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Encapsulation Requirements Of LED Chip

- Mar 01, 2018 -

As the core device of LED street lamp, the performance of LED chip needs to enhance the efficiency of light efficiency, life, stability, optical design and heat dissipation through the LED encapsulation process. Because of the different structure of the chip, the corresponding packaging technology also has great difference.

Light effect enhancement

The chip of the positive and vertical structures is the contact of GaN with fluorescein and silica gel, and in the inversion structure, sapphire (sapphire) is contacted with phosphor and silica gel. The refractive index of GaN is about 2.4, the refractive index of sapphire is 1.8, the refractive index of phosphor is 1.7, and the refractive index of silica gel is usually 1.4-1.5. Sapphire / (silicone + Phosphor) and GaN/ (silicone + phosphors) the critical angle of total reflection are 51.1-70.8 degrees and 36.7-45.1 degrees, emitted by the sapphire surface of the package structure in light through silica gel and phosphor layer interface critical angle of total reflection more light total reflection loss is greatly reduced. At the same time, the design of the chip structure is different, which leads to the difference of current density and voltage, which has an obvious effect on the light efficiency of LED. Such as dress chip usually the traditional voltage above 3.5V, and the structure of flip chip, the design of electrode structure, a more uniform current distribution, the voltage of LED chip is reduced to 2.8V-3.0V, therefore, in the same flux, the flip chip light efficiency than formal chip optical efficiency of about 16-25% higher.

LED chip.jpg

Reliability improvement

The reliability of LED is determined by materials such as LED chip, phosphor, silica gel, bracket and gold wire. The heat generated by LED chip will not directly affect the reliability of LED chip's junction, temperature, phosphor and silica gel. At present, according to the different systems, the ability of high temperature resistance of phosphors is also quite different. Usually, the phosphors begin to decay at above 100-120 degrees. Therefore, how to reduce the surface temperature of LED chips is the key factor to improve the reliability of LED. The vertical structure of chip through the metal substrate will heat quickly export to the bracket, the surface temperature of the chip is low, heat through the sapphire dress chip export to support, because the sapphire thermal conductivity is low (about 20W/mK), the heat can not be quickly export, gradually accumulated on the reliability influence of phosphor. Flip chip structure of the vast majority of heat down through the gold bump into the silicon substrate (rapid thermal conductivity is about 120W/mK), and then by the silicon substrate into the support, and to the low thermal conductivity of sapphire, only a small part of the heat accumulation on sapphire, heat (downward export) and light (to the separation from design, at the same time) sapphire surface temperature is low, can prolong the aging period of the fluorescent powder, greatly improve the reliability and life of LED. Meanwhile, due to the good heat dissipation design of flip chip structure, flip chip 1W has better L-I linear relationship (see Figure 3) and saturation current tolerance and high current endurance. The inverted 1W power chip can support long - term room temperature 780mA large current aging.

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