Aging Phenomena of Epoxy Encapsulated and Their Impacts on LED Performance

 

LED (Light Emitting Diode), as a new type of high-efficiency, energy-saving, and long-life light source, has been widely applied in fields such as lighting and display. Due to its good optical performance, electrical insulation performance, and mechanical performance, epoxy resin has become a commonly used material for epoxy encapsulated LED. However, during long-term use, epoxy resin will inevitably undergo aging phenomena, which will have a significant impact on the performance of LEDs. Conducting in-depth research on the aging phenomena of epoxy resin and their impacts on LED performance is of great significance for improving the quality and reliability of LED products.

Structure and Principle of Epoxy Encapsulated LEDs

The LED chip is the core component of an LED for emitting light, and the light it generates needs to be protected and optimized optically through the encapsulation material. An epoxy encapsulated LED usually consists of an LED chip, electrodes, a support frame, and an epoxy encapsulation layer. The epoxy encapsulation layer not only plays a role in protecting the chip from the external environment but also can improve the optical performance of the LED, such as increasing the light extraction efficiency and color consistency.

 

Aging Phenomena of Epoxy Resin during Long-term Use

(1) Optical Aging Phenomena

1. Yellowing: During long-term use, especially under the action of factors such as ultraviolet rays and heat, the epoxy resin will undergo a yellowing phenomenon. This is because the chemical bonds in the epoxy resin molecules are broken and reorganized, generating some chromophoric substances, which cause the color of the epoxy resin to turn yellow. Yellowing will reduce the light transmittance of the epoxy resin, affecting the luminous efficiency and color characteristics of the LED.
2. Increased Light Scattering: As the aging progresses, some tiny cracks, bubbles, or impurity particles may be generated inside the epoxy resin. These defects will lead to an increase in the scattering of light in the epoxy resin. The increase in light scattering will make the light emitted by the LED more divergent, reducing the directivity and brightness of the light.

(2) Physical Aging Phenomena

1. Decrease in Hardness and Strength: The long-term action of thermal cycles, mechanical stress, etc., will cause the molecular chains of the epoxy resin to relax and break, resulting in a decrease in its hardness and strength. The decrease in hardness and strength will weaken the protective ability of the epoxy encapsulation layer for the LED chip, increasing the risk of the chip being mechanically damaged by the outside world.
2. Dimensional Change: The epoxy resin will expand and contract under different temperature and humidity conditions. The long-term thermal expansion and contraction cycles will cause internal stress in the epoxy encapsulation layer, thus leading to dimensional changes. Dimensional changes may cause gaps to appear at the interfaces between the encapsulation layer, the chip, and the support frame, affecting the electrical performance and sealing of the LED.

(3) Chemical Aging Phenomena

1. Hydrolysis Reaction: In a humid environment, chemical bonds such as ester bonds in the epoxy resin are prone to undergo hydrolysis reactions. The hydrolysis reaction will break the molecular chains of the epoxy resin, reducing its molecular weight and performance. The acidic substances generated by the hydrolysis may also corrode the LED chip and electrodes, affecting the electrical performance of the LED.
2. Oxidation Reaction: The epoxy resin will undergo an oxidation reaction under the action of high temperature and oxygen, generating some functional groups such as carbonyl groups and carboxyl groups. The oxidation reaction will change the chemical structure and performance of the epoxy resin, making it more brittle and unstable.

 

Impacts of Epoxy Resin Aging on LED Performance

(1) Impacts on Optical Performance

1. Decrease in Luminous Efficiency: The yellowing and increased light scattering of the epoxy resin will lead to more light being absorbed and scattered, thus reducing the luminous flux emitted from the LED and decreasing the luminous efficiency. Research shows that when the yellowing of the epoxy resin is severe, the luminous efficiency of the LED may decrease by more than 10%.
2. Color Drift: The aging of the epoxy resin will change its transmittance and scattering characteristics for light of different wavelengths, causing the color of the light emitted by the LED to drift. Color drift will affect the color consistency and accuracy of the LED in lighting and display applications.

(2) Impacts on Electrical Performance

1. Decrease in Electrical Insulation Performance: Aging reactions such as the hydrolysis and oxidation of the epoxy resin will generate some ionic substances in it, which will reduce the electrical insulation performance of the epoxy resin. The decrease in electrical insulation performance may lead to leakage between the LED chip and the support frame, affecting the normal operation of the LED.
2. Increase in Contact Resistance: The dimensional changes of the encapsulation layer and the generation of interface gaps caused by the aging of the epoxy resin may lead to poor contact between the chip and the electrodes, increasing the contact resistance. The increase in contact resistance will not only increase the power consumption of the LED but may also cause local overheating of the chip, accelerating the aging of the LED.

(3) Impacts on Thermal Performance

1. Deterioration of Heat Dissipation Performance: After the epoxy resin ages, the internal heat conduction paths may be damaged, resulting in a decrease in thermal conductivity. The deterioration of heat dissipation performance will make it difficult for the heat generated by the LED chip to be effectively dissipated, increasing the chip temperature, and thus affecting the luminous efficiency and lifespan of the LED.
2. Increase in Thermal Stress: The dimensional changes and decrease in hardness caused by the aging of the epoxy resin will cause greater thermal stress in the LED during thermal cycles. The increase in thermal stress may lead to the appearance of cracks or delamination at the interfaces between the chip, the support frame, and the encapsulation layer, further deteriorating the performance of the LED.

 

Prevention and Mitigation Measures for Epoxy Resin Aging

(1) Optimizing the Epoxy Resin Formula

1. Adding Anti-aging Agents: Adding anti-aging agents such as ultraviolet absorbers, antioxidants, and anti-hydrolysis agents to the epoxy resin can effectively inhibit the aging reactions of the epoxy resin. For example, adding an appropriate amount of ultraviolet absorbers can reduce the damage of ultraviolet rays to the epoxy resin and delay the occurrence of yellowing.
2. Selecting the Appropriate Curing Agent: Different curing agents will affect the curing degree and performance of the epoxy resin. Selecting the appropriate curing agent can increase the cross-linking density and stability of the epoxy resin and enhance its anti-aging ability.

(2) Improving the Encapsulation Process

1. Controlling the Curing Conditions: Precisely controlling the curing temperature, time, and pressure, etc., of the epoxy resin can ensure that the epoxy resin is fully cured and reduce the generation of internal defects. Optimized curing conditions are helpful for improving the quality and performance of the epoxy encapsulation layer.
2. Improving the Sealing of the Encapsulation: Adopting advanced encapsulation processes and sealing materials to improve the sealing of the LED encapsulation, preventing external environmental factors such as moisture and oxygen from entering the epoxy encapsulation layer, thus slowing down the aging rate of the epoxy resin.

(3) Optimizing the Usage Environment

1. Controlling Temperature and Humidity: Try to control the temperature and humidity of the LED working environment within an appropriate range, and avoid the LED working in a high-temperature and high-humidity environment for a long time. Heat dissipation design and moisture-proof measures can be adopted to improve the usage environment of the LED.
2. Reducing Ultraviolet Irradiation: In the application of LEDs, try to reduce the irradiation of ultraviolet rays on the epoxy encapsulation layer. For example, an ultraviolet protection layer can be added to the surface of the LED or encapsulation materials with ultraviolet resistance can be used.

Conclusion

During long-term use, epoxy encapsulated LED will experience various aging phenomena, including optical, physical, and chemical aspects. These aging phenomena will have a significant impact on the optical, electrical, and thermal performance of LEDs. Through measures such as optimizing the epoxy resin formula, improving the encapsulation process, and optimizing the usage environment, the aging of the epoxy resin can be effectively prevented and mitigated, and the reliability and service life of LEDs can be improved. In the future, with the continuous development of LED technology, the performance requirements for epoxy encapsulation materials will become higher and higher. It is necessary to conduct further in-depth research on the aging mechanism and anti-aging technology of epoxy resin to meet the development needs of the LED industry. At the same time, it is also necessary to strengthen the aging monitoring and evaluation of LED products during actual use to provide a more accurate basis for the quality control and performance optimization of LED products.

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