Packaging Process

1. The task of advanced packaging for LEDs

is to connect the external leads to the electrodes of the LED chip, while protecting the LED chip and improving the light extraction efficiency. The key processes include die mounting, wire bonding, and encapsulation.

2. LED packaging forms

LED packaging forms come in a wide variety, mainly adopting different form factors, thermal management solutions, and light output effects depending on the specific application requirements. LEDs are classified by packaging form into Lamp-LED, TOP-LED, Side-LED, SMD-LED, High-Power-LED, and more.

3. LED packaging process flow

4. Packaging process description

1. Chip inspection

Microscopic inspection: checking whether the material surface has mechanical damage or pitting (lockhill), whether the chip size and electrode dimensions meet process requirements, and whether the electrode pattern is complete.

2. Wafer expansion

Since LED chips remain closely arranged after dicing with very small spacing (approximately 0.1mm), which is not conducive to subsequent processing operations, we use a wafer expander to stretch the adhesive film bonded to the chips, increasing the LED chip spacing to approximately 0.6mm. Manual expansion is also possible, but it easily causes chips to fall off and result in waste and other defects.

3. Dispensing

Applying silver adhesive or insulating adhesive at the corresponding positions on the LED lead frame. (For GaAs and SiC conductive substrates with back electrodes used in red, yellow, and yellow-green chips, silver adhesive is used. For sapphire insulating substrates used in blue and green LED chips, insulating adhesive is used to secure the chips.) The process difficulty lies in controlling the dispensing volume, with detailed process requirements for adhesive height and dispensing position. Since silver adhesive and insulating adhesive have strict requirements for storage and usage, the awakening, mixing, and usage time of silver adhesive are all critical process considerations.

4. Back coating

In contrast to dispensing, back coating uses a coating machine to first apply silver adhesive on the back electrode of the LED, then mount the silver-adhesive-coated LED onto the LED lead frame. The efficiency of back coating is much higher than dispensing, but not all products are suitable for the back coating process.

5. Manual die attach

The expanded LED chips (with or without back coating) are placed on the fixture of the die attach station, with the LED lead frame positioned underneath the fixture. Under a microscope, a needle is used to place each LED chip one by one onto the corresponding position. Manual die attach has one advantage over automatic die attach: it allows convenient switching between different chips at any time, making it suitable for products that require multiple types of chips.

6. Automatic die attach

Automatic die attach actually combines two major steps: adhesive application (dispensing) and chip placement. First, silver adhesive (insulating adhesive) is dispensed onto the LED lead frame, then a vacuum nozzle picks up the LED chip and places it at the corresponding position on the lead frame. For automatic die attach, the key is to master the equipment programming and to adjust the adhesive application and placement accuracy. In terms of nozzle selection, bakelite nozzles should be used as much as possible to prevent damage to the LED chip surface, especially for blue and green chips where bakelite nozzles are mandatory. This is because steel nozzles will scratch the current spreading layer on the chip surface.

7. Sintering

The purpose of sintering is to cure the silver adhesive. Sintering requires temperature monitoring to prevent batch defects. The silver adhesive sintering temperature is generally controlled at 150 degrees Celsius with a sintering time of 2 hours. Depending on actual conditions, it can be adjusted to 170 degrees Celsius for 1 hour. Insulating adhesive is generally cured at 150 degrees Celsius for 1 hour. The silver adhesive sintering oven must be opened every 2 hours (or 1 hour) according to process requirements to replace the products being sintered, and must not be opened at random in between. The sintering oven must 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 connection between the internal and external leads of the product. LED wire bonding processes include gold wire ball bonding and aluminum wire wedge bonding. First, the first point is bonded on the LED chip electrode, then the aluminum wire is drawn to the corresponding position above the lead frame, the second point is bonded, and the wire is severed. The gold wire ball bonding process first forms a ball before bonding the first point; the rest of the process is similar. Wire bonding is a critical step in LED packaging technology. The key process parameters to monitor include the gold/aluminum wire loop shape, bond pad shape, and pull strength. In-depth research on wire bonding process involves multiple aspects, such as gold (aluminum) wire materials, ultrasonic power, bonding pressure, capillary (steel nozzle) selection, and capillary (steel nozzle) movement trajectory.

LED encapsulation mainly includes three methods: dispensing, potting, and transfer molding.

The key process control difficulties are bubbles, short shots, and black spots. Design considerations mainly focus on material selection, choosing epoxy and lead frames with good bonding compatibility. Manual dispensing encapsulation requires a high level of operational skill (especially for white LEDs), with the main difficulty being control of the dispensing volume, as the epoxy thickens during use. White LED dispensing also faces the issue of phosphor sedimentation causing color temperature variation in light output.

10. Potting encapsulation

Lamp-LED packaging uses the potting method. The potting process involves first injecting liquid epoxy into the LED molding cavity, then inserting the wire-bonded LED lead frame, placing it in an oven for the epoxy to cure, and finally demolding the LED from the cavity.

11. Transfer molding encapsulation

The wire-bonded LED lead frames are placed into the mold, the upper and lower mold halves are closed using a hydraulic press under vacuum, solid epoxy is placed at the entrance of the injection channel and heated, then pushed by hydraulic plungers into the mold runner system. The epoxy flows through the runners into each LED molding cavity and cures. (12) Curing and post-curing

Curing refers to the curing of the encapsulation epoxy. General epoxy curing conditions are 135 degrees Celsius for 1 hour. Transfer molding encapsulation is typically at 150 degrees Celsius for 4 minutes.

13. Post-curing

Post-curing is to ensure complete curing of the epoxy and to perform thermal aging on the LEDs. Post-curing is very important for improving the bonding strength between the epoxy and the lead frame (PCB). General conditions are 120 degrees Celsius for 4 hours.

14. Trim and singulation

Since LEDs are connected together during production (not individual units), Lamp-packaged LEDs use a trim machine to cut the connecting tabs of the lead frame. SMD-LEDs are on a PCB panel and require a dicing saw to complete the singulation process.

15. Testing

Testing the optoelectronic parameters of LEDs, inspecting dimensional specifications, and sorting LED products according to customer requirements.

16. Packaging

Counting and packaging the finished products. Ultra-high brightness LEDs require anti-static packaging.