At present, the main packaging technology of LED lamps and lanterns is compared

1) LED single chip package

LED has achieved rapid development in the past 30 years. The first batch of products appeared in 1968. The luminous flux of the LED with a working current of 20mA was only a few thousandths of a lumens, the corresponding luminous efficiency was 0.1 lm/W, and there was only one red light with a light color of 650 nm. In the early 1970s, the technology progressed rapidly, with the luminous efficiency reaching 1 lm/W, and the colors expanded to red, green, and yellow. With the invention of new materials and the improvement of light efficiency, the power and luminous flux of a single LED light source are also increasing rapidly. Originally, the driving current of a general LED was only 20 mA. In the 1990s, the drive current of a LED light source code-named “piranha” increased to 50-70mA, while the drive current of the new high-power LED reached 300-500 mA. In particular, the successful development of white LEDs in 1998 made the application of LEDs take a substantial step from a mere sign display function to a lighting function. Figure 2-1 to Figure 2-4 describe the development of LED.

A. Status of power LED packaging technology

Power LED is divided into two types: power LED and watt (W) power LED. The input power of the power LED is less than 1W (except tens of milliwatts power LED); the input power of the W-class power LED is equal to or greater than 1W.

The earliest HP company introduced LEDs with a “piranha” package structure in the early 1990s, and an improved “Snap LED” in 1994. There are two operating currents, 70mA and 150mA, and input power up to 0.3 W. Then OSRAM company launched “Power TOP LED”, and then some companies launched a variety of power LED package structure. The input power of the power LED of these structures is several times higher than that of the original bracket packaged LED, and the thermal resistance is reduced to a fraction of the past.

W-class power LED is the core of future lighting. Major companies in the world have invested a lot of effort to research and develop its packaging technology. The single-chip W-class power LED was first launched by Lumileds in 1998 as the LUXEON LED. The package structure is characterized by the use of thermoelectric separation. The flip chip is directly soldered to the heat sink with a silicon carrier, and a reflective cup, With new structures and new materials such as optical lenses and flexible transparent adhesives, single-chip 1W, 3W and 5W high-power LEDs are now available. Lumileds has a number of patented technologies for power white light diode packaging. OSRAM launched the single-chip Golden Dragon” series of LEDs in 2003. Its characteristic is that the heat sink is in direct contact with the metal circuit board, which has good heat dissipation performance, and the input power can reach 1W. Nichia’s 1W LED has a working current of 350 mA. , The luminous fluxes of white light, blue light, blue-green light and green light are 23, 7, 28 and 20 lumens respectively, and its life expectancy is 50,000 hours.

B Overview of Power LED Packaging Technology

If semiconductor LEDs are to be used as lighting sources, the luminous flux of conventional products is far from that of general-purpose light sources such as incandescent lamps and fluorescent lamps. Therefore, the key to the development of LED in the lighting field is to increase its luminous efficiency and luminous flux to the level of existing lighting sources. Due to the continuous improvement of the input power of LED chips, the power LED packaging technology should mainly meet the following two requirements: ①The package structure must have high light extraction efficiency; ②The thermal resistance should be as low as possible, so as to ensure the photoelectric performance of the power LED. reliability.

The epitaxial material used in the power LED adopts MOCVD epitaxial growth technology and multiple quantum well structure. Although its internal quantum efficiency needs to be further improved, the biggest obstacle to obtaining high luminous flux is still the low light extraction efficiency of the chip. The design of the existing power LED adopts a new structure of flip-chip welding 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 working current. With higher luminous flux, in addition to the chip, the packaging technology of the device also plays an important role.

Key technologies of power LED packaging:

a. Heat dissipation technology

The traditional indicator-type LED packaging structure is generally made by using conductive or non-conductive glue to mount the chip in a small-sized reflector cup or on a stage, and then the internal and external connections of the device are completed by gold wire and then encapsulated with epoxy resin. Its thermal resistance is as high as 150～250℃/W. If the new power chip adopts the traditional LED packaging form, the chip junction temperature will rise rapidly and the epoxy carbonization will turn yellow due to poor heat dissipation, which will cause the accelerated light of the device. Decay to failure, or even failure due to an open circuit caused by the stress generated by rapid thermal expansion.

For high-current power LED chips, a new package structure with low thermal resistance, good heat dissipation and low stress is the key to the technology of power LED devices. The chip can be bonded with materials with low resistivity and high thermal conductivity; a copper or aluminum heat sink is added to the bottom of the chip, and a semi-encapsulated structure is adopted to accelerate heat dissipation; even a secondary heat dissipation device is designed to reduce the thermal resistance of the device; The inside of the device is filled with flexible silica gel with high transparency. The colloid will not cause the device to open or turn yellow due to sudden temperature changes. The material of the part should also fully consider its thermal conductivity and heat dissipation characteristics to obtain good overall heat. characteristic.

The packaging structure of ordinary LED and high-power LED is shown in Figure 2-5 and Figure 2-6 respectively. The thermal resistance reference value is shown in Table 2-1.
2-5 General LED package structure diagram
2-5 General LED package structure diagram

Figure 2-6 High-power LED package structure diagram
Figure 2-6 High-power LED package structure diagram

Table 2-1 Comparison of thermal resistance reference values ??between ordinary LEDs and high-power LEDs

LED power thermal resistance reference (℃/W)

Ordinary LED 150～250

1W LED ＜ 50

3W LED ＜ 30

5W LED ＜ 18

10W LED ＜ 9

b Secondary optical design technology

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

c. Power LED white light technology

There are three common process methods to realize white light as follows:

① The blue chip is coated with YAG phosphor, and the blue light excites the yellow-green light emitted by the phosphor and the blue light to synthesize white light. This method is relatively simple, efficient and practical. The disadvantage is that the consistency of the cloth glue is poor, the phosphor is easy to precipitate, which leads to poor uniformity of the light-emitting surface and poor color uniformity; the color temperature is too high and the color rendering is not ideal.

② RGB three primary colors, multiple chips or multiple devices emit light and mix into white light, or use blue + yellow dual-chip complementary colors to produce white light. As long as the heat dissipation is effective, the white light produced by this method is more stable than the previous method, but the drive is more

③ Coat RGB phosphor powder on the ultraviolet light chip, and use purple light to excite the phosphor powder to produce three-primary light mixing to form white light. Due to the low efficiency of current UV chips and RGB phosphors, they have not yet reached the practical stage.

Backlight

To realize the industrialization of W-class power LED products for lighting, the following technical problems must be solved:

① Phosphor coating amount and uniformity control: The glue coating method used in the LED chip + phosphor process is usually to mix the phosphor and glue and apply it to the chip with a dispenser. In the process of operation, because the viscosity of the carrier glue is a dynamic parameter, the phosphor has a larger specific gravity than the carrier glue and the precipitation and the accuracy of the dispenser are affected, it is difficult to control the uniformity of the phosphor coating amount in this process, resulting in white light. The color is uneven.

② Coordination of chip photoelectric parameters: The characteristics of semiconductor technology determine that 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. This is especially true of RGB three-color chips, which have a great impact on white light chromaticity parameters, which is one of the key technologies that must be solved in industrialization.

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

d. Testing technology and standards

With the development of W-class power chip manufacturing technology and white light LED technology, LED products are gradually entering the lighting market, and traditional LED product parameter testing standards and testing methods for display or indication can no longer meet the needs of lighting applications. Semiconductor equipment and instrument manufacturers at home and abroad have also launched their own test instruments. The test principles, conditions, and standards used by different instruments have certain differences, which increase the difficulty and complexity of test applications and product performance comparisons. LED is going to expand into the lighting industry, and the establishment of led tape lighting product standards is an important means of industry standardization.

e. Screening technology and reliability guarantee

Due to the limitation of the appearance of the lamps, the assembly space of the lighting LED is sealed and limited, which is not conducive to the heat dissipation of the LED, which means that the use environment of the lighting LED is inferior to the traditional display and indication LED products. In addition, lighting LEDs work under high-current driving, which puts forward higher reliability requirements for them. In 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 ensure product reliability.

f. Electrostatic protection technology

Since GaN is a wide band gap material and has a high resistivity, the induced charges generated by static electricity during the production process of this type of chip are not easy to disappear, and accumulate to a considerable extent, which can generate a high electrostatic voltage. When the capacity of the material is exceeded, breakdown will occur and discharge will occur. The positive and negative electrodes of the blue chip on the sapphire substrate are located on the chip with very small spacing; for the InGaN/AlGaN/GaN double heterojunction, the InGaN active layer is only a few tens of nanometers, and the ability to withstand static electricity is very small. The device is broken down by static electricity, causing the device to fail. Compared with traditional LEDs, GaN-based LEDs have poor antistatic ability as a distinct disadvantage. The failure problem caused by static electricity has become a very difficult problem that affects the product qualification rate and use promotion. Therefore, in industrial production, whether the prevention of static electricity is proper or not directly affects the yield, reliability and economic benefits of the product.

Static electricity prevention technologies are as follows: ①Precautions are taken from the human body, platform, ground, space, product transmission, and stacking in the production and use places. ②The electrostatic protection circuit is designed on the chip. ③Assemble electrostatic protection devices on the LED.

2) Multi-chip integrated packaging

In order to avoid problems such as the decline of luminous efficiency caused by large-size chips, small-size chip integration can be used to increase the maximum luminous flux of a single tube. Due to the relatively mature chiplet technology, various high thermal conductivity insulating sandwich aluminum substrates have good heat dissipation, are good for improving light efficiency and increasing device stability, and are convenient for chip integration and heat dissipation. The effect is good, and there are many structures and packaging forms. However, the inherent shortcomings of the formal small chip, such as the shading of the electrode lead, will be aggravated when multiple chips are integrated and affect the luminous efficiency. The design of “leadless” chip integration on the substrate can avoid the lead problem and improve the light efficiency of the small chip integration. One way.

NorLux series developed by UOE company in the United States. This series uses a hexagonal aluminum plate as the substrate. The diameter of the substrate is 1.25 inches. The light-emitting area is located at the center of it. The diameter is about 0.375 inches. It can accommodate 40 light-emitting diode chips. The bonding wires of the chips are made on the substrate. The two contact points are connected with the positive electrode and the negative electrode. The chip structure can determine the number of dies arranged on the substrate according to the required output light power. In 2003, Lamina Ceramics launched a high-power LED array packaged with the company’s unique low-temperature sintered ceramic (LTCC-M) technology on a metal substrate. Panasonic launched a high-power white LED packaged by a combination of 64 chips in 2003. Hebei Lide Company currently has multi-chip integrated LED power light source products of various colors, various working voltages and various powers, such as monochrome, multi-primary color, and white. The maximum integrated power has reached 12W (color) and 6W (white). ).
Figure 2-7 Multi-chip integrated packaging products

3) Phosphor

In the preparation of white light LEDs, phosphor is a very critical material, and its performance directly affects the brightness, color coordinates, color temperature and color rendering properties of white light LEDs. The method of using LED chips to cooperate with specific phosphors to generate white light is simple in process and low in cost. The current commercial white LED products and the future development trend are still in the mainstream of single-chip type, and the development of phosphors with good luminous characteristics is the key to obtaining high-brightness, high luminous efficiency, and high color rendering white LEDs.

Generally, the criteria for selecting phosphors for LEDs are: ①The phosphor can be effectively excited by the matching LED chip; ②It has high quantum efficiency; ③The chemical properties are stable.

There are currently three ways to use phosphors to generate white light: blue LEDs with yellow phosphors; blue LEDs with red and green phosphors; UV-LEDs with red, green and blue phosphors. Currently commercial white LEDs are mostly single-chip blue LEDs combined with yellow phosphors. The white light generation method of blue LEDs combined with red and green phosphors has only been reported in patents by companies such as OSRAR and Lumileds, but has not yet been commercialized. Products have appeared, and the way of UV-LED with tri-color phosphors is currently under development. The advantages and disadvantages of different phosphors for white LED are shown in Table 2-3.

Phosphors produce white LEDs

A. Rare earth yellow phosphor

In the single-chip method of using LEDs to generate white light, the technology of using blue LED chips with yellow-emitting phosphors is relatively mature. At present, most commercial white-light LEDs use this combination; the yellow phosphors used are cerium activated Yttrium aluminum garnet (hereinafter referred to as “YAG:Ce”). It can emit broadband yellow light under the excitation of the blue LED chip and mix with the blue light emitted by the chip to form white light. At the same time, according to the needs of different chips and applications, by adjusting the molar ratio of Y3+, Gd3+ or Al3+, Ga3+, yellow phosphors of the required wavelength can be obtained.

B. Rare earth red phosphor

Although the use of blue light in conjunction with yellow-emitting phosphors has achieved great success in generating white light, this method still has two disadvantages: the color rendering needs to be further improved, especially it is difficult to prepare a single yellow phosphor with a low correlated color temperature ( For white light LEDs below 3500 K, the shortcomings of these two aspects can be improved by adding red phosphor; at the same time, in other methods of generating white light, red phosphor also plays a pivotal role, for example, it can be combined with blue LED and Green phosphors can produce white light in combination with green, blue phosphors and purple or ultraviolet LEDs to produce white light. For a long time, red phosphors have low efficiency and become LED phosphors and even the bottleneck of the development of white LEDs.

C. Rare earth green phosphor

The green phosphor for LED is divalent europium-activated calcium magnesium chlorosilicate (CMSC), which has a strong emission peak, and its excitation spectrum is very wide, suitable for ultraviolet, violet or blue LED excitation

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