LED has the advantages of small volume, low power consumption, long life and environmental protection. In the actual production and development process, it is necessary to evaluate the reliability level of LED chip through life test and improve the reliability level of LED chip through quality feedback, To ensure the quality of LED chips.
● Determination of life test conditions
Electronic work in the provisions of the work and environmental conditions, the work of the test called life test, also known as durability test.
With the LED production technology to improve the level of product life and reliability greatly improved, LED theoretical life of 100,000 hours, if you still use the normal normal stress under the normal life test, it is difficult for product life and reliability To make a more objective evaluation, and we test the main purpose is to master the life of the LED chip light output attenuation, and then infer its life.
According to the characteristics of LED devices, after comparative tests and statistical analysis, the final provisions of the 0.3 × ~ 0.3mm2 chip life test conditions:
Samples randomly selected, the number of 8 to 10 chips, made ф5 single lamp;
Working current is 30mA;
The ambient conditions are room temperature (25 ℃ ± 5 ℃);
The test period is 96 hours, 1000 hours and 5000 hours.
Working current is 30mA is the rated value of 1.5 times, is to increase the life of the electrical stress test, although the results can not represent the real life of the situation, but there is a great reference value; life test to the extension film production batch for the mother, random Extraction of one of the epitaxial wafers in the 8 to 10 chips, encapsulated into ф5 single lamp device, for 96 hours life test, the results represent the production of all the epitaxial wafers.
It is generally believed that a test cycle of 1000 hours or more is called a long-term life test. Production process is stable, 1000 hours of life test frequency is low, 5000 hours of life test frequency can be lower.
● Process and precautions
For LED chip life test samples, you can use the chip, commonly known as die, can also be used after the package device. The use of bare crystal form, the external stress is small, easy to heat, so light fades small, long life, and the actual application of the gap between the larger, although the current can be adjusted to increase, but not as direct use of single-lamp device intuitive.
The use of single-lamp device for life test, resulting in device light aging factors are complex, there may be chip factors, there are packaging factors. In the course of the experiment, a variety of measures are taken to reduce the influence of the factors of the package, and the details of the accuracy of the test results may be improved one by one to ensure the objectivity and accuracy of the life test results.
1, sample extraction method
Life test can only be used to assess the sampling method, with a certain degree of risk.
First of all, the product quality with a certain degree of uniformity and stability is the prerequisite for sample assessment, only that product quality is uniform, sampling is representative;
Secondly, due to the actual product quality there is a certain degree of dispersion, we take a random sampling method to improve the accuracy of life test results. We through the search for relevant information and a large number of comparative tests, proposed a more scientific sample extraction methods: the chip according to its location in the epitaxial wafer is divided into four areas, each area 2 to 3 chips, a total of 8 to 10 chips, For the different device life test results vary widely, and even contradictory situation, we set the strict life test method, that is, each area 4 to 6 chips, a total of 16 to 20 chips, according to the normal conditions of life test, but the number of plus Strict, rather than rigorous test conditions;
Thirdly, in general, the more the number of samples, the less risk, the more accurate the results of the life test results, but the more the number of samples, the more the number of samples will inevitably lead to human, material and time waste, test costs. How to deal with the relationship between risk and cost has always been the content of our research, our goal is to take a scientific sampling method, at the same test costs, so that the risk dropped to a minimum.
2, photoelectric parameters test method and device light distribution curve
In the LED life test, the test samples were first tested for photoelectric parameters, and the optoelectronic parameters were eliminated or abnormal. The qualified persons were numbered and put into life test. After the continuous test, the test was carried out to obtain the life test results.
In order to make the life test results objective and accurate, in addition to the measurement equipment to do the measurement, but also in principle before the test is used in the same test instrument to reduce the unnecessary error factors, which is particularly important for optical parameters; In the early stage, we use the change of the light intensity of the device to judge the light failure condition. Generally, the axial light intensity of the device is measured. For the device with small half-angle of the light distribution curve, the magnitude of the light intensity varies abruptly with the geometric position, Affect the objective and accuracy of the results of life test results, in order to avoid this situation, the use of large-angle package, and the use of non-reflective cup bracket, remove the reflective cup light distribution, eliminate the device package light distribution performance, The accuracy of the optical parameter test is subsequently verified by the use of luminous flux measurements.
3, resin degradation on the impact of life test
The existing epoxy resin encapsulated material is less transparent after UV irradiation, is the photoaging of polymer materials, ultraviolet and oxygen is involved in a series of complex reactions under the results, is generally believed that the photo-induced automatic oxidation process. The effect of resin deterioration on the life test results is mainly to reflect the long-term life test of 1000 hours or more. At present, only the irradiation of ultraviolet rays can be reduced as much as possible to improve the objectivity and accuracy of life test results. In the future, you can also select the packaging material, or test the value of the degradation of epoxy resin, and exclude it from the life test.
4, the impact of packaging technology on life testing
The encapsulation process has a great influence on the life test. Although the transparent resin is encapsulated, the internal solid crystal, bonding and so on can be observed directly by the microscope for failure analysis. However, not all of the packaging defects can be observed. For example, The quality of the solder joint and the process conditions are closely related to the temperature and pressure, and the temperature is too high, the pressure is too large will cause the chip to produce deformation, resulting in dislocation, or even split, affecting the luminous efficiency and life.
Wire bonding, resin encapsulation stress changes, such as heat dissipation, expansion coefficient are all important factors affecting the life test, the life test results than the die life test is poor, but for the current low-power chip, increased the assessment of the Quality range, life test results closer to the actual use of the production control have a certain reference value.
The design of the life test bed
Life test bench by the life test unit board, bench and dedicated power supply equipment, can be 550 groups (4400) LED life test.
According to the requirements of life test conditions, LED can be used in parallel and series connection in two forms. Parallel connection with the positive and negative, the negative and the negative connection in parallel, which is characterized by the working voltage of each LED, the total current ΣIfn, in order to achieve the operating current of each LED If consistent, requiring each LED The positive voltage should be consistent.
However, the characteristic parameters between the devices there is a certain difference, and the LED forward voltage Vf decreases with the temperature rise, different LED may be due to differences in heat dissipation, and cause the difference between the working current If, poor heat dissipation conditions, Large, forward voltage Vf drop is also larger, resulting in working current If rise. Although the current limit can be added by adding a series resistor to reduce the above phenomenon, but there are complex lines, the current current If the difference is large, can not be applied to different VF LED and other shortcomings, it should not be used in parallel connection drive.
Series connection form: the positive connection of multiple LEDs to the cathode, the advantages of each LED through the same current, the general should be in the current limiting resistor R, as shown in Figure 2 for the single-string circuit, when an LED open , Will lead to this string of eight LED off, from the principle of LED chip open the possibility of minimal. We believe that life test LED, to constant current drive and series connection work is better. Using the common 78 series power circuit IC constitute the LED constant current drive circuit, which is characterized by low cost, simple structure, high reliability; by adjusting the potentiometer resistance, you can easily adjust the constant current; suitable power supply voltage range, drive The current is more accurate and stable, the power supply voltage changes less affected. We use the two lines as the basic route, parallel to form the life test unit board, each unit board can be 11 groups (88) LED life test.
Bench for the general standard modular shelves, after a reasonable wiring, so that each unit board can be easily loaded and unloaded to achieve online operation. Dedicated power supply equipment, the output is 5-way DC 36V safe voltage, load capacity of 5A, of which 2 with microcomputer timing control function can be automatically turned on or off, 5 input and output are indicated.