In recent years, with the rapid development of LED production technology, the brightness of its LED has been improved and its lifespan has been extended, and the production cost has been greatly reduced, rapidly expanding the LED application market, such as consumer products, signal systems and general lighting, and its global market scale. growing up very fast. In 2003, the global LED market was approximately US$4.48 billion (approximately US$2.7 billion in the high-brightness LED market), 17.3% more than in 2002 (47% in the high-brightness LED market), and continued to grow on the mobile phone market. It is predicted that there will still be in 2004. The growth rate of 14.0% can be expected. {HotTag}
In terms of product development, the development of white LEDs has become a key development project for manufacturers. At present, there are mainly four methods for manufacturing white LEDs:
First, blue LED+ yellow fluorescent powder (such as: YAG)
Second, red LED + green LED + blue LED
Third, ultraviolet LED + red / green / blue fluorescent powder
Fourth, blue LED + ZnSe single crystal substrate
White light LEDs used in mobile phones, digital cameras, PDAs and other backlight sources are made of blue single crystal grains and YAG fluorescent light. With the mobile phone flash, large and medium-sized (NB, LCD-TV, etc.) display light source modules, as well as the application of special lighting systems gradually increased. In the end, it will be extended to use in general lighting system equipment. The high power LED market using white LED technology will gradually emerge. In terms of technology, the biggest challenge at present is to enhance and maintain brightness. If the cooling capacity is further enhanced, the market has great potential for development.
ASM has more than two decades of experience in the development and production of automated optoelectronic component packaging equipment. There are many ways to improve LED brightness in the industry, from the chip and package design level to the packaging process with the goal of improving heat dissipation and increasing luminous efficiency. . In this article, we provide an overview and discussion of the latest developments and results of LED packaging technology.
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 the use of different electrode design to control the current density, the use of ITO thin film technology to make the current through the LED evenly distributed, so that the LED chip in The largest number of photons are produced as much as possible. Then use a variety of different methods to extract each photon emitted by LED, such as the production of different shapes of the chip; the use of the chip around the effective control of light refraction to improve the efficiency of LED light extraction, research and development to expand the size of a single chip surface (> 2mm2) to increase light Area, more use of rough surface to increase the penetration of light and so on. There are some high-brightness LED chips on the location of pn two electrodes close to each other, so that the chip light-emitting efficiency and heat dissipation. Recently, the production of high-power LEDs has used new and improved laser lift-off and metal bonding techniques to remove LED epitaxial wafers from GaAs or GaN growth crystal substrates and bond them. On another metal substrate or other material with high reflectivity and high thermal conductivity to help high-power LED to improve light extraction efficiency and heat dissipation.
Package design
After years of development, vertical LED lamps (φ3mm, φ5mm) and SMD lamps (surface mount LEDs) have evolved into a standard product model. However, with the development and needs of the chip, it has developed a packaged product design that suits high power. In order to reduce manufacturing costs by using automated assembly technology, high-power SMD lamps have also emerged. Moreover, driven by the rapid adoption of the portable consumer product market, the design size of high-power LED packages will be smaller and thinner to provide a broader product design space.
In order to maintain the finished product's brightness after encapsulation, the newly improved high-power SMD device is equipped with a cup-shaped reflective surface, which helps to reflect all light rays out of the package uniformly to increase output lumens. Covering the round optical lens on the LED, the material was changed to use Silicone sealant instead of epoxy resin in the past, so that the package can maintain a certain degree of durability.
Packaging Technology and Solutions
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, moisture, and other external impacts. When selecting the packaging method, material, and application platform, factors such as the shape, electrical/mechanical characteristics, and die bonding accuracy of the LED epitaxy must be considered. Since LEDs have their optical properties, they must also be considered and ensured in their optical characteristics.
Whether it is a vertical LED or SMD package, a high-precision crystal bonder must be selected, because the precise placement of the LED die into the package directly affects the luminous efficacy of the entire packaged device. As shown in Figure 1, if the position of the crystal grains in the reflective cup is deviated, the light cannot be completely reflected, which affects the brightness of the finished product. However, if a crystal-fixing machine has an advanced pre-image recognition system (PR System), the quality of the lead frame can still be accurately welded to a predetermined position in the reflector cup.
General low-power LED devices (such as the lighting of the pointing device and the mobile phone keyboard) are mainly silver solid crystals, but since the silver paste itself cannot withstand high temperatures, at the same time when the brightness is increased, the heating phenomenon also occurs, thus affecting the products. To obtain high-quality, high-power LEDs, a new die-bonding process is developed. One of them is to use eutectic bonding technology to first solder the die to a soubmount or heat sink. Then, the entire die with the heat-dissipating substrate is then soldered to the packaged device, so that the heat-dissipation capability of the device can be enhanced and the light-emitting power can be relatively increased. As for the substrate material, silicon, copper, and ceramic are commonly used heat-dissipating substrate materials.
Eutectic welding
The key to the technology is the choice of eutectic materials and the control of the welding temperature. A new generation of InGaN high-brightness LEDs, such as eutectic soldering, can use pure tin (Sn) or Au-Sn alloy as the contact surface plating at the bottom of the die, and the die can be soldered to a substrate plated with gold or silver. on. When the substrate is heated to a suitable eutectic temperature (Fig. 5), gold or silver penetrates 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 fastened to the heat of the solder. Sink or substrate (Figure 6). The choice of eutectic temperature depends on the heat resistance of the die, substrate, and device materials and the temperature requirements for the subsequent SMT reflow process. When considering the eutectic bonding table, in addition to the high position accuracy, another important condition is the flexible and stable temperature control. The addition of nitrogen or mixed gas devices helps to protect against oxidation in the eutectic process. Of course, like silver paste solid crystal, to achieve high-precision solid crystal, depends on the rigorous mechanical design and high-precision motor movement, in order to make the welding head movement and welding force control just right, without sacrificing high productivity and high yield The request.
Flux can also be added during the eutectic soldering process. The biggest feature of this technology is that it does not require additional additional soldering force. Therefore, excessive eutectic alloy overflow will not occur due to excessive solid-state soldering force, which reduces the chance of LED short-circuiting. .
Flip Chip Soldering
Flip-chip welding has been actively used in high-power LED processes in recent years. The flip-chip method flip-chips the GaN LED die onto a heat-dissipating substrate. This is because there is no obstruction to the gold wire pad and it helps to increase the brightness. As the distance through which the current circulates decreases, the resistance decreases, so the heat generation also decreases. At the same time, such bonding can also effectively transfer heat to the next layer of the heat-dissipating substrate and then to the outside of the device. When this technology is applied to SMD LEDs, not only the light output is increased, but also the overall product area can be reduced, and the product application market can be expanded.
There are two main solutions for the development of flip-chip LED technology: one is solder bump reflow technology; the other is thermosonic welding technology. Lead-tin ball soldering (Fig. 10) has already been used in IC packaging applications and the process technology has matured, so it will not be described in detail here.
For the production of low-cost and low-line-count devices, the Thermosonic flip chip technology (Figure 11) is particularly suitable for high-power LED welding. The interface made of gold is used for soldering. Because the melting temperature of gold itself is higher than that of lead solder and silver paste, it is more flexible in the process design after die bonding. In addition, there are advantages of lead-free process, simple process, and reliable metal contact. After several years of research and experience accumulation, the thermosonic flip chip process has mastered the optimized process parameters, and several major LED manufacturers have successfully put into mass production.
Except for the use of full production lines, the remaining large number of automation equipment (such as die attach machines, lead welding machines, testers, and taping machines) are all dependent on imports.
Development of specific proposals for LDD equipment industry in China
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 follow the path of introduction, digestion, absorption, innovation, and improvement. The specific plan is as follows: Under the support of the state, through the tripartite alliance of the country, LED manufacturing companies and equipment and materials manufacturing, establish China LED equipment, materials, manufacturing and application complex with incubator function.
In order to grasp and improve the level of equipment manufacturing in a short period of time and drive the development of China's medium-to-high-end LED industry, it is recommended that joint ventures raise funds (50% of the country, 15% of equipment development units, 15% of chip manufacturing and packaging technology research units, LED chips 20% of manufacturing and packaging companies) Build a complete LED chip manufacturing and packaging demonstration production line. The demonstration line to solve the equipment development and process testing, LED product manufacturing process technology research and verification, to achieve the complete supply of process technology and process equipment for any. All units participating in the demonstration construction have the right to use the production line for research and testing of related products, technologies, and industrialization. In particular, LED product production units can preferentially receive relevant research and industrialization achievements.
For this 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 is to use some two years or so to localize some key equipment (the state grants certain bonuses to support); for ordinary equipment that already has a certain foundation and a cost-effective advantage in China, it uses market competition principles and adopts an access system. The optimal matching (through the formulation of standards); for some of the more difficult and short-term equipment that cannot be localized, the state should arrange for research and complete the development of production prototypes. The second step, starting from the third year of implementation of this program, the first two types of equipment involved in the first step will focus on solving problems such as the adaptability, production efficiency, reliability, appearance and cost of the production process, with domestic support and batch production. Supply capacity, research and development of equipment to complete the production of prototype commercialization (γ-type machine). The third step is that from the fifth year of implementation of this plan, all the equipment involved in the first and second steps have international competitiveness. In addition to satisfying domestic demand, they must occupy a certain international market; Production requirements and bulk supply capacity.