【Popular knowledge】12 steps of LED packaging

 LED packaging technology is currently developing towards high luminous efficiency, high reliability, high heat dissipation and thinness. The main highlights are ceramic substrate packaging, flip chip technology, phosphor coating technology, and metal wire bonding technology. The steps of LED packaging can be divided into twelve steps.

　　1. 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 on sapphire insulating substrates , insulating glue is used to fix the chips.)

　　2. Automatic mounting

　　Automatic mounting actually combines the two major steps of glue dipping (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 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 adjust the equipment's glue sticking and installation accuracy. When choosing a nozzle, try to use a bakelite nozzle 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 chip surface.

　　3. Sintering

　　The purpose of sintering is to solidify the silver paste. Sintering requires temperature monitoring to prevent batch defects.

　　The sintering temperature of silver glue is generally controlled at 150℃ and the sintering time is 2 hours. It can be adjusted to 170℃ and 1 hour according to actual conditions.

　　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, and it must not be opened at will. The sintering oven must not be used for other purposes to prevent pollution.

　　4. Pressure welding

　　The purpose of pressure 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 pressure welding processes: gold wire ball welding and aluminum wire pressure welding. 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 pressure welding gold wire (aluminum wire), the shape of the solder joint, and the tension.

　　In-depth research on the pressure welding process involves many issues, such as gold (aluminum) wire material, ultrasonic power, pressure welding pressure, selection of chopper (steel nozzle), movement trajectory of chopper (steel nozzle), etc.

　　5. Glue dispensing

　　There are three main types of LED packaging: dispensing, potting, and molding. Basically, the difficulties in process control are bubbles, missing materials, and black spots. The design is mainly about the selection of materials, and the selection of epoxy and brackets with good combination. (General LEDs cannot pass the airtightness test) TOP-LED and Side-LED shown in the right figure are suitable for dispensing packaging. Manual dispensing packaging requires a high level of operation (especially for white light LEDs). The main difficulty is the control of 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 color difference in light output.

　　6. Glue filling and packaging

　　The encapsulation of Lamp-LED is in the form of potting. 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 let the epoxy solidify, and then remove the LED from the cavity to complete the molding.

　　7. Molded packaging

　　Put the welded LED bracket 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 glue channel with a hydraulic push rod, and the epoxy will flow into each LED molding groove along the glue channel and solidify.

　　8. Curing and post-curing

　　Curing refers to the curing of encapsulated epoxy, and the general epoxy curing conditions are 135℃ for 1 hour. Molded encapsulation is generally 150℃ for 4 minutes.

　　9. Post-curing

　　Post-curing is to allow the epoxy to fully cure and thermally age the LED. Post-curing is very important to improve the bonding strength between the epoxy and the bracket (PCB). The general conditions are 120℃, 4 hours.

　　10. Reinforcing steel bars and slicing

　　Since LEDs are connected together during production (not individually), Lamp packaged LEDs use rib cutting to cut off the connecting ribs of the LED bracket. SMD-LEDs are on a PCB board and require a dicing machine to complete the separation work.

　　11. Testing

　　Test the photoelectric parameters of LEDs, inspect their dimensions, and sort LED products according to customer requirements.

　　12. Packaging

　　The finished products are counted and packaged. Blue/white/green super bright LEDs require anti-static packaging.