With the continuous advancement in LED packaging technology and the growing implementation of energy-saving and emission-reduction policies globally, the use of LED light sources in the lighting industry is steadily increasing. New packaging methods are being introduced regularly to meet the evolving demands of efficiency and performance. Despite these developments, key aspects such as heat dissipation, light output, reliability, and cost-effectiveness remain critical concerns for LED technology. If no significant breakthroughs occur in these areas, or if alternative technologies surpass LEDs in the future, the dominance of LEDs in the lighting market could be challenged.
COB (Chip on Board) technology has emerged as a promising solution in this context. Compared to traditional discrete LED packages, COB offers superior thermal management and higher luminous flux density. When designing the package structure, minimizing the junction temperature of the chip is essential. The COB configuration provides the shortest thermal path, allowing direct heat transfer from the chip to the metal substrate and then to the heat sink. This results in better thermal performance compared to conventional assembly methods.
The materials used for COB metal substrates include copper, aluminum, aluminum oxide, and aluminum nitride. Among these, aluminum is commonly chosen due to its balance of cost, thermal conductivity, and corrosion resistance. As LED packaging technology continues to evolve, the importance of efficient heat dissipation becomes even more critical, especially with the increasing demand for high-performance lighting solutions.
In the electronics field, every 10°C rise in ambient temperature can significantly increase the failure rate of devices, following what is known as the "10°C rule." While many current approaches focus on selecting materials with high thermal conductivity—such as copper, aluminum, and ceramics—it is not sufficient to rely solely on the circuit board for heat dissipation. Additional thermal design strategies must be implemented to enhance overall thermal performance.
Heat dissipation is a fundamental aspect of electronic device reliability and performance. Heat transfer occurs through conduction, which involves the movement of energy between particles in direct contact. This is most effective in solids and liquids, while gases rely on diffusion. The basic formula for heat transfer is:
**Q = K × A × ΔT / ΔL**
Where Q represents the heat transferred, K is the thermal conductivity of the material, A is the area of heat transfer, ΔT is the temperature difference, and ΔL is the distance over which the heat is conducted. From this equation, it's clear that higher thermal conductivity, larger surface area, and shorter distances all contribute to more efficient heat dissipation.
As a next-generation light source, LEDs are increasingly used in general lighting applications. To boost luminous flux, manufacturers often increase chip brightness or use dense arrays. However, these methods require more power, and only a small portion of the input energy is converted into light. Most of it becomes heat, leading to increased thermal challenges, especially in compact designs where heat dissipation is limited.
Like traditional light sources, LEDs generate heat during operation. Only about 30%–40% of the electrical energy is converted into light, while the remaining 60%–70% is lost as heat due to non-radiative recombination processes. Higher temperatures accelerate this loss, reducing luminous efficiency and shortening the lifespan of the LED. Effective thermal management is crucial to maintain low junction temperatures and ensure long-term performance.
Figure 1 illustrates how the luminous flux of a Lumileds 1W LED decreases with rising junction temperature, highlighting the direct impact of thermal conditions on LED longevity.
Thermal performance parameters for LEDs primarily include junction temperature and thermal resistance. Junction temperature refers to the temperature at the PN junction, while thermal resistance measures how effectively heat is dissipated from the junction to the environment. Lower thermal resistance means better heat conduction, resulting in lower junction temperatures, improved light output, and extended lifespan.
The PCB (Printed Circuit Board) plays a dual role in LED systems: it serves as both a power carrier and a heat dissipation medium. Therefore, the thermal design of the PCB—such as pad placement, wiring, and plating—is critical to ensuring optimal thermal performance.
Packaging processes also have a significant impact on thermal performance. Traditional single-chip packaging is suitable for low-power applications, but when applied to high-density or long-duration lighting, heat accumulation becomes a major issue. COB technology addresses this by offering a more compact and efficient thermal path.
Table 1 compares the thermal resistance of LEDs packaged using different methods. It shows that COB technology provides the lowest thermal resistance, making it an ideal choice for high-performance lighting applications.
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