With continuous innovation in LED packaging technology and the implementation of energy conservation and emission reduction policies both domestically and internationally, the proportion of LED light sources used in the lighting field is increasing daily, and new packaging forms are constantly being introduced. Ouyang Minghua, a senior engineer at Yuanlei, stated, "The performance of LEDs in terms of heat dissipation, luminous efficacy, reliability, and cost-effectiveness remains a focus. If breakthroughs are not achieved in these areas, or if new products other than LEDs can achieve breakthroughs in the future, then the lighting industry may not choose LEDs." COB (ChiponBoard) is an LED packaging product launched by the industry against this backdrop. Compared to traditional discrete LED packaging products, it has better primary heat dissipation capabilities and high-density luminous flux output. This article, in addition to explaining some characteristics of COB, focuses on exploring how to improve the luminous efficacy of COB from a fundamental perspective, seeking methods to meet the core value points of lighting.
COB has good heat dissipation capabilities.
When designing LED packaging structures, the chip junction temperature should be minimized as much as possible. COB (Chip-on-Board) packaging offers the shortest heat dissipation path, rapidly transferring heat from the chip to the metal substrate and then to the heat sink. Therefore, COB provides better heat dissipation than traditional discrete component assemblies. Currently, COB metal substrates are available in copper, aluminum, alumina, and aluminum nitride. Considering overall cost, heat dissipation, and corrosion resistance, aluminum is the primary choice for COB substrates. The following diagram shows the structure of Yuanlei's COB products.
COB can achieve high-density luminous flux output.
When using LEDs in modular designs, we often have limited space for the light source, yet we want sufficiently high brightness output within a small size. This is almost impossible to find with discrete LED components. While some might choose 3535 ceramic LEDs or other small, high-luminous-flux products, none can match the high-density luminous-flux output of COB or MCOB. This is where the advantage of COB in product modularization becomes apparent: it facilitates modular design while maintaining high luminous-flux.
COB's driver design is very flexible, and the application side can select COB according to the existing driver conditions to meet various solutions from low voltage to high voltage.
COB Three Modular Design Drawings
Disadvantages of COB packaging
While COB (Chip-on-Board) lighting offers many advantages, it's not yet mainstream in the lighting market. While we are optimistic about COB's future prospects, we should also recognize its shortcomings to understand why its application is limited. First, COB's luminous efficacy remains relatively low. Because COB emits light from a single plane, it lacks the PPA (Power Pad) to assist the chip's side light output like discrete LEDs, resulting in some light loss. Second, COB currently lacks a standardized form factor. Domestic manufacturers mostly produce COBs based on their respective partners, matching them to corresponding luminaires and driving methods. The inconsistent form factors among manufacturers further restrict the large-scale use of COB.
Methods to improve COB lighting effects
Given the low luminous efficacy of COB (Chip-on-Board) LEDs, this author believes solutions should be considered from the perspective of substrate principles. Improving LED packaging luminous efficacy typically refers to external quantum efficiency, i.e., how to increase the luminous flux of the packaged product or the entire module while keeping the chip size constant. First, a high-reflectivity substrate material is crucial. The reflectivity of the chip placement area directly affects the overall luminous flux output, regardless of whether the substrate is copper, aluminum, or ceramic. Furthermore, it's essential to consider the areas outside the emitting surface when assembling COBs. If a reflector isn't added to the emitting surface, reflective material should be added to the areas outside. Second, large-particle, high-brightness phosphors and high-transmittance encapsulation silicone are essential, similar to methods used to improve the luminous efficacy of traditional discrete LEDs. Third, the chip spacing and driving method during die bonding must be reasonable. For the same power, a solution with the lowest possible driving current should be used to avoid excessive heat buildup inside the substrate. Additionally, when the emitting surface is small, applying lens-shaped silicone to the phosphor surface can not only increase the angle but also further improve luminous efficacy.
Summarize
Our understanding of LEDs should return to that of ordinary electronic components, just like capacitors and resistors. The only difference is that LEDs emit light, while other components perform different functions, together forming an electronic system. As long as the light output of the entire system meets market value requirements, choosing COB is perfectly acceptable.
