LED released nitrides red powder for high light quantum density white light.

Abstract: white LED is developing towards higher optical efficiency, better light quality, higher encapsulation density and higher reliability. The performance of nitride red phosphors directly affects the luminous efficiency, color temperature, color rendering index and service life of white LED, especially its high temperature and humidity resistance. It plays a vital role in maintaining the light efficiency and anti bleaching performance of medium and high power devices. The company has released a new series of red powder products, showing good stability against high temperature and high humidity environment. It has an important supporting role in improving the reliability of high power white LED devices.

1. Reliability challenges of phosphors due to changes in the working environment of high power devices.

Throughout the development of white LED technology, the packaging structure is from direct insertion, plastic half package to surface mount (SMD, subdivided into PPA, PCT and EMC, etc.) to integrated (COB) and high-power ceramic packages. At the same time, in order to meet the requirements of general lighting, the color rendering index is constantly rising. White LED is developing towards higher optical efficiency, better light quality, higher packaging density and higher reliability.

Phosphors and chips constitute the core part of white LED devices. Especially with the increasing power density of white LED devices, the reliability of nitride red powder is very critical. This performance will significantly affect the maintenance efficiency and color shift performance of white LED, which will affect the service life of finished products. With the continuous development of new packaging structures such as EMC, WLP and CSP, and with the increase of packaging density and input power, the blue light photon density of chips has increased dramatically. The heat released by the non radiative transition during the excitation of phosphors has led to a dramatic rise in the temperature of phosphor particles. According to the previous study of this research group, only one factor may cause the phosphor particle temperature to rise to 200 degrees Celsius, far higher than the chip junction temperature (120 degree). Considering that the phosphor is also affected by high density blue light irradiation and chip heat conduction, it will further increase the temperature of the phosphor particle itself (about 220 degrees Celsius). That is to say, there is a very steep temperature gradient between the interface between the particle and the colloid of the phosphor and the colloid. As a result of the thermal quenching of phosphor itself, the luminous efficiency of the thermal equilibrium state will be greatly reduced by more than 15%. With the continuous improvement of chip technology, chip size will continue to decrease, and the further improvement of optical efficiency and power density will further exacerbate these problems.

Fig. 1 Schematic diagram of temperature gradient around phosphor particles

At the same time, what is more noteworthy is that the high temperature and humidity environment caused by the immersion of water vapor in the encapsulated colloid and the high temperature of the phosphor itself is the more severe test that the phosphor must face. Aluminosilicate yellow green powder (including LuAG and Ga-YAG) has good chemical stability in the present high color white fluorescent phosphor technology. LuAG is the first choice in making high power devices or in highly reliable field because of its excellent thermal quenching characteristics. LuAG green powder is the first choice. However, the red nitrides (including SCASN and CASN two series), which play an important role in enhancing indication, are facing severe challenges under the action of high temperature and humidity. In the paper published by J. Zhu Chem. in 2015, Jie Zhu of NIMS, Japan, proposed the reaction mechanism and the deterioration mechanism of CASN red powder under the action of water vapor, that is, under the action of H2O, the elements of the Sr in Ca (Sr, Ca) were immersed in the oxidizing reaction, and at the same time, the ammonia reaction was also released when the formation of "(", ") and" (")) was carried out. As follows: [1], namely, (Sr, Ca) AlSiN3:Eu red powder changes in the matrix phase under the action of water vapor, while the activator ion Eu2+ is oxidized to Eu3+, resulting in serious deterioration of the luminescent properties of the phosphor. 2 (Sr, Ca) AlSiN3 (s) + 10H2O (g) to (Sr, Ca) Al2Si2O8 (s) + 6NH3 (s) + ((2))

Fig. 2 Schematic diagram of failure mechanism of (SrCa) AlSiN3:Eu caused by water vapor interaction [1]

2, high temperature and humidity performance test and evaluation

In order to accurately evaluate the dependability of red powder, the experimental conditions of high temperature and high humidity cooking were adjusted in this study. The heating temperature was controlled at about 125 degrees Celsius, which made the phosphor slowly deteriorate under relatively mild cooking conditions, and it could be more suitable by prolonging cooking time. In order to study the deterioration of red powder carefully. The specific cooking conditions are 0.18MPa, 100%RH and 125 degrees. The evaluation includes two parts: one is the direct cooking of phosphors, and the other is the removal of some phosphor samples at different intervals for the comparison of the micro morphology and color drifting. The second part is the encapsulation of several phosphors to be tested. The lamp is made into a lamp bead, then placed in the above environment for aging, and the index of the lamp bulb after different processing time is tested. Finally, the above two aspects of the test data are integrated to evaluate the reliability and reliability of nitride powder. The following table lists the red powder products collected from several major phosphor enterprises at home and abroad.

Three, analysis and results

1) micromorphology

Figures 2a, 2b, and 3a and 3b respectively show the morphology of sample1 and sample2 red samples and their morphology after 48h cooking. The morphology of phosphor can be changed to different degrees by comparing the morphology. After sample1 48h cooking, the crystal of the phosphor was seriously cracked and showed a lamellar cleavage, indicating serious deterioration of the crystal, while the morphology of sample2 did not change. In fact, we can see that the body color of sample1 is obviously weakened by observing the powder color, while the sample2 sample is basically unchanged.  

2) phosphor packaging before and after cooking

Figure 4 shows that two samples of sample1 and 2 can be packaged after cooking. The reliability of the samples can be evaluated by comparing the size of the color floating. As shown, sample2 has almost no color floating, and sample1 has severe color bleaching, which is consistent with the change rule of microtopography.

3) light and color indicators after cooking bulbs

As shown in Fig. 5, after being treated with sample1, the color bleaching of 36h beads is 1%, which is more than 6% at 72h, while Sample3 is very stable until the color drift of 72h is no more than 1%. Of course, from the perspective of color drift, the 48h drift of the lamp beads is smaller than that of the beads made of phosphor after cooking. This is mainly due to the fact that the phosphors are encapsulated in silica gel and are affected by the environment in which the water vapor is partly immersed.

Four. Reliability evaluation of new upgrade products of Bo Rui photoelectric.

Through the comparative tests above and the analysis of the deterioration mechanism, the author believes that the main reasons leading to serious deterioration of the red powder come from two aspects, one is the crystallinity of the red powder itself, which is related to the purity of the raw materials, the formulation design and the synthesis process. For example, some impurities in raw materials may cause a large number of defects inside the crystal; and whether the control of the synthesis process is appropriate will affect the morphology of the phosphor crystal; two, the surface state of the crystal. If the surface structure can be adjusted reasonably by surface modification technology, the immersion process of the external water vapor can be generated. A certain inhibitory effect. After nearly a year and a half of technical research, the team successfully overcome the problem of color bleaching of nitride red powder in high temperature and high humidity environment.

1) micromorphology

In order to further evaluate the reliability of sample2, this study extended the cooking time to 168h and found no serious deterioration. As shown in Fig. 6, from the microscopic morphology, the phosphor crystal has not changed significantly.

Fig. 7 shows the microstructure of the latest product before and after cooking. As shown, the upgraded red powder maintained a good state in 168h without cracking or cleavage, and it could be preliminarily judged that the stability of phosphor crystal reached a level comparable to that of sample2.

2) anti bleaching performance

Figure 8 shows the floating color curve of the product and sample2 phosphor after cooking. As shown in the picture, the packaged phosphors can be used to encapsulate the products. The results show that the products of this study are quite stable with sample2. When 168h is used, the range of color floating is about 1%. Similarly, it can be seen from the color drift curve of the beads after cooking in Fig. 9 that both of them show a good stability in the color range of 168h at 0.8%.

Five, conclusion

With the development of white LED towards higher optical efficiency, better quality of light and color, higher packaging density and higher stability. The new structure based on flip chip CSP and WLP structure is becoming more mature or brings more challenges to phosphors. The research work mainly includes two aspects. On the one hand, it aims to provide a reliable method for evaluating the reliability of phosphors for packaging enterprises. On the other hand, by developing red powder with excellent high temperature and humidity resistance, it can provide strong support for light-emitting materials to enhance the reliability of high power and high color white light devices. In fact, this research product has been mass produced and applied to some high-end customers. From the feedback data of long-term aging performance provided by these users, we can see that there is a good correlation between the accelerated degradation model of nitrided red powder and its long-term aging behavior under actual working conditions. This study will report relevant research findings in subsequent research reports.

Of course, the factors that affect the working condition of the lamp are very complicated. It will be influenced by many factors, such as the material of the bracket, the type of the support structure, the sealing characteristics of the encapsulation glue, and the input power density of the device. We also sincerely hope that we can carry out in-depth technical exchanges and discussions with industry experts, especially the technical experts of packaging enterprises, and jointly promote research on related technical problems.

References

1., Jie Zhu, Le Wang, Tianliang Zhou et al. Moisture-induced degradation and and, Moisture-induced, ","

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