LED lumen attenuation is the weakening of the light signal during transmission. LED products made by global leading manufacturers at the present stage have lumen attenuation in different grades after used for a period of time and this phenomenon also exists in high power LEDs. Then, what causes LED attenuation? Lumen attenuation is directly related to the temperature, and is mainly determined by the chip, phosphor used and the packaging technology. At present, lumen attenuation has become the chief problem for white LED to march into civil lighting field. There are two main problems that cause LED lumen attenuation:
 

Ⅰ The quality of the LED product itself
1.  The brightness of LED will attenuate quickly if the quality of the adopted LED chip is not good.
2. The production technology has some defects. If the heat generated by LED chips can not be exported out of the chip smoothly by the PINs, LED chips may be over heated and thus causing chip attenuation more serious.
Ⅱ The operating condition
1. LED should be drove by constant current, if drove by voltage, LED lumen attenuation will be caused.
2. If the driving current is larger than the rated value, LED lumen may easily get attenuated.
Among the various causes of LED products lumen attenuation, “heat” is the pivotal one. Many manufacturers pay little attention to the heat dissipation of the products, leading to a much more serious lumen attenuation compared to those products that dissipate heat properly. Besides, thermal resistance of the LED chip itself, silver colloid, the heat dissipation effect of the substrate and golden wire problems are also related closely to LED lumen attenuation.

Classification of LED products
LED which is becoming widespread owns various kinds of product. LED products can be classified into different types according to light emitting colors, outer surface trait and structure of the diodes, luminous intensity, operating current, chip material, function and etc. This passage is going to introduce four types of LED products classification.
1. According to light emitting colors of the diode:
LEDs are available in red, orange, green (subdivide into yellow-green, standard green and pure green) and blue and so on.
Some light emitting diodes have chips that bear two or three colors. Besides, the package (the plastic body) of LED may have light scattering agent or not, be colored or colorless, thus, LED can also be classified into the following four types: colored transparent, colorless transparent, colored scattering and colorless scattering.
2. According to the outer surface (plastic body) traits of the diode, LED can be classified into round LED, square LED, rectangular LED, surface LED, side LED, surface mount subminiature LED and so on.
Round LED can be divided into following types according to its diameter: φ2mm、φ4.4mm、φ5mm、φ8mm、φ10mm and φ20mm and so on. Round LED with diameter of φ3mm is generally noted as T-1 in abroad, φ5mm noted as T-1(3/4) and φ4.4mm noted as T-1(1/4).
The angle distribution condition of round LED’s luminous intensity can be evaluated by the value of its viewing angle. The following three types are classified according to the angle distribution of the luminous intensity:
(1) High directivity LED: This kind of LED generally adopts sharp head epoxy packaging or metal reflection cavity packaging, usually without light scattering agent. The viewing angle of this kind of LED varies between 5and 20or even smaller. With high directivity, it can be used as local illumination source or to build up an automated detecting system together with a light detecting machine.
(2) Standard type: It is generally used as indicator lamp. The viewing angle varies between 20and 45(3) Scattering type: Its is usually applied as indicator lamp that has comparatively bigger viewing angle which varies between 45and 90or even larger. It has more light scattering agent than the other types.
3. According to the structure, LED can be classified into full epoxy resin packaging, metal base, ceramic base epoxy resin packaging and glass packaging.
4. According to the luminous intensity and operating current.
According to luminous intensity, there are standard brightness LED (with luminous intensity less than 10mcd), high brightness LED (with luminous intensity between 10mcd and 100mcd) and ultrahigh brightness LED (with luminous intensity larger than 100mcd).
LED can also be divided into two types according to its operating current: standard type which runs at an operating current from more than ten mA to tens mA; low current LED which runs at an operating current below 2mA (however, with the same brightness as standard LED).

A Brief Discussion on LED Packaging Technology
LED is the acronym of Light Emitting Diode. It is a kind of semiconductor device that is commonly used in indicator light and display board. LED is capable of transforming electric energy into light energy directly in high efficiency and its life-span can reach as long as tens of thousands hours to one hundred thousand hours. Compared to traditional bulbs, LED also bears the advantages of non-friable and power-saving and so on.
However, only well-packaged LEDs can be applied as terminal products in practical use. And the packaging types should be designed according to the requirements of each distinct type of LED.
Firstly, we need to figure out why should LEDs be packaged?
Through packaging, outer leads can be linked to the electrode of LED chips, thereby to protect the chips and to improve its luminescence efficiency. More important than the electrical signal output function, packaging makes sure that the diode chips to operate normally and to export visible light successfully. So the packaging is not an easy job as electric parameters and optical parameters requirements should be count on at the same time when doing the design.
Secondly, the packaging equipments used will be introduced here.
The requirements for LED packaging are strict. It is essential to use high-precision crystal solid machine to package no matter Lamp-LED or Surface Mount Device LED (SMD-LED). If LED chips were not placed into the package precisely, the luminescence efficiency of the overall packaging device will be influenced directly. Any deviation from the established position will prevent LED light to be reflected fully from the reflective cup, affecting LED’s brightness. So an advanced crystal solid machine with a PR System should be used in order to weld LED chips into the package precisely, without considering the quality of the lead frame.
The latter part of this passage is going to introduce different kinds of LED packages
LEDs applied in different occasions and LEDs with different sizes, heat-dissipation methods and luminescence efficiency will have different types of LED packages. Currently, LED packages can be divided into the following types: Lamp-LED, TOP-LED, Side-LED, SMD-LED, High-Power-LED, Flip Chip-LED and so on.
1. Lamp-LED
Lamp-LED adopts potting process. Firstly to inject liquid epoxy resin into the LED molding cavity, then to insert LED stent which is well pressed and welded into the liquid, finally to put it into the drying oven to solidify the epoxy resin. LED pared from the cavity will be the final product. As the manufacturing technique for Lamp-LED is comparatively simple and its cost is lower, its market share is comparatively high.
2. SMD-LED
SMD-LED is mounted to the surface of the circuit board. SMT (Surface Mount Technology) is usually adopted. This type of LED can use reflow soldering, and problems related to luminance, visual angle, smoothness, reliability and consistency can be well solved in SMD-LED. Meanwhile, lighter PCB and reflector material is used in SMD-LED. What’s more, heavy carbon steel chip pins used in Lamp-LED is got rid of after improvement, enabling a less amount of epoxy resin to  be required in the filling of the display reflector, thus the size and weight of the LED is reduced. Generally, SMD-LED can cut product’s weight by 50% easily, making terminal application perfectly.
3. Side-LED
Currently, one of the focused problems for LED packaging is related to side-emitting. It should be made sure that the side-emitting intensity is the same with that of the surface emitting when LED is used as the backlight source of LCD. Although side-emitting can be realized through the design of a lead frame, its heat dissipation effect is not ideal. Thus the invention of a specially designed reflector by Lumileds Company comes out. Using the reflector’s principle, surface emitting LED can radiate side light, thus high power LED can be applied to large size LCD backlight modules successfully.
4. TOP-LED
TOP-LED is a commonly seen surface mount light-emitting diode. It is mainly used as the backlight or status indicator light of multi-functional thin profile handsets and PDA.
5. High-Power-LED
In order to get high power, high brightness LED light sources, manufacturers is now making efforts to produce high power LED with proper packages. At present, LED packages that can endure W scale power have been invented, such as Norlux Series high power LED package. It is a multi-chip assembly with a hexagon aluminum base board that functions as heat sink at the same time. Base board’s diameter amounts to 31.75mm. The emitting zone is located in its central part, with diameter of about (0.3755.4)mm. Forty LED chips can be contained in one package. Generally, the future outlook for high-power-LED package is promising due to its high density chip assembly, high luminescence efficiency, low thermal resistance and high output power under large current flow.
It is obvious that the thermal characteristics of power LED can directly influence its working temperature, luminescence efficiency, wavelength, life-span and other factors. So it is important to focus on the packaging design process, manufacturing technology of power LED chips.
6. Flip Chip-LED
There are couple numbers of perforation in the PCB base board in Flip Chip-LED. Every perforation in one side of the base board is set with a kind of conductive material that is spread on the surface of the base board. Then couple numbers of unpackaged LED chips are fixed on the perforations on the side that has conductive material. The anode and the cathode of every single LED chip are connected to the conductive material on the surface of the base board through a tin-ball. Meanwhile, LED chips are adhered with transparent sealing glue that is a hemisphere shape on the side that faces with the perforation.
Conclusion
According to the solid-state lighting principle, the luminescence efficiency of LEDs can reach approximately 100%. So, it is called the new light source in the 21st century, and is possible to become the fourth generation light source after incandescence lamp, fluorescent lamp and high-intensity discharge lamp. In the future, manufacturers should focus on the development of high-power, high brightness LED. As packaging is the process to link the preceding and the following parts in LED industry chain, attentions should be specially paid to it.