How to test the life of LED chips part two

Wire bonding and resin encapsulation induced stress changes, such as heat dissipation, expansion coefficient, etc., are all important factors that affect the life test. The life test results are worse than the bare die life test, but for the current low-power chips, the assessment is increased The quality range and life test results are closer to actual use conditions, which have a certain reference value for production control.

Design of life test bench

The life test bench is composed of a life test unit board, a bench and a special power supply device, and can perform 550 groups (4400) LED life tests at the same time.

According to the requirements of life test conditions, LEDs can be driven in parallel and in series. Parallel connection form: Connect the positive and negative poles of multiple LEDs in parallel. The feature is that the working voltage of each LED is the same, and the total current is ΣIfn. In order to achieve the same working current If for each LED, each LED is required The forward voltage should also be consistent.

However, there are certain differences in the characteristic parameters between the devices, and the forward voltage Vf of the LED decreases as the temperature rises. Different LEDs may cause differences in the operating current If due to the difference in heat dissipation conditions. LEDs with poor heat dissipation conditions have a higher temperature rise. If it is large, the forward voltage Vf also drops greatly, causing the operating current If to rise. Although the above phenomenon can be alleviated by adding series resistors to limit current, there are disadvantages such as complicated wiring, large difference in operating current If, and inability to apply LEDs of different VFs, so it is not suitable to use parallel connection drive mode.

Series connection form: connect the anode and cathode of multiple LEDs into a string. The advantage is that the working current of each LED is the same. Generally, a current limiting resistor R should be connected in series, as shown in Figure 2 for a single string circuit. When an LED is open , Will cause this string of 8 LEDs to go out. In principle, the possibility of an open circuit of the LED chip is extremely small. We believe that the life test of the dot matrix LED, the working mode of constant current drive and series connection is better. The LED constant current drive circuit composed of common 78 series power circuit ICs is characterized by low cost, simple structure and high reliability; by adjusting the resistance of the potentiometer, the constant current can be easily adjusted; the applicable power supply voltage range is large, and the drive The current is more accurate and stable, and the influence of power supply voltage changes is small. We use the circuit shown in Figure 2 as the basic route to form life test unit boards in parallel, and each unit board can perform 11 groups (88) LED life tests at the same time.

The rack is a general standard combined rack. After reasonable wiring, each unit board can be easily loaded and unloaded, realizing online operation. Dedicated power supply equipment, the output is 5 channels of DC 36V safety voltage, the load capacity is 5A, of which 2 channels have a microcomputer timing control function, which can be automatically turned on or off, and 5 channels of input and output are separately indicated.