Borui Optoelectronics Releases Nitride Red Powder for High-Gloss Quantum Density White LEDs

Abstract: White LEDs are moving toward higher light efficiency, better light color quality, higher package density and higher reliability. The performance of the nitride red phosphor directly affects the light efficiency, color temperature, color rendering index and service life of white LEDs , especially the advantages and disadvantages of its high temperature and high humidity resistance for the light efficiency maintenance and color resistance of medium and high power devices. Drift performance plays a vital role. Borui Optoelectronics has released a new series of red powder products, which exhibits good stability against high temperature and high humidity environment erosion and plays an important supporting role in improving the reliability of medium and high power white LED devices.

1. Phosphorus reliability challenge caused by changes in the working environment inside high-power devices

Throughout the history of white LED technology, package structures range from in-line, plastic semi-packaged 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 continues to increase. White LEDs are moving toward higher light efficiency, better light color quality, higher package density and higher reliability.

Phosphors and chips are the core part of white LED devices, especially as the power density of white LED devices continues to increase, and the reliability of nitride red powder is critical. This performance will be good for white LED efficacy and resistance. The color drift performance has a significant impact, which in turn affects the life of the finished product. With the continuous development of new packaging structures such as EMC, WLP, CSP, and the large increase in package density and input power, the blue photon density emitted by the chip increases sharply, and the heat released by the non-radiative transition during the excitation process of the phosphor This causes the temperature of the phosphor particles themselves to rise sharply. According to the previous research of this group, only one factor may cause the temperature of the phosphor particles to rise to about 200 °C, which is much higher than the junction temperature of the chip (120 °C), considering that the phosphor is also irradiated by high-density blue light. The effect of heat conduction of the chip further pushes up the temperature of the phosphor particles themselves (about 220 ° C), that is, a very steep temperature gradient from the phosphor particles to the colloid interface to the inside of the colloid. Therefore, due to the thermal quenching of the phosphor itself, the light efficiency in the thermal equilibrium state is greatly reduced, up to 15% or more. With the continuous improvement of chip technology, the chip size will continue to decrease, and the further improvement of light efficiency and power density will further aggravate the above problems.

Figure 1 Schematic diagram of the temperature field gradient around the phosphor particles

At the same time, it is more worth noting that the high temperature and high humidity environment formed by the water vapor immersed in the encapsulant and the high temperature of the phosphor itself is a more severe test that the phosphor must face. In the current high-color white phosphor technology solution, aluminate yellow-green powder (including LuAG and Ga-YAG) has good chemical stability, and LuAG is produced due to its excellent characteristics in thermal quenching characteristics. LuAG green powder is the first choice for high-power devices or where reliability is particularly high. Nitride red powder (both SCASN and CASN series), which is crucial for the promotion of the indicator, is extremely challenging under the action of high temperature and high humidity. In the paper published by J. Mater. Chem. in NIMS, NIMS, Japan, the reaction mechanism and degradation mechanism of CASN red powder under water vapor were proposed. Under the action of H2O, (Sr, Ca)AlSiN3:Eu The N element in the oxidized H2O is oxidized. At the same time as the formation of (Sr, Ca)Al2Si2O8 and Ca(OH)2, ammonia gas is also released. The specific reaction formula is as follows [1], namely (Sr, Ca)AlSiN3:Eu Under the action of water vapor, the activator ion Eu2+ is also oxidized to Eu3+ while the matrix phase changes, resulting in severe deterioration of the luminescent properties of the phosphor.
2(Sr,Ca)AlSiN3(s) + 10H2O(g) → (Sr,Ca)Al2Si2O8(s) + 6NH3(g) + Ca(OH)2 (s)

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

2, high temperature and high humidity performance test evaluation

In order to accurately evaluate the performance of red powder reliability, the experimental conditions of high temperature and high humidity cooking were adjusted in this study, that is, the heating temperature was controlled at about 125 °C, so that the phosphor slowly degraded under relatively mild cooking conditions. By appropriately prolonging the cooking time, the deterioration behavior of the red powder can be studied in more detail. The specific cooking treatment conditions are 0.18 MPa, 100% RH and 125 ° C. The evaluation includes two parts: one part is to directly cook the phosphor, and some phosphor samples are taken at intervals to perform microscopic morphology and color drift comparison test; The second part is to package several kinds of phosphors to be tested in the same package form, make them into lamp beads, then place the lamp beads in the above environment for aging, and test the indicators after the lamp beads pass different processing time. Finally, the rapid evaluation of the reliability of nitride red powder by combining the test data of the above two aspects. The following table lists the red powder products collected by several major phosphor companies at home and abroad.

Third, analysis and results

1) Microscopic appearance

Figures 2a, 2b and 3a, 3b show the initial morphology of the sample1 and sample2 red powder samples and the morphology after 48 hours of cooking. By comparing the topography, it is very intuitive to judge the degree of change in the appearance of the phosphor. After the sample1 sample was cooked for 48 hours, the phosphor crystals were severely cracked and appeared as layered cleavage, indicating that the crystals were seriously deteriorated; while the morphology of sample2 did not change. In fact, by observing the body color of the powder, we can also very intuitively see that the body color of sample1 is obviously lighter, while the sample2 sample has basically no change.

2) Phosphor packaging before and after cooking

Figure 4 shows that the samples 1 and 2 are subjected to cooking after encapsulation, and the reliability can be evaluated by comparing the magnitude of the color drift. As shown in the figure, sample2 has almost no color drift, and sample1 has a very serious color drift, which is consistent with the change of microscopic morphology.

3) Light color index after lamp bead cooking

As shown in Fig. 5, the lamp beads made by sample1 are subjected to cooking treatment, and the color drift amplitude reaches 1% at 36h, and exceeds 6% at 72h; while sample3 is very stable, and the color drift is not up to 72h. More than 1%. Of course, from the aspect of color drift, the color drift of the lamp bead is smaller than that of the phosphor after the cooking process, which is mainly because the phosphor is encapsulated in the silica gel and is immersed in the outside. The role of water vapor is related to a certain degree of protection of the environment in which it is located.

Fourth, the reliability evaluation of the new upgraded products of Bo Rui Optoelectronics

Through the above comparative test and analysis of its degradation mechanism, the author believes that the main cause of serious deterioration of red powder comes from two aspects, one is the crystallinity of red powder itself, which is related to raw material purity, formulation design and synthesis process. For example, some impurities in the raw material may cause a large number of defects inside the crystal; whether the control of the synthetic process is appropriate will affect the crystal morphology of the phosphor; the second is the surface state of the crystal, such as the surface layer can be properly adjusted by surface modification technology. The structural state can have a certain inhibitory effect on the immersion process of the external water and gas. Through nearly one and a half years of technical research, our research team successfully overcome the problem of color drift of nitride red powder in high temperature and high humidity environment.

　 1) Microscopic appearance

In order to further evaluate the reliability level of sample2, this study continued to extend the cooking time to 168h, and found that there was no serious deterioration. As shown in Fig. 6, from the viewpoint of its microscopic morphology, the phosphor crystals did not change significantly.

Figure 7 shows the microscopic morphology of the latest products before and after cooking in this study. As shown in the figure, the upgraded red powder remained in good condition at 168h without cracking or cleavage, and it was preliminarily judged that the crystal stability of the phosphor reached a level comparable to that of sample2.

2) Anti-color drift performance

Fig. 8 also shows the lamp-color floating curve of the research product and the sample2 phosphor after cooking. As shown in the figure, using the cooked phosphor to encapsulate, it can be seen that the research product and sample2 show considerable stability. At 168h, the amplitude of both colors is about 1%. Similarly, it can be seen from the color drift curve of the lamp bead after cooking in Fig. 9 that the color drift of both at 168 h is about 0.8%, and both show good stability.

V. Conclusion

As white LEDs continue to evolve toward higher light efficiency, better light color quality, higher package density and higher stability. The gradual maturity of new structures such as flip chip-based CSP and WLP structures poses more challenges for phosphors. The research work mainly includes two aspects. On the one hand, it provides a quick method for evaluating the reliability of phosphors for packaging companies. On the other hand, it develops high-power and high-sensitivity by developing red powder with excellent high temperature and high humidity resistance. The reliability of the color white light device provides a strong support on the luminescent material. In fact, the research products have been mass-produced and applied to some high-end customers. From the long-term aging performance feedback data provided by these users, this study has a good correlation between the accelerated degradation model proposed for nitriding red powder and the long-term aging behavior under actual working conditions. This study will be studied in the follow-up study. The report reports relevant research results.

Of course, the factors affecting the color drift of the lamp bead are very complicated, and are affected by many factors such as the material of the bracket, the type of the bracket structure, the sealing property of the encapsulant, and the input power density of the device. We also sincerely hope to conduct in-depth technical exchanges and discussions with industry experts, especially the technical experts of packaging companies, to jointly promote research on related technical issues.

references

1. Jie Zhu, Le Wang, Tianliang Zhou et al. Moisture-induced degradation and its mechanism of (Sr,Ca)AlSiN3:Eu2+, a red-color-converter for solid state lighting. J.Mater.Chem.C, 2015 ,3,3181

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