Four trends in the development of power white LED packaging technology

1996, Nichia, Nakamura for the first time to use the blue LED with yellow phosphor converted white LED synthesis. The yellow fluorescent powder he uses for Y3Al5O12:Ce3+ (YAG:Ce3+), the phosphor has strong broadband absorption at the wavelength around 470nm, then stimulate 540nm near the yellow light, blue light and fluorescent powder with LED emitted excited yellow light to form white light. Later, because of its high conversion efficiency, good thermal stability and wide band width, YAG:Ce3+ became the mainstream technology of white light.

Trend 1: development of new phosphors

YAG:Ce3+ is one of the earliest widely used in white light LED technology of fluorescent powder, but because the red component of the emission spectrum of white LED is less, it is difficult to obtain high color index and low color temperature; on the other hand, the sustainable development of the semiconductor lighting to promote people to develop a higher conversion efficiency of phosphor. Early, by adding in YAG:Ce3+ (Ca, Sr), S:Eu2+ (Ca, Sr) Ga2S4:Eu2+ red and green phosphors to achieve high color index, low color temperature requirements, but because of its physical and chemical properties of alkaline earth metal sulfide is not stable, easy deliquescence, volatile and corrosive problems, can not meet the LED lighting industry demand. Recently, people have developed a red phosphor thermal stability and excellent chemical stability, can completely replace the alkaline earth metal sulfide to achieve high color index, low color temperature of white LED, because of its high silicon nitrogen (oxygen) tetrahedral structure, called nitrogen oxides, with higher excitation efficiency. At present, foreign companies in the LED phosphor technology is mature, and holds most of the important patents. While they occupied LED market through control of the fluorescent powder patent, YAG:Ce3+ phosphor patent is mainly composed of Nichia [U.S. 5998925] Osram share, occupy the [U.S. Tb3Al5O12: Ce3+ fluorescent powder patent 6812500, 6060861, 65276930], TG, LWB and Tridonic doped Eu2+ (SrBaCa) holds 2SiO4Si:Al, B, P, Ge.. [U.S. 6809347] patent Intematix, Eu2+ doped (SrBaMg) holds 2SiO4O:F, Cl, N, S.. Patent [U.S. 20060027781, 200627785, 200628122]. In contrast, most of the research on LED phosphor used in scientific research institutes, mainly on the synthesis and luminescence of phosphors and other physical properties of the research, but not enough to do in the development of industrial technology.

Trend two: the establishment and development of white LED optical model

Phosphor for white LED, but also according to the specific needs of the LED, such as the size of the phosphor particles, etc.. The study of fluorescent powder mainly focused on the influence of the optical properties of phosphors on the performance of white LED, such as the efficiency of [1-3], the color space distribution [4,5] and the optical quality [6,7]. In these studies, using the method of Monte Carlo ray tracing simulation of the optical properties of the LED package structure by using optical software, the fluorescent powder layer into Mie scattering material, which can simulate by optical excitation and emission characteristics of white LED, but the simulation does not take into account the scattering characteristics of fluorescent powder concrete, lack of experiment. Narendran[8], Zhu[9], Kang[10] and Z. Liu[11] were used to study the optical properties of phosphors.

Trend three: new phosphor coating method

The traditional phosphor coating method is a point powder mode, namely, a mixture of fluorescent powder and colloid is filled into the cup bowl of the chip holder, and then heated and solidified. This coating method is difficult to control the amount of phosphor, and because of the different excitation, so that white LED prone to macular or blue spots, such as uneven color. Philips Lumileds company put forward the method of conformal coating phosphor powder coating, they cover the surface of the flip chip LED with a uniform thickness of the phosphor film, improve the light stability of white LED. There are also companies to use the chip surface deposition of a layer of phosphor to achieve excitation. These coating methods are to contact the chip and phosphor. The optical simulation results of H. Luo et al. Show that the near field excitation method of the phosphor and the chip increases the back scattering loss and reduces the light extraction efficiency of the device [1]. The Australian Sommer uses numerical simulation method to simulate the Philips Lumileds phosphor conformal coating structure, the results show that this coating method can not provide a better angle uniformity [4]. With the further development of the optical simulation of white LED, the scheme of far field excitation of phosphor shows more superiority.

Trend four: high current injection and heat dissipation structure

In order to meet the demand of high luminous flux of general illumination, people have increased the driving power of single chip, the former 1W high power chip is injected into 3W, 5W, or even higher. This makes the heat problem of white LED is more and more serious, people use a variety of heat dissipation technology, such as heat pipe, micro heat pipe [12], water cooling, air cooling and other methods to implement LED.