With the continuous improvement of LED light efficiency in recent years, the life and reliability of LEDs have received more and more attention from the industry, and it is one of the most important performances of LED products . Every component and link in the manufacture of LED products has an impact on its reliability and longevity. The reliability of solder joints between LED devices and printed circuit boards (PCBs) is critical to ensure the overall reliability of LED luminaires. This article describes how to use thermal shock testing to analyze the solder joint reliability of Cree XLamp® high-power LED devices. It is designed to help Cree users master the application and design considerations of Cree LED devices and improve product development efficiency and quality. .
First, the factors affecting the reliability of solder joints
In the LED application design, several solder joints are very important. As shown in Figure 1, the reliability of the three solder joints is the key to ensuring heat transfer from the LED junction to the heat sink.
Figure 1 Important solder joints that ensure heat transfer from the LED junction to the heat sink
1. Solder joints of LED chips to LED substrates
This is usually ensured by the LED manufacturer to be complete and reliable, and the following factors should be considered when designing the LED package . 1) physical properties of the LED chip and the LED substrate; 2) material selection; 3) solder joint geometry (pad size and shape, placement of the pad relative to the solder mask); 4) mechanical properties of the bulk solder alloy; 5) The nature and structure of the intermetallic compound formed at the joint of the solder joint/heat sink.
2, LED substrate to PCB solder joints and PCB to heat sink solder joints
The luminaire manufacturer ensures the reliability and integrity of the two solder joints in the application and design. The integrity of the solder joint between the LED substrate and the PCB is one of the main determinants of long-term lumen maintenance and reliability of LED products. Solder joint reliability depends not only on the solder alloy, but also on the metal coating of the LED device and PCB. In addition, the reflow profile has a significant impact on the performance of lead-free solder joints because it affects the wettability and microstructure of the solder joint. Damage or defective solder joints can lead to open circuit failure conditions, which can result in complete failure of the electrical performance of the luminaire.
A key feature to consider when soldering LED devices to PCB reliability is the difference in thermal expansion coefficient between the LED device and the PCB material. Changes in operating conditions can result in different forces due to mismatch in expansion coefficients. These forces may be amplified by some mechanical action (such as bending of the LED substrate). For larger package size LEDs soldered on a rigid PCB, the solder joints furthest away from the center of the LED package have the greatest stress due to expansion mismatch.
Figure 2 Factors affecting the reliability of LED solder joints
In summary, Figure 2 shows the various influencing factors affecting LED solder joints, some of which are highlighted in the text. This study used thermal shock to evaluate the solder joint reliability of selected high power XLamp® LED devices. The rate of temperature rise in the thermal shock test is much higher than in the thermal cycle test, so the damage to the solder joint is much more severe. Through the thermal shock test results, you can gain a deeper understanding of the reliability of solder joints.
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