1. Thermal Resistance Rth
Thermal resistance, denoted as Rth, is the ratio of the temperature difference across a heat flow path to the power dissipated along that path. When an LED is operating and reaches a steady thermal state, if the chip surface dissipates 1 watt of power, the temperature difference between the pn junction of the chip and the connected bracket or aluminum substrate is defined as the thermal resistance Rth in °C/W. A lower Rth value indicates more efficient heat transfer from the LED chip to the mounting structure, which helps maintain better performance and longevity.
2. LED Storage and Operating Temperatures
The recommended storage temperature for LEDs under normal conditions ranges from -40°C to +100°C. However, during the packaging process, some manufacturers may store LEDs at 150°C for 1-2 hours to accelerate the drying of the encapsulant or phosphor. The long-term impact of this practice on LED performance is still under investigation. In terms of operation, the typical working temperature range is -30°C to +80°C. It's important to note that the actual operating temperature depends on the thermal resistance. To ensure optimal performance and lifespan, it's advisable to keep the pn junction temperature below 100°C during operation.
3. Electrostatic Discharge (ESD) Protection
Proper anti-static measures are crucial when handling LED devices. Static electricity can cause damage during transportation or assembly. Special attention must be given to ESD protection. After manufacturing, most LEDs should have an ESD protection rating of at least 500V when the device is in an open-circuit condition. This ensures that the LED can withstand common static charges without sustaining damage.
4. Failure Rate λ
The failure rate λ is a key metric used to assess the reliability of LED devices. It refers to the number of "dead lights" that occur within a batch after being turned on. A low failure rate indicates high-quality production. Ideally, if no dead lights appear within the first 10 hours of operation, the failure rate is considered excellent, often reaching zero. This metric is essential for evaluating the overall quality and durability of LED products.
5. Lifespan
The lifespan of an LED is primarily determined by its half-life decay time—the period it takes for the light output to drop to 50% of its initial level. Theoretically, LEDs can last over 100,000 hours. However, due to material limitations and manufacturing processes, commercially available LEDs typically have a lifespan ranging from 20,000 to 30,000 hours. The actual life of an LED is heavily influenced by the system’s heat dissipation and light extraction efficiency during use.
6. Additional Performance Indicators
LEDs are often encapsulated using epoxy or other adhesives. Over time, exposure to environmental factors and chemical reactions can degrade the optical properties of the encapsulant, leading to discoloration, cloudiness, or even cracking. These changes can reduce light transmission and negatively affect both the performance and service life of the LED. Therefore, maintaining proper storage conditions and minimizing exposure to harsh environments is essential to preserving the original technical specifications of the device.
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