As electronic components, leds (Light Emitting Diode, led) has been more than 40 years, but for a long time, limited to the luminous efficiency and brightness, only for use by Light, until the end of last century broke through the technical bottleneck, produce the high brightness leds and lincom led with high efficiency, extended to the use of Light, city Light engineering, full color, etc., provided the possibility for the lighting Light source. With the expansion of LED application range, it is more important to improve the reliability of LED. Leds have the advantages of high reliability and long life, in the process of actual production research and development, need to life test to evaluate the reliability of the LED chip level, and through the quality feedback to improve the reliability of the LED chip level, to ensure the quality of the LED chip, so at the same time of realizing industrialization of full color LED, developed the LED chip life test conditions, methods, means and devices, etc., in order to improve the accuracy of the science and the results of the life test.
Electronic products in the specified working and environmental conditions, the work of the test is called life test, also known as durability test. With the improvement of LED production technology level, the life and reliability of the product improved, LED the theory of life of 100000 hours, if still adopts the conventional life test under normal stress rating, it is very hard to the life and reliability of the product to make relatively objective evaluation, and the main purpose of our experiment is to, through the life test master LED chip light output attenuation situation, thus infer that its life. According to the characteristics of LED devices, through comparative test and statistical analysis, the life test conditions of chips below 0.3× ~ 0.3mm2 are finally specified:
[1]. Random sample, the amount of 8 ~ 10 chips, made ф 5 single lamp;
[2]. The operating current is 30mA;
[3]. The environmental conditions were at room temperature (25℃±5℃).
[4]. The test cycle is 96 hours, 1000 hours and 5000 hours.
The working current of 30mA is 1.5 times of the rated value, which is the life test of increasing electric stress. Life test for epitaxial wafer production batch of mother sample, a random sample of a piece of 8 ~ 10 particles in the epitaxial wafer chip, encapsulated into a ф 5 single lamp device, life test for 96 hours, the results on behalf of all the extension of the production batch. Generally, the test cycle of 1000 hours or more is called long life test. When the production process is stable, the test frequency of 1000 hours' life is lower, and the test frequency of 5000 hours' life is lower.
For LED chip life test samples, either chip, commonly called bare crystal, or packaged device can be used. The use of bare crystal form, the external stress is small, easy to heat, so the light decline is small, long life, and the actual application of a large gap, although can be adjusted by increasing the current, but not directly using a single lamp device form intuitive. Using single lamp device for life test, the factors that cause the photoaging of the device are complex, which may include chip factor and package factor. In the process of test, a variety of measures were taken to reduce the influence of packaging factors and improve the details that may affect the accuracy of life test results one by one, so as to ensure the objectivity and accuracy of life test results.
Life test can only adopt the evaluation method of sampling test, which has certain risk. First of all, product quality has a certain degree of uniformity and stability is the premise of sampling evaluation, only if the product quality is considered to be uniform, sampling is representative; Secondly, due to the discreteness of actual product quality, we adopted the method of partition random sampling to improve the accuracy of life test results. We by looking for related information and a lot of contrast test, and put forward a more scientific way of sample extraction, the chip according to its position in the epitaxial wafer is divided into four area, see figure shown in a partition, were 2 ~ 3 chip, a total of 8 ~ 10 chips, for different device life test results of six to one, and even contradictory situation, we stipulated the strict life test method, namely were 4 ~ 6 chip, a total of 16 ~ 20 grains chips, according to the normal conditions of life test, just number add yan, rather than add strict test conditions; Third, generally speaking, the larger the number of samples, the smaller the risk and the more accurate the results of the life test. However, the larger the number of samples, the larger the number of samples. How to deal with the relationship between risk and cost has always been our research content. Our goal is to minimize the risk under the same test cost by adopting scientific sampling method.
In the LED life test, the test samples are firstly tested and screened for photoelectric parameters, and the devices with abnormal photoelectric parameters are eliminated. The qualified ones are numbered one by one and put into the life test. 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 in principle the same test instrument is used before and after the test to reduce unnecessary error factors, which is particularly important for optical parameters; Early we use measuring device to judge light failure condition, the change of light intensity the general test device of the axial intensity, half Angle smaller devices for light distribution curve, the size of the intensity values along with the geometric position and a sharp change, repeatability is poor, affect the life test result of the objectivity and accuracy, in order to avoid this kind of situation, take the form of encapsulation large Angle, and choose no reflection cup holder, eliminate reflection cup light distribution, eliminate the influence of device packaging format with optical performance and improve the accuracy of the optical parameters testing, follow-up have been verified by means of luminous flux measurement.
Encapsulation process on a greater influence on the life test, although use transparent resin encapsulation, available microscope to directly observe the internal solid crystal, bonding, etc., for failure analysis, but not all of the encapsulation process defects can be observed, such as: bonding solder joint quality and process conditions are closely related to temperature and pressure, while the temperature is too high, too much stress can produce stress and deformation in the chip to the introduction of dislocation, appear even dark crack, affecting the luminous efficiency and life span. Wire bonding, resin encapsulation is the stress changes, such as heat dissipation, expansion coefficient are the important factors influencing the life test and life test results more naked crystal life test is poor, but for the small power chip, increase the quality of the assessment scope, the life test result is more close to the actual usage, has certain reference value to the production control.
The transparency reduction of the existing epoxy resin packaging materials after ultraviolet irradiation is the result of photoaging of polymer materials and a series of complex reactions involving ultraviolet and oxygen. The influence of resin deterioration on the life test results mainly reflects the long-term life test of 1000 hours or more. At present, the objectivity and accuracy of the life test results can only be improved by minimizing the exposure of ultraviolet radiation. In the future, the light decay value of epoxy resin can be determined by selecting the packaging material, or it can be excluded from the life test. 4. Design of life test bed
The service life test bench is composed of service life test unit board, bench and special power supply equipment. It can conduct service life test of 550 groups (4400 LED) at the same time.
According to the requirements of life test conditions, LED can be used in parallel and series two connected drive forms. Parallel connection form: the multiple leds of the positive and the positive and negative and negative in parallel connection, its characteristic is the working voltage of each LED, the total current of Σ interferons, in order to achieve the working current of each LED the If consistent, requires each LED forward voltage also should agree. However, there are some differences in the characteristic parameters between devices, and the forward voltage Vf of LED decreases with the rise of temperature. Different leds may cause the difference of working current If due to the difference in heat dissipation conditions. For leds with poor heat dissipation conditions, the temperature rise is large, and the forward voltage Vf decreases, causing the working current If to rise. Although the above phenomenon can be alleviated by adding series resistance current limiting, there are disadvantages such as complex circuit, large If difference of working current, and not applicable to LED with different VF, so it is not suitable to adopt parallel connection driving form.
In the form of series connection, the positive and negative electrodes of multiple leds are connected into a series. Its advantages are the same as the working current of each LED. Generally, the current limiting resistance R should be connected into a series. We believe that the life test LED, with constant current drive and series connection working mode is better. The LED constant current drive line composed of IC of 78 series power supply circuit is characterized by low cost, simple structure and high reliability. By adjusting the resistance value of potentiometer, it is convenient to adjust the constant current. Applicable to a wide range of power supply voltage, drive current is more accurate and stable, power supply voltage changes little impact. We take FIG. 2 circuit as the basic route, and make up the life test unit board in parallel. Each unit board can carry out 11 groups (88 LED) life test at the same time.
The shelf is a general standard modular shelf, which is wired reasonably, so that each unit plate can be loaded and unloaded easily, and the online operation can be realized. Special power supply equipment, the output is 5 dc 36V safety voltage, load capacity of 5A, 2 of which have microcomputer timing control function, can be automatically opened or closed, 5 input, output instructions, figure 3 is the life test bed system wiring diagram.
Advantages of the design scheme of this life test bench:
[1]. The life test current is accurate, adjustable and constant;
[2]. With microcomputer timing control function, it can be opened or closed automatically;
[3]. LED with different VF can be used at the same time without additional adjustment;
[4]. The unit combination structure can increase the life test unit at any time and realize online operation;
[5]. Low voltage power supply is adopted to ensure safety performance.
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