Improve the light extraction efficiency of thermal resistance power LED packaging technology

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Ultra-high brightness LED applications continue to expand, first into the special lighting market areas, and to the general lighting market. As the LED chip input power continues to increase, these power LED packaging technology put forward higher requirements. Power LED packaging technology should meet the following two requirements: First, the package structure to have a high light extraction efficiency, the second is the thermal resistance to be as low as possible, so as to ensure the power LED photovoltaic performance and reliability.

Semiconductor LEDs To act as a light source for illumination, the luminous flux of conventional products is far away from the versatility of light sources such as incandescent and fluorescent lamps. Therefore, the LED to the field of lighting development, the key is to its luminous efficiency, luminous flux increased to the level of existing lighting sources. Epitaxial materials used in power LED epitaxial growth by MOCVD technology and multi-quantum well structure, although its quantum efficiency needs to be further improved, but the biggest obstacle to obtain high luminous flux is still chip low light extraction efficiency. The design of the existing power LED adopts a new flip-chip structure to improve the light extraction efficiency of the chip, improve the thermal characteristics of the chip, and increase the photoelectric conversion efficiency of the device by increasing the chip area and increasing the operating current so as to obtain the Higher luminous flux. In addition to the chip, the device packaging technology also holds a pivotal position. The key packaging technology processes are:

Cooling technology

The traditional LED-type LED package structure, the general is conductive or non-conductive plastic chip will be mounted in the small size of the reflective cup or on the stage, the device to complete the connection between the inside and outside the device after the epoxy resin package, The thermal resistance of up to 250 ℃ / W ~ 300 ℃ / W, the new power chip if the traditional LED package, will result in poor heat dissipation caused by the rapid rise of the chip junction temperature and carbonization of epoxy yellow, resulting in the device Of the accelerated light failure until failure, and even failure because of the stress caused by rapid thermal expansion.

Therefore, for a large current LED chip power, low thermal resistance, good heat dissipation and low stress of the new packaging structure is the key technology of power LED devices. Can be low resistivity, high thermal conductivity of the material bonding chip; the lower part of the chip plus copper or aluminum heat sink, and the use of semi-encapsulated structure to accelerate the heat; or even design the second heat sink to reduce the thermal resistance of the device. Inside the device, filled with highly transparent flexible silicone rubber, silicon rubber in the temperature range (typically -40 ℃ ~ 200 ℃), the colloid will not be due to sudden changes in temperature caused the device open, it will not turn yellow phenomenon. Part material should also give full consideration to its thermal conductivity, thermal characteristics, in order to obtain good overall thermal characteristics.

Secondary Optical Design Technology

In order to improve the light extraction efficiency of the device, an additional reflective cup and multiple optical lenses are designed.

Power LED white light technology

Common methods to achieve white light in the following three ways:

(1) YAG phosphor is coated on the blue chip, and the blue light of the chip excites the yellow-green light of 540 nm-560 nm and the yellow-green light and the blue light. The preparation method is relatively simple, efficient and practical. Disadvantage is the poor consistency of the amount of cloth, phosphor precipitated easily lead to poor gloss uniformity, color consistency is not good; high color temperature; color is less than ideal.

(2) RGB trichromatic multiple chips or multiple devices glow mixed color into white, or use blue + yellow-green double-chip color to produce white light. As long as the cooling method, the white light generated by this method than the previous method is stable, but the drive is more complicated, in addition to consider the different color chips of different light failure rate.

(3) Apply RGB phosphor on the UV chip, and use the violet light to excite the phosphor to produce the three primary colors mixed light to form white light. Due to the low efficiency of current UV chips and RGB phosphors, the practical stage has not yet been reached.

We believe that lighting W-class power LED products to achieve industrialization must also solve the following technical problems:

1, the amount of powder coating control: LED chip + phosphor coating method used in the process, the phosphor is usually mixed with plastic and then use the dispenser to the chip. In operation, due to the fact that the viscosity of the carrier gel is a dynamic parameter, the specific gravity of the phosphor is greater than that of the carrier gel, and the accuracy of the dispenser and the like, the control of uniformity of coating amount of the fluorescent powder is difficult, resulting in white light Uneven color.

2, with the photoelectric parameters of the film: the characteristics of semiconductor technology, the same material used to determine the same wafer between the chip there may be optical parameters (such as wavelength, light intensity) and electrical (such as forward voltage) parameter differences. RGB three-color chip is the case, a great influence on the chroma parameters of white light. This is one of the key technologies industrialization must solve.

3, According to the requirements of the application of optical chromaticity parameter control: different uses of products, the white LED color coordinates, color temperature, color rendering, optical power (or light intensity) and the spatial distribution of light and other requirements are different. Control of the above parameters involves product structure, process methods, materials and many other factors. In industrialized production, it is very important to control the above factors and get the products that meet the application requirements and have good consistency.

Detection technology and standards

With the development of W-class power chip manufacturing technology and white LED technology, LED products are gradually entering the (special) lighting market. The traditional LED product parameter testing standards and testing methods for displaying or indicating can no longer meet the needs of lighting applications. Semiconductor equipment manufacturers at home and abroad have also launched their own test equipment, test instruments used by different instruments, conditions, standards there are some differences, an increase of test applications, the performance comparison of the work more difficult and complex issues.

China Optics and Optoelectronics Industry Association Optoelectronic Devices Branch Association according to the needs of the development of LED products, in 2003 released the "light-emitting diode test method (Trial)", the test method to increase the LED chrominance parameters. However, LED lighting industry to expand, the establishment of LED lighting product standards is an important means of industrial standardization.

Screening Technology and Reliability Assurance

Due to the appearance of the lamps and lanterns, LED lighting assembly space sealed and limited, sealed and limited space is not conducive to LED cooling, which means that the use of lighting LED environment is inferior to the traditional LED display and instruction. In addition, the lighting LED is driven by high current drive, which put forward higher reliability requirements. In industrial production, for different product applications, appropriate heat aging, temperature cycling impact, load aging screening process, eliminate early failure, to ensure the reliability of the product is necessary.

Electric protection technology

Since GaN is a wide bandgap material, the resistivity is high, and the induced charges due to static electricity in the production process of such chips are not easily disappeared and accumulate to a considerable extent, which can generate high electrostatic voltages. Breakdown occurs and discharges when the material's capacity is exceeded. The blue chip of the sapphire substrate has positive and negative electrodes on the chip with very small spacing. For the InGaN / AlGaN / GaN double heterojunction, the active layer of InGaN is only tens of nanometers and has a small electrostatic capacity, Electrostatic breakdown, the device failure.

Therefore, in the industrial production, the prevention of static electricity is appropriate, a direct impact on product yield, reliability and economic benefits. Static prevention techniques are as follows:

1, The production and use of places from the human body, Taiwan, and space and product transmission, stacking and other aspects of the implementation of preventive measures are anti-static clothing, gloves, bracelets, shoes, mats, boxes, ion fans, testing equipment.

2, chip design static protection circuit.

3, LED assembly protection devices.

Current Status of Power LED Packaging Technology

Power LED is divided into power LED and W-class power LED two. Power LED input power is less than 1W (except for a few milliwatts of power LED); W-class power LED input power equal to or greater than 1W.

Foreign power LED packaging technology

(1) Power LED

The earliest HP company in the early 1990s launched the "Piranha" package structure of the LED, and in 1994 introduced an improved "SnapLED", there are two operating currents, respectively, 70mA and 150mA, the input power up to 0.3W . Then OSRAM introduced "PowerTOPLED." Then some companies introduced a variety of power LED package structure. The power LED of these structures is several times higher than the input power of the LED of the original bracket package, the thermal resistance drops a fraction.

(2) W-class power LED

W-class power LED is the core of the future part of the lighting, so the world's major companies have invested a lot of power, W-class power LED packaging technology research and development.

Single-chip W-class power LED was first introduced by Lumileds in 1998 LUXEONLED, the packaging structure is characterized by the use of thermoelectric separation form, the flip-chip silicon carrier directly soldered to the heat sink and the use of reflective cup, optical Lenses and flexible materials such as transparent plastic structure and new materials are now available single-chip 1W, 3W and 5W high-power LED. OSRAM company launched in 2003 single-chip "GoldenDragon" series of LEDs, the structure is characterized by heat sink and metal Direct contact with the circuit board, with good thermal performance, and the input power up to 1W.

Multi-chip package of high-power LED package, the structure and package more. UOE company in the United States in 2001 launched a multi-chip packaging package Norlux series LED, the structure is the use of hexagonal aluminum plate as a substrate. In 2003, LaninaCeramics introduced a high-power LED array packaged with the company's proprietary low-temperature sintered ceramic (LTCC-M) technology on metal substrates. Panasonic introduced a high-power white LED packaged in a 64-chip package in 2003. Nichia introduced the brightest white LED in the world in 2003 with a luminous flux of 600 lm and a power output of 1000 lm 30W, the maximum input power of 50W, white LED display module provides luminous efficiency of 33lm / W.

For multi-chip high-power LEDs, many companies continue to develop many new packages with new structure based on the actual market demand. Their development and development are very fast.

Domestic power LED packaging technology

Domestic LED packaging products more complete varieties, according to preliminary estimates, the country more than 200 LED packaging plant, packaging capacity of more than 200 billion / year, the package is also very strong supporting capacity. However, many packaging plants for the private sector, small-scale. However, China's Taiwan UEC Corporation (UF) using metal bonding (MetalBonding) technology package MB series of high-power LED is characterized by the use of Si instead of GaAs substrate, good heat dissipation, and metal bonding layer for the light reflection layer, improve light output .

For the research and development of high-power LED packaging technology, at present the country has not officially supported the investment, the domestic research units seldom intervene, and the efforts of the packaging enterprises in R & D (human and financial resources) are still not enough, forming a weak domestic development environment for the packaging technology , Packaging technology level compared with foreign countries there is still a considerable gap.