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 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.
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.
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 removed 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 reduce product’s weight by 50% easily, making terminal application perfectly.
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.
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.
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 (0.375×25．4)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.
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.