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Packaging Elements Affecting the Efficiency of LED

With the development of LED chip technology and packaging technology, adapt to the demand of high luminous flux LED products in the field of lighting, this power type LED is gradually entering into the market. This type of power leds are generally put the light emitting chip on the cooling heat sink, the above assembly optical lenses to achieve a certain optical space distribution, the internal Lens filled low stress flexible silica gel.

Power type LED wants to enter the field of lighting, home lighting, there are many problems to be solved, and the most important one is the luminous efficiency. Currently, power type LED is reported the highest luminous efficiency is about 50lm/W on the market, far away from the daily household lighting requirements. In order to improve the efficiency of the power types LED, the efficiency of the chip is to be improved. On the other hand, the packaging technology of power type LED should be further improved. From the structure design, material technology and process technology and other aspects start, improve the packaging efficiency of the product.

What are the factors that affect the packaging elements of the luminance efficiency?

1. Heat Dissipation Technology

For the light emitting diode is composed of a PN junction, when the forward current through the PN junction flow, PN junction has heat loss, the heat through the adhesive, potting materials, heat sink, radiation to the air, in the process of each part of the materials have thermal impedance, thermal resistance is determined by the size of the device, the structure and the material of the fixed value. If a luminous diode thermal resistance is Rth(℃/W), heat dissipation power as PD (W), at this time due to the heat loss of the current caused by the PN junction temperature rise: T(℃)=Rth×PD PD, PN junction temperature: TJ=TA+Rth X PD

And, TA is the environmental temperature. Due to the rising of junction temperature will make the PN junction recombination decreased, the LED luminance will drop. At the same time, due to the heat loss cause the temperature rise, the LED brightness will not continue to increase with the current into a proportional increase, which shows the thermal saturation phenomenon. Also, with junction temperature rising, lighting peak wavelength also will shift to long wave direction, about 0.2-0.3nm/℃, this for white LED, Blue light wavelength drift will cause and fluorescent powder excitation wavelength mismatch, thereby reducing the white led the overall luminous efficiency, which lead to change the color temperature of the white light.

For encapsulation and application, how to reduce the thermal resistance of the products, make the heat generated by the p-n junction can send out as soon as possible, not only can enhance the saturation current of the product, improve the luminous efficiency of the product, but also improves the reliability and life of the product. In order to reduce the thermal resistance of the products, first of all, it is particularly important to the choice of packaging materials, including heat sink, adhesive glue, etc., the lower thermal resistance of each material, requiring heat conduction performance is good. Second structural design is reasonable, the thermal performance between different materials match in a row, the material between the thermal conductivity of connection is good, to avoid heat in heat conduction channel bottleneck, ensure heat from escaping through the inner to the outer layer. At the same time, to ensure that from the process, according to the preliminary design of the cooling channels timely heat out.

2. The choice of filled rubber

Based on the law of refraction, light from the optically denser medium incident to optically thinner medium, when the incident angle reaches a certain value, that is greater than or equal to a critical angle will occurs complete emission. In terms of GaN blue chip, GaN materials refractive index is 2.3, when light from inside crystal fired into the air, according to the law of refraction, the critical angle θ0=sin-1 (n2 /n1)

Where n2 is equal to 1, i.e. air refractive index, n1 is GaN refractive index, so the calculated critical angle θ0 is about 25.8°. In this case, can only send out the incident angle is less than or equal to 25.8 degrees spatial angle light, reportedly, at present GaN chip external quantum efficiency at about 30%-40%. Therefore, due to the absorption of the interior of the crystal chip, shoot to the outside lens light of the crystal are very few. Likewise, the light emitted by the chip is transmitted through the packaging material, and the material is transmitted to the space, and also need to consider the effect of the material on the light efficiency.

Therefore, in order to improve the efficiency of the LED product packaging, it is necessary to increase the value of n2, which is to improve the refractive index of the packaging material, so as to improve the critical angle of the product. Therefor to improve the efficiency of packaging. At the same time, the packaging material for light absorption is relative small. In order to improve the proportion of incident light, the package shape best is arched or domed, so, light from packaging materials fired into the air, almost vertical to the interface and will not produce a full reflection.

3. Reflection Processing

Reflection processing mainly has two aspects: One is reflection processing of the inside chip; One is packaging materials relative to the light reflection, through the inner and outer reflection processing to improve the proportion of luminous flux where emitted from inside the chip, reduce the inner absorption of the chip and improve the luminous efficiency of the LED power product. From the package, power LED is usually to make the power chip assembly in the metal bracket or substrate with reflective cavity, the bracket type of reflective cavity is generally taken electroplating way to improve the reflection effect, and the base plate of the reflection cavity is generally used in polishing method, some are use electroplating way, but above two kinds of processing method affected by mold accuracy and process. At present, the domestic production base of the reflecting cavity because of the oxidation polishing precision insufficient or metallic coatings, reflection effect is poor, this has led to a lot of light shooting to the reflection area be absorbed, not according to the expected target reflection to the light emitting surface, which leads to the final package light extraction efficiency is low.

4. Fluorescent Powder and Coating Selection

For the white power LED, the enhancement of the luminous efficiency is related to the choice of the phosphor and the process. In order to improve the efficiency of blue chip, we should choose the suitable phosphor powder, including the excitation wavelength, particle size, excitation efficiency and so on. Secondly, the fluorescent powder coating should be uniform, it is best to relative luminescence chip each of the light-emitting surface layer of uniform thickness, in case of the thickness caused by uneven local light can not emit and improve the light spot quality.

Good thermal design has a significant effect on improving the luminous efficiency of LED products, but also is the premise to ensure the product life and reliability. And the design of a good lighting optical channel, here focused on the reflection cavity, filling plastic, etc. the structure design, material selection and processing, can effectively improve the efficiency of power LED. For power type white LED, the selection and process design of the phosphor powder is very important for the improvement of the spot and the enhancement of the luminous efficiency.

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