LED Encapsulation Technology

02/01 112

First, Production Process

a)      Cleaning: use ultrasonic cleaning PCB or LED bracket, and then drying.

b) Mounting: make adhesive at the electrode of LED chip (wafer) bottom, then place the chip (wafer) in thorn station, under the microscope, mount the chip one by one on corresponding pad of PCB or LED’s bracket with thorn crystal pen, then sintering the adhesive and make it curing.

c) Bonding: with aluminum wire or gold wire welder connect electrode to the LED chip, leading current injection. LED mounted directly on the PCB, it generally uses aluminum wire welding. (Making white TOP-LED, gold wire welder is needed.)

d) Encapsulating: By sticking adhesive, protect LED chip and wire with epoxy. Sticking adhesive on the PCB board, there are strict requirements for colloidal shape after curing, which is directly related to the brightness of the backlight. This process will also assume the task of making phosphor (white LED).

e) Welding: If the backlight is SMD-LED or other encapsulated LED, it needs to solder LED to the PCB board before the assembly process.

f) Cutting film: punch all kinds of backlight diffusion film, reflective film.

g) Assembly: According to drawings, install all materials of backlight manually in the right way.

h) Testing: Check the backlight optical parameters and light uniformity.

i) Packaging: packaging the finished product as requires and warehousing.

 

Second, Encapsulation Process

1.      LED Encapsulation Task

To connect the outer lead to the electrode of the LED chip and protect the LED chip, also improve the light extraction efficiency. The key steps have mounting, bonding and encapsulating.

2.      LED Encapsulation Form

There are various forms of LED Encapsulation, use appropriate dimensions, the heat dissipation measures and the light effect depending on different application,. According to forms of LED encapsulation, there are Lamp-LED, TOP-LED, Side-LED, SMD-LED, High-Power-LED and the like.

3.      LED Encapsulation Process

4. Encapsulation Process Instruction

1). Chip Inspection

Microscopy:

Whether there is mechanical damage and pitting on the surface (lockhill)?

Do chip size and electrode size meet the process requirements?

Is electrode pattern complete?

2). Extend Slice

Since the LED chip still has close array and small space (about 0.1mm) after slicing, it is not good for the following process. We use machine to extend bonding film of the chip, which makes the space of the LED chip stretched to about 0.6mm. It can also be expanded manually, but it will lead to chip falling waste and other problems easily.

3). Sticking Adhesive

Stick adhesive or insulating adhesive at the corresponding place of LED bracket (For GaAs, SiC conductive substrate, red, yellow, yellow-green chip having back electrode uses adhesive. For insulating sapphire substrate, blue, green LED chip, uses insulating adhesive to fix the chip)

The difficulty is to control the amount of adhesive, and it has specific requirements for colloid height, stick adhesive position.

Since the adhesive and insulating adhesive have strict requirements for the storage and use, material, stirring and using time of adhesive are something needs to be noticed.

4). Prepared Adhesive

Different from sticking adhesive, prepared adhesive is to coat adhesive at the LED back electrode by machine first, and then mount LED with adhesive on the LED bracket. Efficiency of prepared adhesive is much higher than sticking adhesive, but not all products are suitable for prepared adhesive.

5). Stab Sheet Manually

Place the extended LED chip (prepared adhesive or non-prepared adhesive) on the fixture of sheet station, and put LED bracket under the fixture. Under the microscope stab LED chips with a needle one by one to the corresponding position. Compare with mounting automatically, stab sheet manually has a good advantage, which is to replace different chips easily at any time. It can be applied for products that need to be installed with a variety of chips.

6). Assembly Automatically

Actually assembly automatically includes two steps: stick adhesive and install chip, first stick adhesive (insulating adhesive) on the LED bracket, then suck up the LED chip with vacuum suction, and then place it in the corresponding position.

Assembly automatically is to get well know of equipment operation process, make adjustment for sticking adhesive and installation precision. For the choice of nozzle, it should use bakelite nozzle to prevent damage for the surface of the LED chip, especially for blue, green chip, it must use bakelite. Because the steel tips will scratch the current diffusion layer on the surface of chip.
7). Sintering

Sintering aims to make adhesive curing, it requires monitoring temperature, in case of inferior.

For adhesivesintering temperature controls in 150 , 2 hours. According to actual conditions, it can be adjusted to 170 , 1 hour.

For insulating adhesive, generally it is 150 , 1 hour.

When sintering, the sintering oven needs to be opened every 2 hours (or 1 hour) to replace sintered products. During this process, it is forbidden to be opened. Also, sintering oven can’t be used for other purposes in case of pollution.

8). Bonding

The purpose of bonding is to lead the electrode to the LED chip and complete the connection work of products inside and outside leads.

There are two types of bonding: gold ball bonding and aluminum wire bonding. For aluminum wire bonding process, after the first bonding on the electrode of LED chip, pull the aluminum wire to the above of bracket; after the second bonding pull apart aluminum wire. The process of gold ball bonding is similar to it, except that sintering the ball before the first bonding.

Bonding is the key for LED encapsulation technology. It needs to monitor the bonding shape of gold (aluminum) wire, welding spot shape and tension.

Study of the bonding process involves many problems, such as gold (aluminum) wire material, ultrasonic power, bonding pressure, chopper (steel tip) selection, chopper (steel tip) trajectory,

9). Adhesive Encapsulation

There are three types of LED encapsulation: sticking, potting and molding.

Difficulties in process control are bubble, lack of material, dark spots. For material selection, it’s   good to choose epoxy and bracket. (General LED can’t pass air-tight test). TOP-LED and Side-LED is suitable for adhesive encapsulation. Manually adhesive encapsulation has strict requirements for operation (especially for white LED), the main difficulty is the control for the amount of adhesive, because the epoxy will thicken during the process. For white LED, it will cause chromatic aberration because of phosphor sediment.

10). Potting Encapsulation

Potting process: inject liquid epoxy into LED molding cavity, and then insert bonding LED bracket, put it into oven to curing the epoxy, after that take out LED from the mold cavity, it is done.

11). Molding Encapsulation

Put the bonding LED bracket in a mould, clamping the upper and lower mold and vacuumed by hydraulic machine, put the solid epoxy into inlet of adhesive injection and heat it, mould it by hydraulic rod, the epoxy has access to all LED molded groove and curing.

12). Curing and Post Curing

Curing means curing epoxy encapsulation, epoxy curing is at 135 , 1 hour. Molding Encapsulation is at  150 , 4 minutes.

13). Post Curing

Post curing is to make curing epoxy fully and make LED thermal aging. Post curing is very important for improving epoxy and the adhesive strength of the bracket (PCB). Generally it is at 120 , 4 hours.

14). Cutting and Slicing

Since the LED in production are linked together (not individual), it needs to separate them by cutting. SMD-LED is on the PCB board, it needs to be separated by slicing.

15). Testing

Test LED photoelectric parameters, inspect dimensions and sort the LED product according to customer requirements.

16). Packaging

Package the finished products. Super bright LED needs anti-static packaging.


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