Comprehensive analysis of LED chip life test

As electronic components , LEDs have emerged 40 years, but has long been subject to restrictions luminous efficiency and brightness , only indicator used until the end of the century to break through the technical bottleneck to produce high brightness and high efficiency led indoor lighting wholesale, so the range of applications to the lights, urban landscape projects, such as full- color screen that provides a lighting possibilities. With the increasing range of LED applications , improving LED reliability has greater significance . The LED has advantages of high reliability and long life , in the actual production process of the development , through the life test for the level of reliability of the LED chips * price level and to improve the reliability of the LED chip through quality feedback to ensure that the LED chip quality , for at the same time to achieve full color LED industry , the development of the LED chip life test conditions , methods, tools and equipment , so as to improve the accuracy of the science and life test results .
Comprehensive analytical LED chip life test

    Determine the lifetime test conditions

Electronics in the specified operating and environmental conditions , working life test trials referred to , also known as a durability test . With the improvement of LED production technology , product life and reliability greatly improved , LED theoretical life of 100,000 hours , if you are still using conventional normal rated life test under stress , difficult life and reliability of the product * price to make a more objective , and the main purpose of our experiment is mastered through life test LED chip light output attenuation conditions , and then infer its life. According to the characteristics of LED devices , through comparative testing and statistical analysis, and ultimately provides life test conditions 0.3 × ~ 0.3mm2 chip the following :

[ 1 ] . Random sample , the number of 8 to 10 chips, made ​​ф5 single lamp ;

[ 2 ] . Operating current of 30mA;

[ 3 ] . Environmental conditions at room temperature (25 ℃ ± 5 ℃);

[ 4 ] . Test period of 96 hours , 1,000 hours and 5,000 hours of three;

Rated operating current of 30mA is 1.5 times , is to increase the life of electrical stress test , although the results do not represent the real life situation , but there is a great reference value ; life test in wafer production batch for the mother -like , random which an epitaxial film extract 8 to 10 chips, packaged into a single lamp device ф5 conducted for the 96 hours life test , the results of all on behalf of the epitaxial wafer production batch . Generally, the test period of 1000 hours or more are called long- life test. When the production process is stable , 1,000 hours of life testing frequency is low, 5,000 hours of life testing frequency may be lower .

Process and Precautions

For the LED chip life test sample , the chip may be used , generally referred to as die , after the device is encapsulated may be used . Using die form , external stress is small, easy to heat , so the light fades , long life , the larger the gap between the actual application , although can be adjusted by increasing the current , but not as a single lamp device in the form of direct and intuitive . Form single lamp life test device , resulting in aging of the optical device of complex factors , there may be factors chip package also factors . During the test , take a variety of measures to reduce the impact factors of the package, the details of life on the accuracy of the test results may be affected by one improvement to ensure the objectivity and accuracy of the life test results .

Sample extraction methods

Life test using a sample test only * way to estimate , with a certain risk. First, the quality of products have a certain degree of uniformity and stability is the premise of sampling * estimate , only that the product quality is uniform, only a representative sample ; Second, due to the presence of certain discrete actual product quality, we take the partition random sampling approach to improve the life of the test results accuracy. We find a lot of relevant information and comparison test , proposed a more scientific sample extraction method: according to their position in the chip wafer is divided into four areas , partitioning see Figure 1 shows , each district two to three chips , a total of 8 to 10 chips , device life test results for different disparities , even contradictory , we tightened the provisions of the life test approach , ie every four to six chip area of 16 to 20 chips, the normal life test conditions , but the number of tightening , rather than tightening the test conditions ; Third , generally, the more the number of samples , the smaller the risk , the life test results , the more accurate the result , however , the more the number of samples number of samples too , must build ** force , wasting resources and time trial costs. How to deal with the relationship between risk and cost , has been the content of our study , our goal is by taking a scientific sampling methods , under the same test costs, risk down to a minimum.

Optical parametric test methods and devices with light curve

In the LED life test , the test sample first screening test optical parameters , out over the optical parameters or abnormal regulation device for each number and passer into the life test , the continuous retest after the test to obtain the results of the life test . To make life test result is objective , accurate, in addition to doing the measurement test instrument , but also provides before and after the test principle used is the same tester , to reduce unnecessary error factor , which is particularly important optical parameters ; initial measurement devices we use to determine the change in light intensity light failure conditions, test devices generally axial light intensity , light distribution curve for half-size smaller device , with the dramatic changes in the size and geometry of the light intensity values ​​, measurement repeatability is poor, affect the life of the objectivity and accuracy of the test results , in order to avoid this situation , the use of the package in the form of wide-angle , and optional non-reflective cup holder , remove the cup with light reflection effect , eliminate the impact of the performance of the device package in the form of light distribution , increase precision optical parametric testing , follow-up is verified through the use of flux measurements.

Impact resin deterioration of life test

After existing epoxy encapsulated material by ultraviolet radiation reduce transparency, is light aging of polymer materials, is the result of a complex series of reactions under ultraviolet light and oxygen participation is generally considered to be light -induced auto-oxidation process . Impact resin deterioration of life test results , mainly 1000 hours or more long-term life test, currently only possible by reducing ultraviolet radiation, to improve the life of the test results if objectivity and accuracy. Future also by selecting packaging materials, or the value of the test light fades epoxy , and exclude them from the life test .

Impact of packaging technology on life test

Encapsulation process on a larger life test , although the use of transparent resin package , available microscope to directly observe the internal solid crystal, bonding , etc., for failure analysis, but not all of the defects in the packaging process can be observed , for example: key the quality of bonding pads and the temperature and pressure conditions are closely related , and the temperature is too high , too much pressure will cause the wafer to stress deformation , thereby introducing dislocations, dark crack even affect the luminous efficiency and lifetime. Wire bonding , the introduction of the resin package stress , such as heat expansion coefficient are the important factors of life test , the life test results of the life test than the difference between the die , but low-power chips for the current increase of the assessment mass range , life test results closer to the actual use of the production control has a certain reference value.

Life test bed design

Life test bed by the life test unit board , bench and a dedicated power equipment components , can be 550 group ( 4400 ) LED life test simultaneously.

Life test conditions as required , led outdoor lighting wholesale can be connected in parallel and in series two drive form . Parallel connection: a plurality of positive electrode connecting the LED coming with the positive electrode , negative electrode and the negative electrode in parallel , each of which is characterized as the LED voltage , total current ΣIfn, each LED in order to achieve consistent operating current If, requiring each LED also consistent with the forward voltage . However, there are some differences between the device parameters , and the LED forward voltage Vf decreases with increasing temperature , probably because of the different thermal conditions LED difference, and the difference between operating current If triggered , the poor heat dissipation of the LED, the temperature rise over the large, forward voltage drop Vf is larger, resulting in an increase of the operating current If . Although it is possible to reduce the current limiting resistor in series by adding the above phenomenon , but there are circuit complexity , current If the difference is large, can not apply different LED VF 's other shortcomings , it should not be used in parallel drive form .

Series connection: When the LED cathode to the anode is about to connect multiple clusters , the advantage of operating current through each LED , as should normally limiting resistor in series with R, as shown in two single- string circuit , open circuit when there is an LED , will lead this string of eight LED goes out , the possibility of an open LED chip from the principle of minimal. We believe LED life test , constant current drive to work and better connected in series . LED constant current driver circuit using a common power supply circuit IC constitute 78 series , which is characterized by low cost, simple structure, high reliability ; by adjusting potentiometer resistance , you can easily adjust the constant current ; Big Power supply voltage range, drive more accurate and stable current , supply voltage variation less affected. We have two circuit diagram for the basic line , parallel constituting life test unit board, each cell board can be 11 groups ( 88 ) LED life test simultaneously.

Stand for the general standard modular shelves , after a reasonable layout , so that each cell board can be easily loaded and unloaded realize online operation . Dedicated power supply , 36V DC output for the 5-way safe voltage , load capacity of 5A, 2 of them with a microcomputer timing control feature that automatically turn on or off , 5 inputs, outputs indicate , Figure 3 shows the life test bed system wiring diagram .

The advantages of life test station design options :

[ 1 ] . Life test current and accurate , adjustable, constant ;

[ 2 ] . With a microcomputer timing control feature that automatically turn on or off ;

[ 3 ] . Can also apply different VF of the LED, and do not require additional adjustments ;

[ 4 ] . Use of modular structures , which can increase the life test unit , to achieve online operation ;

[ 5 ] . Low-voltage power supply, safety performance.

More info you can visit:http://www.lighting-ledlight.com/index.php/tag/led-outdoor-lighting-wholesale/