How to improve the reliability of LED lighting

The application of LED products is getting wider and wider. To meet the market demand, the industry is aiming at various product lines, including lighting LEDs that can achieve high color rendering and high reliability, small thin LEDs that are represented by PICOLED products, and automotive customers. Systemized color LEDs, etc. have devoted considerable R&D resources.

Rapid growth of white LED output for lighting

Due to the global energy conservation trend and the rising awareness of energy conservation caused by the Great East Japan Earthquake, the demand for white LEDs for lighting in the Japanese market has greatly increased, and the output value of the LED lighting market is rapidly increasing. However, if the lighting source is to be completely removed from the traditional Incandescent light bulb lighting is converted to LED lighting. There are still some problems to be solved in the product characteristics. Among them, the industry's R&D focus is mainly on the high color rendering and high reliability of LED lights. The following will focus on high Ra values ​​for employees respectively. Share technology with luminous efficiency.

Take into account high Ra values ​​and luminous efficiency

The average color rendering index (Ra) is an index by which the light source causes the object to represent or reproduce the true color. The higher the index, the better the color reproducibility (Ra of the sun is 100). The market expects white light for illumination. LED can have high luminous efficiency and high color rendering (Ra≧80), but there is a trade-off characteristic between luminous efficiency and Ra value. Due to the higher requirements for luminous efficiency in the market, the high luminous efficiency LED of Ra≒70 is currently on the market.

The package structure of the general white LED lamp is to mount the blue LED chip on the substrate and encapsulate it with the resin containing the phosphor. After the LED element's luminescent color (blue) and the phosphor's luminescent color (yellow, red, green, etc.) are mixed, white light is formed.

From the viewpoint of luminous efficiency, white light is generally formed with blue light and yellow light, but this results in poor reproducibility of red light and is therefore not suitable for lighting applications. The commonly used solution is to increase the red light composition, thereby improving the reproducibility of red light, but this may lead to problems with unsatisfactory luminous efficiency.

In order to achieve a combination of high luminous efficiency and high Ra value, the industry has efficiently deployed the phosphors inside the package and used the best of both worlds as a countermeasure. The company successfully developed Ra≧80 and has a very high luminous efficiency. This series of products performs very well on either the Ra or R9 (red) index. Compared with other brands with the same Ra value, the series has higher R9 values ​​and better red reproducibility. . With the breakthrough of this technology, LED lamps can not only reduce the uneven color, but also can meet the more detailed color grade.

High reliability

In recent years, the demand for reliability in the market has also become increasingly high. In particular, due to the high reflectivity of LED packages, silver plating is generally used, but silver will degrade the LED beam due to vulcanization (a phenomenon of blackening due to reaction with sulfur). This phenomenon is caused by outdoor LED lamps. Due to serious problems, various manufacturers have come up with countermeasures against various silver plating solutions. However, this problem still cannot be completely improved.

In view of this, the industry abandoned the silver plating method and switched to nickel/gold plating. Changing the LED package from silver to nickel/gold plated leads to an increase in cost and poor luminous efficiency due to the decrease in reflectivity. However, these problems have been successfully overcome by improving the package structure.

The new package structure can not only maintain high luminous efficiency, but also achieve high-reliability LED luminous performance. This series of products exhibit excellent performance even in the vulcanization test and can completely avoid the phenomenon of beam degradation.

LED small/thin

As mobile devices are becoming smaller and lighter, the demand for small-pitch products in the market is increasing year by year, and parts are also facing more requirements for lowering the height and reducing the size. In addition, since outdoor full-color display devices mostly use LEDs, in order to improve performance, full-color LED packages are also facing higher densities.

Component Technology

In order to make the package smaller and thinner, the internal LED components must also be small and thin. Therefore, the industry has adopted the self-developed process technology from the formation of the light-emitting layer on the wafer to the waferization, and finally succeeded in reducing the element size of the AlGaInP light-emitting LED to 0.13 millimeters (mm). Thickness t = 50 micrometers (μm), achieving miniaturization goals at one go.

Mold Technology

In order to ensure the strength of the product, the industry has proposed a method of sealing the semiconductor element with a resin. The resin sealing process adopts the Transfer Mold method. However, the mold cavity of the mold becomes thinner and thinner (cavity thickness is 0.10 mm). Therefore, the fluidity of the resin must be ensured. In addition, in order to ensure the optical characteristics of the LED, it is not possible to add a filler material for ensuring the strength of the component, and as a result, the mechanical strength of the product is reduced, but the above problems have been solved at present.

Assembly technology

In the manufacture of LED chips, the LED elements must be soldered in a sealing resin with a thickness of t=0.10 mm. Therefore, the industry has used a wire bonding machine developed by the industry to successfully reduce the pitch and reduce the circuit.

At present, the world's smallest ultra-small LED is only 1006 in size and only 0.2 mm in thickness. This product is not limited by the installation space, and uses high-brightness LED components to emit light from the inside of the housing of the mobile phone through the LED light. Transparent lighting.

Not only that, ultra-small LED can also be applied to the dot matrix display. The traditional 1608-size products have a minimum pitch of 2mm, while ultra-small LEDs can be mounted with a minimum pitch of 1.5mm for high-density mounting, thus enabling more detailed performance.

In other aspects, due to the small package size of the solution, it can be used on seven-segment displays and dot matrix display modules, and die-bonding (Die-bonding) necessary for direct wafer packaging (COB) technology is omitted. ), bonding wire, bonding (Bonding) and other processes.

LED lighting for vehicles

With the rapid popularization of LED bulbs and lighting applications, automotive LED lighting has become more popular in the market than ever before. In terms of vehicle interior use, LED backlights have been used almost everywhere, whether it is car stereos, car navigation systems or air conditioning panels. Next, interior lights and warning lights, such as those still using traditional light bulbs, and instrument panel backlights using cold-cathode tubes, will gradually face the destiny of replacement.

In the exterior of vehicles, traditional light bulbs such as taillights, turn signals, and position lights have been gradually replaced in recent years. Even headlights have been switched from conventional halogen lamps and high-intensity discharge (HID) lamps to LED lamps. Replaced by. From the point of view of environmental visibility, the trend of using LED lamps as Daytime Running Lamps (DRLs) is worth paying attention to.

In response to the diversified needs of vehicles, the industry will focus on R&D of automotive LED technology with the following two priorities.

Customization of color and brightness

In terms of automotive interiors, most of the light sources around the dashboard such as air-conditioning panels are designated by the OEM. The aluminum phosphide, gallium, indium, and elemental LED series products introduced by the industry have the advantages of self-made components. Both color and luminosity can be customized on customer's request.

Others, such as the use of InGaN and phosphor-containing resins, have succeeded in creating white and pink LED series, which also provide color customization capabilities. Like the main button backlight and other frequently used buttons, the difference between the buttons and the adjacent buttons can be highlighted by subtle color differences, thereby eliciting the user's attention. This aluminum-phosphide-gallium-indium element is a technology that suppresses the wavelength difference at the epitaxial growth stage, so it can meet the strict specifications of customers.

R & D vulcanization measures / new products

On the other hand, the market demand for LED lamps for vehicle exterior applications such as taillights is greater than heat resistance and weather resistance. However, since the lead frame of conventional LED packages is silver plated, vulcanization and light beams are easily generated. Deterioration, the current issue has also begun to receive attention.

In view of this, the industry has changed nickel plating/palladium/gold plating as leadframe materials and successfully solved the problem of beam degradation caused by vulcanization. In addition, for the shortcomings of photometric reduction caused by nickel/palladium/gold plating, the industry has also developed a new series of products that can be solved by increasing the output efficiency of the device itself, exhibiting no less than the beam intensity of the silver plated product. . In the future, the industry will adopt nickel plating/palladium plating/gold plating as a countermeasure to improve the encapsulation and vulcanization, and actively expand new product lines to meet the diverse needs of customers.