Analysis of the color shift mechanism of warm white LED light sources

　　1. Introduction

　　With the continuous improvement of environmental protection requirements, lighting products have also begun to develop in the direction of energy saving and high efficiency. Traditional lighting products have low luminous efficiency and short life, which is not conducive to long-term development. This has prepared the market for LED lighting. The luminous efficiency of LED lamps can reach 8 to 10 times that of incandescent lamps and 2 times that of energy-saving lamps. The life span can also reach 50,000-100,000 hours, which has great advantages over traditional lighting products. With the continuous development of technology and the continuous promotion of products, the price of LED lighting products has continued to decline, and they have begun to enter the general lighting market.

　　Different spaces require different colors of lighting, and choosing the right light source is also an important part of choosing lamps. Warm white light has a color temperature of about 3000K and a yellowish color, which can create a warm lighting environment. Therefore, it has been increasingly used in home lighting and some engineering lighting fields.

　　Warm white LED can be realized by the following methods: (1) blue LED + yellow phosphor + nitride red phosphor; (2) blue LED + single YAG orange-yellow powder; (3) blue LED + yellow phosphor + silicate orange phosphor; (4) blue chip + yellow phosphor + red chip. The first solution has a high color rendering index and high stability, but the overall luminous efficiency is relatively low and the cost is high; the second solution has high stability and low cost, but the color rendering index is low. The third solution has low cost and high luminous efficiency, which is the most advantageous among all the solutions at this stage, but it is affected by the chemical stability of the silicate phosphor itself, and the stability is relatively poor and the color rendering index is low. The fourth solution has high luminous efficiency and high color rendering index, but the cost is high and it is difficult to control the light color stability. Combining the advantages and disadvantages of various technical solutions, solutions 1 and 3 have gradually become the mainstream. The former is mainly used in the field of indoor main lighting due to its comprehensive advantages in luminous efficiency and color rendering index, while the latter is currently mostly used in indoor auxiliary lighting or outdoor lighting due to its significant advantages in luminous efficiency.

　　As the mainstream application solution of warm white LED, there are still many problems in the application that need to be continuously improved. One of the most common problems is that the light color gradually drifts during long-term operation, and the color temperature gradually changes from warm white to pure white. This article conducts a systematic analysis of color-changing lamp beads in order to discover the internal mechanism of color drift and provide a reference for proposing solutions.

　　2. Experimental Methods

　　In this study, several 5050 lamp beads that changed color were randomly selected from a batch of soft light strips that changed color. As shown in Figure 1, the color-changing lamp beads in the lower row have changed significantly compared to the non-color-changing lamp beads in the upper row, from the original orange to yellow-green.

　　Figure 1 Comparison between color-changing lamp beads and non-color-changing lamp beads

　　Four light sources were randomly selected from the non-color-changing and color-changing light sources for data testing, and the test data are shown in Table 1. Comparing the two sets of data, it can be seen that the color coordinates x and y values ​​of the lamp beads after color change have changed significantly, the x value has become smaller, and the y value has increased, and the color temperature has also increased from the original 2940K to 4300K, and the improvement in light efficiency is caused by the increase in color temperature.

　　In order to further analyze the internal mechanism of the color change of the lamp beads, this paper uses energy spectrum (EDS, model _______) to perform elemental analysis on the colloid of the color-changing lamp beads, the silver coating in the bowl, the bracket welding feet and other parts, and explores the mechanism of the color change of the lamp beads through the analysis of the element changes in these parts.　　3. Results and discussion

　　3.1 Colloid site

　　As can be seen from Figure 2a-b, the colloid mainly includes elements such as Si, Y, Al, and O. Due to the obstruction of silica gel to X-rays, these elements should correspond to the constituent elements of the YAG powder and silicate powder contained in the colloid.

　　Figure 2 (a) SEM image of colloid

　　Figure 2 (b) EDS analysis spectrum of colloid

　　3.2 Silver coating inside the bracket

　　The spectrum of Figure 3b shows that the bottom of the bracket contains Y, Al, Ga, Si, Sr, Ba, Cl, and S plasma, among which Y, Al, Ga, Si, Sr, and Ba are all constituent elements of the corresponding phosphors, and the appearance of Cl and S may be that the S and Cl elements in the external environment penetrate through the silica gel and are deposited on the surface of the bracket.

　　Figure 3 (a) SEM photo of the internal silver-plated layer

　　Figure 3 (b) EDS analysis spectrum of the silver-plated layer

　　3.3 Pinout

　　Figures 4a~b show the SEM photos and EDS spectra of the pins outside the bracket, respectively. From the spectra, it can be found that the analysis of the pins contains elements such as S, V, Pb, Sn, etc., which are inferred to be the constituent elements of the solder paste.

　　Figure 4 (a) SEM photo of the lead

　　Figure 4 (b) EDS analysis spectrum of the pin part.　　The author believes that the color change of the lamp bead may be caused by the attenuation of the silicate orange powder, which leads to a significant decrease in the proportion of orange-red light, and then manifests itself as a change from warm white light to pure white light. As for its color change mechanism, through the comprehensive analysis data above, the author believes that the reasons for the color deviation may be the following:

　　(1) Due to the water permeability of organic silicon materials, during use, under the action of high temperature, water penetrates into the silica gel in the form of water vapor and undergoes a hydrolysis reaction with the silicate orange powder therein.

　　(2) The solder paste used for welding contains a large amount of S element. During the production process, the S element invades the colloid and meets water vapor. Under high temperature, a reaction occurs. The reaction formula is: S(2-) +H2O →HS(-) +OH(-) [Note: the "-" in the brackets of OH(-) represents a negative valence]. Once HS(-) is formed, it will significantly aggravate the effect on the silicate phosphor.

　　(3) During use, the ambient atmosphere contains Cl(-) ions. Once they invade and form HCl, they will also play a similar role to HS, that is, they will intensify the corrosion of the phosphor.

　　4. Precautions

　　From the above analysis, it can be seen that silica gel, as a coating carrier of phosphor, plays a vital role in the stability of LED lamp beads. This article combines global applications and proposes preventive measures from the following aspects.

　　First of all, it is required that the colloid and the fluorescent lamp must form a good bond to prevent water vapor and external pollutants from invading from the interface. At the same time, the colloid itself is also required to have a good isolation effect. There are two main types of silicone materials on the market, silicone resin and silica gel. Silicone resin has a higher degree of cross-linking of its internal long-chain macromolecules, and its sealing is significantly better than silica gel, which has a good isolation effect on the intrusion of water vapor. If the intrusion of water vapor can be controlled, the hydrolysis reaction with the phosphor powder can be avoided, thereby significantly reducing the probability of discoloration of the lamp beads.

　　Secondly, we should try to use solder paste or solder with low S content. At the same time, during the production process, we should try to ensure smooth airflow in the production environment to avoid the enrichment of S element in local space. In addition, the use of casing materials that do not contain S element is also one of the effective protective measures.

　　Thirdly, heat is the catalyst that causes the phosphor to deteriorate. Optimizing the design from aspects such as lamp structure and heat dissipation materials to improve heat dissipation capacity and lower operating temperature is also an important improvement measure to alleviate discoloration.

　　Fourthly, you can choose phosphors with waterproof coating. The silicate orange powder (model BO591 series) developed by Borui Optoelectronics uses a unique patented coating technology, which forms a hydrophobic layer on the powder surface to prevent water molecules from adsorbing on the surface of the phosphor particles. It is also beneficial to promote the dispersion of phosphors in colloids and improve the consistency of light color. It has been widely used in many packaging companies.

　　5. Conclusion

　　In this paper, EDS was used to perform elemental analysis on different parts of the color-changing warm white LED lamp beads. The results showed that the silver-plated layer inside the bracket had reacted with the S element. This shows that the acidic product formed by the S element invading from the environment and combining with water vapor may be the key factor aggravating the silicate phosphor.

　　By selecting silicone materials and phosphors with excellent waterproof properties, the problem of lamp bead discoloration can be greatly alleviated. Of course, if we can start from the source and develop phosphors with excellent chemical stability, we can fundamentally solve this problem. This is also the research direction that we, the phosphor material supplier, need to pay attention to in the long term.