How to check the reliability of LED chip

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 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 chip.

Determination of life test conditions：Electronic products in the specified working and environmental conditions, the work 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 condition, And then deduce its life span.

According to the characteristics of LED devices, through comparative test and statistical analysis, the final provisions of 0.3× 0.3mm2 chip life test conditions: sample random selection, the number of 8 ~ 10 chips, made into the 5 single lamp; The working current is 30mA; The environmental condition is room temperature (25℃±5℃); The test period was 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 electrical stress. Although the results can not represent the real life situation, they have great reference value. Life test to the epitaxial sheet production batch as the master sample, randomly selected one of the epitaxial sheet of 8 to 10 chips, packaged into 5 single lamp device, for 96 hours of life test, the results on behalf of the production batch of all epitaxial sheet.

It is generally considered that the test period of 1000 hours or more is called a long life test.

When the production process is stable, the life test frequency of 1000 hours is low, and the life test frequency of 5000 hours can be even lower.

Process and precautions

For LED chip life test samples, chips can be used, generally referred to as bare crystal, or packaged devices can be used. Using bare crystal form, the external stress is small, easy to heat, so light decay is small, long life, and the actual application of a large gap, although can be adjusted by increasing the current, but not as direct use of single lamp device form intuitive.

The life test is carried out in the form of single lamp device. The factors that cause the photoaging of the device are complicated, including chip factor and package factor. In the process of the test, various measures were taken to reduce the influence of encapsulation 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. Sampling method: Life test can only be evaluated by sampling test, which has certain risk.

First of all, a certain degree of uniformity and stability of product quality is the premise of sampling evaluation, only when the product quality is considered to be uniform, sampling can be representative; 

Secondly, due to the discreteness of the actual product quality, we adopt the method of partition random sampling to improve the accuracy of the life test results. By searching relevant information and conducting a large number of comparative tests, we proposed a more scientific sample extraction method: Chip according to its position in the epitaxial wafer can be divided into four area, 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, which 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 tightening the test conditions;

Thirdly, generally speaking, the greater the number of samples, the lower the risk and the more accurate the result of the life test results. However, the greater the number of samples and the excessive number of samples will inevitably lead to the waste of manpower, material resources and time, and the increase of the test cost. How to deal with the relationship between risk and cost has always been the content of our research. Our goal is to minimize the risk under the same test cost by adopting scientific sampling methods.

2. Photoelectric parameter test method and device light distribution curve

In the LED life test, the photoelectric parameters of the test samples are tested and screened to eliminate the devices whose photoelectric parameters are over the standard or abnormal. The qualified ones are numbered one by one and put into life test. After the continuous test is completed, the retest is carried out 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, also stipulate in principle before and after the test is the same tester test, in order 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. In addition, a bracket without a reflecting cup was selected to exclude the light distribution function of the reflecting cup and eliminate the influence of the device packaging form on the light distribution performance, so as to improve the accuracy of the optical parameter test, which was verified by the light flux measurement.

3. Influence of resin deterioration on life test

The transparency reduction of existing epoxy resin packaging materials after exposure to ultraviolet light is the result of photoaging of polymer materials and a series of complex reactions with the participation of ultraviolet light and oxygen. It is generally considered to be a photoinitiated 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 objectivity and accuracy of the life test results can only be improved by reducing the ultraviolet radiation as far as possible. In the future, it can be excluded from the life test by selecting the packaging material or verifying the light decay value of epoxy resin.

4. The influence of encapsulation process on life test

The packaging process has a great influence on the life test. Although transparent resin packaging is adopted, the internal solid crystal and bonding can be directly observed with a microscope for failure analysis, but not all the packaging process defects can be observed, for example: Bonding solder joint quality is closely related to the process conditions such as temperature and pressure, and too high temperature and pressure will cause chip deformation and stress, thereby introducing dislocation, even dark crack, affecting the luminous efficiency and life.

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.