Microscopic inspection: whether there is mechanical damage and pitting on the surface of the material (whether the chip size and electrode size of Lockhill meet the process requirements and whether the electrode pattern is complete.
2.LED expansion
Because the LED chips are still closely arranged after dicing, the spacing is very small (about 0. 1mm), which is not conducive to the subsequent process operation. We use a film stretcher to expand the film of bonded chips, so as to stretch the spacing between LED chips to about 0.6 mm. Manual expansion is also possible, but it is easy to cause bad problems such as waste of chips.
3.LED dispensing
Apply silver glue or insulating glue to the corresponding position of LED bracket. (For GaAs and SiC conductive substrates, red, yellow and yellow-green chips with back electrodes are made of silver paste. For blue and green LED chips with sapphire insulating substrate, insulating glue is used to fix the chips. )
The process difficulty lies in the control of dispensing amount, and the height and location of colloid have detailed process requirements.
Because there are strict requirements for the storage and use of silver glue and insulating glue, the proofing, stirring and use time of silver glue are all matters that must be paid attention to in the process.
4. Preparation of 4.LED glue
Contrary to glue dispensing, glue making is to apply silver glue to the electrode on the back of LED by glue making machine, and then install the LED coated with silver glue on the back of LED bracket. The efficiency of dispensing is much higher than dispensing, but not all products are suitable for dispensing.
5.LED manual puncture
Put the expanded LED chips (with or without glue) on the fixture of the stabbing table, put the LED bracket under the fixture, and stab the LED chips to the corresponding positions one by one with a needle under the microscope. Compared with automatic mounting, manual punching has an advantage that it is convenient to replace different chips at any time, and it is suitable for products that need to install multiple chips.
6. Automatic 6th gear. LED installation
In fact, automatic mounting is a combination of two steps: gluing (gluing) and chip mounting. First, apply silver glue (insulating glue) on the LED bracket, then suck up and move the LED chip with a vacuum nozzle, and then put it in the corresponding bracket position. In the process of automatic mounting, it is mainly necessary to be familiar with the equipment operation programming, and at the same time, to adjust the gluing and installation accuracy of the equipment. In the selection of suction nozzle, try to use bakelite suction nozzle to prevent damage to the surface of LED chip, especially for blue-green chip. Because the steel nozzle will scratch the current diffusion layer on the chip surface.
7.LED sintering
The purpose of sintering is to solidify the silver paste, and the sintering needs to monitor the temperature to prevent batch defects. The sintering temperature of silver paste is generally controlled at 65438 050℃ and the sintering time is 2 hours. It can be adjusted to 170℃ and 1 hour according to the actual situation. Insulation glue is generally 150℃ 1 hour.
The silver glue sintering furnace must be opened every 2 hours (or 1 hour) according to the process requirements to replace the sintered products, and it is not allowed to be opened at will. The sintering furnace shall not be used for other purposes to prevent pollution.
8.LED 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 bonding processes for LED: gold wire ball bonding and aluminum wire bonding. The picture on the right shows the bonding process of aluminum wire. Press the first point on the electrode of the LED chip, then pull the aluminum wire above the corresponding bracket, press the second point, and then pull the aluminum wire off. The gold wire ball welding process is to burn a ball first and then press the first point, and other processes are similar.
Bonding is a key link in LED packaging process, and the main process to be monitored is the arch wire shape, solder joint shape and tension of bonding gold wire (aluminum wire).
9.LED sealant
There are three kinds of LED packaging: glue, potting and plastic packaging. Basically, the difficulties in process control are bubbles, lack of materials and black spots. The design is mainly about the selection of materials, and the combination of epoxy and bracket is selected. (General LED can't pass the air tightness test)
9. 1LED allocation:
Top LEDs and side LEDs are suitable for distribution packaging. Manual dispensing packaging requires a high level of operation (especially white LED), and the main difficulty is to control the amount of dispensing, because epoxy will thicken during use. The dispensing of white LED still has the problem of color difference caused by phosphor precipitation.
9.2LED potting and packaging
The LED lamp is encapsulated by potting. The process of potting is to inject liquid epoxy resin into the LED molding cavity, then insert the pressure welded LED bracket, put it in the oven to cure the epoxy resin, and then take the LED out of the molding cavity for molding.
9.3LED molding package
Put the pressure-welded LED bracket into the mold, close the upper and lower molds with a hydraulic press and vacuum, put the solid epoxy resin into the rubber channel of the mold with a hydraulic ejector pin, heat it at the entrance of the rubber channel, and the epoxy resin enters each LED molding groove along the rubber channel for curing.
10.LED curing and post-curing
Curing refers to the curing of encapsulated epoxy resin. Generally, the curing conditions of epoxy are 135℃ and 1 hour. Molding and packaging are generally carried out at 150℃ for 4 minutes. Post-curing is to completely cure epoxy resin and thermally age LED. Post curing is very important to improve the bonding strength between epoxy resin and PCB. The general condition is 120℃ for 4 hours.
1 1.LED rib cutting and slicing
Because the LEDs are connected together (not single) in production, the LEDs in the lamp package adopt the method of rib cutting to cut off the ribs of the LED bracket. SMD-LED is on PCB, and it needs dicing machine to complete the separation.
12.LED test
Test the photoelectric parameters of LED, check the overall dimensions, and classify LED products according to customer requirements.
13.LED package
And packaging the finished product. Ultra-bright LEDs need anti-static packaging.