4 Factors Impact LED Package Light Extraction Efficiency
Posted: 2015-09-11 04:07:58  Hits: 927
Conventional LED is generally scaffolding, which uses epoxy resin encapsulation, but its power and total luminous flux is small, and high bright one can only be used as some special lighting. With the development of the LED chip technology and packaging technology, the demand for high flux LED products comply with the lighting area, power-type LED gradually enter into the market. This type of power LED generally put the light emitting chip on the cooling heat sink, with above optical lens assembly in order to achieve a certain optical spatial distribution and internal fill low stress flexible silicone lens.
 
If power type LED really enters into the lighting field to realize household lighting, it still has a lot of problems to solve, one of the most important one is the luminous efficiency. The highest lumen efficiency of power type LED reported on the market at present is about 50 lm/W, which is still far short of household lighting requirements. In order to improve the efficiency of power type LED, on the one hand, the efficiency of the light emitting chip needs to be improved; type power LED packaging technology, on the other hand, also need to further improve, and improve product packaging take light efficiency from the aspects of structure design, material technology and engineering technology.
 
1. Thermal dissipation technology
For LED composed of PN junction, when forward current flow through the PN junction, it has heat loss, and the heat radiate into air through the adhesive glue, potting materials and heat sink, etc. In the process each part of the material has to prevent heat thermal impedance, which is also the thermal resistance. It is determined by the size, structure and the materials of the fixed value. Sets the thermal resistance of LED to Rth (℃ / W), heat dissipation power PD(W), at this time the PN junction temperature rise caused by the heat loss of current should be: T(℃)=Rth×PD.
PN junction temperature is: TJ=TA+Rth×PD
 
TA is ambient temperature. As a result of the junction temperature rise, the probability of PN junction glow will reduce, while its brightness of will fall. Due to the heat loss caused by temperature rise, the light emitting diode brightness will no longer continue with current increase, which shows the saturation phenomenon. In addition, as the junction temperature rise, light emitting peak wavelength will drift towards long wave about 0.2 to 0.3 nm/℃. For LED coated by blue chip mixed YAG phosphors, blue light wavelength drift will cause the mismatch of and phosphor excitation wavelength, thus reduce the whole white LED luminous efficiency and leads to a change in the white light color temperature.
 
2. Filled rubber selection
According to reflection law, when the light enters from optically denser medium to thinner medium and the incident angle reaches a certain value, which is greater than or equal to the critical angle, total reflection will happen. Take GaN blue chips as example, GaN materials of refractive index is 2.3, when the light enters into the air from inside the crystal, according to the law of refraction, the critical Angle θ0=sin-1(n2/n1) in which n2 is equal to 1, namely air refractive index, while n1 is GaN refractive index, thus the critical angle of θ0 is about 25.8 degrees. It is reported that currently GaN chip in the external quantum efficiency is about 30% - 40%, therefore, as a result of the crystal chip internal absorption, the proportion of light which can penetrate outside the crystal light is the very few. In the same way, chip light will sent to the space through the encapsulation material, which also needs considering the material to get the influence of the luminous efficiency.
 
3. Reflection treatment
Reflective processing mainly have two aspects, one is the chip internal reflection processing, another one is packaging material light reflection, through the reflection of both internal and external treatment to improve the light flux ratio from inside the chip, reduce chip internal absorption, and improve power LED luminous efficiency of the finished product. As for packaging, power type LED usually puts power type chip on a reflective cavity metal stents or substrate, scaffolding reflective cavity is generally plating ways to improve the effect of reflection, and a reflection of base plate cavity usually adopts polishing method, while some also use plating processing. But the above two kinds of processing way are affected by the precision mould and process, reflective cavity after processing has a certain effect, but is not ideal. The reflection of the current domestic production base plate cavity, caused by a lack of precision polishing or oxidation of the metal coating, has a poor reflection effect, which leads to a lot of light being absorbed after reaching reflex zones and not as expected reflects to a smooth surface. It will eventually leads to the low encapsulation luminous efficiency.
 
4. Selection and coatings of fluorescent powder
For white power type LED, the improvement of luminous efficiency is related to the selection and process of phosphor. In order to improve the efficiency of phosphor stimulating the blue chips, at first the choice of phosphor should be appropriate, which includes excitation wavelength, granularity, and excitation efficiency, all above need comprehensive evaluation to consider the various performance. Then phosphor powder should be coated uniformly, and the best way is relatively light emitting chip each light rubber thickness below, so as to avoid uneven thickness to cause local light penetrate, and at then same time improve the quality of the light spot.
 
Good heat dissipation design has obvious effect for improving the luminous efficiency power type LED product, and it is also the premise to ensure product life and reliability. And well designed optical channel here mainly refers to the reflective cavity, filling adhesive and another structural design, material selection and processing, which can effectively improve the efficiency of power type LED. For white LED, phosphor selection and process design are also crucial on the improvement of the light spot and light-emitting efficiency.

4 Factors Impact LED Package Light Extraction Efficiency

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