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Thoughts on Reliability Testing of LED Lighting(III)
- May 24, 2017 -

Fourth, LED lighting accelerated life test method

In order to quickly find the failure point and weak point of the LED light source module, the reliability test experiment is usually carried out by the accelerated life test. The so-called accelerated life test is not to change the failure mechanism under the premise of using the method to improve the stress, so that the device failure to speed up in a short period of time to achieve acceleration in the case of failure rate, life and other data, and then calculated in the normal state Reliability Characteristics under Stress Conditions. Under the condition of increasing the stress, the change of the internal chemistry of the LED can be accelerated, and the defects of the device structure design and material are exposed quickly. It provides the basis and reference for the structural design and material optimization of the LED light source module. At present, the commonly used stress conditions include temperature, humidity, vibration and shock, solar radiation (ultraviolet radiation), electromagnetic radiation, air pressure, chemical (corrosion gas), dust, voltage, current, etc., Light source module more effective acceleration stress is mainly temperature, humidity, current and vibration, LED lighting reliability test method is the key to how to use the combination of stress, time and application of the way. In accordance with the way in which stress is applied during the test, the accelerated life test can be divided into the following.

1. Constant stress accelerated life test

The constant stress accelerated life test is to separate the samples into groups, each of which is tested under a fixed stress. The stress of the sample during the test remains constant and the stress level is not less than three. The test time is longer and the number of samples is relatively large. But compared with the other two accelerated life test is the most mature test method, the test equipment is relatively simple, easy to control the test conditions, test results error is small, and thus widely used. At present, the US Energy Star LED lamp life test is to use this method, you must first test the LED chip at 55 , 85 and a manufacturer specified temperature environment 6000 hours of light failure data, and then test the LED chip in the LED lights In the temperature, you can calculate the LED lamp life. But the constant stress acceleration method takes too long and can not meet the needs of the market.

Step - stress accelerated life test

The step-stress accelerated life test is that the stress experienced by the sample during the test is increased step by step at a certain time interval until the sample produces sufficient degradation. The test was able to observe the failure of components in a relatively short period of time and required only one set of test samples. But the time interval between the two sets of stress is not easy to determine. If the time interval is too short, the transition effect on the stress change will have an effect on the aging of the product. The time interval is too long, and there is no essential difference with the constant stress accelerated life test. And the step-stress-accelerated life test to determine the product life-stress relationship, then the error is relatively large.

At present, a research hotspot is to use the step temperature stress and constant high humidity stress on the LED light source subsystem for accelerated degradation test, to predict its life. The use of this method must be based on the following five assumptions:

(1) The performance degradation experienced by the test sample is irreversible, ie the performance degradation process is monotonic.

(2) At each accelerated stress level, the failure mechanism and failure mode of the test sample remained unchanged.

(3) The accelerated degradation data of the test samples have the same distribution pattern at different stress levels, and the pseudo-failure life of the samples obtained by the performance degradation data should be subject to the same distribution type at different stress levels.

(4) The test sample has a "no memory characteristic" and its residual life is independent of the cumulative mode, depending only on the stress level and the accumulated failure.

(5) The performance degradation process can be described by a linear or linearized expression. 

Generally, three step temperature stress levels and a constant humidity stress level are selected. First, calculate the specific temperature stress step time, and then determine the confidence degree, the number of samples, etc., when the test time to reach the termination time, select the higher degree of fitting degradation The model is used to fit the degenerate data to calculate the pseudo-failure life of the sample at different temperature stress levels under constant high humidity stress. Finally, the reliability characteristics of the light source reliability distribution function under normal stress level are obtained and the life is calculated. The use of this method generally test time of 2000 hours to meet the needs of the market, but the final reliable research results have not been compared with the actual lighting life of the certified lamps.

3. Progressive stress accelerated life test

The progressive stress accelerated life test is that the stress experienced by the sample during the test is increased by the time constant until the sample is sufficiently degraded. The advantage of this test is that the acceleration efficiency is the highest and the test time is the shortest. But in the course of the test, the stress increases continuously with time. In order to determine the degree of degradation of the component and the stress-time dependency, it is necessary to repeat the test several times at several different stress-time rate of change. Statistical analysis is very complex, and the test device is more expensive and therefore less used.

4. High accelerated life test

Now an LED reliability enhancement test is based on the step-by-step stress acceleration and progressive stress acceleration, combined with other stress conditions of the high accelerated life test (HALT). According to the standard GWM8287, the first step in the high accelerated life test is to carry out the step temperature test.

The second step is to carry out rapid temperature change test.

The third step of the step vibration test.

The fourth step of the step vibration and rapid temperature changes at the same time the test.

Finally, according to the actual sample conditions in the above four steps on the basis of progressive stress accelerated cycle test, several stress conditions continue to increase until the product failure so far. The main purpose of this method is to find the product stress limit, unpredictable life, the general test time is less than 100 hours.

 

These four accelerated life tests have their own advantages and disadvantages. In the actual reliability test, the test method should be selected for the characteristics of different test items.

V. Conclusion

At present, LED lighting reliability and accelerated life detection methods are in the study, different research institutions have their own research direction and point of view. I believe that about 2000 hours of accelerated test time the market can accept the test costs will not be too high, stress accelerated test time after the end, in addition to testing commonly used LED lamps luminous flux, color coordinates and other indicators, it is recommended to add LED electrical noise power Spectrum, etc. can reflect the micro-physical mechanism to change the reliability index. Because the change in luminous flux is not sensitive, the actual reliability structure and micro-physical mechanism of some LED lamps have changed after accelerated life testing, but the luminous flux and color coordinates have not changed significantly. If the test with other reliability indicators can be improved Reliability of the reliability test results.

 

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