Is it possible to test out early failing LEDs?
Like other semiconductor devices, LEDs can learn from the screening methods of semiconductor diodes, triodes and integrated circuits to eliminate devices that are prone to early failure, thereby reducing the failure rate of products entering customer applications.
Screening test methods have been developed for various failure models. The commonly used methods are as follows:
(1) Electrothermal accelerated fatigue test
In each production batch of LED products, a certain number of samples are randomly selected according to the specified sampling ratio, and the defective products are exposed under the electric and thermal stress of greater intensity, so as to achieve the purpose of rejection. For example, for conventional low-power LEDs, make IF=30mA, age at a temperature of 85°C for more than 240 hours, and then test and count the failure rate to see if it exceeds the specified ratio.
(2) Environmental test
The environmental test is to simulate the invasion of various natural phenomena encountered in the application of LEDs, and test the performance of LEDs.
Affordability. For example, if the LED is applied in a rocket device, when the rocket is launched, the LED will be subjected to various invasions such as gravitational acceleration, shock vibration, and temperature change, and the materials that constitute the LED will experience various stress shocks. If the processing process is not adequately prevented, it is likely to cause the LED to fail.
Generally speaking, environmental tests are not all tests of the LEDs produced, because some tests are destructive tests, and the tested samples will have changes in appearance and performance. The product can no longer be shipped from the factory. Therefore, the environmental test adopts the periodic sampling method. Such tests generally include:
(A) High and low temperature impact test - multiple shocks from high temperature to low temperature.
(B) Temperature cycling test - high temperature low temperature high temperature low temperature cycle.
(C) Tide temperature test - store the LED for a specified time under the specified temperature and humidity.
(D) Salt spray test—stored for a specified period of time in an atmosphere of specified salinity.
(E) Sand and Dust Test - Simulates storage or work in a desert environment.
(F) Irradiation test - various ray irradiation to observe the photoelectric performance of LED.
(G) Vibration and Shock Tests—Tests to simulate transportation of LEDs at specified amplitudes and frequencies.
(H) Drop test—drops from a certain height several times.
(I) Lead Tensile and Bending Tests - Tensile strength tests and bending tests are performed on the lead wires of the LEDs.
(J) Centrifugal acceleration test—simulates the ability to withstand the rotating state of the LED.
Etc. to simulate various natural phenomena and use environments that LEDs may encounter, and test their bearing capacity and stress matching conditions of various components.
(3) Life test
In order to observe the change law of the light performance of the LED in the case of long-term continuous use, the LED system
Manufacturers must conduct long-term energization aging on a sample of each category to examine the "life" time of that particular type of product. Through the tracking and observation of thousands or even tens of thousands of hours for each product of different processes and materials. Data are accumulated to make statistics on the "average lifespan" of their expected jobs.
In the life test, the rated power is generally added to the LED under the specified environmental conditions, and the LED is energized and aged for a long time.
