Computer controlled life testing of Light-Emitting Diodes (LED's)

An automated, computer-controlled, test complex was designed and built to life test a large number of light-emitting diodes (LED's). One of the objectives of the test was to determine the minimum burn-in time required to estimate the lifetime of the LED's. The test setup, experimental hardware, operating details, and the problems that arose during the test program are described. Experimental and theoretical results of attempts to estimate the lifetimes of the devices are presented and discussed. It was found that if the light output power degraded by more than 15 percent of the initial light output power and if there was a voltage change exhibited by the LED's of more than 100 mv, within a period of 200 hr, then the light output power of the LED:s would fall below 50 percent of the original light output power within 2,000 hr of operation. The initial degradation rate was found to be proportional to the operating temperature and current density, whereas the degradation rate of those devices that passed the screening criteria appeared to be independent of these parameters after 2,000 hr of operation. The double-heterojunction devices had a longer mean time to failure than did the homojunction devices. The time to obtain 50 percent of the initial light output power was used as a criterion of failure.