In recent years, all countries in the world, such as European countries, the United States, Japan, Korea and China, all have LED lighting related projects. Among them, the plan of "ten cities and thousands of cities" popularized by our country is the most notable. Street lamp is an indispensable part of urban lighting. Traditional street lamps usually use high-pressure sodium lamp or metal halide lamp. The two kinds of light sources are characterized by their small size, large output and high luminous efficiency. However, only about 40% of these light sources are used in road lamps and lanterns, and only 60% of the light is reflected through the lamp reflector and then emitted from the lamps and lanterns. At present there are two basic traditional lamps, a lamp is directly irradiated on the direction of illumination is very high, the road can reach more than 50Lx, this region is the excessive lighting obviously, while the two lamps light at the intersection of the center position of light illumination was only under the illumination of 20%-40% light distribution low degree of homogeneity; two is the reflector efficiency of this lamp is 50%-60%, so the light loss in the process of reflection, so the traditional high pressure sodium lamps or halogen lamps in the overall efficiency of 70-80%, low degree of homogeneity, and excessive waste of illumination. In addition, high pressure sodium lamp and metal halide lamp life is usually less than 6000 hours, and the color index is less than 30; LED has high efficiency, energy saving, long life (50 thousand hours), environmental protection, high color index (75 ") and other significant advantages, how to effectively apply the LED and become the most popular lamp manufacturers the topic of LED in road lighting. Generally speaking, according to the usage environment of street lamps, there are strict requirements for optical design, life support, dustproof and waterproof capacity, heat treatment and light efficiency of LED. As the core of the LED street lamp, the manufacturing technology of LED chip and the corresponding packaging technology jointly determine the future of the application of LED in the lighting field.
1) enhancement of luminescence efficiency of LED chip
The improvement of the luminous efficiency of LED chip determines the energy saving capacity of LED street lamps. With the development of epitaxial growth technology and multiple quantum well structure, the internal quantum efficiency of epitaxial wafers has been greatly improved. How to meet the standard of street lighting depends largely on how to extract the most light from the chip with the least power. In a simple sense, it is to reduce the driving voltage and improve the light intensity. The LED chip structure of the traditional dress, usually coated with a layer of translucent conductive layer on the p-GaN makes the current distribution is more uniform, and the conductive layer of light emitted by the LED have some absorption, and p electrode will block part of the light, the light efficiency which limits the LED chip. While the use of LED flip chip structure, not only can avoid the conductive layer on P electrode and the electrode pad absorbs light shading problems, but also through the reflective layer of low ohmic contact to the light guide to set down on the surface of p-GaN, it can also reduce the driving voltage and high light intensity. (see Fig. 1) on the other hand, the technology of patterned sapphire substrate (PSS) and chip surface roughening can also increase the efficiency of LED chip by more than 50%. PSS structure is mainly to reduce the total reflection of photons in the device and increase the efficiency of light. The chip surface roughening technology can reduce the loss of light reflected at the interface when the light is emitted from the chip to the chip. At present, LED chip uses inverted structure and graphical technology, 1W power chip white light package, 5000K color temperature, the highest light effect of 134lm/W.
2) life and reliability of LED chip
The nodal and heat dissipation of the chip
Heat dissipation is a technical problem that the power type white light LED needs to focus on. The heat dissipation effect is directly related to the life of the street lamp and the energy saving effect. LED generates light by the transition between electrons in the interband, and there is no infrared part in the spectrum, so the heat of the LED can not be emitted by radiation. If the heat in the LED chip can't be sent out in time, the accelerator parts will be aging. Once the temperature of the LED exceeds the maximum critical temperature (according to the different epitaxy and process, the chip temperature is about 150 degrees C), the permanent failure of the LED is often caused. It is important to effectively solve the heat dissipation of LED chip and to improve the reliability and life of the LED street lamp. To do this, the most direct way is to provide a good thermal channel heat dissipated from node to. Compared with the traditional positive structure, the vertical and flip chip structure has better heat dissipation ability on the chip level compared with the traditional positive structure. The copper alloy is used directly as the substrate for the vertical structure chip, which effectively improves the cooling capacity of the chip. The flip chip (Flip-Chip) technology through the LED eutectic solder flip chip to a silicon substrate with a higher thermal conductivity (on thermal conductivity of about 120W/mK, traditional dress chip sapphire thermal conductivity is about 20W/mK), gold bump and the silicon substrate and the substrate between the chip and improve the heat dissipation capability of LED chip, LED heat protection can quickly export from the chip.
3) an example of the stability of the flip chip
LED street is usually around 60-200W, the two main ways to achieve, one is through the "multi chip gold series parallel module and a plurality of LED through the PCB serial parallel" approach to achieve high wattage. No matter what kind of way, are required by the welding line in the process of encapsulation (Wire-bonding) mode to realize the circuit chip and the bracket is connected, and the welding process of LED ceramic nozzle chip impact is the main cause of LED leakage, weld, traditional dress and vertical structure of LED, the electrode surface is located on the light emitting chip. Therefore, the positive impact of the welding line in the process of ceramic nozzle can easily lead the light emitting region and electrode metal layer damage in LED chip by flip chip structure, electrode on the silicon substrate, no chip welding line in the process of impact, greatly improve the reliability and production yield.