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LED Packaging Technology of Improving the Efficiency of Light Extraction and Reducing Thermal Resistance


The application of ultra-high brightness LED is expanding. First of all, it enters the market field of special lighting and strides forward to the general lighting market. Due to the increasing input power of LED chips, higher requirements are put forward for the packaging technology of these power LED chips. The packaging technology of power LED should mainly meet the following two requirements: first, the packaging structure should have high optical efficiency; second, the thermal resistance should be as low as possible, so as to ensure the photoelectric performance and reliability of power LED.

If semiconductor LED is to be used as lighting source, the luminous flux of conventional products is far away from the common light sources such as incandescent lamp and fluorescent lamp. Therefore, the key to the development of LED in the field of lighting is to improve its luminous efficiency and luminous flux to the level of existing lighting sources. The epitaxial materials used in power LED are MOCVD epitaxial growth technology and multi quantum well structure. Although the internal quantum efficiency needs to be further improved, the biggest obstacle to obtain high luminous flux is still the low efficiency of chip. The existing power LED design adopts a new flip chip structure to improve the chip's optical efficiency, improve the chip's thermal characteristics, and increase the chip area, increase the working current to improve the device's photoelectric conversion efficiency, so as to obtain high luminous flux. In addition to chips, the packaging technology of devices is also very important. Key packaging technologies include:

Heat dissipation technology

The traditional LED packaging structure of indicator light type is to use conductive or non-conductive adhesive to install the chip in the small size reflecting cup or the chip carrier. After the gold wire completes the internal and external connection of the device, it is encapsulated with epoxy resin. Its thermal resistance is as high as 250 ℃ / W ~ 300 ℃ / W. if the new power type chip adopts the traditional LED packaging form, the chip junction temperature will rise rapidly due to poor heat dissipation And epoxy carbonization turns yellow, resulting in accelerated light failure of the device until failure, and even failure due to open circuit caused by the stress generated by rapid thermal expansion.

Therefore, for the power LED chip with large working current, the new packaging structure with low thermal resistance, good heat dissipation and low stress is the key technology of the power LED device. The chip can be bonded with materials with low resistance and high thermal conductivity; copper or aluminum heat sink is added at the lower part of the chip, and the semi encapsulated structure is adopted to accelerate the heat dissipation; even the secondary heat dissipation device is designed to reduce the thermal resistance of the device. In the inner part of the device, the flexible silicone rubber with high transparency is filled. In the temperature range (generally - 40 ℃ ~ 200 ℃) that the silicone rubber bears, the colloid will not cause the device to open circuit or turn yellow due to the sudden change of temperature. The part material should also fully consider its heat conduction and heat dissipation characteristics to obtain good overall thermal characteristics.

Secondary optical design technology

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

Power LED white light technology

There are three common methods to realize white light:

(1) the blue chip is coated with YAG phosphor, the blue light of the chip excites the phosphor to emit 540nm ~ 560nm yellow green light, and the yellow green light and the blue light synthesize white light. The method is relatively simple, efficient and practical. The disadvantages are poor consistency of glue distribution, easy precipitation of fluorescent powder, poor uniformity of glossy surface, poor color consistency, high color temperature, and unsatisfactory color rendering.

(2) the RGB three primary colors of multiple chips or devices are mixed into white light, or the blue + yellow green dual chips are used to complement the color to produce white light. As long as the heat dissipation is appropriate, the white light produced by this method is more stable than the former method, but the driving is more complex. In addition, different light decay rates of different color chips should be considered.

(3) the RGB phosphor is coated on the UV chip, and the phosphor is excited by the violet light to produce the white light. Due to the low efficiency of UV chips and RGB phosphors, they have not yet reached the practical stage.

We believe that the following technical problems must be solved in order to realize industrialization of W-class power LED products for lighting:

1. Powder coating amount control: the coating method used in the LED chip + fluorescent powder process is usually to mix the fluorescent powder and glue and then apply it to the chip with a distributor. In the process of operation, because the viscosity of carrier glue is a dynamic parameter, the proportion of fluorescent powder is larger than carrier glue, resulting in precipitation and distributor accuracy and other factors, it is difficult to control the uniformity of the coating amount of fluorescent powder in this process, resulting in the uneven color of white light.

2. Chip photoelectric parameter matching: due to the characteristics of semiconductor technology, there may be differences in optical parameters (such as wavelength, light intensity) and electrical parameters (such as forward voltage) between the same material and the same wafer chip. RGB three primary color chip is more like this, which has a great influence on white light chroma parameters. This is one of the key technologies that industrialization must solve.

3. Control of the light chromaticity parameters generated according to the application requirements: for products of different purposes, the requirements for the color coordinates, color temperature, color rendering, light power (or light intensity) and light spatial distribution of white LED are different. The control of the above parameters involves the coordination of product structure, process method, material and other factors. In the industrial production, it is very important to control the above factors and get products that meet the application requirements and have good consistency.

Testing technology and standard

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 detection standards and test methods for display or indication can not meet the needs of lighting applications. Semiconductor equipment and instrument manufacturers at home and abroad have also launched their own test instruments. There are certain differences in the test principles, conditions and standards used by different instruments, which increases the difficulty and complexity of test application and product performance comparison.

According to the needs of LED product development, the optoelectronic branch of China Optoelectronic Industry Association issued "LED test method (Trial)" in 2003, which added the regulation of LED chroma parameters. But led needs to expand to the lighting industry, the establishment of LED lighting product standards is an important means of industrial standardization.

Screening technology and reliability assurance

Due to the limitation of the lamp appearance, the assembly space of LED for lighting is sealed and limited, and the sealed and limited space is not conducive to LED heat dissipation, which means that the use environment of LED for lighting is inferior to that of traditional LED products for display and indication. In addition, LED lighting is driven by high current, which puts forward higher reliability requirements. In the industrial production, it is necessary to carry out appropriate heat aging, temperature cycle impact, load aging process screening tests for different product uses, to eliminate early failure products, and to ensure the reliability of products.

Electric protection technology

Because Gan is a wide band gap material with high resistivity, the induced charge generated by static electricity in the production process of this kind of chip is not easy to disappear, and can accumulate to a considerable extent, which can generate a high electrostatic voltage. When the material's bearing capacity is exceeded, breakdown and discharge will occur. For the InGaN / AlGaN / GaN double heterojunction, the InGaN activation layer is only tens of nanometers, which has a very small capacity to bear static electricity, and is very easy to be broken down by static electricity, making the device invalid.

Therefore, in the industrial production, whether the prevention of static electricity is appropriate directly affects the product yield, reliability and economic benefits. Electrostatic prevention technologies are as follows:

1. Prevent the production and use places from the aspects of human body, platform, ground, space, product transmission and stacking by means of anti-static clothing, gloves, bracelets, shoes, pads, boxes, ion fans, testing instruments, etc.

2. Electrostatic protection circuit is designed on the chip.

3. LED is equipped with protection devices.

Current status of power LED packaging technology

Power LED is divided into power LED and W-class power LED. The input power of power LED is less than 1W (except for tens of milliwatt power LED); the input power of class W power LED is equal to or greater than 1W.

Power LED packaging technology abroad

(1) power LED

The first HP company launched the "piranha" led in the early 1990s, and the improved "snapled" in 1994. There are two kinds of working current, 70ma and 150mA, with input power up to 0.3w. Then OSRAM company launched "powertop led". After that, some companies launched a variety of power LED packaging structures. The power LED of these structures is several times higher than that of the original bracket package, and the thermal resistance is reduced to a fraction.

(2) class W power LED

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

Single chip W-class power LED was first introduced by Lumileds company in 1998. The packaging structure is characterized by the use of thermoelectric separation, the flip chip silicon carrier is directly welded on the heat sink, and the use of reflective cup, optical lens and flexible transparent rubber and other new structures and materials. Now we can provide single chip 1W, 3W and 5W high-power led.osram company launched single-core in 2003 The structure of "Golden Dragon" LED is characterized by direct contact between heat sink and metal circuit board, good heat dissipation performance, and input power up to 1W

There are many structures and packaging forms of high-power LED in multichip package. UOE company launched norlux series LED in 2001. The structure of the LED is hexagonal aluminum plate. Lanina ceramics introduced a high-power LED array in 2003, which is packaged with LTCC-M technology. In 2003, Panasonic launched a high-power white LED packaged with 64 chips. In 2003, Riya launched what it called the world's brightest white LED, with a light flux of 600LM, a power consumption of 30W and a maximum input power of 50W when the output beam is 1000lm, and a luminous efficiency of 33lm / w White LED module for exhibition

As for the high-power LED with multi chip combination, many companies have developed many new products with new structure package according to the actual market demand, and the development speed is very fast.

Domestic power LED packaging technology

Domestic LED packaging products have a wide range. According to preliminary estimates, there are more than 200 LED packaging plants in China with a packaging capacity of more than 20 billion / year, and the packaging supporting capacity is also very strong. However, many of the packaging plants are private enterprises with small scale. However, the features of MB Series high-power LED packaged by UEC company (Guolian) in Taiwan, which adopts metal bonding technology, are that Si is used instead of GaAs substrate, good heat dissipation, and metal bonding layer is used as light reflecting layer to improve light output.

For the research and development of high-power LED packaging technology, at present, the state has not officially supported the investment, domestic research units rarely intervene, and the investment of packaging enterprises in research and development (human and financial) is not enough, forming a weak situation of domestic development of packaging technology, and there is a considerable gap between packaging technology level and foreign countries.

Yiwu Fuwei e-commerce firm (IACS Co., Limited) is a very professional Led manufacturer who produces high quality outdoor led light. Our popular products are solar led lights, UFO lights, underwater lights and so on. If you have any question about the led product, please feel free to contact us.