High-power LED package has been a hot research topic in recent years, due to their complex structures and production process and directly effects on the performance and lifetime of LED chips / LED lamps. The selection of LED packaging methods, materials, structures and production process is mainly determined by chip constructions, optical / mechanical characteristics, special applications and costs. After 40 years of development, LED package technology has gone through four major stages: Lamp LED, SMD LED, Power LED Tubes and COB LED. With the continuous increasing of chip power, especially the developing demand of solid-sate lighting technology, high-power LED package need to meet more restrict requirements in the aspect of optical parameters, thermal parameters, electrical parameters and mechanical structures. In order to effectively improve the luminous efficiency, new ideas and design concepts need to be adopted.
Phosphor powder plays a role of color mixing and white-ray creating in the LED package. Their major characteristics are size, shape, luminous efficiency, conversion efficiency, stability (thermal and chemical) and so on. Luminous efficiency and conversion efficiency are key points. It has been proved that, with the raise of temperature, the quantum efficiency of phosphor powder will be reduced, as well as the output luminance. In the mean time, the wavelength will be changed, leading to variations in color temperatures. High working temperature will also speed up the deterioration of phosphor powder. Phosphor powder is coated by epoxy and silicone rubber mixtures, the thermal conductivity of which is relatively poor. When exploded under violet rays or UV rays, phosphor powder will be easily deteriorated. Besides, high temperature also exerts bad influences on thermal stability of encapsulants and phosphor powder. Generally, the refractive index of phosphor powder is larger than or equal to 1.85, while the refractive index of silica is around 1.5. Due to the unmatched refractive index, light scattering phenomenon occurs on the surface of phosphor particles. This problem can be improved by adding nano phosphor in the silica gel, increasing the refractive index to 1.8 or more for the later and increasing the luminous efficiency by 10%-20%.
Overall, in order to improve the luminous efficiency and reliability of LED chips, especially high-power LED package, encapsulation layers tend to be gradually replaced by phosphor practices with high refractive index. By doing so, both the uniformity and packaging efficiency can be improved dramatically.