1. Thermal Resistance Rth
Thermal resistance Rth refers to the ratio of the temperature difference across a heat flow path to the power dissipated along that path. When an LED is turned on and reaches a thermal steady state, if the chip dissipates 1 W of power, the temperature difference between the pn junction and the attached heatsink or aluminum substrate is defined as the thermal resistance Rth in °C/W. A lower Rth value indicates more efficient heat transfer from the chip to the mounting structure, which is essential for maintaining optimal performance and longevity.
2. Storage and Operating Temperatures of LEDs
Under normal conditions, the storage temperature for LEDs should range from -40°C to +100°C. However, during packaging, LEDs may be briefly stored at 150°C for 1–2 hours to accelerate the drying of encapsulants or phosphors. The long-term impact of this process on LED performance remains under investigation. The operating temperature range for LEDs is typically -30°C to +80°C, though this can vary depending on the thermal resistance. It's recommended to keep the pn junction temperature below 100°C during operation to ensure reliability and prevent premature failure.
3. Electrostatic Discharge (ESD) Protection
Proper ESD protection is crucial when handling LED devices, as static electricity can damage them both during transportation and assembly. After manufacturing, the ESD protection level of a typical LED should be above 500V to prevent electrical damage. Ensuring proper grounding and using anti-static tools are essential practices to maintain product integrity.
4. Failure Rate λ
The failure rate λ measures how many LEDs in a batch become non-functional after being powered on. This is a critical metric for assessing the quality and reliability of LED products. If no "dead lights" appear within the first 10 hours of operation, it suggests a low failure rate, ideally zero, indicating high-quality manufacturing and consistent performance.
5. Lifespan of LEDs
The lifespan of an LED is primarily determined by the time it takes for its light output to decrease to half its initial brightness. Theoretically, LEDs can last over 100,000 hours, but in practice, due to material limitations and manufacturing processes, most commercially available LEDs have a lifespan of around 20,000 to 30,000 hours. The actual lifetime is significantly influenced by the system's heat dissipation and light extraction efficiency during use.
6. Additional Performance Indicators
LEDs are typically encapsulated with epoxy or other adhesives. Over time, exposure to environmental factors and chemical reactions can degrade the light transmittance of the encapsulant. In some cases, the material may yellow or become cloudy, reducing light output. In extreme cases, the encapsulant may crack or vitrify, leading to performance degradation. These issues can cause the LED to deviate from its original specifications, affecting both its light extraction efficiency and overall service life.
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