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LED Packaging Process Analysis

1. The task of LED packaging

　　 is to connect the outer lead to the electrode of the LED chip, while protecting the LED chip and improving the light extraction efficiency. The key processes are rack mounting, pressure welding, and packaging.

　　2, LED package form

　　 LED packaging forms can be said to be various, mainly according to different application occasions to adopt the corresponding external dimensions, heat dissipation countermeasures and light effect. LEDs are classified into Lamp-LED, TOP-LED, Side-LED, SMD-LED, High-Power-LED, etc. according to the packaging form.

　　3, LED packaging process

　　1) Chip inspection

　　 Microscopic inspection: Whether there is mechanical damage on the surface of the material and whether the lockhill chip size and electrode size meet the process requirements. Whether the electrode pattern is complete

　　2) Expansion

　　Because the LED chips are still closely arranged after dicing, the pitch is very small (about 0.1mm), which is not conducive to the operation of the subsequent process. We use a film expander to expand the film of the bonding chip, and the distance between the LED chips is stretched to about 0.6mm. Manual expansion can also be used, but it is easy to cause undesirable problems such as chip drop and waste.

　　3) Dispensing

　　Put silver glue or insulating glue on the corresponding position of the LED bracket. (For GaAs, 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 substrate, insulating glue is used to fix the chip.) Process difficulties It lies in the control of the amount of glue, and there are detailed process requirements for the height of the colloid and the position of the glue. Since silver glue and insulating glue have strict requirements for storage and use, the waking, mixing, and use time of silver glue are all matters that must be paid attention to in the process.

　　4) Prepare glue

　　In contrast to the glue dispensing, the glue preparation is to use the glue preparation machine to first coat the silver glue on the back electrode of the LED, and then install the LED with the silver glue on the back of the LED bracket. The efficiency of glue preparation is much higher than that of glue dispensing, but not all products are suitable for glue preparation process.

　　5) Hand-stabbed piece

　　 Place the expanded LED chips (with or without glue) on the fixture of the piercing table, place the LED bracket under the fixture, and use a needle to pierce the LED chips one by one to the corresponding position under the microscope. Compared with automatic rack mounting, manual piercing piece has an advantage, it is convenient to change different chips at any time, and it is suitable for products that need to install multiple chips.

　　6) Automatic mounting

Automatic mounting is actually a combination of the two steps of glue dip (glue) and chip mounting. First, silver glue (insulating glue) is applied on the LED bracket, and then the LED chip is sucked up to the moving position with a vacuum nozzle, and then placed in the The corresponding bracket position.

　　 In the process of automatic racking, it is mainly necessary to be familiar with the operation and programming of the equipment, and at the same time adjust the glue and installation accuracy of the equipment. In the selection of nozzles, try to use bakelite nozzles to prevent damage to the surface of the LED chip, especially bakelite must be used for blue and green chips. 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 paste, and sintering requires temperature monitoring to prevent poor batch performance.

　　The sintering temperature of silver paste is generally controlled at 150°C, and the sintering time is 2 hours. According to the actual situation, it can be adjusted to 170°C for 1 hour.

　　 Insulating glue is generally 150°C, 1 hour.

　　The silver glue sintering oven must be opened every 2 hours (or 1 hour) according to the process requirements to replace the sintered products, and not open at will. The sintering oven must not be used for other purposes to prevent pollution.

　　 8) Pressure welding

　　The purpose of pressure welding is to lead the electrode to the LED chip to complete the connection of the inner and outer leads of the product.

　　 The pressure welding process of LED has two types: gold wire ball bonding and aluminum wire bonding. The picture on the right is the process of aluminum wire bonding. First, press the first point on the electrode of the LED chip, then pull the aluminum wire to the top of the corresponding bracket, press the second point and then tear the aluminum wire. The gold wire ball bonding process burns a ball before pressing the first point, and the rest of the process is similar.

　　 Pressure welding is a key link in the LED packaging technology. The main things that need to be monitored in the process are the shape of the pressure-bonded gold wire (aluminum wire), the shape of the solder joint, and the tensile force. The in-depth study of the pressure welding process involves many issues, such as gold (aluminum) wire material, ultrasonic power, pressure welding pressure, selection of the wedge (steel nozzle), and the motion trajectory of the wedge (steel nozzle), etc. There are differences in the microstructure of the solder joints pressed by the two different wedges, which affects the quality of the product. We will not repeat it here.

　　9) Dispensing package

　　 There are mainly three types of LED packaging: glue, potting, and molding. Basically, the difficulties of process control are bubbles, lack of material, and black spots. The design is mainly the selection of materials, and the choice of epoxy and brackets that are well-combined. (General LEDs cannot pass the airtightness test) The TOP-LED and Side-LED shown in the right picture are suitable for dispensing packages. Manual dispensing packaging requires high levels of operation (especially white LEDs). The main difficulty is the control of the dispensing volume, because the epoxy will thicken during use. The dispensing of white light LEDs also has the problem of phosphor precipitation causing color difference in light output.

　　10) Glue encapsulation

　　 Lamp-LED package adopts potting form. The potting process is to first inject liquid epoxy into the LED molding cavity, then insert the pressure-welded LED bracket, put it into the oven to cure the epoxy, and then take the LED out of the cavity to form.

　　11) Molded package

Put the pressure-welded LED bracket into the mold, clamp the upper and lower molds with a hydraulic press and vacuum, put the solid epoxy into the entrance of the injection channel, and press the hydraulic ejector into the mold channel for heating. The glue channel enters each LED molding groove and cures.

　　12) Curing and post curing

"Cure" refers to the curing of the encapsulation epoxy. The general epoxy curing conditions are at 135°C for 1 hour. Molded packages are generally at 150°C for 4 minutes.

　　 post-curing is to fully cure the epoxy and thermally aging the LED at the same time. Post-curing is very important to improve the bonding strength between epoxy and PCB. The general conditions are 120°C, 4 hours.

　　13) Cutting tendons and dicing

　　Because the LEDs are connected together in production (not individually), Lamp packaged LEDs use cutting ribs to cut the connecting ribs of the LED bracket. SMD-LED is on a PCB board and requires a dicing machine to complete the separation work.

　　14) Test

　　 Test the photoelectric parameters of the LED, inspect the appearance size, and at the same time sort the LED products according to customer requirements.

　　15) Packaging

　　 The finished products are counted and packaged. Super bright LED needs anti-static packaging.