Causes of LED failure in COB package |
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| Reading Volume： 【 1762 】 | Time： 【 2017-08-23 】 | |
| Although the theoretical life of LED chips can reach 100,000H from a technical point of view, due to the impact of packaging, driving, heat dissipation and other technologies, the life of light sources or lamps applied to general lighting can only reach 30,000H, and there are even many on the market that use less than half a year. Products that will go wrong. How to improve the reliability of LED lighting products has always been a problem that companies need to solve. Common root causes are: ① If it appears as bad point B or D, and it mostly appears on the edge of the dam, the red circle in the middle picture in Figure 2. It is probably caused by structural interference. The reasons for the interference need to be further discussed and confirmed from the aspects of whether the mirror aluminum center and the chip layout center deviate and whether the entire lamp structure reserves enough gaps. ②If the defect occurs at point C, and mostly occurs in the middle part, the cause of the shell book will be due to improper protection in the packaging or assembly manufacturing process. (2) Poor welding, which generally occurs at points A, B, D, and E is bad and the force value is qualified during the tensile test, but the breaking point will appear at points A, B, D, and E. For example, Figure 4 shows the disconnection of point D status. In this case, it can be determined that the welding process is improper or the welding equipment is unstable. The temperature, power, pressure, time, etc. of the welding wire can be adjusted according to the specific failure items such as collapsed wire, broken wire, too large or too small gold ball, etc. Process parameters are resolved. (3) The stress damage of the encapsulation adhesive generally occurs at points A, B, D, and E, and the defects will only occur around the junction of the dam adhesive and the encapsulation adhesive. Observe the gaps at the adhesive bonding surface under a high-power microscope. may. The root cause of this failure is often due to the difference in thermal expansion coefficients of the two colloids, which leads to stress concentration and breaks the solder joints. Improvements Through the analysis of the main failure modes of COB introduced above, we can learn the technical methods to improve the actual service life. (1) Structural design A good structure design must not only ensure the realization of functions, but also be error-proof. The LED based on COB packaging technology is itself a vulnerable device. When installing and fixing the secondary optical light distribution components in the structural design, it is necessary to reserve enough gaps to ensure that the error operation is guaranteed under the limit tolerance and during the assembly process. The risk of damage to COB cannot be reduced or minimized. (2) Packaging technology In the packaging process, we use the wire bond tension test as an important indicator for measuring the bond and qualification. And in the process of quality control, it is often considered that the greater the pulling force, the better the reliability of the product. Practice has proved that this is a wrong approach, especially in the measurement of edge keys and quality in the COB wire bonding process, too large or too small will lead to defects. The specific tensile specification limits need to be researched and discussed according to product characteristics. On the other hand, choose the appropriate encapsulation and sealing process. Use glues with similar thermal expansion coefficients as much as possible, and it is necessary to determine the appropriate drying temperature and time to ensure consistent solidification speed and prevent internal stress. (3) Process protection Because the COB device is arranged in an array, the exposed surface of the light-emitting device is large, the bonding wire and the chip have poor resistance to external forces, and the packaging gel itself cannot play a good protective role. Therefore, the use of suitable packaging, transfer carriers, pick-up tools, and fixed pick-up methods in the manufacturing process can prevent or reduce mechanical damage. (4) Reasonable selection Although screening cannot ultimately improve product quality, it can prevent the outflow of substandard products. In order to screen out defective COB devices, aging and testing at a certain temperature can be used. COB devices can be tested at high temperature, and the temperature should not be higher than the junction temperature of PN. If the whole lamp is aging, the recommended temperature should not exceed the declared maximum ambient temperature, and the time can be reduced as the temperature increases. Specific parameters need to be researched and determined according to product characteristics. (5) Optimize the reliability verification scheme Whether the product can meet the requirements for use needs to be fully verified. Considering that there is no reliability test specification for LED products, whether it is international or domestic, the current conventional practice is to conduct reliability tests such as high temperature, low temperature, alternating heat and humidity, switch impact, etc. according to GB/T2423, GB/T24825 and other standards. . But in practice, we found that such a method is not enough to detect failures, that is, products that have been determined to have reliability failures can also be verified by such tests. Therefore, we need to develop a reliability test plan that meets the characteristics of LEDs based on COB packaging technology for the product working environment and special purpose demonstration. Combining the rapidly changing market, adjust reliability test conditions such as temperature, humidity, installation method, customer's usage habits and other factors to innovate and select reliability verification solutions. Achieve the launch of reliable products in the shortest possible time. |
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