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How to test the life of LED chips part two?

How to test the life of LED chips part two?

Oct 10, 2022

The use of single-lamp devices for life testing has complex factors that cause photoaging of the devices, which may include chip factors and packaging factors. In the test process, various measures were taken to reduce the influence 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.

1. Sample extraction method

Life test can only adopt the evaluation method of sampling test, which has certain risks.

First of all, a certain degree of uniformity and stability of product quality is the prerequisite for sampling evaluation. Only when product quality is considered uniform can sampling be representative;

Secondly, because there is a certain degree of dispersion in the actual product quality, we adopt the method of random sampling in districts to improve the accuracy of the LED display module life test results. By searching for relevant information and conducting a large number of comparative experiments, we proposed a more scientific sample extraction method: Divide the chip into four areas according to its position on the epitaxial wafer, each area has 2 to 3 chips, and 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 the method of tightening the life test, that is, 4 to 6 chips per zone, a total of 16 to 20 chips, and the life test is carried out under normal conditions, but the number is increased. Strict, not stricter 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 excessive number of samples will inevitably cause waste of manpower, material resources and time, and increase the cost of testing. 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 under the same test cost by adopting a scientific sampling method.

2. Photoelectric parameter test method and device light distribution curve

In the LED life test, the test samples are tested and screened for photoelectric parameters, and the devices with out-of-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 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 instrument, it is also stipulated in principle that the same test instrument is used before and after the test to reduce unnecessary error factors. This is particularly important for the optical parameters; In the initial stage, we used the change of the light intensity of the measuring device to judge the light decay status. Generally, the axial light intensity of the device was tested. For the device 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. Affect the objectivity and accuracy of the life test results. In order to avoid this situation, a large-angle package is used, and a non-reflective cup holder is selected to eliminate the light distribution effect of the reflective cup, eliminate the influence of the light distribution performance of the device package form, and improve The accuracy of the optical parameter test is verified by subsequent use of luminous flux measurement.

3. The influence of resin deterioration on life test

The transparency of the existing epoxy resin encapsulating materials decreases after being irradiated by ultraviolet rays, which is the photo-aging of the polymer materials and the result of a series of complex reactions involving ultraviolet rays and oxygen. It is generally considered to be an auto-oxidation process initiated by light. The impact of resin deterioration on the results of the life test is mainly reflected in the long-term life test of 1000 hours or more. At present, the only way to improve the objectivity and accuracy of the life test results is to reduce the exposure of ultraviolet rays as much as possible. In the future, it is also possible to select the packaging material or verify the light attenuation value of the epoxy resin and exclude it from the custom LED screen life test.

4. The influence of packaging process on life test

The packaging process has a greater impact on the life test. Although transparent resin packaging is used, the internal solid crystal bonding 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 solder joints and process conditions are closely related to temperature and pressure. Too high temperature and too much pressure will cause chip deformation and stress, which will introduce dislocations and even dark cracks, which will affect luminous efficiency and life.

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