Bo Rui photoelectricity releases nitride red powder for high light quantum density white light LED

Abstract: white LED is developing towards higher light efficiency, better light color quality, higher packaging density and higher trust direction. The performance of nitrides red phosphor directly affects the light effect, color temperature, color index and service life of white light LED, especially its high temperature and high humidity performance, which plays an important role in the maintenance and anti color drift performance of the medium and high power devices. Bo Rui optoelectronic published a new series of red powder products, showing good stability in resisting high temperature and high humidity environment erosion, and has an important support for improving the reliability of medium and high power white LED devices.

1. The reliability challenge of phosphor caused by changes in the working environment of high power devices

In the process of the development of white LED technology, the packaging structure is from straight plug, plastic half package to surface mount type (SMD, subdivided into PPA, PCT and EMC) to integrated (COB) and high-power ceramic packaging. At the same time, in order to meet the requirements of general lighting, the color rendering index is continuously raised. White LED is developing towards higher light efficiency, better light color quality, higher packaging density and higher trust direction.

Phosphor and chips are the core parts of the white light LED devices, especially with the increasing power density of the white light LED devices. The reliability of the nitride red powder is very critical. The performance will affect the maintenance rate of the white light LED and the performance of the color resistant bleaching, and affect the service life of the finished product. With the continuous development of new packaging structures such as EMC, WLP and CSP, with the increase of packaging density and input power, the blue light photon density of the chip is increasing rapidly. The heat of phosphor powder released by non radiation transition in the excitation process leads to a sharp increase in the temperature of the phosphor particles. According to the previous study of our group, only this one factor may cause the phosphor particle temperature to rise to about 200 centigrade, far higher than the chip junction temperature (120 degrees C). Considering that the phosphor is also irradiated by high density blue light and the heat conduction of the chip, the temperature of the phosphor particles is further higher (about 220 degrees C). That is to say, a very steep temperature gradient is formed from the phosphor particles and the colloid interface to the interior of the colloid. As a result of the thermal quenching of the phosphor itself, the light effect of the heat balance will be greatly reduced, up to 15%. With the continuous improvement of chip technology, chip size will continue to decrease, and further enhancement of light and power density will further exacerbate the above problems.

Figure 1 Diagram of temperature field gradient around phosphor particles

At the same time, it is more worthy of concern that the high temperature and high humidity environment formed by the water gas of encapsulated colloid and the high temperature of the phosphor itself is a more severe test that the phosphor must face. In the current high color white light phosphor technical scheme, aluminate yellow green powder (including LuAG and Ga-YAG) has good chemical stability, in which LuAG has excellent properties in thermal quenching characteristics, so LuAG green powder is the first choice in the production of high power devices or high reliability requirements. As for ascension, the vital nitride red powder (including two series of SCASN and CASN) is facing a very severe challenge under the action of high temperature and humidity. The Jie Zhu of NIMS in Japan, published on the J. Mater. Chem. in 2015, proposed the reaction mechanism and degradation mechanism of CASN red powder under the action of water vapor, that is, the N elements in the AlSiN3:Eu (Sr, Ca) AlSiN3:Eu under the action of H2O are submerged and oxidize, and the ammonia gas is also released, and the specific reaction formula is also released at the same time. The following [1], (1), (Sr, Ca) AlSiN3:Eu red powder under the action of water vapor, the activator ion Eu2+ is also oxidized to Eu3+ while the matrix phase changes, which leads to the serious degradation of the luminescent properties of the phosphor. 2 (Sr, Ca) AlSiN3 (s) + 10H2O (g) - (Sr, Ca) Al2Si2O8 (s) + 6NH3 (g) + 2

Figure 2 Schematic diagram [1] of (SrCa) AlSiN3:Eu failure mechanism caused by water gas action

2, test and evaluation of high temperature and high humidity performance

In order to evaluate the reliability of the red powder accurately, the experimental conditions of high temperature and high humidity cooking were adjusted in this study. The heating temperature was about 125 degrees centigrade, which made the phosphor deteriorate slowly in the relatively mild cooking condition, and then prolong the cooking time, so that it can be more suitable. It is a careful study of the deterioration of red powder. The specific cooking conditions are 0.18MPa, 100%RH and 125 C. The evaluation includes two parts: one part is directly cooking the phosphor, taking out some samples of phosphor every time, and comparing the micromorphology and color drift. The second part is to encapsulate several kinds of phosphor powder in the same form. The lamps are made into beads, and then the bulbs are placed in the above environment for aging, and the indexes of the lamp after different treatment time are tested. Finally, the reliability of nitrides red powder is evaluated quickly by combining the two aspects of the test data. In the following table, this study collects the red powder products of several major phosphor enterprises at home and abroad.

Three, analysis and results

1) micromorphology

Figures 2a, 2b, and figure 3a and 3b respectively show the initial morphology of sample1 and sample2 red powder samples and the morphology after 48h cooking. The morphology of the phosphor can be very intuitively determined by the contrast morphology. After the sample1 sample was cooked by 48h, the crystal of the phosphor was seriously cracked and presented as a layered cleavage, indicating that the crystal had a serious deterioration, while the morphology of the sample2 had not changed. In fact, by observing the color of the powder, we can also be very intuitive to see that the body color of sample1 is obviously pale, and that the sample2 sample is basically unchanged.

2) encapsulation of phosphor before and after cooking

Figure 4 shows that two samples of sample1 and 2 are packaged after cooking, and the reliability is evaluated by comparing the range of color drift. As shown in the picture, sample2 has almost no color drift, and sample1 has very serious color drift, which is in accordance with the change law of Micromorphology.

3) the light and color index of the lamp after steaming

As shown in Figure 5, the color drifting of the sample1 made by cooking is 1% at 36h, more than 6% at 72h, while Sample3 is very stable until the 72h color drift is not more than 1%. Of course, from the range of color drift, the 48h color drift amplitude of the lamp is smaller than that made by the cooked phosphor, mainly because the phosphor is enveloped in silica gel, which is related to the environment in which the outside water is immersed in a certain degree of protection.

Four, trustworthiness evaluation of the brand new upgrade product of Bo Rui photoelectric

Through the analysis of the above comparison test and its deterioration mechanism, the author thinks that the main cause of the serious deterioration of the red powder comes from two aspects, on the one hand, the crystallinity of the red powder itself, which is related to the purity of the raw material, the design of the formula and the synthesis process. For example, some impurities in the raw material may cause a large number of defects in the crystal, and whether the control of the synthetic process will have an effect on the crystal morphology of the phosphor; two is the surface state of the crystal. If the surface modification technology can be used to regulate the structure state of the surface, it can produce the immersion process of the outside water and gas. A certain inhibitory effect. The problem of color drift of nitrides red powder in high temperature and high humidity environment has been successfully overcome by the technical breakthrough of nearly one and a half years.

1) micromorphology

In order to evaluate the trust level of sample2, this study continued to extend the cooking time to 168h, and found no serious deterioration. As shown in Figure 6, there is no obvious change in the crystal of the phosphor from its microscopic appearance.

Figure 7 shows the micromorphology of the latest products before and after cooking. As shown in the picture, the upgrading of the red powder in 168h is in good condition without cracking or cleavage. It can be found that the crystal stability of the phosphor can reach the same level as that of the sample2.

2) anti color drifting performance

In Figure 8, the lamp - colored float curve made of this research product and sample2 phosphor after cooking is also shown. As shown in the picture, using the phosphor after cooking to package, it can be seen that both the research product and the sample2 show considerable stability. At 168h, the range of both colors is about 1%. In the same way, the color drift curve of the lamp after steaming in Figure 9 shows that both the color drift amplitude of both at 168h is about 0.8%, and both show good stability.

Five. Conclusion

With the white light LED developing towards higher light efficiency, better light color quality, higher packaging density and higher stability direction. The new structure, such as the CSP and WLP structure based on the flip chip, is becoming more and more mature or more challenging to the phosphor. This research work mainly includes two aspects. On the one hand, it aims at providing a fast method for evaluating the reliability of phosphor powder for packaging enterprises. On the other hand, by developing red powder with excellent high temperature and high humidity characteristics, it is a powerful support for improving the reliability of high power and high color white light devices. In fact, this research product has achieved mass production and has been applied to some high-end customers. According to the long-term aging performance feedback data provided by these users, this study has a better correlation between the accelerated deterioration model of nitrided red powder and the long-term aging behavior under the actual working state. This study will report the related research results in the follow-up study.

Of course, the factors that affect the color drift of the lamp work are very complex, which will be influenced by many factors, such as the scaffold material, the type of the support structure, the sealing characteristics of the package, the input power density of the device and so on. We also sincerely hope to be able to conduct in-depth technical exchanges and discussions with the experts of the industry, especially the technical experts of the packaging enterprises, and promote the research on related technical problems.

References

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

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