Systematic evaluation and analysis method for LED product quality

Solid-state lighting (SSL) product development is more than just LEDs and driver electronics. Richie Richards explains how to evaluate the complex design factors, including material selection, thermal management, compatibility, etc., which, when handled properly, will produce the best lighting products.

　　As the old saying goes, “You know something when you see it, but sometimes you can’t tell.” Design factors that affect solid-state lighting performance are, as such, often difficult to describe. The light produced by an LED is only a small part of the overall evaluation system. The installation process and the product life cycle are equally important for a complete evaluation. Let’s look at some examples of areas that are prone to doubt.

　　LED technology is developing rapidly, and many lighting manufacturers like to make unfounded claims about the color quality, brightness, luminous efficiency, durability, and even the entire product. Lack of proper thermal design or improper circuit design will cause excessive electrical stress. If chemical compatibility issues arise during the production process, it will lead to quality degradation of LED products and even lighting failures. Figure 1 collects the process causes of quality degradation and failure.

Figure 1. Solid-state lighting product quality issues can result from thermal, chemical, or driver problems.

　　Overall Assessment

　　In order to ensure the quality of solid-state lighting products, quantitative testing is essential. This will determine the strength of the design and the long-term reliability of the product. It is not just the LED part, but the entire solid-state lighting product is about quality. Cree has launched a comprehensive test of thermal, electrical, mechanical, photometric and optical parameters (TEMPO) for LED lamps. The TEMPO service helps LED component customers solve problems. The project also enables product developers to quickly respond to challenges, save development costs, and gain more market time for new products.

　　The system evaluation of lighting products is a multi-point testing and analysis process. Engineers must conduct a series of thermal, electrical, mechanical, and photometric tests and submit a comprehensive report of data that can determine the performance of the product. The toolbar at the end of this article is attached with a detailed test catalog of the TEMPO service. In addition, the test results will be applied to the TM-21 standard adopted by the Illuminating Engineering Society (IES) to design the life of the LED. Testing and evaluation are very challenging, especially for those lighting manufacturers who are just engaged in designing LEDs . Solid-state lighting products contain a large number of components: LEDs, circuit boards, optical components, current drivers, power supplies, heat sinks, and mechanical housings. Any component will affect the performance, quality, and life of the product.

　　The lighting system performance needs to be evaluated in terms of mechanical construction and long-term reliability. In addition, the test results report needs to be based on LM-79-08. The evaluation needs to provide comparative results of Energy Star, DLC and UL standard tests.

　　Thermal and mechanical testing

　　Most LED failure issues are temperature related; performance and designed life are closely related to thermal management and the junction temperature of the LED semiconductor chip. Increased junction temperature reduces light output and shortens service life. Proper thermal management and mechanical structure are critical to performance. Technical means such as x-ray photography of LED soldering can be used to evaluate mechanical structure and evaluate actual heat dissipation performance to verify the assumed plan of heat dissipation design, which is necessary to ensure quality and reliability.

　　Thermal testing is challenging for products. Misplaced thermocouples, too much photon energy illuminating the thermocouples are all examples of incorrect temperature testing. These mistakes can reduce the life of the product.

Thermal and mechanical testing of solid-state lighting products can reveal design or manufacturing defects

　　Proper testing requires accurate verification of the solder joints for junction temperature monitoring. TEMPO testing involves verifying the solder joints for junction temperature monitoring, solid-state infrared thermal imaging, and an estimate of the junction temperature based on the test. As shown in Figure 2, the thermal performance of LED luminaires can be measured and described using temperature testing and infrared thermal imaging.

　　Please note the infrared photo on the right hand side of Figure 2, which shows the difference in heat dissipation between two identical PCBs. The PCB on the left is cooler and appears yellow in the photo, while the one on the right is bright red and the LED is white-hot, indicating that the PCB heat sink should be installed on the right.

　　The mechanical interface between the LED and the printed circuit board has a profound impact on the thermal performance of the solid-state lighting system. It can be analyzed using x-rays. This is very useful for verifying the quality of the soldering and determining whether it is ineffective or over-soldering.