LED has the advantages of small size, low power consumption, long life and environmental protection. In the actual production and development process, it is necessary to pass the life test to the reliability level of the LED chip, and improve the reliability level of the LED chip through quality feedback. To ensure the quality of LED chips.
As an electronic component, Light Emitting Diode-led has appeared for more than 40 years, but it has been limited by luminous efficiency and brightness for a long time. It is only used by the indicator light. It broke through the technical bottleneck at the end of the last century, producing high brightness and high efficiency. The LED and Languang LEDs extend the range of applications to signal lights, urban nightscape projects, full color screens, etc., providing the possibility of being an illumination source. As the range of LED applications increases, improving LED reliability is of even greater significance. LED has the advantages of high reliability and long life. In the actual production and development process, the reliability level of the LED chip needs to be priced through the life test, and the reliability level of the LED chip is improved by quality feedback to ensure the LED chip. For this reason, in order to realize the industrialization of full-color LEDs, the conditions, methods, means and devices for the life test of LED chips have been developed to improve the scientificity of the life test and the accuracy of the results.
2. Determination of life test conditions
The work test performed by an electronic product under specified working and environmental conditions is called a life test, also known as a durability test. With the improvement of LED production technology, the life and reliability of the product are greatly improved. The theoretical life of LED is 100,000 hours. If the life test under normal normal stress is still used, it is difficult to maintain the life and reliability of the product. A more objective price is made, and the main purpose of our experiment is to grasp the light output attenuation of the LED chip through the life test, and then infer its life. According to the characteristics of the LED device, after comparative test and statistical analysis, the life test conditions of the chip below 0.3Ã—~0.3mm2 are finally specified:
. Samples are randomly selected, the number is 8 ~ 10 chips, made of Ñ„5 single lamp;
. Working current is 30mA;
. Ambient conditions are room temperature (25 Â° C Â± 5 Â° C);
. The test period is three hours, 96 hours, 1000 hours and 5000 hours;
The working current is 30 mA, which is 1.5 times of the rated value. It is a life test to increase the electrical stress. Although the result does not represent the real life condition, it has great reference value. The life test uses the epitaxial wafer production batch as the mother sample and is random. 8 to 10 chips in one of the epitaxial wafers were taken and packaged into a Ñ„5 single-lamp device for a 96-hour life test, and the results represent all epitaxial wafers of the production batch. It is generally considered that a test period of 1000 hours or more is called a long-term life test. When the production process is stable, the 1000-hour life test frequency is lower, and the 5000-hour life test frequency can be lower.
3, process and precautions
For the LED chip life test sample, a chip, generally referred to as a bare crystal, or a packaged device may be used. Adopting the bare crystal form, the external stress is small, and it is easy to dissipate heat. Therefore, the light decay is small and the service life is long. The difference from the actual application is large. Although it can be adjusted by increasing the current, it is better to use the single-lamp device directly. The life test is carried out in the form of a single lamp device, which causes the factors of the light decay aging of the device to be complicated, and may have chip factors as well as packaging factors. During the test, various measures are taken to reduce the influence of packaging factors, and the details that may affect the accuracy of the life test results are improved one by one, ensuring the objectivity and accuracy of the life test results.
3.1 sample extraction method
The life test can only be carried out by the method of sampling test, which has certain risks. First of all, the product quality has a certain degree of uniformity and stability is the premise of sampling * estimation, only the product quality is considered to be uniform, sampling is representative; secondly, due to the existence of certain discreteness in the actual product quality, we take the partition Random sampling methods to improve the accuracy of life test results. We searched for relevant data and conducted a large number of comparative experiments, and proposed a more scientific sample extraction method: the chip is divided into four regions according to its position in the epitaxial wafer. For the partitioning situation, as shown in Figure 1, 2 to 3 chips per region. A total of 8 to 10 chips, for the difference in life test results of different devices, even contradictory situations, we have specified a method of tightening the life test, that is, 4 to 6 chips per area, a total of 16 to 20 chips, as normal Conditional life test, but the quantity is tightened, not the test conditions are tightened; thirdly, the more the sample quantity, the lower the risk, the more accurate the result of the life test result, but the more the sample quantity, the more the sample quantity Too much will inevitably lead to waste of manpower, material resources and time, and the cost of trials will rise. How to deal with the relationship between risk and cost has always been our research content. Our goal is to minimize the risk at the same test cost by adopting a scientific sampling method.
3.2 Photoelectric parameter test method and device light distribution curve
In the LED life test, the test sample is firstly tested and screened by photoelectric parameters, and the photoelectric parameters are over-regulated or abnormal. 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 result. 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 the same tester test is used before and after the test to reduce unnecessary error factors, which is especially important for optical parameters; In the early stage, we used the change of the light intensity of the measuring device to judge the light decay condition. Generally, the axial light intensity of the device was tested. For a device with a small half angle of the light distribution curve, the magnitude of the light intensity value changes abruptly with the geometric position, and the measurement repeatability is poor. To influence the objectivity and accuracy of the life test results, in order to avoid this situation, a large angle package is adopted, and a non-reflective cup holder is selected to eliminate the light distribution effect of the reflector cup, thereby eliminating the influence of the light distribution performance of the device package form and improving The accuracy of the optical parameter test is subsequently verified by the use of luminous flux measurements.
3.3 The effect of resin deterioration on life test
The existing epoxy resin encapsulating material is reduced in transparency after being irradiated by ultraviolet rays, is a photoaging of a polymer material, and is a result of a series of complicated reactions involving ultraviolet rays and oxygen, and is generally considered to be a photo-induced auto-oxidation process. The effect 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 be improved only by reducing the ultraviolet radiation as much as possible. In the future, the epoxy resin's light decay value can be determined by selecting the packaging material and removing it from the life test.
3.4 Influence of packaging process on life test
The packaging process has a great influence on the life test. Although it is packaged in a transparent resin, the internal solid crystal, bonding, etc. can be directly observed by the microscope for failure analysis, but not all packaging process defects can be observed, for example: The quality of the joints and the process conditions are closely related to temperature and pressure. When the temperature is too high and the pressure is too high, the chip will be deformed to generate stress, which will lead to the introduction of dislocations and even cracks, which will affect the luminous efficiency and life. The stress changes caused by wire bonding and resin encapsulation, such as heat dissipation and expansion coefficient, are all important factors affecting the life test. The life test results are worse than the bare crystal life test, but for the current low power chips, the assessment is increased. The quality range and the life test result are closer to the actual use, and have certain reference value for production control.
4, the design of the life test bench
The life test bench consists of a life test unit board, a gantry and a dedicated power supply unit, and can perform 550 sets (4400 pieces) of LED life test at the same time.
According to the requirements of the life test conditions, the LEDs can be connected in parallel and in series. Parallel connection form: the positive pole of multiple LEDs is connected in parallel with the positive pole, the negative pole and the negative pole. The characteristic is that the working voltage of each LED is the same, and the total current is Î£Ifn. In order to achieve the consistent working current If of each LED, each LED is required. The forward voltage should also be the same. However, the characteristic parameters of the device are different, and the forward voltage Vf of the LED decreases with the temperature rise. Different LEDs may cause the difference of the working current If due to the difference of the heat dissipation conditions, and the temperature of the LED with poor heat dissipation is higher. Large, the forward voltage Vf drops also large, causing the operating current If to rise. Although the above phenomenon can be alleviated by adding a series resistor current limiting, there are disadvantages such as complicated wiring, large difference in operating current If, and inability to apply LEDs of different VFs, and therefore it is not preferable to adopt a parallel connection driving form.
Series connection form: the positive pole and the negative pole of multiple LEDs are connected into a string. The advantage is that the operating current of each LED is the same. Generally, the current limiting resistor R should be connected in series. As shown in Fig. 2, a single string circuit is used. This will cause the string of 8 LEDs to go out. From the principle, the possibility of opening the LED chip is extremely small. We believe that life-testing LEDs work better with constant current and series connections. The LED constant current driving circuit formed by the common 78 series power circuit IC is characterized by low cost, simple structure and high reliability; the constant current can be conveniently adjusted by adjusting the resistance of the potentiometer; The current is more accurate and stable, and the influence of the power supply voltage change is small. We take the circuit of Figure 2 as the basic route and form a life test unit board in parallel. Each unit board can simultaneously perform 11 sets (88 pieces) of LED life test.
The gantry is a general standard modular shelf. After reasonable wiring, each unit board can be easily loaded and unloaded for online operation. Dedicated source device, the output is 5 DC 36V safe voltage, load capacity is 5A, 2 of them have microcomputer timing control function, can be automatically turned on or off, 5 input and output respectively indicate, Figure 3 is the life test bench system wiring diagram .
Advantages of this life test bench design:
. Life test current is accurate, adjustable and constant;
. With microcomputer timing control function, it can be turned on or off automatically;
. It can be applied to different VF LEDs at the same time without additional adjustment;
. With the unit combination structure, the life test unit can be added at any time to realize online operation;
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