LED packaging structure and technology

LED is a type of light-emitting device that can directly convert electrical energy into visible light and radiation energy. It has a series of characteristics such as 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, etc. It has developed by leaps and bounds and can now mass-produce high-brightness and high-performance products of various colors in the entire visible spectrum. The 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 achieve an annual production capacity of 30 billion units, realize the large-scale 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. It is predicted that by 2005, the international market demand for LEDs will be about 200 billion units, with sales reaching 80 billion US dollars.

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

The particularity of LED packaging

Most of the LED packaging technology has developed and evolved on the basis of discrete device packaging technology, but it has great particularity. In general, the tube core of the discrete device is sealed in the package body, and the main function of the package is to protect the tube core and complete the electrical interconnection. The LED package is to complete the output of electrical signals, protect the normal operation of the tube core, and output: the function of visible light. It has both electrical parameters and optical parameter design and technical requirements. It is impossible to simply use the packaging of discrete devices for LEDs.

The core light-emitting part of the 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 connected to a pin through a spherical contact point and a gold wire, and 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) to play the role 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 very 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 easily absorbed after multiple reflections inside the tube core, which is prone to total reflection and excessive light loss. Select epoxy resin with corresponding refractive index 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. Selecting packaging materials with different refractive indices, the packaging geometry has different effects on the photon escape efficiency, and 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 round or flat, the corresponding viewing angle will increase.

In general, the wavelength of LED light changes with temperature by 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 luminous intensity of the LED will decrease by about 1% for every 1℃ increase in temperature. The package dissipates heat; it is very important to maintain color purity and luminous intensity. In the past, the method of reducing the driving current was mostly used to reduce the junction temperature. The driving current of most LEDs is limited to about 20mA. However, the light output of the LED will increase with the increase of current. At present, the driving current of many power LEDs can reach 70mA, 100mA or even 1A. It is necessary to improve the packaging structure, a new LED packaging design concept and a low thermal resistance packaging structure and technology to improve 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 the application design, the thermal design and thermal conductivity of the PCB circuit board are also very important.

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

Product packaging structure type

Since the 1990s, the research and development of LED chip and material manufacturing technology has made many breakthroughs. Transparent substrate trapezoidal structure, textured surface structure, chip flip-chip structure, commercialized ultra-high brightness (above 1cd) red, orange, yellow, green, and blue LED products have been launched one after another. In 2000, they began to be used in special lighting with low and medium light flux. 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 combinations, a variety of series, varieties, and specifications of products can be produced.

The types of LED product packaging structures are also 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 also uses multiple tube cores, which are combined with appropriate connections (including series and parallel connections) of the tube cores and suitable optical structures to form the light-emitting segments and light-emitting points of the light-emitting display. Surface mount LEDs can gradually replace pin-type LEDs. They are more flexible in application design and have already occupied a certain share in the LED display market. There is an accelerating development trend. Some solid lighting sources have been launched, becoming the medium- and long-term development direction of LEDs in the future.

Pin-type packaging

LED foot-type 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 operating 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, which 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, which 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 have been developed for point and surface light sources for market and customer application.

LED light-emitting displays can be composed of various multi-digit products such as digital tubes or M-shaped tubes, symbol tubes, and matrix tubes, and can be 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 digit 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, low material consumption, 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 leads are 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 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 light 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 packaged. 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 is placed on a 106mm long circuit board, with 101 tube cores (up to 201 tube cores). It is a high-density package. It uses the principle of optical refraction to make the point light source pass through the 13-15 gratings of the transparent cover to form a point-to-line display for each tube core. The packaging technology is relatively complex.

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

Surface mount package

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 packages to SMDs is in line with the general development trend of the entire electronics industry, and many manufacturers have launched such products.

Most early SMD LEDs used an improved SOT-23 with a transparent plastic body, with an outer dimension of 3.04×1.11mm and reel-type container taping packaging. Based on SOT-23, the SLM-125 series and SLM-245 series LEDs with high brightness SMD lenses were developed. The former is single-color luminescence, and the latter is two-color or three-color luminescence. 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 product weight 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 heat dissipation. The data of SMD LED provided by manufacturers are based on 4.0×4.0mm 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 package with lead chip carrier)-2 package, with an outer size of 3.0×2.8mm. The high-brightness tube core is assembled by a unique method. The product thermal resistance is 400K/W. It can be welded in CECC mode. Its luminous intensity reaches 1250mcd under a driving current of 50mA. The character height of the seven-segment one-, two-, three- and four-digit digital 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 multicolor PLCC package has an external reflector, which can be easily combined with the light-emitting tube or light guide, and the current transmission optical design is replaced by the reflective type to provide uniform lighting for a large area. The power SMD LED package that works under 3.5V and 1A driving conditions is developed.

The power package

LED chip and package are developing towards high power. Under high current, the luminous flux is 10-20 times larger than that of Φ5mmLED. Effective heat dissipation and non-degradation packaging materials must be used to solve the light decay problem. Therefore, the tube shell and packaging are also its key technologies. LED packages that can withstand several W power have appeared. The 5W series of white, green, blue-green and blue power LEDs have been supplied since the beginning of 2003. The white light LED light output reaches 1871m, and the light efficiency is 44.31m/W. The green light decay problem has been solved. The LED that can withstand 10W power has been developed. The large area tube size is 2.5×2.5mm, which 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 LED is a flip chip soldering of A1GalnN power flip chip die on a silicon carrier with solder bumps, and then the flip chip soldered silicon carrier is loaded into the heat sink and tube shell, and the bonding leads are packaged. 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 stable flexible gel, 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 output light power, external quantum efficiency and other performance are excellent, which has brought the development of LED solid light sources to a new level.

The packaging 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 31.75mm, 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 be combined. The emitted light is monochrome, color or synthetic white, 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, with high light extraction efficiency, low thermal resistance, better protection of the tube core and bonding wires, and high light output power under high current. It is also a promising LED solid light source.

In application, the packaged products can be assembled on a metal core PCB with an aluminum interlayer to form a power density LED. The PCB is used as the wiring for connecting the device electrodes, and the aluminum core interlayer can be used as a heat sink to obtain a 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 reflective type.

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