1. The task of LED packaging
The external lead is connected to the electrode of the LED chip, and the LED chip is protected at the same time, and the light extraction efficiency is improved. 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. Led is classified into Lamp-LED, TOP-LED, Side-LED, SMD-LED, High-Power-LED, etc. according to the package form.
3. LED packaging process
a) Chip inspection
Microscopic examination: 1. Whether there is mechanical damage on the surface of the material and the lockhill;
2. Whether the chip size and electrode size meet the process requirements;
3. Whether the electrode pattern is complete.
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.
Silver glue or insulating glue is placed at 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 with sapphire insulating 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.
d) Preparation glue
In contrast to dispensing, the glue is prepared by first applying silver glue to the LED on the back side of the LED, and then mounting the LED with the silver back on the led bracket. The efficiency of the preparation glue is much higher than that of the dispensing, but not all products are suitable for the preparation process.
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.
f) 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.
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.
h) 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.
i) 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.
j) 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-welded led bracket, put it into the oven to cure the epoxy, and then pull the LED out of the cavity to form.
k) 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 path enters each of the led molding grooves and solidifies.
l) 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.
m) post curing
Post-cure is to fully cure the epoxy while heat-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.
n) cutting and dicing
Since the LEDs are connected together in the production (not single), the Lamp package led uses a rib to cut the ribs of the led bracket. SMD-led is on a PCB board and requires a dicing machine to complete the separation.
Test the photoelectric parameters of led, check the external dimensions, and sort the LED products according to customer requirements.
The finished product is counted and packaged. Ultra-bright LEDs require anti-static packaging.
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