Analyze how to scientifically conduct LED chip life test

1. Introduction

As an electronic component, light emitting diodes ( LEDs ) have been around for more than 40 years, but for a long time, they were limited by luminous efficiency and brightness and were only used for indicator lights. It was not until the end of the last century that the technical bottleneck was broken and high-brightness and high-efficiency LEDs and blue light LEDs were produced, expanding their application range to signal lights, urban night scene projects, full-color screens, etc., providing the possibility of using them as lighting sources. With the expansion of the application range of LEDs, improving the reliability of LEDs has become more important. LEDs have the advantages of high reliability and long life. In the actual production and research and development process, it is necessary to evaluate the reliability level of LED chips through life tests, and improve the reliability level of LED chips through quality feedback to ensure the quality of LED chips. For this reason, while realizing the industrialization of full-color LEDs, conditions, methods, means and devices for LED chip life tests have been developed to improve the scientific nature of life tests and the accuracy of results.

2. Determination of life test conditions

The working test of electronic products under the specified working and environmental conditions is called life test, also known as durability test. With the improvement of LED production technology, the life and reliability of products have greatly improved. The theoretical life of LED is 100,000 hours. If the conventional life test under normal rated stress is still used, it is difficult to make a more objective evaluation of the life and reliability of the product. The main purpose of our test is to grasp the light output attenuation of LED chips through life tests, and then infer their life. According to the characteristics of LED devices, after comparative tests and statistical analysis, the life test conditions for chips below 0.3×～0.3mm2 are finally specified:

[1]. Samples are randomly selected, with the number of chips being 8 to 10, and made into a ф5 single lamp;

[2]. The operating current is 30mA;

[3]. The environmental condition is room temperature (25℃±5℃);

[4] The test period is 96 hours, 1000 hours and 5000 hours;

The working current of 30mA is 1.5 times the rated value. It is a life test with increased electrical stress. Although the result cannot represent the actual life situation, it has great reference value. The life test uses the epitaxial wafer production batch as the master sample, randomly selects 8 to 10 chips from one of the epitaxial wafers, packages them into ф5 single-lamp devices, and conducts a 96-hour life test. The result represents all epitaxial wafers in this production batch. It is generally believed that a test cycle of 1000 hours or more is called a long-term life test. When the production process is stable, the frequency of 1000-hour life tests is low, and the frequency of 5000-hour life tests can be even lower.

3. Process and precautions

For LED chip life test samples, chips, generally called bare crystals, or packaged devices can be used. In the bare crystal form, external stress is small and heat is easy to dissipate, so the light decay is small and the life is long, which is quite different from the actual application situation. Although it can be adjusted by increasing the current, it is not as intuitive as directly using a single lamp device. When using a single lamp device for life test, the factors that cause the light aging of the device are complex, which may be chip factors and packaging factors. During the test, a variety of measures were taken to reduce the impact of packaging factors, and the details that may affect the accuracy of the life test results were improved one by one to ensure the objectivity and accuracy of the life test results.

3.1 Sample extraction method

Life test can only adopt the evaluation method of sampling test, which has certain risks. First, the premise of sampling evaluation is that the product quality has a certain degree of uniformity and stability. Only when the product quality is considered to be uniform, the sampling is representative; second, due to the certain discreteness of the actual product quality, we adopt the method of random sampling in the partition to improve the accuracy of the life test results. By searching for relevant information and conducting a large number of comparative tests, we have proposed a more scientific way of sampling: the chip is divided into four zones according to its position on the epitaxial wafer. The partition situation is shown in Figure 1. There are 2 to 3 chips in each zone, a total of 8 to 10 chips. For the situation where the life test results of different devices are very different or even contradictory, we have stipulated a method of tightening the life test, that is, 4 to 6 chips in each zone, a total of 16 to 20 chips, and the life test is carried out under normal conditions. It is just the number that is tightened, not the test conditions; third, generally speaking, the more samples are taken, the smaller the risk, and the more accurate the results of the life test results are. However, the more samples are taken, the more samples are taken, which will inevitably cause waste of manpower, material resources and time, and increase the test cost. How to deal with the relationship between risk and cost has always been the subject of our research. Our goal is to reduce the risk to the lowest level at the same test cost by adopting scientific sampling methods.

3.2 Photoelectric Parameters Test Methods and Devices Light Distribution Curve

In the LED life test, the test samples are first screened by photoelectric parameter testing, and the devices with over-specification or abnormal photoelectric parameters are eliminated. The qualified ones are numbered one by one and put into the life test. After the continuous test is completed, the test is repeated to obtain the life test results. In order to make the life test results objective and accurate, in addition to the measurement of the test instrument, it is also stipulated that the same tester is used for testing before and after the test in principle to reduce unnecessary error factors. This is especially important for light parameters. In the early stage, we used to measure the change of the light intensity of the device to judge the light decay condition. Generally, the axial light intensity of the device is tested. For devices with a small half-angle of the light distribution curve, the light intensity value changes sharply with the geometric position, and the measurement repeatability is poor, which affects the objectivity and accuracy of the life test results. In order to avoid this situation, a large-angle packaging form is used, and a non-reflector cup bracket is selected to eliminate the light distribution effect of the reflector cup, eliminate the influence of the light distribution performance of the device packaging form, and improve the accuracy of the light parameter test. It was verified by the subsequent use of luminous flux measurement.

3.3 Impact of packaging process on life test

The packaging process has a great influence on the life test. Although transparent resin packaging is used, the internal solid crystal and bonding can be directly observed with a microscope for failure analysis, but not all packaging process defects can be observed. For example, the quality of the bonding solder joints is closely related to the process conditions of temperature and pressure. Excessive temperature and pressure will cause the chip to deform and generate stress, thereby introducing dislocations and even dark cracks, affecting the luminous efficiency and life. The stress changes introduced by wire bonding and resin packaging, such as heat dissipation and expansion coefficient, are important factors affecting the life test. The life test results are worse than those of bare crystal life tests, but for the current low- power chips, the quality range of the assessment has been increased, and the life test results are closer to the actual use, which has a certain reference value for production control.

3.4 Effect of resin deterioration on life test

The existing epoxy resin packaging materials lose transparency after being exposed to ultraviolet rays. This is the light aging of polymer materials. It is the result of a series of complex reactions involving ultraviolet rays and oxygen. It is generally considered to be a light-induced auto-oxidation process. The impact of resin deterioration on the life test results is mainly reflected in long-term life tests of 1,000 hours or more. At present, the objectivity and accuracy of the life test results can only be improved by reducing ultraviolet radiation as much as possible. In the future, it is also possible to select packaging materials or determine the light decay value of epoxy resin and exclude it from the life test.

4. Design of life test bench

The life test bench consists of a life test unit board, a test stand and a dedicated power supply device, and can perform 550 groups (4400 pieces) of LED life tests at the same time.

According to the requirements of the life test conditions, LEDs can be connected in parallel and in series . Parallel connection: multiple LEDs are connected in parallel with each other, and with each other. The characteristics are that the working voltage of each LED is the same, and the total current is ΣIfn. In order to achieve the same working current If for each LED, the forward voltage of each LED must also be the same. However, there are certain differences in the characteristic parameters between devices, and the forward voltage Vf of LEDs decreases with the increase in temperature. Different LEDs may have different working currents If due to different heat dissipation conditions. LEDs with poor heat dissipation conditions have a larger temperature rise and a larger drop in forward voltage Vf, causing the working current If to increase. Although the above phenomenon can be alleviated by adding a series resistor to limit the current, there are disadvantages such as complex circuits, large differences in working currents If, and inability to apply LEDs with different VFs. Therefore, it is not suitable to use a parallel connection drive.

Series connection form: multiple LEDs are connected in series from positive to negative. The advantage is that the working current of each LED is the same. Generally, a current limiting resistor R should be connected in series. As shown in Figure 2, a single-string circuit is shown. When one LED is open-circuited, the string of 8 LEDs will go out. In principle, the possibility of an open circuit in the LED chip is extremely small. We believe that the LED life test is best done with constant current drive and series connection. The LED constant current drive circuit composed of the common 78 series power supply circuit IC is characterized by low cost, simple structure, and high reliability. The constant current can be easily adjusted by adjusting the resistance value of the potentiometer. The applicable power supply voltage range is large, the drive current is relatively accurate and stable, and the power supply voltage change has little effect. We use the circuit in Figure 2 as the basic route and connect it in parallel to form a life test unit board. Each unit board can simultaneously perform 11 groups (88) of LED life tests.

The test bench is a general standard modular shelf. After reasonable wiring, each unit board can be easily loaded and unloaded to achieve online operation. The dedicated power supply equipment outputs 5-way DC 36V safe voltage with a load capacity of 5A. Two of them have microcomputer timing control function and can be automatically turned on or off. The 5-way input and output are indicated separately. Figure 3 is the wiring diagram of the life test bench system.

Advantages of this life test bench design:

[1]. Life test current is accurate, adjustable and constant;

[2]. With microcomputer timing control function, it can automatically open or close;

[3] Can be used with LEDs of different VFs at the same time without the need for additional adjustments;

[4]. The unit combination structure allows the addition of life test units at any time to achieve online operation;

[5]. Use low voltage power supply to ensure safety performance.