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What is the Highest LED Tj
The Highest Tj refers to the highest temperature condition under which LEDs can work normally. Working under the highest temperature condition, LED light efficiency shall cause reversible changes and lead to irreversible failure. That is to say, LED light efficiency and phosphor conversion efficiency will decrease under the highest Tj.
Two reasons can be taken into consideration when measuring the causes of temperature rising problem. One point is the LED raw material. Super brightness LED adopts MOCVD technique and epitaxy growth technique as the core to improve the light conversion efficiency. Since epitaxial materials are multi-layered, where lattice mismatches to some degree; there exists somewhat structural defects. While under the highest Tj, structure fault will be worsened, eventually, leading to light decay. The other one is the impurity elements intervene, which results in large quantity of deep level and gives rise to light failure.
In essence, LED encapsulation epoxy is characterized by qualitative changes under high temperature. According to the statistics, at the point of Tg=125℃, epoxy would be transformed from glassy state into rubber state, where epoxy expansion factor shall increase greatly. When temperature rises (higher than Tg=125℃) to the highest temperature, epoxy will expand tremendously and finally generate the greater pressure between LED chip electrode and lead.
Furthermore, epoxy could turn to yellowing when temperature is higher than Tg=125℃. Higher is the temperature, sooner epoxy is yellowing. To solve this problem, high-power LED has given up adopting epoxy as encapsulated material and used glass, PC as optical lens, thus improving LED performance in a better way.