Discussion on innovation in LED packaging technology for lighting

1. Conventional existing packaging methods and application fields

At present, there are several LED packaging methods: bracket packaging, SMD packaging, and module packaging. These packaging methods are common and commonly used.

The bracket row package was first adopted to produce a single LED device. This is the common lead-type light-emitting diode (including piranha package). It is suitable for instrument indicator lights, urban lighting projects, advertising screens, guardrails, traffic lights, and some products and fields that are currently widely used in my country.

SMD is a leadless package with small size and thinness. It is very suitable for keyboard, display, lighting of mobile phones , backlighting of TV sets, and electronic products that need lighting or indication. In recent years, SMD packaging has developed in the direction of large size and high power . Three or four LED chips are packaged in one SMD , which can be used to assemble lighting products. Module packaging is also a multi-chip package. Dozens or hundreds of LED chips are packaged on an alumina or aluminum nitride substrate with a smaller size and high packaging density. The internal connection is a mixed type, that is, there are multiple chips in series and several parallel connections. This type of package is mainly used to expand power and is used for lighting products. Due to the high density of the module package, the heat generated during application is large, and heat dissipation is the primary problem to solve the application. Devices produced by the above packaging methods have a common feature for the production of lighting fixtures: the number of thermal resistance channels is large, it is difficult to produce high-quality lighting fixtures, and the connection processing requirements between the module itself and the radiator are relatively high. At present, all packaging methods are to mix yellow phosphor ( YAG ) and epoxy resin in different proportions, directly apply them to the LED chip that emits blue light , and then heat and cure. The advantage of this common method is that it saves materials, but the disadvantage is that it is not conducive to heat dissipation and the phosphor will also age. Because epoxy resin and phosphor are not good thermal conductive materials, and wrapping the entire chip will affect heat dissipation. This method is obviously not the best solution for manufacturing LED lighting fixtures.

At present, high-power chips produced abroad, white light chips above 0.5 watts, are coated with a uniform layer of YAG phosphor paste on the blue light chip, and the appearance is a yellow cube. (Except for the two gold pads used for welding, there is no phosphor. This method can improve the light effect compared with the previous commonly used method , so it is widely used abroad.) When packaging, just weld this white light chip on the designed circuit board, eliminating the process of coating phosphor. It brings convenience to lamp manufacturers, but domestic chip suppliers are currently unable to mass-produce such LED white light chips.

China is one of the earliest countries to develop LED street lights . At present, it is also used well in China. The reason is that the country attaches great importance to the " low-carbon economy ". In 2009, China implemented the " 10,000 LED street lights in 10 cities". Many cities have experimental sections to test the feasibility of LED street lights. China is based on the application of street lights, while foreign countries ( Osram , Nichia , Samsung and other companies) are based on indoor lighting. Which of these two routes has more advantages is still unclear. As far as China is concerned, it is due to national conditions that LED street lighting is used as the application direction. The reason is that China's national income is low, and the cost of LED indoor lighting is high, which is unacceptable to the people. The use of LED street lights is paid by the government, and LED street light manufacturers have taken a fancy to this point. In fact, the working conditions of LED street lights are more stringent and demanding than indoor LED lighting fixtures. If the quality can be passed (heat dissipation, service life, color rendering, reliability, etc.), it will be easier to make indoor LED lighting. At present, foreign LED giants are launching hundreds or even thousands of LED indoor lighting fixtures, with prices ranging from US$20 to US$75 and power ranging from a few watts to 20 watts. However, the packaging methods they use are all mentioned above. The only one is Philips , which uses phosphor coating on the LED lampshade, which was rated as one of the most creative LED lighting products in 2009.

The author believes that all LED lighting fixtures should be manufactured using multi-chip packaging and module packaging (module packaging is a high-density multi-chip packaging), and it is best to package the LED chip directly on the main body of the lamp, so that the number of thermal resistance is the least and a better heat dissipation effect can be achieved. Or a circuit body coated with copper foil is made on the main body of the lamp, and its thermal resistance is also low. The power of LED lighting must be at least several watts, so multiple chips are used, and the previous packaging process is not applicable. New methods and processes must be used. It is difficult to produce high-quality and highly reliable LED lamps by assembling multiple packaged LED devices . I hope that technicians engaged in the manufacture of LED lamps can understand this.

2. Innovation of phosphor coating process

The phosphor coating of the module package is currently applied directly on the chip. The phosphor of the same module is relatively consistent, but for mass production, different modules may have color differences. The better way is to use LED phosphor film or diaphragm. Film and diaphragm can be mass-produced with good consistency. LED lamps are multi-chip packaged, and the light emitted is mixed. After the phosphor film or diaphragm is converted into white light, the color difference can be eliminated. The requirements for films and diaphragms are:

1. Able to transmit light, with a thickness between 0.1 and 0.5 mm, uniform phosphor and smooth appearance.

2. The light conversion efficiency should be high, the stability should be good, the life should be long and the anti-aging performance should be good.

3. It can be made with or without a base, or made into a thin sheet, depending on the implementation conditions and cost. The base should be colorless, transparent and aging resistant.

4. Easy to process and shape, can be cut to any size, and has low cost.

Another method is to mix fluorescent powder and transparent plastic in proportion, and directly produce a lampshade with fluorescent powder through an injection molding machine and a mold, so that the lampshade can convert blue light into white light. This is more convenient and easier, because the lampshade converts mixed blue light, so the output white light has no color difference, and the light is softer and does not produce glare.

3. In order to better solve the problem of LED heat dissipation , the lamp design and packaging should be considered together, and the packaging and the heat sink for LED heat dissipation should be made into a whole to effectively reduce the number of thermal resistances. This is a very effective way to improve the heat dissipation of lamps.

Currently, the LED fluorescent lamps on the market do not have heat sinks. Such lamps cannot achieve high power, high quality and long life. The correct product design should take the heat sink into consideration. The industrial production is to use a mold to extrude a semicircular aluminum profile with fins for the heat sink of the LED fluorescent lamp, and then cut the required length according to the power. Then, an aluminum-based copper foil circuit is made, and the chip is fixed on the copper foil, connected with gold wire or bonded with a bonding machine. Such lamps have good heat dissipation effect, only two thermal resistances, one or two less thermal resistances than the commonly used packaging method, which effectively reduces the temperature of the chip and plays a role in improving the quality and life of LED fluorescent lamps.

Another method is to design a protrusion on the aluminum profile, mill out many rectangles as needed, and use a common (0.8-1.0mm thick) PCB circuit board to open a rectangular hole according to the rectangle of the aluminum profile, paste or rivet the PCB circuit board on the aluminum profile, and fix the LED chip on the rectangular protrusion on the aluminum profile, and then use gold wire to connect the circuit on the PCB board and the chip. This production process is the best, with only one thermal resistance and the best heat dissipation effect. Manufacturers of LED lamps should give priority to this solution, followed by the method of making copper foil circuits on aluminum profiles. Only such innovation can effectively solve the heat dissipation problem of LED long strip lamps and improve the quality and life of LED fluorescent lamps.