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  • What are the integration technologies for high-power LED multi-function packages?
    Oct 27, 2018

    diode


    Diode, (English: Diode), a device with two electrodes that allows only current to flow in a single direction, many of which are applied to its rectification function. The varactor diode (Varicap Diode) is used as an electronically tunable capacitor. The current directionality of most diodes is often referred to as the "Rectifying" function. The most common function of a diode is to allow only current to pass in a single direction (called forward bias) and reverse in reverse (called reverse bias). Therefore, the diode can be thought of as an electronic version of the check valve.


    An early vacuum electronic diode; it is an electronic device that conducts current in one direction. Inside the semiconductor diode, there is a PN junction and two lead terminals. The electronic device has a unidirectional current conductivity according to the direction of the applied voltage. Generally, a crystal diode is a p-n junction interface formed by sintering a p-type semiconductor and an n-type semiconductor. A space charge layer is formed on both sides of the interface to form a self-built electric field. When the applied voltage is equal to zero, the diffusion current is equal to the drift current caused by the self-built electric field due to the difference in the concentration of carriers on both sides of the p-n junction, and is in an electric equilibrium state, which is also a diode characteristic under normal conditions.


    Early diodes included "Cat's Whisker Crystals" and vacuum tubes (known as "Thermionic Valves" in the UK). Most of the most common diodes today use semiconductor materials such as silicon or germanium.


    characteristic


    Positive


    When the forward voltage is applied, the forward voltage is small at the beginning of the forward characteristic, which is insufficient to overcome the blocking effect of the electric field in the PN junction. The forward current is almost zero. This segment is called the dead zone. This forward voltage that does not turn the diode on is called the deadband voltage. When the forward voltage is greater than the deadband voltage, the electric field in the PN junction is overcome, the diode is forward-conducting, and the current rises rapidly as the voltage increases. In the current range of normal use, the terminal voltage of the diode remains almost constant during turn-on. This voltage is called the forward voltage of the diode. When the forward voltage across the diode exceeds a certain value, the internal electric field is quickly weakened, the characteristic current increases rapidly, and the diode is conducting. It is called the threshold voltage or threshold voltage, the silicon tube is about 0.5V, and the manifold is about 0.1V. The forward voltage drop of the silicon diode is about 0.6-0.8V, and the forward voltage drop of the germanium diode is about 0.2-0.3V.


    Reverse


    When the applied reverse voltage does not exceed a certain range, the current through the diode is the reverse current formed by the minority carrier drift motion. Since the reverse current is small, the diode is in an off state. This reverse current is also called reverse saturation current or leakage current, and the reverse saturation current of the diode is greatly affected by temperature. Generally, the reverse current of a silicon tube is much smaller than that of a tantalum tube. The reverse saturation current of a small power silicon tube is on the order of nA, and the low power tube is on the order of μA. When the temperature rises, the semiconductor is excited by heat, the number of minority carriers increases, and the reverse saturation current also increases.


    breakdown


    When the reverse voltage exceeds a certain value, the reverse current suddenly increases. This phenomenon is called electrical breakdown. The threshold voltage that causes electrical breakdown is called the diode reverse breakdown voltage. The diode loses unidirectional conductivity during electrical breakdown. If the diode does not cause overheating due to electrical breakdown, the unidirectional conductivity may not be permanently destroyed. After the voltage is removed, the performance can be restored, otherwise the diode is damaged. Therefore, the reverse voltage applied to the diode should be avoided.


    A diode is a two-terminal device with unidirectional conduction. It has an electronic diode and a crystal diode. Because the heat loss of the filament is lower than that of the crystal diode, the electronic diode is rarely seen. It is more common and commonly used. It is a crystal diode. The unidirectional conduction characteristics of diodes, semiconductor diodes are used in almost all electronic circuits. It plays an important role in many circuits. It is one of the earliest semiconductor devices, and its application is also very extensive.


    The tube voltage drop of the diode: the silicon diode (non-illuminated type) has a forward voltage drop of 0.7V, and the forward tube voltage drop of the neon tube is 0.3V. The forward tube voltage drop of the LED varies with different illuminating colors. There are three main colors. The specific voltage drop reference values are as follows: the voltage drop of the red LED is 2.0--2.2V, the voltage drop of the yellow LED is 1.8-2.0V, and the voltage drop of the green LED is 3.0-3.2V, the rated current is about 20mA when it is normally illuminated.


    The voltage and current of the diode are not linear, so the resistors should be connected when the different diodes are connected in parallel.


    Characteristic curve


    Like the PN junction, the diode has unidirectional conductivity. Typical volt-ampere characteristics of silicon diodes. When the diode is applied with a forward voltage, when the voltage value is small, the current is extremely small; when the voltage exceeds 0.6V, the current begins to increase exponentially, which is usually referred to as the turn-on voltage of the diode; when the voltage reaches about 0.7V When the diode is fully turned on, this voltage is usually referred to as the turn-on voltage of the diode, indicated by the symbol UD.


    For a germanium diode, the turn-on voltage is 0.2V and the turn-on voltage UD is about 0.3V. A reverse voltage is applied to the diode, and when the voltage value is small, the current is extremely small, and the current value is the reverse saturation current IS. When the reverse voltage exceeds a certain value, the current begins to increase sharply, which is called reverse breakdown. This voltage is called the reverse breakdown voltage of the diode and is represented by the symbol UBR. The breakdown voltage UBR values of different types of diodes vary widely, from tens of volts to several thousand volts.


    Reverse breakdown


    Zener breakdown


    Reverse breakdown is divided into two cases: Zener breakdown and avalanche breakdown. In the case of high doping concentration, the width of the barrier region is small, and when the reverse voltage is large, the covalent bond structure in the barrier region is destroyed, and the valence electrons are decoupled from the covalent bond to generate an electron-hole pair. , causing a sharp increase in current, this breakdown is called Zener breakdown. If the doping concentration is low, the barrier region is wide, and it is not easy to cause Zener breakdown.



    Avalanche breakdown


    Another type of breakdown is an avalanche breakdown. When the reverse voltage is increased to a large value, the applied electric field accelerates the electron drift speed, colliding with the valence electrons in the covalent bond, and knocking the valence electrons out of the covalent bond to generate a new electron-hole pair. The newly generated electron-hole is accelerated by the electric field and then knocks out other valence electrons. The carrier is avalancheally increased, causing a sharp increase in current. This breakdown is called avalanche breakdown. Regardless of the breakdown, if the current is not limited, it may cause permanent damage to the PN junction.


    With the increasing global energy shortage trend, green energy-saving and environmentally-friendly LEDs have attracted much attention. All countries in the world have formulated their own LED lighting development plans. China’s “Twelfth Five-Year Plan” also clearly describes the LED lighting development goals, and lists LEDs as key energy-saving projects during the “Twelfth Five-Year Plan” period, ranking seven national strategies. Energy-saving and environmental protection industries and new materials industries in emerging industries.


    With the development of the LED lighting industry, from the production of LED chips to the lighting market, a relatively complete industrial chain has been formed. But for traditional LED lighting, from chip, package, circuit board to application, all aspects are relatively independent. The lighting requirements of different places have put forward various new requirements for the packaging of LEDs. How to integrate multiple technologies in the module and make the LED module package become smaller, more multifunctional and intelligent through the system packaging method has become a problem we need to explore. From a technical point of view, LED is a semiconductor device that is easy to combine with other semiconductor-related technologies to develop products with higher added value, opening up new markets that traditional lighting cannot reach. LED multi-function system 3D package can integrate light source, active, passive electronic components, sensors and other components, and integrate them into a single miniaturized system, which is a new technology with great market potential.


    LED multi-function package integration technology


    There are some simple LED integrated packaging products on the market, but the integration is low, which can not meet the needs of LED lighting modules for LED packaging products in the future. The development trend of chip module light source reflects the requirements of the lighting market for technology development: portable products require more integrated light sources; in commercial lighting, road lighting, special lighting, flashlights, etc., integrated LED light sources have great applications. market. Compared with package-level modules, chip-level modules are smaller, save space, and save packaging costs, and because of the high integration of the light source, it is convenient for secondary optical design.


    Three-dimensional packaging is an electronic packaging technology developed in recent years. In general, the important factors that accelerate the application of 3D integration technology to microelectronic systems include the following:


    1. The overall shape of the system: reduce system size, reduce system weight and reduce the number of pins;


    2. Performance: Increase integration density and shorten interconnect length to increase transfer speed and reduce power consumption;


    3. High-volume, low-cost production: reduce process costs, such as integrated package and PCB hybrid use; multi-chip simultaneous packaging;


    4. New applications: such as ultra-small wireless sensors;


    There are a number of different advanced system integration methods, including: package stacking technology on the package; chip stacking on PCB (wire bonding and flip chip), stacked flexible functional layers with embedded devices; with or without Advanced printed circuit board (PCB) stacking of embedded electronics; wafer level chip integration; TSV based vertical system integration (VSI). The advantages of 3D integrated packaging include: device integration using different technologies (such as CMOS, MEMS, SiGe, GaAs, etc.), ie "hybrid integration", which usually replaces long two-dimensional interconnects with shorter vertical interconnects, thus Reduce system parasitics and power consumption. Therefore, 3D system integration technology has great advantages in performance, function and shape. In recent years, various key universities and R&D institutions are developing different types of low-cost integration technologies.


    The State Key Laboratory of Semiconductor Lighting Joint Innovation has also conducted systematic research on LED system integrated packaging. The research aimed at LED downlights, through the development of wafer-level packaging technology, plans to integrate some of the drive components and LED chips into the same package. Among them, the die required for the LED and the linear constant current driving circuit is the main component of the circuit heating, and the volume is relatively small, and it is easy to integrate, but the main heating element needs to consider the heat dissipation design. Other components are bulky and not easy to integrate. Inductors, sampling resistors, and fast recovery diodes, although there is a certain amount of heat generation, do not require a special heat dissipation structure.


    Based on the above considerations, we have designed the assembly of the light-emitting module as follows:


    1. The driving circuit die and the LED chip are integrated in the package, and the remaining circuit components are integrated on the PCB board;


    2. The PCB board surrounds the integrated package for easy connection;


    3. The PCB and the integrated package are placed on the heat sink;


    The advantage of this structure is that the volume is small; the main heating element is directly in contact with the heat sink through the package, and is easy to dissipate heat; the component that does not require special heat dissipation is placed on the ordinary PCB. Compared with the MCPCB, the cost is saved; when needed, the components can be designed on the back side of the PCB and hidden in the empty area of the heat sink to avoid the influence of the components on the light.