Packaging technology and structure of LED display screen

LED packaging technology is mostly developed and evolved on the basis of discrete device packaging technology, but it has great particularity. In general, the die of the discrete device is sealed in the package. The function of the package is to protect the die and complete the electrical interconnection. LED packaging is to complete the output of electrical signals, protect the normal work of the die, output: visible light function, both electrical parameters, and optical parameters of the design and technical requirements, can not be simply used in the packaging of discrete devices for LED.

The core of LED is a PN junction core composed of p-type and n-type semiconductors. When the minority carriers injected into the PN junction combine with the majority of carriers, they will emit visible light, ultraviolet light or near-infrared light. However, the photons emitted from PN junction are non directional, that is, they have the same probability to emit in all directions. Therefore, not all the light generated by the core can be released, which mainly depends on the quality of semiconductor material, the structure and geometry of the core, the internal structure and packaging material of the package. The application requires improving the internal and external quantum efficiency of LED. The conventional Φ 5mm LED package is to bond or sinter a square tube core with a side length of 0.25mm on the lead frame. The positive pole of the die is bonded to the gold wire through the spherical contact point, and the inner lead is connected with a pin. The negative electrode is connected with another pin of the lead frame through the reflecting cup, and then the top of the lead frame is wrapped with epoxy resin. The function of the reflecting cup is to collect the light emitted from the side and interface of the tube core, and emit the light to the desired direction angle. The epoxy resin coated on the top has several functions: protecting the tube core from external erosion; using different shapes and material properties (with or without dispersing agent) to act as lens or diffuse lens to control the divergence angle of light; the correlation between the refractive index of the tube core and the air refractive index is too large, resulting in the small critical angle of total reflection inside the tube core, and the light generated by its active layer Only a small part is taken out, most of them are easy to be absorbed by multiple reflection inside the tube core, which is easy to cause excessive light loss due to total reflection. Epoxy resin with corresponding refractive index is selected as transition to improve the light emission efficiency of the core. The epoxy resin used to form the shell must have moisture resistance, insulation, mechanical strength, and high refractive index and transmittance of light emitted from the tube core. With different refractive index packaging materials, the influence of packaging geometry on photon escape efficiency is different. The angular distribution of luminous intensity is also related to the core structure, light output mode, material and shape of packaging lens. If a sharp resin lens is used, the light can be focused on the axis direction of the LED, and the corresponding viewing angle is small; if the resin lens at the top is circular or planar, the corresponding viewing angle will be increased.

In general, the wavelength of LED varies from 0.2 nm / C to 0.3 nm / ℃ with temperature, and the spectral width increases, which affects the color brightness. In addition, when the forward current flows through the PN junction, the thermal loss causes the temperature rise in the junction area. At room temperature, when the temperature increases by 1 ℃, the luminous intensity of LED will be reduced by about 1%, and the encapsulation heat dissipation; when maintaining the color purity and luminous intensity is very important, in the past, the driving current of most LED is limited to about 20mA. However, the light output of LED will increase with the increase of current. At present, the driving current of many power LED can reach 70ma, 100mA or even 1a. It is necessary to improve the packaging structure, brand-new LED packaging design concept and low thermal resistance packaging structure and technology to improve the thermal characteristics. For example, using large area chip flip chip structure, choosing silver glue with good thermal conductivity, increasing the surface area of metal support, and directly installing silicon carrier with solder bumps on heat sink. In addition, the thermal design and thermal conductivity of PCB are also very important in application design.

After entering the 21st century, LED’s high efficiency, ultra-high brightness and panchromatic are developing and innovating continuously. The luminous efficiency of red and orange LED has reached 100 im / W, that of green LED is 501m / W, and the luminous flux of a single LED has reached dozens of IM. LED chips and packages no longer follow Gong’s traditional design concept and manufacturing mode. In terms of increasing the light output of chips, research and development is not limited to changing the amount of impurities in materials, lattice defects and dislocations to improve the internal efficiency. At the same time, how to improve the internal structure of the die and package, enhance the probability of photon emission inside led, improve the light efficiency, solve the problem of heat dissipation, light extraction and heat sink Optimization of design, improvement of optical performance and acceleration of SMD process are the main directions of industry research and development.

Product packaging structure type

Since the 1990s, many breakthroughs have been made in the research and development of LED chip and material manufacturing technology. Transparent substrate trapezoidal structure, texture surface structure, chip flip chip structure, commercialized ultra-high brightness (more than 1 CD) red, orange, yellow, green and blue LED products have been on the market, as shown in Table 1. Since 2000, they have been applied in special lighting with low and medium light flux. The upstream and midstream industry of LED has received unprecedented attention, which further promotes the development of downstream packaging technology and industry. By adopting different packaging structure forms and sizes, different luminous color die and its two-color or three-color combination mode, a variety of series, varieties and specifications of products can be produced.

The types of packaging structure of LED products are shown in Table 2. They are also classified according to the characteristics of luminous color, chip material, luminous brightness, size, etc. A single core generally constitutes a point light source, and the assembly of multiple cores can generally form a surface light source and a line light source for information, status indication and display. The light-emitting display is also composed of multiple cores, which are combined with the appropriate optical structure through the appropriate connection (including series and parallel) of the tube cores to form the light-emitting section and luminous point of the light-emitting display. Surface mount LED can gradually replace the pin led, and its application design is more flexible. It has occupied a certain share in the LED display market and has the trend of accelerating development. Some products of solid-state lighting source are on the market, which will become the medium and long-term development direction of LED in the future.

Pin type package

Led pin type package uses lead frame as pins of various package shapes, which is the first package structure successfully developed and put into the market. There are many varieties and high technology maturity, and the internal structure and reflective layer of LED pin package are still improving. The standard LED is considered by most customers as the most convenient and economical solution in the display industry. The typical traditional LED is placed in a package that can withstand 0.1 W input power. 90% of the heat is emitted from the negative pin rack to the PCB board and then to the air. How to reduce the temperature rise of PN junction during operation is a must for packaging and application. High temperature curing epoxy resin is usually used as the encapsulation material. It has excellent optical properties, good process adaptability and high product reliability. It can be packaged into colored transparent or colorless transparent lens and colored scattering or colorless scattering lens packaging. Different lens shapes form a variety of shapes and sizes. For example, the circle is divided into several types according to the diameter, such as Φ 2mm, Φ 3mm, Φ 4.4mm, Φ 5mm, Φ 7mm, etc The components can produce different luminous effects. There are a variety of packaging structures for Decor point light source: the epoxy resin package with ceramic base has good working temperature performance, the pin can be bent into the required shape, and the volume is small; the plastic reflective cover package with metal base is a kind of energy-saving indicator lamp, which is suitable for power supply indication; the combination of CMOS oscillator circuit chip and led core can produce strong visual impact flicker by itself The two-color type consists of two different light-emitting colors of the core, packaged in the same epoxy resin lens, in addition to the two-color can also get a third mixed color, widely used in large screen display system, and can be packaged to form a two-color display device; the voltage type combines the constant current source chip and led core, which can directly replace 5-24 V voltage indicator lights. The surface light source is formed by using plastic reflection frame cover and epoxy resin at the specified position of several LED tube cores glued to CB board. The arrangement and connection mode of external leads are determined by different design of PCB board, including double in-line insertion and single in-line insertion. Hundreds of package shapes and sizes have been developed for the market and customers.

LED light-emitting display can be made up of digital tube or meter tube, symbol tube and rectangular tube. It can be designed into various shapes and structures according to actual requirements. Take the nixie tube as an example, there are three kinds of packaging structures, such as reflective cover type, single chip integrated type and single seven section type. There are two kinds of connection modes: common anode and common cathode. One digit is commonly referred to as digital tube, and more than two digits are generally called display. The reflective cover type has the characteristics of large font size, low material consumption and flexible assembly. Generally, a seven segment shell with a reflection chamber is made of white plastic. A single LED core is bonded to the PCB board which is aligned with the seven reflecting cavities of the reflector. The central position of the bottom of each reflection chamber is the luminous area formed by the tube core. The lead wire is bonded by pressure welding, and epoxy resin is dripped into the reflector The resin is bonded with the PCB board of the die, and then cured. There are two kinds of reflecting cover type: empty sealing and solid sealing. The former uses epoxy resin of scattering agent and dye, which is mostly used for unit and double position devices; the latter is covered with color filter and uniform film, and coated with transparent insulating glue on the tube core and bottom plate to improve light efficiency, which is generally used for digital display above four digits. Monolithic integrated mode is to make a large number of seven segment digital display die on the luminescent material chip, and then slice into a single chip graphics die, bonding, pressure welding and packaging the shell with lens (commonly known as fisheye lens). In single seven segment mode, the large area LED chips that have been made are cut into light-emitting strips containing one or more core, so the same seven pieces are bonded on the digital shape of the felling frame, and are composed of pressure welding and epoxy resin packaging. The characteristics of single chip and single strip are miniaturization and can be packaged in dual in-line package, most of which are special products. LED light column display is a kind of high-density package with 101 tube cores (up to 201 tubes) on the 106mm long circuit board. The point light source is imaged through 13-15 gratings of transparent cover by using the principle of optical refraction. The packaging technology is complex.

The electroluminescence mechanism of semiconductor pn junction determines that led can not produce white light with continuous spectrum, and a single LED can not produce more than two kinds of high brightness monochromatic light. We can only use fluorescent material to coat blue or ultraviolet LED core with phosphor to indirectly generate broad band spectrum and synthesize white light; or use several (two or three or more) different color light emitting core sealing Installed in a component shell, white LED full-color screen is formed by mixing color light. In 2000, Japan produced 100 million white LEDs, which developed into a kind of stable white light emitting products. Many white LEDs were designed and assembled into pairs with low luminous flux requirements. They were mainly used for local decoration and pursued fashionable electric light sources.

Surface mount package

In 2002, surface mount LED (SMD LED) was gradually accepted by the market, and gained a certain market share. The transition from pin type packaging to SMD is in line with the development trend of the whole electronic industry. Many manufacturers have launched such products.

Most of the early SMD LEDs were modified SOT-23 with transparent plastic body, the overall dimension was 3.04 × 1.11mm, and the package was wrapped in reel containers. Based on SOT-23, slm-125 series and slm-245 series of high brightness SMD with lens are developed. The former is monochromatic and the latter is bicolor or tricolor. In recent years, SMD LED has become a development hotspot, which can solve the problems of brightness, viewing angle, flatness, reliability, consistency, etc. with lighter PCB and reflective layer materials, less epoxy resin needs to be filled in the display reflection layer, and the heavier carbon steel material pin is removed. By reducing the size and weight, the product weight can be easily reduced by half, and finally the application is more perfect It is especially suitable for indoor and semi outdoor full-color display.

Table 3 shows several sizes of common SMD LEDs and the best viewing distance calculated from the dimensions (plus the necessary clearance). The data of SMD LED provided by the manufacturer are all based on 4.0 × 4.0 mm pad. Reflow soldering can be designed to make the pad equal to the pin. Ultra high brightness LED products can be packaged with PLCC (plastic packaging lead chip carrier) – 2, with the overall dimension of 3.0 × 2.8mm. The high brightness LED core is assembled by unique method. The thermal resistance of the product is 400k / W, which can be welded according to CECC method. The luminous intensity is 1250 MCD at 50mA driving current. The character height of seven segment one digit, two digit, three digit and four digit digital SMD LED display device is 5.08-12.7mm, and the display size range is wide. PLCC package avoids the manual insertion and pin alignment process required by the seven segment digital display. It meets the production requirements of automatic pick-up and mount equipment. The application design space is flexible and the display is bright and clear. The multi-color PLCC package has an external reflector, which can be easily combined with LED or photoconductor. The reflective type replaces the current transmission type optical design to provide uniform lighting for large areas. A power type SMD LED package operating under 3.5V and 1A driving conditions is developed.

Power package

LED chip and package are developing towards high power, and the luminous flux is 10-20 times larger than that of Φ 5mm led under high current. Effective heat dissipation and non degradation packaging materials must be used to solve the problem of light attenuation. Therefore, tube shell and package are also the key technologies. LED packaging which can withstand several W power has appeared. 5W series white, green, blue-green, blue-green power LED has been supplied since the beginning of 2003. The output of white LED is 1871m, the luminous efficiency is 44.31m/w, and the green light fading problem. We have developed a large-area led with 10W power. The size of the LED is 2.5 × 2.5mm, which can work under 5A current, and the light output reaches 2001m. As a solid-state lighting source, it has a great development space.

Luxeon series power LED is to flip weld the a1galnn power flip chip on the silicon carrier with solder bumps, and then install the silicon carrier to heat sink and tube shell, and bond the lead wire for packaging. This kind of package is the best for the design of light extraction efficiency, heat dissipation performance and increasing working current density. Its main features: low thermal resistance, generally only 14 ℃ / W, only 1 / 10 of the conventional LED; high * performance, the package is filled with stable flexible cementitious body, in the range of – 40-120 ℃, there will be no internal stress caused by sudden change of temperature, which will break the gold wire from the lead frame, prevent the epoxy resin lens from yellowing, and the lead frame will not be stained due to oxidation; the best design of reflector cup and lens The radiation pattern is controllable and the optical efficiency is the highest. In addition, the LED solid-state light source is developed to a new level due to its excellent output power and external quantum efficiency.

The package structure of norlux series power LED is a multi chip combination with hexagonal aluminum plate as the base (making it non-conductive). The base diameter is 31.75mm, and the luminous area is located in the center of the base. The diameter is about (0.375 × 25.4) mm, which can accommodate 40 led cores. The aluminum plate is used as heat sink at the same time. The bonding leads of the die are connected with the positive and negative electrodes through two contact points made on the base, and the number of the cores arranged on the base is determined according to the required output light power. The ultra-high brightness AlGaInN and AlGaInP die can be assembled, and the emitting light is monochromatic, colored or synthetic white respectively. Finally, the high refractive index material is used to encapsulate according to the optical design shape. This kind of package adopts conventional die high-density combination package, which has high light extraction efficiency, low thermal resistance, better protection of the core and bonding lead, and high light output power under high current. It is also a promising solid-state LED light source.

In the application, the packaged products can be assembled on a metal core PCB board with aluminum interlayer to form power density LED. The PCB board is used as the wiring of device electrode connection, and the aluminum core sandwich can be used as heat sink to obtain high luminous flux and photoelectric conversion efficiency. In addition, the packaged SMD LED is very small in size and can be combined flexibly to form various lighting sources such as module type, light guide plate type, concentrating type and reflection type.

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

Development and application prospect of LED

In recent years, the luminous efficiency of LED has increased by 100 times, and the cost has decreased by 10 times. It is widely used in large area graphic display, full-color screen, status indication, sign lighting, signal display, backlight of LCD, combination tail light and interior lighting of automobile. Its development prospect has attracted large lighting manufacturers all over the world to join in LED light source and market development. White LED, which has great development and application prospect, is becoming more and more popular as a solid-state lighting device. It is gradually replacing the traditional incandescent lamp. The annual growth rate of the world is more than 20%. Semiconductor lighting plans have been launched in the United States, Japan, Europe and Taiwan Province of China. At present, the luminous efficiency of ordinary white LED is 251 M / W, which is expected to exceed 3001 M / W in 2005. The excellent heat dissipation and optical characteristics of power LED can better adapt to the field of general lighting, which is considered as the only way for led to enter the lighting market. In order to replace fluorescent lamp and white LED, it must have the light efficiency of 150-2001m / W, and the price per im should be obviously lower than 0.015/im (the current price is about 0.25 $/ IM, and the red LED is 0.065/im). There are still many technical problems to be studied, but it is not very far away to overcome these problems. According to the principle of solid-state luminescence physics, the luminous efficiency of LED is about 100%. Therefore, LED is known as the new light source in the 21st century, and is expected to become the fourth generation light source after incandescent lamp, fluorescent lamp and high intensity gas discharge lamp.