1. LED Production Process

a) Cleaning: Utilize ultrasonic cleaning for PCBs or LED brackets, followed by drying.

b) Mounting: Apply silver glue to the bottom electrode of the LED chip (large wafer) for expansion. Place the expanded chip onto a crystal stabbing platform, then, under a microscope, use a crystal stabbing pen to mount each chip onto the corresponding solder pad of the PCB or LED bracket. Subsequently, perform sintering to solidify the silver glue.

c) Wire bonding: Use aluminum or gold wire bonding machines to connect the electrodes to the LED chip for current injection. Aluminum wire bonding machines are generally used for directly mounting LEDs onto PCBs. (Gold wire bonding machines are needed for producing white TOP-LEDs.)

d) Encapsulation: Protect the LED chip and bonding wires using epoxy through dispensing. Dispensing on the PCB requires strict control of the cured gel shape, directly affecting the brightness of the backlighting. This process also involves applying phosphor (for white LEDs).

e) Soldering: If SMD-LEDs or other pre-packaged LEDs are used for backlighting, they need to be soldered onto the PCB before assembly.

f) Film cutting: Use a punching machine to cut various diffusion films, reflective films, etc., needed for backlighting.

g) Assembly: Manually install various materials of the backlighting according to the drawings.

h) Testing: Check the photometric parameters and uniformity of the backlighting.

i) Packaging: Package the finished products according to requirements for storage.

2. LED Packaging Process

a) The task of LED packaging is to connect the external leads to the electrodes of the LED chip, protect the LED chip, and improve light extraction efficiency. Key processes include mounting, wire bonding, and encapsulation.

b) LED packaging forms: LED packaging forms vary widely, mainly depending on different application scenarios, dimensions, heat dissipation strategies, and light extraction effects. LED classification by packaging form includes Lamp-LED, TOP-LED, Side-LED, SMD-LED, High-Power-LED, etc.

c) LED packaging process flow

d) Packaging process description:

1. Chip inspection – visual inspection:

• Check for mechanical damage and spots on the material surface.
• Ensure chip dimensions and electrode sizes meet process requirements.
• Verify the integrity of electrode patterns.

2. Wafer expanding: LED chips remain closely spaced after dicing (about 0.1mm), which is not conducive to subsequent operations. We use wafer expanders to stretch the bonded chip film, increasing the chip spacing to about 0.6mm. Manual expansion can also be used but may lead to chip loss and waste.
3. Dispensing: Dispense silver or insulating adhesive at corresponding positions on the LED bracket. (Silver adhesive is used for GaAs, SiC substrates with back electrodes for red, yellow, and yellow-green chips. Insulating adhesive is used to fix blue and green LED chips on sapphire insulating substrates.) The dispensing process is challenging due to the control of the dispensing amount, with detailed process requirements for gel height and dispensing position. Strict requirements exist for the storage and use of silver and insulating adhesives.
4. Pre-bonding: In contrast to dispensing, pre-bonding involves applying silver adhesive to the back electrodes of LEDs using a pre-bonding machine, then mounting LEDs with adhesive on the LED bracket. Pre-bonding efficiency is much higher than dispensing but may not be suitable for all products.
5. Manual die bonding: Place the expanded LED chip (pre-bonded or unbonded) on the die bonding platform fixture, with the LED bracket placed under the fixture. Use a needle under a microscope to manually bond each LED chip to the corresponding position. Manual die bonding allows for easy replacement of different chips and is suitable for products requiring multiple chip installations.
6. Automated die bonding: Automated die bonding combines adhesive application (dispensing) and chip mounting steps. First, silver adhesive (or insulating adhesive) is dispensed onto the LED bracket, then LEDs are picked up and moved to the appropriate position using vacuum nozzles before being placed on the bracket. Automated die bonding requires familiarity with equipment operation and programming and adjustments to adhesive and chip placement accuracy. It’s preferable to use rubber suction nozzles to prevent damage to LED chip surfaces, especially for blue and green chips.
7. Sintering: The purpose of sintering is to cure the silver adhesive. Sintering requires temperature monitoring to prevent batch defects. The temperature for silver adhesive sintering is generally controlled at 150°C for 2 hours. For insulating adhesive, it’s typically 150°C for 1 hour. Sintering ovens must be opened every 2 hours (or 1 hour) according to process requirements to replace sintered products, and should not be used for other purposes to prevent contamination.
8. Wire bonding: The purpose of wire bonding is to connect the electrodes to the LED chip, completing the internal and external lead connection work. LED wire bonding technology includes gold wire ball bonding and aluminum wire wedge bonding. The image on the right shows the process of aluminum wire wedge bonding, where the first point is pressed onto the LED chip electrode, then the wire is pulled to the corresponding bracket position, and after the second point is pressed, the wire is broken. The key aspects to monitor in wire bonding technology are wire shape, bond shape, and tension. In-depth research on wire bonding technology involves various issues such as wire material, ultrasonic power, bonding pressure, wedge (steel nozzle) selection, and wedge (steel nozzle) movement trajectory.
9. Dispensing encapsulation: LED encapsulation mainly includes dispensing, potting, and molding. The main challenges in process control are bubbles, missing materials, and black spots. Design mainly involves material selection, choosing epoxy and brackets with good adhesion. (Ordinary LEDs cannot pass the airtightness test). As shown in the image on the right, TOP-LED and Side-LED are suitable for dispensing encapsulation. Manual dispensing encapsulation requires high operational skills (especially for white LEDs). The main challenge is controlling the dispensing amount because epoxy thickens during use. Dispensing white LEDs also faces issues such as fluorescence powder precipitation leading to color deviation.
10. Potting encapsulation: Lamp-LED encapsulation adopts potting. The potting process involves injecting liquid epoxy into the LED molding die cavity, then inserting the pre-bonded LED bracket, placing it in an oven to cure the epoxy, and removing the LED from the die cavity to complete the molding.
11. Molding encapsulation: Place the pre-bonded LED bracket into the mold, use hydraulic pressure to close the upper and lower molds while vacuuming, inject solid epoxy into the injection port of the mold, heat the injection port with a hydraulic plunger to push the epoxy into the LED molding groove, and cure it.
12. Curing and post-curing: Curing refers to the curing of encapsulation epoxy, generally at 135°C for 1 hour. Molding encapsulation is typically cured at 150°C for 4 minutes. Post-curing is important for ensuring sufficient epoxy curing and for LED thermal aging. Post-curing conditions are typically 120°C for 4 hours.
13. Deflashing and dicing: As LEDs are produced