With the continuous improvement of LED device manufacturing technology, its luminous efficiency, brightness, and power have been significantly improved. However, the photoelectric conversion efficiency of LEDs is still only about 20%, with the remaining electrical energy being converted into heat energy, causing the component temperature to rise and the luminous efficiency to decrease. As an integral part of the component, the encapsulation material is even more sensitive to high temperatures. Therefore, failure caused by the encapsulation material is one of the main reasons affecting the lifespan of the entire LED module.
This paper focuses on LED modules using common silicone and phosphor encapsulation materials. Representative samples were selected and subjected to aging tests under high-temperature conditions. The aim is to analyze the failure behavior of the encapsulation materials and find their failure mechanisms. By measuring the illuminance of the samples online, the impact of the failure law of the encapsulation material on the reliability of the LED samples under high-temperature conditions was obtained.
1. Experiment As a typical high-reliability electronic product, LEDs can have a lifespan of several years at room temperature. Testing under conventional conditions would be too time-consuming and costly. According to the Arrhenius model, the lifespan of LED modules decreases with increasing temperature. Therefore, increasing the ambient temperature can accelerate the failure of LED modules. Based on the relevant performance parameters of the LED samples selected in this experiment and the results of previous tests, a constant-temperature high-temperature aging test was conducted at 125℃. The main manifestations of LED failure include: a 30% decrease in illuminance, flickering, and complete LED failure (i.e., complete extinguishing). Therefore, to explore the failure behavior of LED modules under high-temperature conditions, it is necessary to understand the pattern of LED illuminance change over time. Traditional offline testing methods require removing the sample for testing, which interrupts the experiment and significantly affects the accuracy of the data. Therefore, this paper adopts an online measurement method to monitor the change of illuminance over time in real time.
1.1 Experimental Procedure
The experimental procedure is shown in Figure 1. The sample is placed in the test chamber for power-on testing. Its illuminance signal is transmitted to an illuminance meter via optical fiber. The illuminance meter converts the light signal into an electrical signal and transmits it to the acquisition device. The acquired data is collected in a computer using sampling software. This system can detect changes in module illuminance in real time without interrupting the experiment; therefore, the accuracy of the experimental data is higher than that of interrupted testing methods.
Figure 1 - Study on Failure of LED Module Packaging Materials under High-Temperature Aging Conditions
The data acquisition equipment included a fully digital multi-channel illuminance meter and supporting software, optical fiber, and optical fiber clamps. The power supply was a constant current source, providing 350mA of current to the LED samples. The high-temperature aging test chamber used was the Ruikai Instruments RK-TH-408UF high and low temperature cycling test chamber, with the temperature controlled at 125℃.
1.2 Test Samples
There were four types of test samples, as shown in Figure 2. From left to right, they are: a blue LED pure chip sample (hereinafter referred to as the pure chip sample), a blue LED chip with silicone (hereinafter referred to as the silicone sample), a white LED sample with phosphor and silicone (hereinafter referred to as the phosphor silicone sample), and a white LED sample with phosphor (hereinafter referred to as the phosphor sample). These samples are all LED modules with sapphire as the substrate, encapsulated on a conductive substrate using silicone or phosphor.
Figure 1 - Study on Failure of LED Module Packaging Materials under High Temperature Aging Conditions
2. Results and Discussion
2.1 Illuminance Monitoring
No flickering or dead LEDs were observed during the experiment. Therefore, an illuminance decrease of more than 30% in an LED sample was considered a failure. Four types of samples were tested simultaneously at 125℃, with five samples selected for each type. The illuminance of the five samples for each type was averaged and then normalized, as shown in Figure 3. The figure shows that after approximately 120 hours of testing, the illuminance of the pure chip sample decreased by about 8%, while the illuminance decrease of the other three samples exceeded 30%. According to the criteria for judging LED failure, the silicone sample, the phosphor silicone sample, and the phosphor sample failed.
Figure 1 - Illumination Curve
2.2 Appearance Changes
The appearance of the samples was observed after the experiment. The appearance of the samples after the experiment is shown in Figure 4.
Figure 1 (with accompanying image)
Post-Experiment
The image shows different appearance changes in the four samples: the pure chip sample showed little change, with only slight deformation of the outermost epoxy resin lens; the silicone sample showed obvious carbonization and bubbles in the middle; the phosphor silicone sample showed obvious bubbles and some less obvious carbonization in the middle; and the epoxy resin lens of the phosphor sample showed obvious deformation.
2.3 Results Analysis
Before the experiment, the test samples were inspected and found to be free of carbonization and bubbles, and the chip and lens were clean and free of foreign matter. After a high-temperature aging test at 125 ℃, carbonization and bubbles appeared in the silicone sample, and the epoxy resin lens of the sample without silicone deformed. The pure chip sample, which did not use silicone or phosphor, showed the least change and the least light attenuation. After 120 h of aging, the light attenuation was less than 10%. According to the failure judgment criteria, this type of sample has not yet failed. Silicone samples using only silicone and phosphor samples using only phosphor failed after approximately 36 hours of testing. The difference lay in the following: before failure, the illuminance decay rate of the silicone sample was lower than that of the phosphor sample; however, after failure, the illuminance decay rate of the silicone sample accelerated significantly, resulting in a much greater illuminance decay after 120 hours compared to the phosphor sample. Phosphor-silicone samples using both silicone and phosphor failed after approximately 12 hours, with a illuminance decay reaching 90% after 120 hours. In summary, the following conclusions can be drawn:
① Pure chip samples had the longest lifespan. A possible reason is that the chip samples used a sapphire substrate without silicone or phosphor filling, meaning they contained no encapsulation material other than epoxy resin lenses. Therefore, under the same testing time and temperature conditions, silicone samples filled with encapsulation material, phosphor samples, and phosphor-silicone samples all failed, while the illuminance of the chip samples, although decreasing, did not reach 30%.
② Silicone and phosphor contribute to accelerated illuminance decay in the module. Silicone carbonizes under high temperatures, producing gas, which is why noticeable bubbles are visible in the tested samples. In the blue light samples, noticeable carbonization is observed because the sapphire substrate exposes the entire chip, making the carbonization directly observable. However, in the white light samples, a phosphor coating on the outer layer of the chip obscures the carbonization process, resulting in noticeable bubbles and less obvious carbonization. Furthermore, the phosphor coating may hinder heat dissipation from the LED sample, leading to increased temperature and decreased illuminance. Therefore, the illuminance decrease in the phosphor sample is significantly greater than that in the chip sample.
③ At 125°C, epoxy resin expands due to heat. When the test is stopped and the samples are cooled to room temperature, the epoxy resin contracts due to the temperature drop, causing lens deformation on the removed samples. Lens deformation reduces light transmission, but this does not cause fatal light attenuation.
3. Conclusion Common encapsulation materials (such as silicone and phosphor) have a significant impact on the reliability of LED modules. To investigate the influence of encapsulation materials, 125℃ was selected as the ambient temperature. An online measurement method was used to conduct constant-temperature aging tests on four different samples simultaneously in a high-temperature test chamber. The results show that at 125℃, the LED module without silicone and phosphor has the longest lifespan and high reliability. However, the carbonization of silicone and the resulting gases, as well as the phosphor hindering heat dissipation, accelerate the decay of illuminance. Using both silicone and phosphor simultaneously will cause rapid illuminance decay, leading to module failure.
