In the use of uv-led, the losses caused by the emission of photons generated by the radiation composite mainly include three aspects: the internal structure defects of the chip and the absorption of materials, the reflection loss caused by the refractive index difference of photons on the exit plane and the total reflection loss caused by the incident Angle greater than the critical Angle of total reflection. As a result, much light cannot escape from the chip to the outside. The transparent rubber layer with relatively high refractive index (potting rubber) is injected on the chip surface. As the rubber layer is between the chip and the air, the loss of photons in the interface is effectively reduced and the light extraction efficiency is improved. In addition, the role of uv-led potting adhesive also includes the chip for mechanical protection, stress release, and as a photoconductive structure.
Therefore, the choice of packaging adhesive and lens material directly affects the luminous efficiency and service life of uv-led devices, requiring high light transmittance, high refractive index, good thermal stability, good liquidity, and easy spraying. In order to improve the reliability of uv-led package, it is also required to have the characteristics of low moisture absorption, low stress, weather resistance and environmental protection. Currently commonly used packaging adhesives include epoxy resin and silica gel.
It can be seen from the transmittance diagram of different wavelengths in figure 1 that, for ultraviolet LED package, quartz glass has the highest transmittance (up to 95%), followed by silicone resin, and the uv transmittance of epoxy resin decreases sharply with the wavelength getting shorter. However, although quartz glass has a high ultraviolet light transmittance, its thermal processing temperature is high, and it is not suitable for sealing in the LED core area. Therefore, quartz glass is generally only used as the outermost lens material in the LED packaging process.