Twelve steps to analyze the manufacturing process of LED chips
The manufacturing process of LED chips can be summed up in twelve steps:
LED chip inspection
Microscopic inspection: Whether there is mechanical damage on the surface of the material and whether the size of the lockhill chip and the size of the electrode meet the process requirements. Whether the electrode pattern is complete.
LED expansion
Since the LED chips are still arranged closely with a small spacing (about 0.1 mm) after dicing, it is not conducive to the operation of the subsequent process. Use a chip expander to expand the film bonding the chips, so that the spacing of 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.
LED dispensing
Apply silver glue or insulating glue to the corresponding position of the LED bracket. For GaAs and SiC conductive substrates, red light, yellow light, and yellow-green chips with back electrodes use silver glue. 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, and there are detailed process requirements in the height of the glue and the position of the glue. Since silver glue and insulating glue have strict requirements for storage and use, the wake-up, stirring and use time of silver glue are all matters that must be paid attention to in the process.
LED glue preparation
Contrary to dispensing, glue preparation is to use a glue preparation machine to first apply silver glue on 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 much higher than that of dispensing, but not all products are suitable for glue preparation.
LED handmade thorns
Place the expanded LED chips (with glue or without glue) on the jig of the thorn table, place the LED bracket under the jig, and use a needle to pierce the LED chips to the corresponding positions one by one under the microscope. Compared with automatic racking, manual thorn chips have an advantage, which is easy to replace different chips at any time, and is suitable for products that need to install multiple chips.
LED automatic rack
The automatic racking actually combines the two steps of gluing (dispensing) and installing the chip. First, put silver glue (insulating glue) on the LED bracket, and then use the vacuum nozzle to suck up the LED chip to move the position, and then place it on the LED bracket. in 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 suction nozzles, try to use bakelite suction nozzles to prevent damage to the surface of LED chips, especially blue and green chips must use bakelite. Because the steel nozzle will scratch the current spreading layer on the surface of the chip.
LED sintering
The purpose of sintering is to solidify the silver paste, and sintering requires monitoring of temperature to prevent batch failure. The temperature of silver glue sintering is generally controlled at 150 ° C, and the sintering time is 2 hours. According to the actual situation, it can be adjusted to 170℃ for 1 hour. Insulation glue is generally 150 ℃, 1 hour.
The silver glue sintering oven must be opened to replace the sintered product every 2 hours (or 1 hour) according to the process requirements, and it must not be opened at will. The sintering oven shall not be used for other purposes to prevent pollution.
LED pressure welding
The purpose of pressure welding is to lead the electrodes to the LED chip to complete the connection of the inner and outer leads of the product.
There are two types of LED pressure welding processes: gold wire ball welding and aluminum wire pressure welding. The process of aluminum wire pressure welding is to first press a point D on the LED chip electrode, then pull the aluminum wire to the top of the corresponding bracket, and then tear the aluminum wire after pressing the second point. The gold wire ball bonding process burns a ball before pressing D a little, and the rest of the process is similar.
Pressure welding is a key link in LED packaging technology. The main process to monitor is the shape of the pressure welding gold wire (aluminum wire) arch wire, the shape of the solder joint, and the tension.
LED encapsulant
There are three main types of LED packaging: dispensing, potting, and molding. Basically, the difficulty of process control is air bubbles, lack of material 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 air tightness test)
LED dispensing TOP-LED and Side-LED are suitable for dispensing packaging. The manual dispensing package requires a high level of operation (especially white LEDs), and the main difficulty is the control of the amount of glue dispensed, because the epoxy will thicken during use. The dispensing of white LEDs also has the problem of chromatic aberration caused by the precipitation of phosphor powder.
LED potting encapsulation Lamp-LED is encapsulated in the form of potting. The process of potting is to first inject liquid epoxy into the LED molding cavity, then insert the pressure-welded LED bracket, put it in an oven to cure the epoxy, and then remove the LED from the cavity to form.
LED molding package Put the pressure-welded LED bracket into the mold, close the upper and lower molds with a hydraulic press and vacuumize, put the solid epoxy into the entrance of the injection channel, and press the hydraulic ejector into the mold rubber channel for heating. The epoxy enters each LED molding tank along the glue channel and cures.
LED curing and post curing
Curing refers to the curing of the encapsulated epoxy, and the general epoxy curing conditions are 135 ° C for 1 hour. Molded packaging is generally at 150 ℃, 4 minutes. Post-curing is to allow the epoxy to fully cure while thermally aging the LED. Post-curing is very important to improve the bond strength of the epoxy to the bracket (PCB). The general conditions are 120°C, 4 hours.
LED Ribs and Dicing
Since the LEDs are connected together (not individually) in production, the Lamp packaged LEDs use cutting ribs to cut off the connecting ribs of the LED bracket. SMD-LED is on a PCB board and requires a dicing machine to complete the separation work.
LED test
Test the optoelectronic parameters of the LED, inspect the external dimensions, and sort the LED products according to customer requirements.
