## 2 Materials and Methods
### Accelerated Degradation Test  
#### 2.2.1 LED Failure Definition  
There are two types of LED failures: accidental catastrophic failures and degradation failures. With the improvement of LED manufacturing technology, the proportion of accidental failures in LEDs has gradually decreased, and degradation failures are the main failure mode of LEDs today.  
Light Output Performance (LOP) refers to the optical power that an optoelectronic component can output under a certain current. This concept is usually used to evaluate the performance of light-emitting diodes (LEDs). According to IEC International Electrotechnical Commission standards and relevant Chinese standards, when the light power of LED lighting decays to 70% of the rated light power, it is usually judged that the product has reached the end of its life. This standard is widely used to measure the performance and durability of LED lamps. Specifically, the time for the light power to decay to 70% (usually marked as L70) refers to the time required for the light output of an LED lamp to drop to 70% of its initial light output under specified conditions of use. This time is usually used to evaluate the service life of LED lamps. Since the initial brightness of LEDs is different, this article selects LEDs with similar initial brightness as a group, and LOP is judged to be 70% of its initial LOP. BE (Basic Error) is the deviation between the measured LOP and the initial LOP, defined as follows in Equation (14):  
\[BE=\frac{LOP_{test}-LOP_{\text{int}}}{LOP_{\text{int}}} \tag{14}\]  
where \(LOP_{int}\) is the initial light output performance indicating the initial value of a single LED before the test and \(LOP_{test}\) is the testing light output performance indicating the value of one observation during the test. For example, under the conditions of 95 \({}^{\circ}\)C, 45% humidity, and 525 mA current, the initial LOP of the first LED in the first group of experiments was 510.5 mw, and it was 483.9 mw after 24 h of testing; that is, the rate of change in 24 h was \(-\)5%. The initial LOP values of the 10 LEDs in the first group of experiments were 510.5, 522.9, 527.1, 527.1, 506.3, 516.3, 530.4, 524.6, 522.1, and 522.1, with the maximum being 530.4 and the minimum being 506.3. The LOP attenuation of the 24 h test was compared with its initial LOP. The BE value is very important when judging LED failure. According to the L70 standard, the LED is declared failed when the difference between the LOP of the LEDs after degradation and the initial LOP of the LEDs accounts for \(-\)30% (\(-\)0.3) of the initial LOP of the LEDs. Many faults can cause LED LOP degradation, such as extreme thermal shocks that can break the dies of LEDs. The high electrical stress and extreme thermal shock are the causes of die cracking [28; 29], as shown in Figure 1a,b. However, in many cases, the LED has no physical damage or burn marks. The life and performance of LEDs are limited by crystal defect formations in the epitaxial layer structure of the die [30]; electrostatic discharge (ESD) causes LED failure [31], etc. In this case, the cause of failure can only be seen after the test is completed and the sol is used under a microscope, such as in Figure 2.  
Figure 1: Appearance of failed LEDs.  
Figure 1 shows the cracking of the colloid in the middle of LEDs. The reason is that the stress of the colloid is released after the LED chip is burned, forming cracks.