LED guardrail lamp diode packaging structure and technology

1. Introduction

LED is a light-emitting device that can directly convert electrical energy into visible light and radiation energy. Due to its low operating voltage, low power consumption, high luminous efficiency, extremely short luminous response time, pure light color, firm structure, impact resistance, vibration resistance, stable and reliable performance, light weight, small size, low cost and other characteristics, its development has been rapid, and it is now possible to mass-produce high-brightness, high-performance products of various colors in the entire visible spectrum. Domestic production of red, green, orange and yellow LEDs accounts for about 12% of the world's total. The industry goal during the "15th Five-Year Plan" period is to reach an annual production level of 30 billion units, realize the mass production of ultra-high brightness AiGslnP LED epitaxial wafers and chips, and produce more than 1 billion red, orange and yellow ultra-high brightness LED tube cores per year, break through the key technology of GaN materials, and realize the medium-volume production of blue, green and white LEDs.

In the LED industry chain, the upstream is LED substrate wafers and substrate production, the midstream industrialization is LED chip design and manufacturing production, and the downstream is LED packaging and testing. The research and development of low thermal resistance, excellent optical properties, and high-reliability packaging technology is the only way for new LEDs to become practical, go to the market, and achieve industrialization. In a sense, it is the link between the industry and the market. Only packaged LEDs can become terminal products, be put into actual market applications, and provide services to customers, making the industrial chain interconnected and seamless.

2. The particularity of LED packaging

LED packaging technology is mostly developed on the basis of discrete device packaging technology, but it also has great particularity. Generally speaking, the die of discrete devices is sealed in the package body, and the main function of the package is to protect the die and complete the electrical interconnection. However, LED packaging is to complete the output of electrical signals, protect the normal operation of the die, and output: visible light. It has both electrical and optical parameter design and technical requirements, and it is impossible to simply use the packaging of discrete devices for LEDs.

The core light-emitting part of LED is a pn junction die composed of p-type and n-type semiconductors. When the minority carriers injected into the pn junction recombine with the majority carriers, visible light, ultraviolet light or near-infrared light will be emitted. However, the photons emitted by the pn junction area are non-directional, that is, there is an equal probability of emission in all directions. Therefore, not all the light generated by the die can be released. This mainly depends on the quality of the semiconductor material, the structure and geometry of the die, the internal structure of the package and the encapsulation material. The application requires improving the internal and external quantum efficiency of the LED. The conventional Φ5mm LED package is to bond or sinter a square die with a side length of 0.25mm on the lead frame. The positive electrode of the die is bonded to the gold wire through the spherical contact point as an inner lead and connected to a pin. The negative electrode is connected to another pin of the lead frame through a reflective cup, and then the top is encapsulated with epoxy resin. The function of the reflective cup is to collect the light emitted from the side and interface of the die and emit it in the desired direction angle. The epoxy resin encapsulated on the top is made into a certain shape, which has the following functions: protecting the tube core from external erosion; using different shapes and material properties (with or without dispersion agent), it has the function of lens or diffuse lens to control the divergence angle of light; the refractive index of the tube core is too related to the refractive index of air, resulting in a small critical angle of total reflection inside the tube core, and only a small part of the light generated by its active layer is taken out, and most of it is left inside the tube core and absorbed after multiple reflections, which is prone to total reflection and leads to excessive light loss. Epoxy resin with corresponding refractive index should be used as a transition to improve the light emission efficiency of the tube core. The epoxy resin used to form the tube shell must have moisture resistance, insulation, mechanical strength, and high refractive index and transmittance of the light emitted by the tube core. The selection of packaging materials with different refractive indices and different packaging geometries has different effects on the photon escape efficiency. The angular distribution of luminous intensity is also related to the tube core structure, light output mode, and the material and shape of the packaging lens. If a pointed resin lens is used, the light can be concentrated in the axial direction of the LED, and the corresponding viewing angle is smaller; if the top resin lens is circular or flat, the corresponding viewing angle will be increased.

In general, the wavelength of LED light changes with temperature at 0.2~0.3nm/℃, and the spectrum width increases accordingly, affecting the color vividness. In addition, when the forward current flows through the pn junction, the heat loss causes the junction area to rise in temperature. Near room temperature, the LED's luminous intensity will decrease by about 1% for every 1℃ increase in temperature. It is very important to maintain color purity and luminous intensity when encapsulating heat. In the past, the method of reducing its driving current was often used to reduce the junction temperature. The driving current of most LEDs is limited to about 20mA. However, the light output of LEDs will increase with the increase of current. At present, the driving current of many power LEDs can reach 70mA, 100mA or even 1A level, which requires improving the packaging structure, adopting a new LED packaging design concept and low thermal resistance packaging structure and technology, and improving thermal characteristics. For example, adopting a large-area chip flip-chip structure, selecting silver glue with good thermal conductivity, increasing the surface area of ​​the metal bracket, and directly mounting the silicon carrier of the solder bump on the heat sink. In addition, in application design, thermal design and thermal conductivity of PCB circuit boards are also very important.

After entering the 21st century, LEDs have been continuously developed and innovated in terms of high efficiency, ultra-high brightness, and full color. The luminous efficiency of red and orange LEDs has reached 100lm/W, and that of green LEDs is 501m/W. The luminous flux of a single LED has also reached tens of lm. LED chips and packages no longer follow the traditional design concepts and manufacturing production models. In terms of increasing the light output of chips, research and development is not limited to improving the internal efficiency by changing the number of impurities, lattice defects, and dislocations in the materials. At the same time, how to improve the internal structure of the tube core and package, increase the probability of photon emission inside the LED, improve the light efficiency, solve the heat dissipation, optimize the design of light extraction and heat sink, improve the optical performance, and accelerate the process of surface mount SMD is the mainstream direction of research and development in the industry.

3. Product packaging structure type

Since the 1990s, the research and development of LED chip and material manufacturing technology has achieved many breakthroughs, transparent substrate trapezoidal structure, textured surface structure, chip flip-chip structure, commercial ultra-high brightness (1cd or more) red, orange, yellow, green, blue LED products have been launched one after another, and have been used in special lighting with low and medium light flux since 2000. The upstream and midstream industries of LED have received unprecedented attention, further promoting the development of downstream packaging technology and industry, using different packaging structure forms and sizes, tube cores with different luminous colors and their two-color or three-color combination methods, and can produce a variety of series, varieties and specifications of products.

The types of LED product packaging structures are classified according to the characteristics of luminous color, chip material, luminous brightness, size, etc. A single tube core generally constitutes a point light source, and multiple tube cores can generally constitute a surface light source and a line light source for information, status indication and display. The light-emitting display is also composed of multiple tube cores, which are appropriately connected (including series and parallel) with a suitable optical structure to form the light-emitting segments and light-emitting points of the light-emitting display. Surface mount LEDs can gradually replace pin-type LEDs, and the application design is more flexible. They have occupied a certain share in the LED display market and have a trend of accelerated development. Some solid lighting sources have been launched on the market, becoming the direction of medium- and long-term development of LEDs in the future.

4. Pin-type package

LED pin package uses lead frame as pins of various package shapes. It is the first package structure successfully developed and put on the market. There are many varieties and the technology is mature. The structure and reflective layer of the package are still being improved. Standard LED is considered by most customers to be the most convenient and economical solution in the display industry. The typical traditional LED is placed in a package that can withstand 0.1W input power. 90% of its heat is dissipated from the negative lead frame to the PCB board and then to the air. How to reduce the temperature rise of the pn junction during operation is a must for packaging and application. The encapsulation material is mostly high-temperature curing epoxy resin, which has excellent optical properties, good process adaptability, and high product reliability. It can be made into colored transparent or colorless transparent and colored scattering or colorless scattering lens packages. Different lens shapes constitute a variety of shapes and sizes. For example, the circle is divided into several types according to the diameter, such as Φ2mm, Φ3mm, Φ4.4mm, Φ5mm, Φ7mm, etc. Different components of epoxy resin can produce different luminous effects. There are many different packaging structures for colorful point light sources: ceramic base epoxy resin packaging has good working temperature performance, the pins can be bent into the required shape, and the volume is small; metal base plastic reflective cover packaging is an energy-saving indicator light, suitable for power indication; the flashing type combines the CMOS oscillation circuit chip with the LED tube core and can generate flashing light with strong visual impact; the two-color type is composed of two tube cores with different luminous colors, which are packaged in the same epoxy resin lens. In addition to the two colors, a third mixed color can be obtained. It is widely used in large-screen display systems and can be packaged to form a two-color display device; the voltage type combines the constant current source chip with the LED tube core and can directly replace various voltage indicator lights of 5~24V. The surface light source is formed by bonding multiple LED tube cores to the specified position of the micro PCB board, using a plastic reflective frame cover and potting epoxy resin. The different designs of the PCB board determine the arrangement and connection of the external leads, and there are structural forms such as dual-row in-line and single-row in-line. Hundreds of package shapes and sizes of point and surface light sources have been developed for the market and customers to choose.

LED light-emitting displays can be composed of various multi-digit products such as digital tubes or ferroelectric tubes, symbol tubes, and matrix tubes, and are designed into various shapes and structures according to actual needs. Taking digital tubes as an example, there are three packaging structures: reflective cover type, monolithic integrated type, and single seven-segment type. There are two connection methods: common anode and common cathode. One is usually called a digital tube, and more than two digits are generally called displays. The reflective cover type has the characteristics of large fonts, less materials, and flexible assembly. It is generally made of white plastic into a seven-segment shell with a reflective cavity. A single LED tube core is bonded to a PCB board that is aligned with the seven reflective cavities of the reflective cover. The center position of the bottom of each reflective cavity is the light-emitting area formed by the tube core. The lead is bonded by pressure welding, and epoxy resin is dripped into the reflective cover. It is aligned and bonded with the PCB board with the tube core, and then cured. The reflective cover type is divided into two types: empty seal and solid seal. The former uses epoxy resin with scattering agent and dye, and is mostly used for single-bit and double-bit devices; the latter is covered with color filter and uniform light film, and transparent insulating glue is applied on the tube core and the bottom plate to improve the light output efficiency. It is generally used for digital display with more than four digits. The monolithic integrated type is to make a large number of seven-segment digital display graphic tube cores on the luminous material wafer, and then cut and divide them into single-chip graphic tube cores, bond, press weld, and package the shell with lens (commonly known as "fisheye" lens). The single-strip seven-segment type cuts the already made large-area LED chip into luminous strips containing one or more tube cores, so that the same seven strips are bonded to the digital font frame, and then pressed and epoxy resin encapsulated. The characteristics of the monolithic type and the single-strip type are miniaturization, and dual-in-line packaging can be used. Most of them are special products. The LED light column display places 101 tube cores (up to 201 tube cores) on a circuit board with a length of 106 mm. It is a high-density package that uses the principle of optical refraction to make a point light source form an image through 13 to 15 gratings in a transparent cover to complete the display of each tube core from point to line. The packaging technology is relatively complex.

The electroluminescent mechanism of semiconductor pn junction determines that LED cannot produce white light with continuous spectrum, and a single LED cannot produce more than two high-brightness monochromatic lights. It can only use fluorescent materials during packaging, and apply fluorescent powder on the blue or ultraviolet LED tube core to indirectly produce broadband spectrum and synthesize white light; or use several (two or three or more) tube cores that emit different colors to be packaged in a component shell, and form white light LED by mixing the colors. Both methods have been put into practical use. In 2000, Japan produced 100 million white light LEDs, which developed into a stable white light product, and designed and assembled multiple white light LEDs into electric light sources that do not require high luminous flux, mainly for local decoration, and pursue new trends.

5. Surface mount packaging

In 2002, surface mount packaged LEDs (SMD LEDs) were gradually accepted by the market and gained a certain market share. The shift from pin-type packaging to SMD was in line with the general development trend of the entire electronics industry, and many manufacturers launched such products one after another.

Most of the early SMD LEDs used the improved SOT-23 with a transparent plastic body, with an outer size of 3.04×1.11mm and a reel-type container with braided packaging. Based on the SOT-23, the SLM-125 series and SLM-245 series LEDs with high brightness SMD lenses were developed. The former is a single-color light emitter, and the latter is a two-color or three-color light emitter. In recent years, SMD LEDs have become a hot spot for development, and have well solved the problems of brightness, viewing angle, flatness, reliability, consistency, etc., using lighter PCB boards and reflective layer materials, requiring less epoxy resin to be filled in the display reflective layer, and removing the heavier carbon steel material pins. By reducing the size and weight, the weight of the product can be easily reduced by half, and ultimately the application is more perfect, especially suitable for indoor and semi-outdoor full-color display applications.

The soldering pad is an important channel for SMD LED heat dissipation. The data of SMD LED provided by manufacturers are based on 4.0mm×4.0mm soldering pads. Reflow soldering can be used to design the soldering pads to be equal to the pins. Ultra-high brightness LED products can use PLCC (plastic encapsulated leaded chip carrier)-2 package, with an outer size of 3.0mm×2.8mm. The high-brightness tube core is assembled by a unique method. The product thermal resistance is 400℃/W. It can be welded in CECC mode. Its luminous intensity reaches 1250mcd at a driving current of 50mA. The character height of the seven-segment one-, two-, three- and four-digit SMD LED display devices is 5.08~12.7mm, and the display size selection range is wide. PLCC packaging avoids the manual insertion and pin alignment process required for the pin seven-segment digital display, meets the production requirements of automatic pick-and-place equipment, has flexible application design space, and displays bright and clear. The multi-color PLCC package has an external reflector and can be easily combined with a light-emitting tube or light guide, replacing the current transmissive optical design with a reflective type, providing uniform lighting for a large area, and providing favorable conditions for the development of power SMD LED packages working under 3.5V, 1A driving conditions.

6. Power type packaging

LED chips and packages are developing towards high power. Under high current, the light flux is 10~20 times greater than that of Φ5mm LED. Effective heat dissipation and non-degrading packaging materials must be used to solve the light decay problem. Therefore, tube shell and packaging are also key technologies. LED packages that can withstand several watts of power have appeared. The 5W series of white, green, blue-green and blue power LEDs have been supplied since the beginning of 2003. The light output of white LEDs reaches 1871m, and the light efficiency is 44.31m/W. As for the light decay problem, a large-area LED tube that can withstand 10W power has been developed; its size is 2.5mm×2.5mm, it can work under 5A current, and the light output reaches 2001m. As a solid lighting source, there is a lot of room for development.

Luxeon series power LEDs are A1GalnN power flip-chip tube cores, flip-chip soldered on a silicon carrier with solder bumps, and then the flip-chip soldered silicon carrier is loaded into a heat sink and tube shell, and the lead wires are bonded for packaging. This package is the best design for light extraction efficiency, heat dissipation performance, and increased working current density. Its main features are: low thermal resistance, generally only 14℃/W, only 1/10 of conventional LEDs; high reliability, the package is filled with a stable flexible gel, and in the range of -40~120℃, the internal stress caused by sudden temperature changes will not cause the gold wire to disconnect from the lead frame, and the epoxy resin lens will not turn yellow, and the lead frame will not be stained by oxidation; the optimal design of the reflector cup and lens makes the radiation pattern controllable and the optical efficiency the highest. In addition, its excellent performance in output light power and external quantum efficiency has brought the development of LED solid light sources to a new level.

The package structure of the Norlux series power LED is a multi-chip combination with a hexagonal aluminum plate as the base (making it non-conductive). The base has a diameter of 31.75 mm, and the light-emitting area is located in the center of the base. The diameter is about 0.375×25.4 mm, which can accommodate 40 LED tube cores. The aluminum plate also serves as a heat sink. The bonding wires of the tube core are connected to the positive and negative electrodes through two contact points made on the base. The number of tube cores arranged on the base is determined according to the required output light power. The ultra-high brightness AlGaInN and AlGaInP tube cores that can be packaged can emit monochrome, color or synthetic white light, and finally encapsulated with high refractive index materials according to the optical design shape. This package uses a conventional tube core high-density combination package, which has high light efficiency and low thermal resistance. It can better protect the tube core and bonding wires, has a higher light output power under high current, and is also a promising LED solid light source.

In application, the packaged product can be assembled on a metal core PCB board with aluminum interlayer to form a power density LED. The PCB board is used as the wiring for connecting the device electrodes, and the aluminum core interlayer can be used as a heat sink to obtain higher luminous flux and photoelectric conversion efficiency. In addition, the packaged SMD LED is very small and can be flexibly combined to form a variety of lighting sources such as module type, light guide plate type, focusing type, and reflection type.

The thermal characteristics of power LEDs directly affect the operating temperature, luminous efficiency, luminous wavelength, service life, etc. of the LEDs. Therefore, the packaging design and manufacturing technology of power LED chips are particularly important.

7. LED development and application prospects

In recent years, the luminous efficiency of LED has increased 100 times, and the cost has dropped to one-tenth of the original. It is widely used in large-area full-color screens for graphic display, status indication, sign lighting, signal display, backlight source of LCD, automobile combination taillight and interior lighting. Its development prospects have attracted major lighting manufacturers around the world to join its light source and market development. White light LED has great development and application prospects. It is economical and environmentally friendly when used as solid lighting devices. It is gradually replacing traditional incandescent lamps. The annual growth rate in the world is more than 20%. China, the United States, Japan and some European countries have launched semiconductor lighting plans. At present, the luminous efficiency of ordinary white light LED is 25lm/W. The excellent heat dissipation and optical properties of power LED are more suitable for the field of ordinary lighting. It is considered by academia and industry as the only way for LED to enter the lighting market. To replace fluorescent lamps, white LEDs must have a luminous efficiency of 150~200lm/W, and the price per lm should be significantly lower than $0.015/lm (current price is about $0.25/lm, red LED is $0.065/lm). There are still many technical problems to be solved to achieve this goal, but overcoming these problems is not a very distant thing. According to the principles of solid-state luminescence physics, the luminous efficiency of LEDs can be close to 100%. Therefore, LEDs are hailed as the "new light source of the 21st century" and are expected to become the fourth generation of light sources after incandescent lamps, fluorescent lamps, and high-intensity gas discharge lamps.

8. Conclusion

There are more than 20 upstream and midstream research and production units and more than 150 packaging companies in the domestic LED industry, and high-end packaging products have not yet been launched on the market. At present, the industrialization research of GaN-based blue-green LED midstream process technology has been completed, and efforts are being made to make the performance indicators of products reach the level of similar products of the same period abroad in a short period of time, and to achieve a monthly production capacity of 10kk in a relatively short period of time, and to develop new products such as power-type LED chips for white light sources. The Ministry of Science and Technology will invest 80 million yuan to launch the National Semiconductor Lighting Project, pay attention to terminal products, start with special products, take automobiles and urban landscape lighting as market breakthroughs, and put high-power and high-brightness LEDs in a prominent position. Its achievements will serve the Beijing Olympics and the Shanghai World Expo. There is no doubt that the substrate, epitaxy, chip, packaging, and application in the industrial chain need to develop together and be cultivated interactively by multiple parties. Packaging is the link between the upper and lower parts of the industrial chain and needs attention and attention.