What causes the junction temperature of surface-mount LED chips to rise?

Release Date:

2020-11-30

　　When SMD LED chip During operation, junction temperature rise can occur. Let’s now examine the causes of this phenomenon.

 

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　　1. Chip-mounted LED die often feature suboptimal electrode structures, and the window-layer substrate, bonding-region materials, and conductive silver adhesive all exhibit finite resistances. These resistances are in series, contributing to the overall series resistance of the LED assembly. As current flows through the P–N junction, it also passes through these resistive elements, generating Joule heating that raises the die temperature or junction temperature.

　　2. Because the P–N junction cannot be perfectly ideal, the injection efficiency of a surface-mount LED chip assembly will never reach 100%. In other words, in addition to the charge injected from the P region into the N region during operation, the N region also injects some charge back into the P region. Under normal conditions, this reverse injection does not produce a photoelectric effect; instead, it is dissipated as heat. Even the useful portion of the injected charge is not entirely converted into light, and when it recombines with impurities or defects in the junction region, it is further converted into heat.

　　3. Practice has proven that the limitation of light extraction efficiency is the cause of SMD LED chip The primary cause of junction temperature rise. Currently, advanced material growth and device fabrication techniques can convert most of the input electrical power of surface-mount LED chips into optical radiation. However, compared with the surrounding medium, the refractive index of the LED chip material is much higher, resulting in the vast majority of photons (>90%) being unable to escape the interface smoothly and undergoing total internal reflection at the interface between the chip and the surrounding medium. These photons are reflected back into the chip, where they undergo multiple internal reflections before being absorbed by the chip material or the substrate and converted into lattice vibrations. This process leads to an increase in the junction temperature of the surface-mount LED chip.

　　4. Clearly, the thermal dissipation capability of a surface-mount LED package is another critical factor determining the junction temperature. When thermal dissipation is strong, the junction temperature decreases; conversely, when thermal dissipation is poor, the junction temperature rises. Because epoxy encapsulant has low thermal conductivity, heat generated at the P–N junction struggles to radiate upward through the transparent epoxy into the surrounding environment. Most of the heat is conducted away via the substrate, silver paste, package housing, and epoxy adhesive layer, with additional conduction paths downward through the PCB and heat sink. It is evident that the thermal conductivity of these materials directly affects the thermal management efficiency of the surface-mount LED package. For standard surface-mount LEDs, the total thermal resistance from the P–N junction to ambient is typically between 300 and 600 °C/W. In contrast, well-designed LED assemblies can achieve a total thermal resistance of only about 15 to 30 °C/W. This substantial difference in thermal resistance means that conventional LED packages can operate reliably only under very low input-power conditions, whereas power-grade LED modules may dissipate several watts or even more.

　　The above is SMD LED chip Possible causes of junction temperature: Shenzhen Guangtai Optoelectronics Co., Ltd. is a supplier of chip LEDs, butterfly LEDs, through-hole LEDs, multi-color chip LEDs, and RGB chip LEDs, as well as high-brightness LEDs and side-view full-color chip LEDs. If you have any needs, please feel free to call us for more information.

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