What is LED?
LED is Light Emitting Diode in English, which is a semiconductor solid light-emitting device. It uses solid semiconductor chips as light-emitting materials. When a forward voltage is applied to both ends, the carriers in the semiconductor recombine to cause photon emission and generate light. LED can directly emit red, yellow, blue, green, cyan, orange, purple, and white light. The first commercial diode was produced in 1960. Its basic structure is a piece of electroluminescent semiconductor material, placed on a rack with leads, and then sealed with epoxy resin around to protect the internal core wire, so LED has good seismic resistance. 2. Why is LED the fourth generation light source (green lighting )?
Classification by the light-emitting mechanism of electric light sources:
First-generation light sources: resistor light-emitting such as incandescent lamps.
Second-generation light sources: arc and gas light-emitting such as sodium lamps.
Third-generation light sources: phosphor light-emitting such as fluorescent lamps.
Fourth-generation light sources: solid-state chip light-emitting such as LEDs.
What are the light-emitting mechanisms and working principles of LEDs?
Light-emitting diodes are made of III-IV group compounds, such as GaAs (gallium arsenide), GaP (gallium phosphide), GaAsP (gallium arsenide phosphide) and other semiconductors, and their core is a PN junction. Therefore, it has the I-N characteristics of a general P-N junction, that is, forward conduction, reverse cutoff, and breakdown characteristics. In addition, under certain conditions, it also has light-emitting characteristics. Under forward voltage, electrons are injected from the N region into the P region, and holes are injected from the P region into the N region. A part of the minority carriers (minority carriers) that enter the other area recombine with the majority carriers (majority carriers) to emit light.
What are the optical properties of LED?
(1) The light emitted by LED is neither monochromatic nor broadband, but a balance between the two.
(2) The LED light source is similar to a point light source but not a point light source.
(3) The color of the light emitted by LED varies with the spatial direction.
(4) The junction temperature of the LED under constant current operation strongly affects the forward voltage VF.
What are the different ways of constructing LED?
LEDs have different chemical compositions due to their different colors:
For example, red: aluminum-indium-gallium-phosphide
Green and blue: indium-gallium-nitride
White and other colors are made by mixing the three primary colors RGB in appropriate proportions. The manufacturing process of LED is similar to that of semiconductors, but the processing accuracy is not as good as that of semiconductors, and the current cost is still relatively high.
What are the wavelengths of various colors?
The spectral wavelength distribution of several commonly used ultra-bright LEDs in China is 460-636nm, and the wavelengths are blue, green, yellow-green, yellow, yellow-orange, and red from short to long. The typical peak wavelengths of several common color LEDs are:
Blue – 470nm,
Blue-green – 505nm,
Green – 525nm,
Yellow – 590nm,
Orange – 615nm,
Red – 625nm.
What are the packaging methods of LED?
Packaging method:
(1) Pin-type (Lamp) LED packaging,
(2) Surface mount (SMD) type ( SMT-LED) packaging,
(3) Chip-on-Board (COB) LED packaging,
(4) System-in-Pack (SiP) LED packaging
(5) Wafer bonding and chip bonding.
What are the classification methods of LED?
1. According to the color of the light-emitting tube
According to the color of the light-emitting tube, it can be divided into red, orange, green (further subdivided into yellow-green, standard green and pure green), blue light, etc. In addition, some light-emitting diodes contain two or three colors of chips.
According to whether the light-emitting diode is doped with scattering agent or not, and whether it is colored or colorless, the above-mentioned light-emitting diodes of various colors can also be divided into four types: colored transparent, colorless transparent, colored scattering and colorless scattering. Scattering type light-emitting diodes and are used as indicator lights.
2. According to the characteristics of the light-emitting surface of the light-emitting tube
According to the characteristics of the light-emitting surface of the light-emitting tube, it can be divided into round lamps, square lamps, rectangular lamps, surface light-emitting tubes, lateral tubes, surface-mounted micro tubes, etc. Circular lamps are classified into φ2mm, φ4.4mm, φ5mm, φ8mm, φ10mm and φ20mm according to their diameters. In foreign countries, φ3mm LEDs are usually recorded as T-1; φ5mm as T-1(3/4); and φ4.4mm as T-1(1/4). The half-value angle can be used to estimate the angular distribution of circular luminous intensity. There are three types based on the angular distribution of luminous intensity:
(1) High directivity. Generally, it is a pointed epoxy package or a package with a metal reflective cavity, and no scattering agent is added. The half-value angle is 5°～20° or less, with high directivity. It can be used as a local lighting source, or used in conjunction with a light detector to form an automatic detection system.
(2) Standard type. Usually used as an indicator light, its half-value angle is 20°～45°.
(3) Scattering type. This is an indicator light with a larger viewing angle, a half-value angle of 45°～90° or more, and a larger amount of scattering agent.
3. According to the structure of the light-emitting diode
According to the structure of the light-emitting diode, there are full epoxy encapsulation, metal base epoxy encapsulation, ceramic base epoxy encapsulation and glass encapsulation.
4. According to the luminous intensity and working current
According to the luminous intensity and working current, there are ordinary brightness LEDs (luminous intensity>10mcd); ultra-high brightness LEDs (luminous intensity<100mcd); the luminous intensity between 10 and 100mcd is called High brightness light emitting diode. The working current of general LED is between tens of mA and tens of mA, while the working current of low current LED is below 2 mA (the brightness is the same as that of ordinary light emitting tube).
In addition to the above classification methods, there are also classification methods by chip material and by function.
What are the production process steps of LED?
1. Process:
a) Cleaning: Use ultrasonic cleaning of PCB or LED bracket and drying.
b) Mounting: Prepare silver glue on the bottom electrode of LED tube core (large wafer) and expand it. Place the expanded tube core (large wafer) on the crystal table. Use a crystal pen to install the tube core one by one on the corresponding pad of PCB or LED bracket under a microscope, and then sinter to solidify the silver glue.
c) Pressure welding: Use aluminum wire or gold wire welding machine to connect the electrode to the LED tube core as a lead for current injection. LED is directly mounted on PCB, generally using aluminum wire welding machine. (Gold wire welding machine is needed to make white light TOP-LED)
d) Packaging: Use epoxy to protect LED core and welding wire through dispensing. Dispensing on PCB board has strict requirements on the shape of colloid after curing, which is directly related to the brightness of the finished backlight source. This process will also undertake the task of dispensing phosphor (white light LED).
e) Welding: If the backlight source is SMD-LED or other packaged LED, the LED needs to be welded to the PCB board before the assembly process.
f) Film cutting: Use punching machine to die-cut various diffusion films, reflective films, etc. required for the backlight source.
g) Assembly: According to the requirements of the drawing, manually install various materials of the backlight source in the correct position.
h) Testing: Check whether the photoelectric parameters and light uniformity of the backlight source are good.
2. The task of LED packaging
is to connect the external lead to the electrode of the LED chip, protect the LED chip at the same time, and play a role in improving the light extraction efficiency. The key processes are mounting, pressing and packaging.
3. LED packaging Form
LED packaging forms can be said to be varied, mainly according to different application scenarios with corresponding external dimensions, heat dissipation measures and light output effects. LEDs are classified into Lamp-LED, TOP-LED, Side-LED, SMD-LED, High-Power-LED, etc. according to packaging forms.
4. LED packaging process flow
5. Packaging process description
(1). Chip inspection
Microscopic inspection: whether there is mechanical damage and pits (lockhill) on the surface of the material, whether the chip size and electrode size meet the process requirements, and whether the electrode pattern is complete.
(2). Chip expansion
Since LED chips are still closely arranged and the spacing is very small (about 0.1mm) after dicing, it is not conducive to the operation of the subsequent process. We use a film expander to expand the film of the bonded chip, so that the spacing of the LED chip is stretched to about 0.6mm. Manual expansion can also be used, but it is easy to cause problems such as chip falling and waste.
(3). Glue dispensing
Apply silver glue or insulating glue to the corresponding position of the LED bracket. (For GaAs and SiC conductive substrates, red, yellow, and yellow-green chips with back electrodes, silver glue is used. For blue and green LED chips with sapphire insulating substrates, insulating glue is used to fix the chips.) The difficulty of the process lies in the control of the amount of glue dispensing. There are detailed process requirements for the height of the colloid and the location of the glue dispensing. Since silver glue and insulating glue have strict requirements for storage and use, the waking up, stirring, and use time of silver glue are all matters that must be paid attention to in the process.
(4). Glue preparation
In contrast to glue dispensing, glue preparation is to use a glue preparation machine to first apply silver glue to the back electrode of the LED, and then install the LED with silver glue on the back on the LED bracket. The efficiency of glue preparation is far Higher than glue dispensing, but not all products are suitable for glue preparation process.
(5). Manual pricking
Place the expanded LED chip (with or without glue) on the fixture of the pricking table, put the LED bracket under the fixture, and use a needle to pierce the LED chips one by one to the corresponding position under a microscope. Compared with automatic mounting, manual pricking has an advantage that it is easy to replace different chips at any time, which is suitable for products that need to install multiple chips.
(6). Automatic mounting
Automatic mounting actually combines the two steps of glue dispensing and chip installation. First, silver glue (insulating glue) is applied to the LED bracket, and then the LED chip is sucked up and moved to the position with a vacuum nozzle, and then placed in the corresponding bracket position. The main process of automatic mounting is to be familiar with the equipment operation programming, and at the same time adjust the glue dispensing and installation accuracy of the equipment. When choosing the nozzle, try to use bakelite nozzles to prevent damage to the surface of the LED chip, especially blue and green chips must be made of bakelite. Because the steel nozzle will scratch the current diffusion layer on the surface of the chip.
(7). Sintering
The purpose of sintering is to solidify the silver glue. Sintering requires temperature monitoring to prevent batch defects. The temperature of silver glue sintering is generally controlled at 150℃ and the sintering time is 2 hours. According to actual conditions, it can be adjusted to 170℃ for 1 hour. The insulating glue is generally 150℃ for 1 hour. The silver glue sintering oven must be opened every 2 hours (or 1 hour) to replace the sintered product according to the process requirements. It must not be opened at will in the middle. The sintering oven must not be used for other purposes to prevent pollution.
(8). Press welding
The purpose of press welding is to lead the electrode to the LED chip and complete the connection of the internal and external leads of the product. There are two types of LED press welding processes: gold wire ball welding and aluminum wire press welding. The right picture shows the process of aluminum wire press welding. First, press the first point on the LED chip electrode, then pull the aluminum wire to the top of the corresponding bracket, press the second point and then break the aluminum wire. In the process of gold wire ball welding, a ball is burned before pressing the first point, and the rest of the process is similar. Pressure welding is a key link in LED packaging technology. The main things that need to be monitored in the process are the arch shape of the gold wire (aluminum wire), the shape of the solder joint, and the tension. In-depth research on the pressure welding process involves many aspects, such as gold (aluminum) wire material, ultrasonic power, pressure welding pressure, selection of splitter (steel nozzle), movement trajectory of splitter (steel nozzle), etc. (The figure below is a microscopic photo of the solder joints pressed by two different splitters under the same conditions. There are differences in the microstructure of the two, which affects the product quality.) We will not repeat it here.
(9). Glue dispensing LED packaging mainly includes glue dispensing, potting, and molding. Basically, the difficulties in process control are bubbles, multiple missing materials, and black spots. The design mainly focuses on the selection of materials and the selection of epoxy and brackets with good combination. (General LEDs cannot pass the airtightness test) As shown in the right figure, TOP-LED and Side-LED are suitable for glue dispensing. Manual dispensing packaging requires a high level of operation (especially for white light LEDs). The main difficulty is to control the amount of dispensing, because the epoxy will thicken during use. The dispensing of white light LEDs also has the problem of phosphor precipitation causing light color difference.
(10). Glue encapsulation
Lamp-LED encapsulation adopts the form of encapsulation. The encapsulation process is to first inject liquid epoxy into the LED molding cavity, then insert the pressed LED bracket, put it in an oven to let the epoxy solidify, and then remove the LED from the cavity to form it.
(11). Molded packaging
Put the pressed LED bracket in Put it into the mold, close the upper and lower molds with a hydraulic press and evacuate them.
Put the solid epoxy into the entrance of the injection channel, heat it, and press it into the mold channel with a hydraulic push rod. The epoxy enters each LED molding groove along the channel and solidifies.
(12). Curing and post-curing
Curing refers to the curing of the encapsulated epoxy. The general epoxy curing conditions are 135℃ for 1 hour. The molded package is generally at 150℃ for 4 minutes.
(13). Post-curing
The purpose of curing is to fully cure the epoxy and thermally age the LED. Post-curing is very important for improving the bonding strength between the epoxy and the bracket (PCB). The general conditions are 120℃ for 4 hours.
(14). Cutting and dicing
Since LEDs are connected together (not individually) during production, Lamp encapsulated LEDs use cutting to cut off the connecting ribs of the LED bracket. SMD-LED is on a PCB board, and a dicing machine is needed to complete the separation work. (15). Testing
Test the photoelectric parameters of LED, check the external dimensions, and sort the LED products according to customer requirements.
(16). Packaging
Count and package the finished products. Ultra-bright LEDs require anti-static packaging.