1 Introduction
As an electronic component, light-emitting diodes (LightEmitTIngDiode-led) 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 by indicator lights. LED and blue light LED, its application range extends to signal lights, urban night scene projects, full-color screens, etc., providing the possibility of being used as a lighting source. With the increase in the scope of LED applications, improving the reliability of LEDs has more important significance. LED has the advantages of high reliability and long life. In the actual production research and development process, it is necessary to price the reliability level of the LED chip through life test, and improve the reliability level of the LED chip through quality feedback to ensure the LED chip For this reason, while realizing the industrialization of full-color LEDs, the conditions, methods, means and devices of LED chip life tests have been developed to improve the scientificity of life tests and the accuracy of results.
2. Determination of life test conditions
Under the specified working and environmental conditions of electronic products, the working test conducted is called the life test, also known as the durability test. With the improvement of LED production technology, the life and reliability of products have improved greatly. The theoretical life of LEDs is 100,000 hours. If the normal life test under normal rated stress is still used, it is difficult to determine the life and reliability of products. To make a more objective price, and the main purpose of our test is to grasp the light output attenuation status of the LED chip through the life test, and then infer its life. According to the characteristics of LED devices, after comparative tests and statistical analysis, the life test conditions of chips below 0.3 & TImes; ～ 0.3mm2 are finally specified:
[1]. Samples are randomly selected, the quantity is 8 ～ 10 chips, made into ф5 single lamp;
[2]. Working current is 30mA;
[3]. The environmental conditions are 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 of the rated value, which is a life test to increase the electrical stress. Although the result does not represent the real life situation, it has a great reference value; Extract 8 to 10 chips in one of the epitaxial wafers, encapsulate them into a Ñ„5 single-lamp device, and perform a 96-hour life test. The results represent all the epitaxial wafers in this production batch. It is generally believed that a test period of 1000 hours or more is called a long-term life test. When the production process is stable, the frequency of 1000-hour life test is lower, and the frequency of 5000-hour life test can be lower.
3. Process and matters needing attention
For the LED chip life test sample, a chip can be used, which is generally called a die, or a packaged device. The die mode is adopted, the external stress is small, and it is easy to dissipate heat. Therefore, the light attenuation is small, the life is long, and the gap between the actual application and the situation is large. Although it can be adjusted by increasing the current, it is better to use a single lamp device directly. The life test is carried out in the form of a single-lamp device, and the factors causing the light aging of the device are complex, which may be caused by chips or packaging. During the test, various 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
The life test can only use the * estimation method of sampling test, which has certain risk. First, a certain degree of uniformity and stability of product quality is the premise of sampling * estimation. Only when the product quality is considered to be uniform, the sampling is representative; secondly, because there is a certain dispersion in the actual product quality, we take the division Random sampling method to improve the accuracy of life test results. We searched for relevant data and conducted a large number of comparative tests, and proposed a more scientific sample extraction method: the chip was divided into four zones according to its position in the epitaxial wafer. For the zoning situation, see Figure 1; 2 to 3 chips per zone There are 8-10 chips in total. For the difference of the life test results of different devices, even the contradictions, we have stipulated the method of tightening the life test, that is, 4-6 chips per zone, a total of 16-20 chips, as normal Conditional life test, only the quantity is tightened, not the test conditions are tightened; third, generally speaking, the greater the number of samples, the lower the risk, the more accurate the results of the life test results, but the more the number of samples Too much will inevitably result in a waste of manpower, material resources and time, and the test cost will rise. How to deal with the relationship between risk and cost has always been the content of our research. Our goal is to reduce the risk to a minimum by adopting a scientific sampling method under the same test cost.
3.2 Photoelectric parameter test method and device light distribution curve
In the LED life test, the test samples are first screened for photoelectric parameters, and the devices whose photoelectric parameters are out of specification or abnormal are eliminated. The qualified ones are numbered one by one and put into the life test. After the continuous test is completed, the retest is performed 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 equipment, it is also stipulated that in principle, the same tester is used before and after the test to reduce unnecessary error factors, which is particularly important for the optical parameters; In the early stage, we used the measurement device to change the light intensity to judge the light attenuation status. Generally, the axial light intensity of the device is tested. For the device with a small half-angle of the light distribution curve, the intensity of the light intensity changes sharply with the geometric position, and the measurement repeatability is poor. It affects the objectivity and accuracy of the life test results. To avoid this situation, a large-angle package is used, and a non-reflecting cup bracket is used to eliminate the light distribution effect of the reflective cup and eliminate the influence of the light distribution performance of the device package. The accuracy of the optical parameter test is subsequently verified by using luminous flux measurement.
3.3 Influence of resin deterioration on life test
Existing epoxy resin encapsulation materials have reduced transparency after being irradiated with ultraviolet rays. It is the photoaging of polymer materials and the result of a series of complex reactions involving ultraviolet rays and oxygen. It is generally considered to be a photo-induced automatic oxidation process. 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 objectiveness and accuracy of the life test results can only be improved by reducing the ultraviolet radiation as much as possible. In the future, the decay value of the epoxy resin can be selected by encapsulating materials, or it can be excluded from the life test.
3.4 Impact of packaging process on life test
The packaging process has a great influence on the life test. Although the transparent resin package 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, such as: bond The quality of the joints and the process conditions are closely related to temperature and pressure. Too high temperature and too high pressure will cause the chip to deform and generate stress, thereby introducing dislocations and even dark cracks, affecting luminous efficiency and life. Wire bonding and stress changes induced by resin packaging, such as heat dissipation and expansion coefficient, are all important factors that affect the life test. The life test results are worse than the bare crystal life test, but for the current low-power chips, the evaluation is increased. The quality range and life test results are closer to the actual use and have a certain reference value for production control.
4. Design of life test bench
The life test bench is composed of a life test unit board, a stand and special power supply equipment, and can perform 550 groups (4400) LED life tests at the same time.
According to the requirements of life test conditions, LEDs can be driven in parallel or series. Parallel connection: the positive and negative poles of multiple LEDs and the negative and negative poles are connected in parallel. The characteristic is that the operating voltage of each LED is the same and the total current is ΣIfn. In order to achieve the same operating current If of each LED, each LED is required. The forward voltage must be consistent. However, there is a certain difference in the characteristic parameters between the devices, and the forward voltage Vf of the LED decreases as the temperature rises. Different LEDs may cause differences in the operating current If due to differences in heat dissipation conditions. LEDs with poor heat dissipation conditions have more If the voltage is large, the forward voltage Vf also drops, causing the operating current If to rise. Although the above-mentioned phenomenon can be alleviated by adding a series resistance current limiting, there are shortcomings such as complicated circuit, large difference in operating current If, LEDs with different VFs, etc., so it is not suitable to use parallel connection driving.
Series connection mode: Connect the positive pole to the negative pole of multiple LEDs in a string. The advantages are the same as the working current of each LED. Generally, the current limiting resistor R should be connected in series. As shown in Figure 2, it is a single string circuit. When one LED opens, , Will cause the string of 8 LEDs to go out, and in principle the possibility of the LED chip opening is extremely small. We believe that the life test of LED is better with constant current drive and series connection. The LED constant current drive circuit composed of common 78 series power circuit ICs is characterized by low cost, simple structure and high reliability; by adjusting the resistance of the potentiometer, the constant current can be easily adjusted; the applicable power supply voltage range is large, the drive The current is more accurate and stable, and the power supply voltage change has less effect. We take the circuit in Fig. 2 as the basic route and form a life test unit board in parallel. Each unit board can simultaneously carry out 11 groups (88) LED life tests.
The bench is a general standard combined shelf. After reasonable wiring, each unit board can be easily loaded and unloaded to achieve online operation. Dedicated power supply, the output is 5 channels of 36V DC safe voltage, and the load capacity is 5A, of which 2 channels have a microcomputer timing control function, which can be automatically turned on or off, and 5 channels of input and output are indicated separately. .
The advantages of this life test bench design scheme:
[1]. Life test current is accurate, adjustable and constant;
[2]. With microcomputer timing control function, it can be turned on or off automatically;
[3]. LEDs of different VF can be applied at the same time without additional adjustment;
[4]. The unit combination structure can increase the life test unit at any time to achieve online operation;
[5]. Use low voltage power supply to ensure safety performance.
Featured by flat surface with measurement 11.5mm width by 21mm height, side bend. This size covers all of our colors and functions including mono, tunable white, RGB, RGBW,DMX and SPI. S15 is the best-selling size among all our models, which are now widely used in high class façade lighting, pool lighting, advertisement and etc.
Pink Neon Light,Red Neon Lights,Led Neon Rope Light,Blue Neon Lights
Tes Lighting Co,.Ltd. , https://www.neonflexlight.com