LEDs produce light when an electrical current passes through their PN junction. During this process, electrical energy is converted into light, while some energy is also released as heat.
As an LED operates over time, heat generated inside the device can accumulate. If the heat is not effectively dissipated, the internal temperature rises and may accelerate the degradation of LED materials and package components.
One visible result of this aging process is lumen degradation, in which the LED's light output gradually decreases over time.
LED lifetime is therefore influenced by more than the LED chip itself. Several factors work together, including:
Junction temperature (Tj): The temperature at the semiconductor junction, which strongly affects the rate of LED aging.
Driving current: The electrical current supplied to the LED, influencing both light output and heat generation.
Thermal management: The ability of the package and surrounding structure to transfer heat away from the LED.
Package design and materials: Internal connections and package construction can affect long-term reliability.
Understanding these factors is important when selecting LEDs for display backlight systems that require stable brightness over extended operating periods.
Junction Temperature: A Key Factor in LED Lifetime
Junction temperature (Tj) refers to the temperature at the active semiconductor junction inside an LED. It is different from ambient temperature (Ta), which describes the surrounding environment, and case temperature (Tc), which refers to the temperature measured at a specified point on the LED package.
When current flows through the PN junction, heat is generated inside the LED. If the heat cannot be transferred away efficiently, the junction temperature increases.
Higher junction temperatures generally accelerate aging mechanisms within the LED. Over time, this can lead to faster lumen degradation and changes in the LED's optical performance.
The relationship can be summarized as:
Higher heat generation or insufficient heat dissipation → Higher junction temperature → Faster aging → Greater long-term brightness degradation.
This is why thermal management is an important part of LED and backlight design.
For display applications, the thermal conditions depend not only on the LED itself but also on the surrounding structure. PCB design, heat-conduction paths, available installation space, and the operating environment can all affect the final junction temperature.
A display designed for continuous operation or high ambient temperatures therefore needs sufficient attention to heat dissipation.
How Does LED Brightness Decrease Over Time?
LED brightness does not necessarily remain at its initial level throughout its operating life. As the device ages, its light output gradually decreases. This process is known as lumen degradation or lumen depreciation.
LED lifetime is often described by the time required for its light output to decrease to a specified percentage of its initial value.
Common lifetime indicators include:
Lifetime Indicator
Meaning
L50
Light output decreases to 50% of its initial value
L70
Light output decreases to 70% of its initial value
L80
Light output decreases to 80% of its initial value
For example, an LED rated at L70 of 50,000 hours is expected to retain approximately 70% of its initial light output after 50,000 hours under the specified conditions.
These indicators describe light output maintenance, not necessarily the point at which an LED stops working. An LED may continue to emit light after reaching its L70 or L50 lifetime point, although its brightness has decreased.
It is also useful to distinguish between two different effects:
Temperature-related light output changes: LED light output can change with operating temperature. Some of these changes are reversible when the temperature returns to its previous level.
Long-term lumen degradation: Prolonged operation and aging can cause a permanent reduction in light output.
For display backlights, both factors matter. Operating temperature affects brightness during use, while long-term degradation influences how well the display maintains its brightness over years of operation.
How to Extend LED Lifetime
LED lifetime can be improved through appropriate electrical and thermal design, together with suitable component selection. The following considerations are particularly relevant to display backlight systems.
Reduce Driving Current
Driving current directly affects LED light output and power consumption. In general, increasing the current produces higher brightness but also increases heat generation.
Reducing the driving current can lower power consumption and help reduce junction temperature. Under suitable operating conditions, this can slow down long-term LED degradation.
However, lower current also means lower initial brightness. The design therefore needs to balance brightness requirements, power consumption, and expected operating life.
For applications requiring high brightness, simply reducing current may not be a practical solution. The LED selection and thermal design must be considered together.
Improve Thermal Management
Effective heat dissipation helps prevent excessive junction temperature during operation.
In an LCD backlight system, thermal performance is influenced by several design factors:
PCB and heat transfer path: The circuit board and surrounding structure should provide an effective path for transferring heat away from the LED.
Structural layout: LED placement and available space affect how heat spreads within the backlight assembly.
Operating environment: High ambient temperatures and continuous operation can increase the thermal load.
Thermal management becomes particularly important in industrial and outdoor display applications, where displays may operate for long periods under demanding environmental conditions.
Select Suitable LED Packages and Bonding Wire
LED package construction is another factor to consider when evaluating long-term reliability.
Inside an LED package, bonding wires provide electrical connections between the semiconductor chip and the package terminals. The material and quality of these connections can influence the electrical and mechanical reliability of the package.
Based on FANNAL's R&D experience, LEDs using alloy bonding wires typically have an estimated lifetime of 20,000–30,000 hours, while LEDs using pure gold bonding wires may reach 50,000–100,000 hours under suitable operating conditions.
These figures represent engineering experience with LED component selection rather than universal lifetime values for every LED package. Actual performance also depends on the chip design, package construction, driving conditions, and thermal management.
When selecting LEDs for display applications, package structure and internal materials should therefore be evaluated alongside brightness, power consumption, and thermal performance.
What Does LED Lifetime Mean for LCD Backlight Design?
In an LCD, the backlight provides the illumination needed to produce a visible image. Its long-term performance directly affects the display's brightness and overall viewing experience.
As the backlight LEDs age, their light output gradually decreases. If the backlight cannot maintain sufficient brightness, the display may no longer meet its original visibility requirements, even if the LCD panel itself remains functional.
This is particularly relevant for displays used in industrial equipment, outdoor terminals, and other systems that operate for extended periods.
When specifying an LCD, engineers should consider more than the initial brightness rating. Important factors include:
Required brightness: The initial backlight output should meet the application's visibility requirements, with appropriate consideration for future degradation.
Operating temperature: Ambient temperature and internal thermal conditions influence LED junction temperature and long-term performance.
Operating hours: Continuous operation places different demands on the backlight compared with intermittent use.
Thermal design: The available installation space and heat dissipation path should be considered during module integration.
LED component selection: LED chip and package characteristics can influence long-term reliability and brightness maintenance.
It is also important to understand that LED lifetime does not directly represent the lifetime of the complete LCD module. The display's overall service life may also be affected by the LCD panel, driver electronics, optical films, connectors, and other components.
For this reason, backlight lifetime should be evaluated as part of the complete display design rather than as an isolated component specification.
Conclusion
LED lifetime and brightness maintenance depend on several interconnected factors. Junction temperature influences the rate of LED aging, driving current affects both brightness and heat generation, and package construction contributes to long-term component reliability.
For LCD backlight applications, effective thermal management and appropriate LED selection help maintain stable brightness during extended operation.
When choosing a display for industrial or outdoor equipment, it is worth evaluating not only the initial brightness but also the operating temperature, expected usage hours, and backlight design. These factors provide a more realistic basis for assessing long-term display performance.
FAQs
1. What causes LED brightness to decrease over time?
LED brightness decreases mainly due to aging-related lumen degradation. Prolonged operation, elevated junction temperature, and component aging can gradually reduce the LED's light output.
2. How does junction temperature affect LED lifetime?
Higher junction temperatures generally accelerate aging mechanisms inside an LED. Keeping the junction temperature within an appropriate range through effective thermal management can help slow down long-term degradation.
3. Does reducing LED driving current extend its lifetime?
Reducing driving current can lower power consumption and heat generation, potentially reducing junction temperature and slowing LED aging. However, it also reduces initial brightness, so the appropriate current depends on the application's brightness and lifetime requirements.
4. What is the difference between L50, L70, and L80?
These indicators describe the time required for an LED's light output to decrease to a specified percentage of its initial value. L50 means 50% remaining output, L70 means 70%, and L80 means 80%.
5. Does LED lifetime determine the lifetime of an LCD?
No. LED lifetime describes the performance of the LED light source under specified conditions. The overall LCD module lifetime also depends on the LCD panel, driver electronics, optical components, and other parts of the display system.
