During the use of UV-LED, the loss of photons generated by radiation recombination when emitted outwards mainly includes three aspects: internal structural defects of the chip and absorption of materials, reflection loss of photons at the exit interface due to refractive index differences, and The total reflection loss caused by the incident angle greater than the critical angle of total reflection. Therefore, a lot of light cannot be emitted from the chip to the outside. By injecting a transparent glue layer (potting glue) with a relatively high refractive index on the surface of the chip, since the glue layer is between the chip and the air, the loss of photons at the interface is effectively reduced and the light extraction efficiency is improved. In addition, the role of UV-LED potting glue also includes mechanical protection of the chip, stress relief, and as a light guide structure.
Therefore, the selection of encapsulant and lens materials directly affects the luminous efficiency and service life of the UV-LED device. The encapsulant is required to have high light transmittance, high refractive index, good thermal stability, good fluidity, and easy spraying. In order to improve the reliability of UV-LED packaging, it is also required to have low moisture absorption, low stress, weather resistance and environmental protection. Currently commonly used encapsulants include epoxy resin and silica gel.
It can be seen from the transmittance graphs of different wavelengths in Figure 1 that for UV LED packaging, quartz glass has the highest transmittance (up to 95%), followed by silicone resin, and epoxy resin has the highest transmittance of ultraviolet light. The shorter the wavelength decreases sharply. However, although quartz glass has a high ultraviolet light transmittance, its thermal processing temperature is high, and it is not suitable for sealing the LED core area. Therefore, quartz glass is generally only used as the outermost lens material in the LED packaging process.
Figure 1 Light transmittance of five different packaging materials
On the other hand, Figure 2 (a) and (b) respectively express the UV resistance and heat resistance curves of epoxy resin and silicone resin. We can know that epoxy resin has good performance in all aspects of the visible wavelength range, but in the ultraviolet band, its UV resistance and heat resistance are significantly worse, while silicone resin has obvious advantages. Therefore, in the packaging of high-power UV-LED, silica gel is significantly better than epoxy resin. Therefore, epoxy resin is generally not used as the packaging lens material for UV-LEDs, but silicone and glass are used as packaging materials.
Figure 2a UV aging of epoxy resin and silicone resin
Figure 2b 150e high temperature aging of epoxy resin and silicone resin
The functions that LED chip packaging materials need to have are also different due to the appearance design of the LED packaging. Generally speaking, as the inner packaging material, silica gel mainly has several specifications such as gel and elastomer. Since silica gel is directly connected to the chip, if it is too soft, it may affect its light-emitting effect, and if it is too hard, it may affect the performance of the chip due to its stress. Therefore, the choice of silica gel should be based on the actual process. If the lens is poured by a traditional process, since there are other molded lens packages on the outer layer, gel or elastic glue is usually used for the inner glue of the lens. In the case of the die glue process, since there is no lens on the outside, the glue directly contacts the chip and the external environment, so we usually choose elastic glue that is resistant to external impact, non-sticky, and high in hardness.
As mentioned earlier, the glass material is not suitable for core area sealing due to the high thermal processing temperature, and is only suitable for making packaged lenses. Common glass materials include K9 and quartz glass. When choosing a glass lens, we must consider its UV transmittance and price. Figure 3 (a) and (b) list the transmittance of K9 and quartz glass. Because quartz glass is expensive, the price of quartz glass on the market is several times higher than that of K9 glass. However, the transmittance in the UVA (350nm~420nm) band is not very different. Therefore, in terms of lens material selection, K9 glass can be considered instead of expensive quartz glass.
Figure 3a UV transmittance of K9 glass
Figure 3 b Ultraviolet transmittance of quartz glass
If conditions permit, quartz glass can be used as the outer packaging lens. Such packaging structure UV-LED has higher luminous efficiency, while quartz glass generally uses JGS2 ultraviolet optical quartz optical glass, which is an optical quartz optical melted with hydrogen and oxygen Glass has good transmittance in the range of 220-2500 nm, while the far ultraviolet optical quartz optical glass (JGS1) and infrared quartz optical glass (JGS3) have higher transmittance in the far ultraviolet and infrared bands respectively.
In conclusion, to obtain a packaging structure with high luminous efficiency and high reliability, the choice of packaging glue and lens materials must have high UV transmittance, good matching refractive index, UV resistance, anti-yellowing, and high The temperature tolerance and good stress and other characteristics.