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Effect of temperature and moisture on LED reliability and its mechanism analysis
Abstract
Temperature and moisture are two key factors to affect LED reliability. The effects of temperature and moisture on the reliability are numerically investigated based on a real LED module from one company. The changes of temperature and moisture with time are given, and the distributions of thermal and moisture stresses are presented. The results demonstrate that temperature of LED module can reach steady after 1.5 minutes while its moisture field needs 46 days to reach balance. The moisture stress is in the magnitude order of 103 Pa, but the magnitude order of thermal stress is about 108 Pa, which means the moisture stress is much smaller than the thermal stress during LED operation process. The results provide the basis for LED packaging design and further study on the effect of thermal coupled with moisture on the LED reliability.
... They found that luminous efficiency of LED would decrease and its reliability would be reduced since thermal stress is enhanced. Luo and Liu et al. [19][20][21] conducted research on heat dissipation and rapid reliability assessment of high-power LED and LED light source, including the defects on thermal behaviors, optical performance, and stress evolutions. Their research results showed that inappropriate process development, moisture and temperature are the dominated reasons leading to LED defects and reliability decrease. ...
... It should be emphasized that all the following calculation is based on a specific sample and temperature values, the obtained values are only suitable for this sample, however, the method is universal and it is used to prove the abovementioned analytical solution. (19) where n is the measurement times, xi is measured value of ith measurement, and x is average of all measured values. (2) PC moisture absorption at 90℃ Referencing data processing of Table 1, D 90 and its uncertainty are obtained as follows by processing data in Table 2. ...
To analyze the effect of moisture on the reliability of LED modules, diffusivity of packaging materials must be provided. Polycarbonate (PC) is an important material for LED lens. Its diffusivity measurement is usually based on the solution of one-dimensional diffusion. In this paper, the diffusivity measurement of PC was conducted. Simulation and analytical close-form solution (CFS) were also used to predict the moisture absorption process. The comparison among the results obtained by analytical solution, simulation and experiments was presented. The results show that they agree well with each other, proving that the present analytical method can be used to calculate the moisture diffusivity coefficient, which is important for LED reliability evaluation.
... The largest thermal stress centers on the LED chip part which is larger than lOMPa. It agrees well with the results in the work [11]. The results are listed in table 1. ...
High power light-emitting diodes (LED) are widely applied in many fields for their special advantages compared with other light sources. Many researches on LED reliability have been conducted in order to build the better LED design. However, seldom detailed reasons for the LED performance degradation are illustrated in the previous research when LED modules work under a certain thermal and moisture condition. This paper aims at studying one kind of LED failure mechanisms caused by moisture diffusion. Through the accelerated experimental test of high power LED modules, we find that the decent speeds of the optical output are different for different LED samples groups. For these tested LED samples, we detect the surfaces of LED chips by Atomic Force Microscope (AFM). The AFM micrographs show that different kinds of chips have their own characteristic surface textures. As the LED modules work under a certain condition, the thermal stress and moisture invasion may cause delaminations inside the modules. This would change the original path of light propagation and result in the degradation of optical output inevitably. For the LED modules packaged by different chips, the forms of delamination occurred at the interfaces between LED chips surface and phosphor layer are not the same. Different chip surface structure has significant impact on the optical output and reliability of LED modules.
High power light emitting diode (HP-LED) as solid-state Lighting (SSL) is a promising light source, and one of the important factors for its wide acceptance is its reliability. [1]Temperature and humidity are two key factors which affect the reliability of HP-LED. Based on the three-dimension model of Multi-chips integrated module LED (MCIM-LED), the effects of temperature and humidity on the reliability of HP-LED are numerically investigated in this paper. The status of thermal conduction and humidity diffusion and the distribution of thermal stress and humidity stress are obtained by the simulation of thermal and humidity respectively. The temperature field reaches equilibrium state in short time, while the stable time of humidity field is dozens of day. In the normal working status, the humidity stress is less than the thermal stress. The results suggest that thermal stress plays a dominant role can be used for the further study on the effects of thermal-humidity coupled on the reliability of HP-LED.
The moisture diffusion phenomenon and moisture stress in a specific LED module was studied in this paper. Firstly, the finite element model of the LED module was built. The moisture diffusion distribution at operation condition with 25°C ~ 85°C, was analyzed. Secondly, the temperature distribution and moisture distribution of the LED module during reflow soldering process were simulated at the initial moisture condition of 85%RH. The thermal stress, moisture stress were analyzed, respectively. The simulation shows that maximum thermal stress in the LED module was 30.1 MPa and the maximum moisture stress was 7.8 MPa. Furthermore, integrated vapor pressure, moisture stress and thermal stress modeling of LED module during reflow were established. The resulting maximum integrated stress was up to 178 MPa. Thirdly, a curing process at the condition of 125°C which was assumed to dry out the moisture in the LED package before reflow process was modeled and the corresponding moisture diffusion distribution was obtained.
This paper designs a 3 × 3 light emitting diode (LED)array with a total power of 9 W, presents a thermal analysis of plate fin, in-line and staggered pin fin heat sinks for a high power LED lighting system, and develops a 3D one-fourth finite element (FE) model to predict the system temperature distribution. Three kinds of heat sinks are compared under the same conditions. It is found that LED chip junction temperature is 48.978 °C when the fins of heat sink are aligned alternately.
A study has been conducted to determine the effects of mechanical stresses induced from the coefficient of thermal expansion (CTE) differential between a light emitting diode (LED) chip, and various substrate materials to which the LEDs were mounted. The LEDs were bonded to typical packaging materials including ceramics, copper and metal matrix composites. The objective of this investigation was to determine the viability of implementing alternate substrate materials for packaging of LED power chips. In particular, thermally induced stresses resulting from the CTE differentials between the alternate substrate materials and the LED sub-mount material were analyzed and compared against the stresses resulting from the nearly ideal CTE match that is realized with traditional ceramic substrates.