from low voltage to high voltage.

Three modular designs of COB

Disadvantages of COB packaging

Although COB has many advantages, it is not yet mainstream in the current lighting market. While we are optimistic about COB, we should also see its shortcomings in order to find the reasons that restrict its application. First of all, the luminous efficiency of COB is still low. Since COB emits light in a plane, it cannot have PPA to assist the side light output of the chip like discrete LEDs, resulting in partial light loss. Secondly, COB currently has no standardized appearance. Most domestic manufacturers manufacture COB based on their respective partners, matching the corresponding lamps and driving methods. The appearance of each company is not uniform, which limits the large-scale use of COB.

Methods to improve COB lighting efficiency

Based on the problem of low COB luminous efficiency, the author believes that the solution should be considered from the principle of substrate. When improving the luminous efficiency of LED packaging, we usually talk about the external quantum efficiency, that is, how to improve the luminous flux of the packaged product or the entire module under the same chip. First of all, the substrate material with high reflectivity and the reflectivity of the chip placement area directly affect the entire luminous flux output. Whether it is a copper substrate, an aluminum substrate, or a ceramic substrate, this rule must be followed. It must also be noted that when assembling and applying COB, the area outside the luminous surface of the substrate must also be considered. If a reflective cup is not added to the luminous surface, reflective materials should be added to the area outside the luminous surface. Secondly, large particles, high brightness phosphors, and high transmittance encapsulation silicone are the same as the traditional discrete LED luminous efficiency improvement method. Thirdly, the chip arrangement spacing and driving method should be reasonable during crystal bonding. Under the same power, a solution with a small driving current should be used as much as possible to avoid too much heat staying inside the substrate. In addition, when the size of the luminous surface is small, applying lens-shaped silicone on the surface of the phosphor can not only increase the angle, but also further improve the luminous efficiency.

Summarize

Our understanding of LED should return to ordinary electronic components, just like capacitors and resistors, except that LED plays the role of emitting light, and other components play their own different functions, together forming an electronic system. As long as the light output of the entire system meets the market value requirements, it is not a bad idea to choose COB.