LED is an emerging lighting energy. Because it is more "squeaky", it is more demanding in the LED packaging environment. The whole process needs to be ESD-free and the temperature should be controlled at 23±2°C, and the humidity should be controlled by 50±10%. In the context of the environment.
1. Process :
a) Cleaning: Ultrasonic cleaning of the PCB or LED holder and drying.
b) Mounting: After the silver electrode is placed on the bottom electrode of the LED die (large wafer), the expansion is performed, and the expanded die (large wafer) is placed on the thorn crystal table, and the die is punched under the microscope with a stylus. One by one is mounted on the corresponding pad of the PCB or LED holder, and then sintered to cure the silver paste.
c) Pressure welding: The electrode is connected to the LED die by an aluminum wire or gold wire welder for current injection. The LED is directly mounted on the PCB, and an aluminum wire welder is generally used. (Making a white light TOP-LED requires a gold wire welder)
d) Packaging: The LED die and bonding wires are protected with epoxy by dispensing. Dispensing on the PCB, there are strict requirements on the shape of the gel after curing, which is directly related to the brightness of the backlight product. This process will also take on the task of a point phosphor (white LED).
e) Soldering: If the backlight is an SMD-LED or other packaged LED, the LED needs to be soldered to the PCB before the assembly process.
f) Film cutting: Various diffusion films, reflective films, etc. required for backlighting are punched by a punch.
g) Assembly: Manually install the various materials of the backlight in the correct position according to the drawings.
h) Test: Check whether the photoelectric parameters of the backlight and the uniformity of light output are good.
I) Packaging: The finished product is packaged and put into storage as required.
Second, the packaging process
1. The task of LED packaging
It is connected to the outer lead electrodes of the LED chip, while protecting the LED chip and acts to improve the light extraction efficiency. The key processes are mounting, pressure welding and packaging.
2. LED package form
LED package form can be said to be varied, mainly according to different applications, the corresponding size, heat dissipation measures and light-emitting effects. LEDs are classified into Lamp-LED, TOP-LED, Side-LED, SMD-LED, High-Power-LED, etc. in package form.
3. LED packaging process flow
4. Package process description
Chip inspection
Microscopic examination: whether there is mechanical damage on the surface of the material and the pithill
Whether the chip size and electrode size meet the process requirements
Is the electrode pattern intact?
2. Expansion
Since the LED chip is still arranged with a small close spacing (about 0.1 mm) after dicing, it is not conducive to the operation of the post process. We use a film expander to expand the film of the bonded chip, and the distance between the LED chips is stretched to about 0.6 mm. It can also be manually expanded, but it is easy to cause problems such as chip falling waste.
3. Dispensing
Place 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 paste is used. For blue and green LED chips of sapphire insulated substrates, insulating paste is used to fix the chips.)
The difficulty of the process lies in the control of the amount of glue, and there are detailed technical requirements in the height of the glue and the position of the glue.
Since silver glue and insulating glue have strict requirements in 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.
4. Preparation glue
In contrast to dispensing, the glue is applied to the back electrode of the LED with a glue machine, and then the LED with silver glue on the back is mounted on the LED holder. The efficiency of the preparation glue is much higher than that of the dispensing, but not all products are suitable for the preparation process.
5. Hand-piercing
The expanded LED chip (with or without glue) is placed on the jig of the lancet table, and the LED holder is placed under the clamp, and the LED chips are punctured one by one under the microscope with a needle. Hand-made thorns have an advantage over automatic loading, making it easy to replace different chips at any time, suitable for products that require multiple chips.
6. Automatic loading
The automatic loading is actually a combination of two steps of glue (dispensing) and mounting the chip. First, put silver glue (insulating glue) on the LED bracket, then use the vacuum nozzle to suck the LED chip to the moving position, and then place it in the The corresponding bracket position.
In the process of automatic loading, the equipment should 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 the nozzle, the bakelite nozzle should be used as much as possible to prevent damage to the surface of the LED chip. In particular, the 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 cure the silver paste, and the sintering requires monitoring of the temperature to prevent batch defects.
The temperature at which the silver paste is sintered 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.
The insulating glue is generally 150 ° C for 1 hour.
The silver-glued sintering oven must be opened and replaced with a sintered product within 2 hours (or 1 hour) according to the process requirements, and should not be opened at will. Sintering ovens must not be used for other purposes to prevent contamination.
8. Pressure welding
The purpose of the pressure welding is to lead the electrode to the LED chip to complete the connection of the inner and outer leads of the product.
There are two kinds of LED pressure welding processes: gold wire ball bonding and aluminum wire pressure welding. 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 over the corresponding bracket, and press the second point to tear the aluminum wire. The gold wire ball bonding process burns the ball before the first point, and the rest of the process is similar.
Pressure welding is a key link in LED packaging technology. The main requirements of the process are the pressure welding of gold wire (aluminum wire) arch shape, solder joint shape, and tensile force.
In-depth research on the pressure welding process involves many problems, such as gold (aluminum) wire material, ultrasonic power, pressure welding pressure, boring tool (steel nozzle) selection, boring tool (steel mouth) trajectory and so on. (The figure below is a micrograph of the solder joints from two different boring tools under the same conditions. The difference in microstructure between the two is related to the quality of the product.) We will not repeat them here.
9. Dispensing package
LED packaging is mainly a little glued, potted, molded. Basically, the difficulty of process control is bubbles, lack of materials, and black spots. The design is mainly for the selection of materials, and the combination of good epoxy and brackets is selected. (General LEDs cannot pass the airtightness test) The TOP-LED and Side-LED shown in the right figure are suitable for dispensing. Manual dispensing requires a high level of operation (especially white LEDs). The main difficulty is the control of the amount of dispensing, as the epoxy thickens during use. The dispensing of white LEDs also has the problem of phosphor precipitation leading to chromatic aberration of light.
10. Potting package
The lamp-LED package is in the form of a potting. The potting process is to first inject liquid epoxy into the LED molding cavity, then insert the pressure-bonded LED bracket into the oven to cure the epoxy, and then remove the LED from the cavity to form.
11. Molded package
The pressure-welded LED bracket is placed in the mold, and the upper and lower molds are clamped by a hydraulic machine and vacuumed, and the solid epoxy is placed in the inlet of the injection molding pipe and pressed into the mold rubber channel by the hydraulic ejector. The glue lane enters each LED forming groove and solidifies.
12. Curing and post curing
Curing refers to the curing of the encapsulating epoxy, which is typically cured at 135 ° C for 1 hour. The molded package is typically at 150 ° C for 4 minutes.
13. Post curing
Post-cure is to allow the epoxy to fully cure while thermally aging the LED. Post-cure is important to increase the bond strength of the epoxy to the support (PCB). The general conditions are 120 ° C for 4 hours.
14. Cutting and dicing
Since the LEDs are connected together in a production process (not a single one), the Lamp packaged LED uses a rib to cut the ribs of the LED holder. SMD-LED is on a PCB board and requires a dicing machine to complete the separation.
15. Testing
Test the photoelectric parameters of the LED, verify the external dimensions, and sort the LED products according to customer requirements.
16. Packaging
The finished product is counted and packaged. Ultra-bright LEDs require anti-static packaging. (small soup)
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