Up Knowledge | What is the efficiency of light extraction in LED packaging?
LED is called the fourth generation lighting source or green light source. It has the characteristics of energy saving, environmental protection, long life and small size. It is widely used in various fields such as indication, display, decoration, backlight, general lighting and urban night scene. According to the different functions, it can be divided into five categories: information display, signal lamp, vehicle lamps, LCD backlight and general lighting.
Conventional LED lamps have shortcomings such as insufficient brightness, which leads to insufficient popularity. Power LED lamp has the advantages of full brightness and long service life, but power LED has some technical difficulties such as packaging. Here is a brief analysis of the factors affecting the efficiency of power LED packaging.
Packaging Elements Affecting Light Collection Efficiency
1. Heat Dissipation Technology
For light emitting diodes (LED) consisting of PN junctions, when the forward current flows through the PN junction, the PN junction has heat loss. The heat is radiated into the air through bonding, encapsulating materials, heat sink and so on. In this process, each part of the material has a thermal impedance, that is, thermal resistance, which is a fixed value determined by the size, structure and material of the device.
Let the thermal resistance of the light emitting diode be Rth (/W) and the thermal dissipation power be PD (W). At this time, the temperature of PN junction caused by the thermal loss of current rises to:
T(?) = Rth & TImes; PD
PN junction temperature is:
TJ = TA + Rth & TImes; PD
TA is the ambient temperature. As the junction temperature rises, the probability of PN junction recombination decreases, and the brightness of light emitting diodes decreases. At the same time, due to the increase of temperature caused by heat loss, the brightness of light-emitting diodes will not continue to increase proportionally with the current, that is to say, it shows Thermal Saturation phenomenon. In addition, with the increase of junction temperature, the peak wavelength of luminescence will also shift to the long-wave direction, about 0.2-0.3 nm/C. For white LED mixed with YAG phosphor coated by blue chip, the shift of blue wavelength will cause mismatch with the excitation wavelength of phosphor, thus reducing the overall luminous efficiency of white LED and leading to the change of white light color temperature.
For power light emitting diodes, the driving current is generally more than several hundred milliamperes. The current density of PN junction is very high, so the temperature rise of PN junction is very obvious. For packaging and application, how to reduce the thermal resistance of the product and make the heat generated by PN junction emit as soon as possible can not only improve the saturated current of the product, improve the luminous efficiency of the product, but also improve the reliability and life of the product. In order to reduce the thermal resistance of products, the choice of packaging materials is particularly important, including heat sink, bonding and so on. The thermal resistance of each material should be low, that is to say, good thermal conductivity is required. Secondly, the structure design should be reasonable, the thermal conductivity of each material should be matched continuously, and the thermal conductivity between the materials should be good, so as to avoid the heat dissipation bottleneck in the heat conduction channel and ensure the heat dissipation from the inside to the outside. At the same time, it is necessary to ensure that the heat is emitted in time according to the pre-designed cooling channel.
2. Selection of Filling Adhesive
According to the law of refraction, when light is incident from a dense medium to a sparse medium, when the incident angle reaches a certain value, that is, when the incident angle is greater than or equal to the critical angle, the total emission will occur. For GaN blue chip, the refractive index of GaN material is 2.3. According to the law of refraction, the critical angle theta 0 equals SIN-1 (n2/n1) when light radiates into air from the inside of the crystal.
Among them, N2 is equal to 1, that is, the refractive index of air and N1 is the refractive index of GaN. From this calculation, the critical angle of theta 0 is about 25.8 degrees. In this case, the only light that can be emitted is the light in the space stereo angle with the incidence angle less than 25.8 degrees. It is reported that the external quantum efficiency of GaN chips is about 30%-40%. Therefore, due to the internal absorption of the chip crystal, the proportion of light emitted outside the crystal is very small. It is reported that the external quantum efficiency of GaN chips is about 30% – 40%. Similarly, the light emitted by the chip should be transmitted to the space through the packaging material, and the influence of the material on the light-taking efficiency should also be considered.
Therefore, in order to improve the light pickup efficiency of LED product packaging, it is necessary to increase the value of n2, that is, to improve the refractive index of packaging materials, so as to improve the critical angle of the product, so as to improve the packaging luminous efficiency of the product. At the same time, packaging materials absorb less light. In order to increase the proportion of light emitted, the shape of the package should be arched or hemispherical, so that when light is emitted from the packaging material to air, it almost vertically reaches the interface, thus no total reflection is generated.
3. Reflection Processing
Reflection processing has two main aspects: one is the reflection processing inside the chip, the other is the reflection of packaging materials to light. Through the reflection processing inside and outside the chip, the ratio of light flux emitted from the chip can be improved, the absorption inside the chip can be reduced, and the luminous efficiency of power LED products can be improved. From the packaging point of view, power-type LED usually assembles power-type chips on metal brackets or substrates with reflective chambers. The bracket-type reflective chambers generally adopt electroplating to improve the reflective effect, while the substrate-type reflective chambers usually adopt polishing method and electroplating treatment when conditions permit. However, the above two methods are affected by the accuracy of the die and process, and the reflected after treatment. The cavity has some reflective effect, but it is not ideal. At present, because of the poor polishing accuracy or the oxidation of metal coating, the reflective effect of the substrate-type reflector in our country is poor, which causes many light rays to be absorbed after they are irradiated into the reflective region, and can not be reflected to the light surface according to the expected target, which leads to the low efficiency of light extraction after final packaging.