LED because of its green, environmental protection and many other advantages, is considered to replace incandescent, fluorescent and other power consumption, pollution of the traditional lighting source of the revolutionary solid light source. Conventional LED is generally bracket, the use of epoxy resin package, the power is small, the overall luminous flux is not large, high brightness can only be used as some special lighting. With the LED chip technology and the development of packaging technology, comply with the field of high luminous flux LED lighting products, power LED gradually into the market. This power-type LED is generally the light-emitting chip on the heat sink, the above assembly of optical lenses to achieve a certain optical spatial distribution, the lens filled with low-stress flexible silicone.
Power LED to really enter the field of lighting, to achieve the daily lighting of the family, its problems to be solved there are many, one of the most important is the luminous efficiency. Currently on the market power LED reported the highest lumen efficiency of 50lm / W or so, far less than the daily requirements of the family lighting. In order to improve the power LED luminous efficiency, on the one hand the efficiency of its light chip to be improved; the other hand, power LED packaging technology also need to further improve, from the structural design, material technology and process technology and other aspects to improve the product Encapsulation efficiency.
2. Encapsulation elements that affect the efficiency of light extraction
1. Cooling technology
For light-emitting diodes consisting of PN junctions, when the forward current flows from the PN junction, the PN junction has heat loss, which is radiated into the air via adhesive, potting material, heat sink, etc., during this process. Part of the material has a thermal resistance to prevent heat flow, that is, thermal resistance, thermal resistance by the device size, structure and material determined by the fixed value. The thermal resistance of the LED is Rth (℃ / W) and the heat dissipation power is PD (W). At this time, the temperature of the PN junction is increased due to the heat loss of the current:
△ T (° C) = Rth × PD.
PN junction temperature is:
TJ = TA + Rth × PD
Where TA is the ambient temperature. Due to the rise in junction temperature will make the probability of PN junction recombination decreased, the brightness of the light-emitting diodes will decline. At the same time, due to heat loss caused by the increase in temperature, light-emitting diode brightness will no longer continue to increase with the proportion of the current, that shows the hot saturation phenomenon. In addition, as the junction temperature rises, the peak wavelength of the light emission will drift to the long wavelength, about 0.2-0.3 nm / ° C. For the white LED obtained by mixing the YAG phosphor coated with the blue chip, Drift, will cause a mismatch with the phosphor excitation wavelength, thereby reducing the overall luminous efficiency of the white LED, and lead to white color temperature changes.
For power light-emitting diodes, the drive current is generally more than a few hundred milliamperes, PN junction current density is very large, so PN junction temperature is very obvious. For the packaging and application, how to reduce the thermal resistance of the product, so that the heat generated by the PN junction can be released as soon as possible, not only can improve the product saturation current, improve product luminous efficiency, but also improve product reliability and life The In order to reduce the thermal resistance of the product, the first choice of packaging materials is particularly important, including heat sink, adhesive, etc., the thermal resistance of each material is low, that requires good thermal conductivity. Second, the structural design should be reasonable, the thermal conductivity of the material between the continuous match, the material between the thermal connection is good, to avoid the heat generated in the heat pipe bottleneck to ensure that heat from the inner to the outer layer of distribution. At the same time, from the process to ensure that the heat in accordance with the pre-designed cooling channel in a timely manner out.