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What causes LED lenses to turn yellow and develop cracks?
Release Date:
2021-11-22 10:37
Source:
LED lens It is a type of lens primarily used in lighting, chemical instruments, and similar applications. So, what causes LED lenses to turn yellow?
1. Discoloration of LED lens coatings due to sulfidation. After the sulfidation reaction in the LED lens, the functional areas of the lamp turn black, the luminous flux gradually declines, and color temperature drift becomes pronounced. The electrical conductivity of silver sulfide increases with rising temperature, making leakage current more likely during operation; in severe cases, the silver layer is completely corroded, exposing the underlying copper. Silver exhibits a strong affinity for sulfur, oxygen, and chlorine. It readily adsorbs water molecules from the air, forming a thin water film on its surface; oxygen, sulfur, and chlorine in the atmosphere then penetrate this film, generating insoluble silver oxide, silver sulfide, and silver chloride, which cause discoloration and corrosion on the silver layer. Rougher silver‑plated surfaces are more prone to condensation than smooth ones, allowing corrosive media to penetrate and leading to discoloration. If the surface is not properly cleaned after plating, residual electroplating solution containing silver ions will rapidly discolor upon contact with humid air.
2. Alter the color of the chip‑bonding adhesive in LED lenses. The LED chip is attached to the leadframe using an adhesive—except for eutectic bonding. As the LED chip serves as the heat source, all thermal dissipation must occur through it. Moreover, because the light emitted from the chip’s active region is blue, with short wavelengths and high energy, the molecular chains of the encapsulant remain in a high‑energy state over time. The encapsulant acts as a thermal barrier surrounding the chip, which is why it gradually discolors.
Epoxy resins tend to carbonize at high temperatures, and prolonged exposure to blue light can cleave molecular bonds, causing yellowing. Silver halogenation, sulfidation, and oxidation reduce the thermal conductivity and bonding strength of the adhesive adhering to the chip.
3. Aging and discoloration of packaging adhesives. Because LED lens They need to be protected with a transparent adhesive, so when selecting an adhesive, pay close attention to its high refractive index and high transmittance. Common choices include silicone adhesives or epoxy resins. However, prolonged exposure to light can cause these adhesives to undergo molecular bond cleavage.
Due to the presence of benzene ring groups, discoloration‑inducing quinone chromophores and yellowing occur under high temperatures and UV exposure. Epoxy materials are more prone to yellowing and cracking when subjected to elevated temperatures and UV irradiation.
LED lens It can enhance light utilization efficiency and luminous efficacy, and can be selected according to different application requirements. Available types include transmissive and refractive lenses. What are the common causes of cracking in LED lenses?
First, cracks in LED lenses may be related to the quality of the adhesive: after curing, the adhesive can form microscopic air bubbles that expand and cause cracking when heated. If the dispensing process is prolonged, or if the epoxy AB adhesive is left to sit for an extended period after mixing, the adhesive layer will thicken, making it increasingly difficult to remove trapped air bubbles.
Second, the amount of adhesive is too large. Epoxy adhesive cures more rapidly when the ambient temperature remains constant. If the adhesive volume is excessive, its viscosity will gradually increase over time, making it progressively thicker and more difficult to degas.
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