About LED packaging process

1. Conductive glue, conductive silver glue
Conductive glue is an indispensable glue in IED production packaging. The requirements for conductive silver paste are good electrical and thermal conductivity, high shear strength, and strong bonding.
UNINWELL International's conductive glue and conductive silver glue have good conductivity, strong shear force, good rheology, and low moisture absorption. It is particularly suitable for the packaging of high-power and high-brightness LEDs.
In particular, UNINWELL's 6886 series conductive silver glue has a thermal conductivity of 25.8 and a shear strength of 14.7, which is the best in the industry.
2. Packaging process
1. The task of LED packaging
It is to connect the external lead to the electrode of the LED chip, protect the LED chip, and improve the light extraction efficiency. The key processes are mounting, pressing welding, and packaging.
2. LED packaging form
LED packaging forms can be said to be varied, mainly based on different application scenarios using corresponding external dimensions, heat dissipation countermeasures and light output effects. LEDs are classified by packaging form into Lamp-LED, TOP-LED, Side-LED, SMD-LED, High-Power-LED, etc.
3. LED packaging process flow
4. Packaging process description
1. Chip inspection
Microscopic inspection: whether there is mechanical damage and pits (lockhill) on the surface of the material .
Whether the chip size and electrode size meet the process requirements
Whether the electrode pattern is complete
2. Die expansion
Since the LED chips are still closely arranged and the spacing is very small (about 0.1mm) after dicing, it is not conducive to the operation of the subsequent process. We use a die expander to expand the film of the bonded chip, so that the spacing of the LED chip is stretched to about 0.6mm. Manual expansion can also be used, but it is easy to cause problems such as chip falling and waste.
3. 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.)
The difficulty of the process lies in the control of the amount of glue dispensing, and there are detailed process requirements for the height of the colloid and the dispensing position.
Since silver glue and insulating glue have strict requirements for storage and use, the waking, stirring and use time of silver glue are all matters that must be paid attention to in the process.
4. Glue preparation
In contrast 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 dispensing, but not all products are suitable for glue preparation.
5. Manual pricking
Place the expanded LED chip (prepared or not prepared) on the fixture of the pricking table, put the LED bracket under the fixture, and use a needle to pierce the LED chips one by one to the corresponding position under a microscope. Compared with automatic mounting, manual pricking has an advantage that it is easy to replace different chips at any time, which is suitable for products that need to install multiple chips.
6. Automatic mounting
Automatic mounting actually combines the two steps of glue dipping (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 to the position 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 use 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 solidify the silver glue. Sintering requires temperature monitoring to prevent batch defects.
The temperature of silver glue sintering is generally controlled at 150℃, and the sintering time is 2 hours. According to actual conditions, it can be adjusted to 170℃, 1 hour.
Insulating glue is generally 150℃, 1 hour.
The silver glue sintering oven must be opened every 2 hours (or 1 hour) according to the process requirements to replace the sintered product, and it must not be opened at will in the middle. The sintering oven must not be used for other purposes to prevent pollution.
8. Pressure welding
The purpose of pressure welding is to lead the electrode to the LED chip to 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. The right picture shows the process of aluminum wire pressure welding. First, press the first point on the LED chip electrode, then pull the aluminum wire to the top of the corresponding bracket, press the second point and then break the aluminum wire. In the process of gold wire ball welding, a ball is burned before pressing the first point, and the rest of the process is similar.
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-welded gold wire (aluminum wire), the shape of the solder joint, and the tension.
In-depth research on the pressure welding process involves many aspects, such as gold (aluminum) wire material, ultrasonic power, pressure welding pressure, selection of splitting knives (steel nozzles), and the movement trajectory of splitting knives (steel nozzles). (The following figure is a microscopic photo of the solder joints pressed by two different splitting knives under the same conditions. There are differences in the microstructure of the two, which affects the product quality.) We will not repeat it here.
9. Glue dispensing packaging
There are three main types of LED packaging: glue dispensing, potting, and molding. Basically, the difficulties in process control are bubbles, multiple 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) As shown in the right figure, TOP-LED and Side-LED are suitable for dispensing packaging. Manual dispensing packaging requires a high level of operation (especially for white light LEDs). The main difficulty is to control 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 light color difference. Welcome to LED World Network ()
10. Glue encapsulation
Lamp-LED encapsulation adopts 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 in an oven to let the epoxy cure, and then remove the LED from the mold cavity to form it.
11. Molding packaging
Put the pressure-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 ejector. The epoxy enters each LED molding groove along the glue channel and cures.
12. Curing and post-curing
Curing refers to the curing of the encapsulated epoxy. Generally, the epoxy curing conditions are 135°C for 1 hour. Molded encapsulation is generally 150°C for 4 minutes.
13. Post-curing
Post-curing is to allow the epoxy to fully cure and to thermally age the LED. Post-curing is very important for improving the bonding strength between the epoxy and the bracket (PCB). The general conditions are 120°C for 4 hours.
14. Rib cutting and dicing
Since LEDs are connected together (not individually) during production, Lamp encapsulated 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.
15. Testing
Test the photoelectric parameters of LEDs, inspect the dimensions, and sort LED products according to customer requirements.
16. Packaging
Count and package the finished products. Super bright LEDs require anti-static packaging