Comparison of the main LED packaging technologies

1) LED single chip packaging

LED has achieved rapid development in the past 30 years. The first batch of products appeared in 1968. The luminous flux of LED with a working current of 20mA was only a few thousandths of a lumen, and the corresponding luminous efficiency was 0.1 lm/W, and there was only one light color, red light at 650 nm. In the early 1970s, the technology progressed rapidly, the luminous efficiency reached 1 lm/W, and the colors were 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 driving current of an LED light source code-named "Piranha" increased to 50-70mA, and the driving current of a new high-power LED reached 300-500 mA. In particular, the successful development of white light LEDs in 1998 has made a substantial step forward in the application of LEDs from simple identification display functions to lighting functions. Figures 2-1 to 2-4 describe the development of LEDs.

Figure 2-1 Ordinary LEDs are mainly used for indicator lights


Figure 2-2 High-brightness LEDs are mainly used for lighting


Figure 2-3 Piranha LED


Figure 2-4 High-power LED

A Status of Power LED Packaging Technology

Power LEDs are divided into two types: power LEDs and watt (W) class power LEDs. The input power of power LEDs is less than 1W (except for LEDs with tens of milliwatts); the input power of W class power LEDs is equal to or greater than 1W.
HP first launched the "piranha" package structure LED in the early 1990s, and launched the improved "Snap LED" in 1994, with two working currents of 70mA and 150mA, and the input power can reach 0.3W. Then OSRAM launched the "PowerTOP LED", and then some companies launched a variety of power LED packaging structures. The power LEDs of these structures have several times higher input power than the original bracket packaged LEDs, and the thermal resistance is reduced to a fraction of the past.

W class power LEDs are the core of future lighting, and major companies around the world have invested a lot of effort in researching and developing their packaging technology. The first single-chip W-class power LED was LUXEON LED launched by Lumileds in 1998. The packaging structure is characterized by the use of thermal and electrical separation, the flip chip is directly soldered to the heat sink with a silicon carrier, and new structures and new materials such as reflective cups, optical lenses and flexible transparent adhesives are used. Now single-chip 1W, 3W and 5W high-power LEDs are available. Lumileds has a number of patented technologies in power-type white light diode packaging. OSRAM launched the single-chip "Golden Dragon" series LED in 2003. Its characteristics are that the heat sink is in direct contact with the metal circuit board, with good heat dissipation performance, and the input power can reach 1W. Nichia's 1W LED has an operating current of 350 mA, and the luminous flux of white light, blue light, blue-green light and green light are 23, 7, 28 and 20 lumens respectively, and its life is expected to be 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 LEDs in the field of lighting is to increase their luminous efficiency and luminous flux to the level of existing lighting sources. Due to the continuous increase in the input power of LED chips, the power LED packaging technology should mainly meet the following two requirements: ① The packaging structure must have a high light extraction efficiency; ② The thermal resistance must be as low as possible, so as to ensure the photoelectric performance and reliability of the power LED.

The epitaxial material used in power LEDs adopts MOCVD epitaxial growth technology and multi-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 existing design of power LEDs uses 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 working current, thereby obtaining a higher luminous flux. In addition to the chip, the device packaging technology is also very important.

Key technologies for power LED packaging:

a. Heat dissipation technology

The traditional indicator LED packaging structure is generally to install the chip in a small-sized reflective cup or on a wafer stage with conductive or non-conductive glue, and then use gold wire to complete the internal and external connections of the device and then encapsulate it 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 will carbonize and turn yellow due to poor heat dissipation, thus causing the device to accelerate light decay until failure, and even fail due to the stress caused by rapid thermal expansion.

For power LED chips with large operating current, low thermal resistance, good heat dissipation and low stress The new packaging structure is the key technology of power LED devices. Low-resistance and high-thermal-conductivity materials can be used to bond the chip; copper or aluminum heat sinks are added to the bottom of the chip, and a semi-encapsulated structure is used 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 highly transparent flexible silicone, which will not cause the device to open due to sudden temperature changes, nor will it turn yellow; the material of the parts should also fully consider its thermal conductivity and heat dissipation characteristics to obtain good overall thermal characteristics.

The packaging structures of ordinary LEDs and high-power LEDs are shown in Figures 2-5 and 2-6 respectively. The reference value of thermal resistance is shown in Table 2-1.

2-5 Ordinary LED packaging structure diagram

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

2-1 Comparison of thermal resistance reference values ​​of 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 collection efficiency of the device, design an additional reflector cup and multiple optical lenses.

c. Power LED white light technology

There are three common process methods for achieving white light:

① YAG phosphor is coated on the blue chip, and the yellow-green light emitted by the blue light excites the phosphor to synthesize white light with the blue light. This method is relatively simple, efficient and practical. The disadvantages are that the consistency of the amount of glue is poor, the phosphor is easy to precipitate, resulting in poor uniformity of the light output surface and poor color consistency; the color temperature is high and the color rendering is not ideal.

② Multiple chips or multiple devices of the three primary colors of RGB emit light and mix colors to produce white light, or use blue + yellow dual chips to complement the colors to produce white light. As long as the heat dissipation is done properly, the white light produced by this method is more stable than the previous method, but the driving is more difficult.③

Apply RGB phosphor on the ultraviolet chip, and use ultraviolet light to excite the phosphor to produce three primary colors to mix and form white light. Due to the low efficiency of the current ultraviolet chip and RGB phosphor, it has not yet reached the practical stage.

Table 2-2 Comparison of three main white light LED preparation routes

Ultraviolet LED + RGB phosphor Blue LED + Yellow phosphor Binary complementary color LED RGB multi-chip combination White light LED chip

Color rendering Best General General Good

Color stability Best Good General General Good

Lumen retention rate No data General Good Good

Fluorescent materials are under research and are relatively mature - - -

-Efficiency Best Good General General Good

Application White light lighting Backlight Special lighting display


Backlight

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

①Control of phosphor coating amount and uniformity: The glue coating method used in the LED chip + phosphor process is usually to mix the phosphor with glue and then apply it to the chip with a dispenser. During the operation, due to the influence of factors such as the viscosity of the carrier glue being a dynamic parameter, the precipitation caused by the phosphor being heavier than the carrier glue, and the precision of the dispenser, it is difficult to control the uniformity of the phosphor coating amount in this process, resulting in uneven white light color.

② Chip optoelectronic parameter matching: The characteristics of semiconductor technology determine that there may be differences in optical parameters (such as wavelength, light intensity) and electrical (such as forward voltage) parameters between chips of the same material and the same wafer. This is even more true for RGB three-primary color chips, which have a great impact on the chromaticity parameters

of white light. This is one of the key technologies that must be solved for industrialization. ③ Control of light chromaticity parameters generated according to application requirements: Products with different uses have different requirements for the color coordinates, color temperature, color rendering, light power (or light intensity) and spatial distribution of white light LEDs. The control of the above parameters involves the coordination of many factors such as product structure, process methods, and materials. In industrial production, it is very important to control the above factors and 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 process technology, LED products are gradually entering the lighting market. The parameter testing standards and test methods of traditional LED products for display or indication can no longer meet the needs of lighting applications. Domestic and foreign semiconductor equipment and instrument manufacturers 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 applications and product performance comparisons. LEDs are to expand into the lighting industry, and establishing LED lighting product standards is an important means of industrial standardization.

e. Screening technology and reliability assurance

Due to the limitations of the appearance of lamps, the assembly space of LEDs for lighting is sealed and limited, which is not conducive to the heat dissipation of LEDs, which means that the use environment of lighting LEDs is inferior to that of traditional display and indication LED products. In addition, lighting LEDs work under high current drive, which puts higher reliability requirements on them. In industrial production, it is necessary to conduct appropriate thermal aging, temperature cycle shock, and 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 bandgap material with high resistivity, the induced charge generated by static electricity in the production process of this type of chip is not easy to disappear. When accumulated to a considerable degree, it can generate a very high electrostatic voltage. When the material's tolerance is exceeded, breakdown and discharge will occur. The positive and negative electrodes of the blue chip on the sapphire substrate are both located on the chip with a very small spacing; for the InGaN/AlGaN/GaN double heterojunction, the InGaN active layer is only tens of nanometers, and the tolerance to static electricity is very small. It is very easy to be broken down by static electricity, causing the device to fail. Compared with traditional LEDs, GaN-based LEDs have poor anti-static ability, which is their distinct disadvantage. The failure problem caused by static electricity has become a very thorny problem affecting the product qualification rate and use promotion. Therefore, in industrial production, whether static electricity prevention is appropriate directly affects the product yield, reliability and economic benefits.

There are several types of static electricity prevention technologies: ① Implement prevention in the production and use places from the aspects of human body, platform, ground, space, product transmission, stacking, etc. ② Design static electricity protection circuits on the chip. ③ Install static electricity protection devices on LEDs.

2) Multi-chip integrated packaging

In order to avoid the problem of reduced luminous efficiency caused by large-size chips, the method of integrating small-size chips can be used to increase the single chip