The latest development trend analysis and results overview of LED packaging technology

At present, white LEDs used in backlights such as mobile phones, digital cameras, and PDAs are formed by using a blue single chip and YAG fluorescent light. With the application of mobile phone flash, large and medium size (NB, LCD-TV, etc.) display light source modules and special-purpose lighting systems, the application has gradually increased. In the end, it will be extended to use in general lighting system equipment, and the high-power LED market using white LED technology will appear one after another. In terms of technology, the biggest challenge now is to enhance and maintain brightness. If the heat dissipation capacity is enhanced, the market has great potential.

ASM has more than 20 years of experience in R&D and production of automated optoelectronic component packaging equipment. There are many ways to improve LED brightness in the industry, from chip and package design to packaging technology to improve heat dissipation and increase luminous efficiency. . In this article, the latest developments and results of the LED packaging process are summarized and discussed.

Chip design

From the evolution of the chip, it is found that major LED manufacturers continue to improve on the upstream epitaxial technology, such as using different electrode designs to control the current density, using ITO thin film technology to make the current distribution through the LEDs evenly distributed, so that the LED chip is in the structure. The most photons are produced as much as possible. Then use a variety of different methods to extract each photon emitted by the LED, such as the production of chips of different shapes; use the periphery of the chip to effectively control the light refraction to improve the light extraction efficiency of the LED, and develop a single chip surface size (> 2mm2) to increase the illumination Area, more use of rough surface to increase the penetration of light and so on. There are some high-brightness LED chips on which the positions of the two electrodes of pn are close to each other, which improves the luminous efficiency and heat dissipation capability of the chip. Recently, the production of high-power LEDs has been to remove the LED epitaxial wafer from the GaAs or GaN long crystal substrate and bond it to another using the newly improved laser lift-off and metal bonding technology. On metal substrates or other materials with high reflectivity and high thermal conductivity, it helps high-power LEDs to improve light extraction efficiency and heat dissipation.

Package design

After years of development, vertical LED lights (φ3mm, φ5mm) and SMD lamps (surface mount LEDs) have evolved into a standard product model. However, with the development and needs of the chip, we have developed a package design that meets high power. In order to reduce manufacturing costs by using automated assembly technology, high-power SMD lamps have emerged. Moreover, driven by the rapid market of portable consumer products, the design of high-power LED package volume is smaller and thinner to provide a wider product design space.

In order to maintain the brightness of the finished product after packaging, a new modified high-power SMD device with a cup-shaped reflecting surface helps to reflect all the light out of the package to increase the output lumen. The LED optical lens on the LED is covered, and the material is changed to use Silicon sealing, instead of the epoxy resin (Epoxy), so that the package can maintain a certain durability.

Packaging process and solution

The main purpose of the semiconductor package is to ensure the correct electrical and mechanical interconnection between the semiconductor chip and the underlying circuit, and to protect the chip from mechanical, thermal, humid and other external impacts. When selecting the packaging method, materials, and operating the machine, factors such as the shape of the LED epitaxial, electrical/mechanical characteristics, and the accuracy of the die attach must be considered. Due to the optical properties of LEDs, packaging must also be considered and ensured in optical properties.

Whether it is a vertical LED or SMD package, a high-precision die bonder must be selected. The accuracy of the placement of the LED chip in the package directly affects the luminous performance of the package. As shown in Figure 1, if the position of the chip in the reflector cup is deviated, the light is not completely reflected, which affects the brightness of the finished product. However, if a solid crystal machine has an advanced pre-image identification system (PRSystem), it can be precisely welded to a predetermined position in the reflector cup despite the poor quality lead frame.

Generally, low-power LED devices (such as the illumination of pointing devices and mobile phone keyboards) are mainly solid crystals with silver paste. However, since the silver paste itself cannot withstand high temperatures, while the brightness is increased, heat is generated, which affects the product. In order to obtain high-quality and high-power LEDs, a new die-hardening process has been developed, one of which is to use eutectic soldering technology to solder the chip to a heat sink (soubmount) or heat sink (heatsink). The whole chip is connected to the heat dissipating substrate and soldered to the package device, so that the heat dissipation capability of the device can be enhanced, and the luminous power is relatively increased. As for the substrate material, silicon (Silicon), copper (Copper) and ceramic (Ceramic) are commonly used heat-dissipating substrate materials.

Eutectic soldering

The most critical technology is the choice of eutectic materials and the control of soldering temperature. A new generation of InGaN high-brightness LEDs, such as eutectic soldering, can be coated with pure tin (Sn) or gold-tin (Au-Sn) alloy as the contact surface. The chip can be soldered to a substrate coated with gold or silver. When the substrate is heated to a suitable eutectic temperature (Fig. 5), gold or silver elements penetrate into the gold-tin alloy layer, the composition of the alloy layer changes to increase the melting point, the eutectic layer is cured and the LED is soldered to the heat. Sink or on the substrate (Figure 6). The choice of eutectic temperature depends on the heat resistance of the chip, substrate and device materials and the temperature requirements of the subsequent SMT reflow process. When considering the eutectic solid crystal machine, in addition to high positional accuracy, another important condition is flexible and stable temperature control. Nitrogen or mixed gas devices are added to help prevent oxidation during the eutectic process. Of course, like the silver paste solid crystal, to achieve high-precision solid crystal, relying on rigorous mechanical design and high-precision motor movement, the welding head and welding force control can be just right, and the high productivity and high yield rate are not damaged. Requirements.

Flux can also be added during the eutectic soldering process. The biggest feature of this technology is that no additional soldering force is required, so there is no excessive eutectic alloy overflow due to excessive bonding force, which reduces the chance of LED short circuit.

FlipChip soldering

Flip chip soldering has been actively used in high-power LED processes in recent years. The flip chip method has reverse-bonded the GaN LED chip to the heat-dissipating substrate. Since there is no gold wire pad hindrance, it will help to improve the brightness. Since the distance through which the current flows is shortened and the resistance is reduced, the generation of heat is relatively lowered. At the same time, such bonding can effectively transfer heat to the heat sink substrate of the next layer and then go outside the device. When this process is applied to SMDLED, not only the light output is increased, but also the overall area of ​​the product can be reduced, and the application market of the product can be expanded.

There are two main solutions in the development of flip chip LED technology: one is the solder ball soldering (Solderbumpreflow) technology; the other is the thermosonic (Thermosonic) soldering technology. Lead solder ball soldering (Fig. 10) has been used in IC packaging applications, and the process technology has matured, so it will not be described in detail here.

Thermosonic flip chip technology (Figure 11) is especially suitable for high power LED soldering for the production of low cost and low line devices. The interface for soldering with gold, because the melting temperature of gold itself is higher than that of lead tin balls and silver paste, it is more flexible for the process design after solid crystal. In addition, there are advantages such as lead-free process, simple process, and reliable metal connection. After many years of research and experience accumulation, the thermosonic flip chip process has mastered the optimized process parameters, and has been successfully put into mass production in several major LED manufacturers.

In addition to the use of the production line, the rest of the automation equipment (such as chip bonding machine, wire bonding machine, testing machine, taping machine) and other automation equipment are all dependent on imports.

Specific proposals for developing China's LDD equipment industry

It is recommended that the state support materials and process equipment as the basis and driving force for development in supporting LED technology and industrial development. In the process of developing LED technology and industry, it is recommended that China take the road of introduction, digestion, absorption, innovation and improvement. The specific plan is as follows: Under the support of the state, through the joint efforts of the state, LED manufacturing enterprises and equipment and materials manufacturing, a Chinese LED equipment, materials, manufacturing and application complex with incubator function will be established.

In order to master and improve the equipment manufacturing level in a short time, and promote the development of China's high-end LED industry, it is recommended that the joint fund raise funds (50% in the country, 15% in the equipment development unit, 15% in the chip manufacturing and packaging technology research unit, LED chip). Manufacturing and packaging companies 20%) to build a complete LED chip manufacturing and packaging demonstration production line. The demonstration line has been responsible for solving the equipment development and process test, LED product manufacturing process technology research and verification, and realizing the complete supply of process technology and process equipment. Any unit participating in the demonstration construction has the right to use the production line for relevant products, technology and industrialization research and testing without charge, especially the LED product production unit, which can obtain relevant research and industrialization results preferentially.

In response to the demonstration line, a three-step strategy was implemented to finally realize the commercialization and localization of LED production equipment (see Table 2 of the equipment localization plan for details). The first step: use two years or so to localize some key equipment (the state gives certain bonus support); for ordinary equipment that has a certain foundation in China and has cost-effective advantages, it uses the principle of market competition and adopts the admission system. Selecting the best supporting facilities (through the formulation of standards); for some of the more difficult equipment that can not be domestically produced in the short term, the state should arrange for the research and development of production prototypes. The second step, from the third year of implementation of the program, the first two types of equipment involved in the first step focus on solving the problems of adaptability, production efficiency, reliability, appearance and cost of the production process, with domestic support and batch Supply capacity, research and development equipment to complete the commercialization of production prototypes (gamma-type machine). In the third step, from the fifth year of implementation of the program, all the equipment involved in the first and second steps have international competitiveness. In addition to meeting domestic demand, it must occupy a certain international market; the equipment can meet industrialization. Production requirements and volume supply capabilities.