1. LED chip type

LED chip types are mainly divided into three categories from the structural point of view: horizontal electrode chips, flip chips and vertical electrode chips.

2. LED package

Packaging is the necessary intermediate link to realize the LED from the chip to the final product, the function of the package is to provide sufficient protection of the chip, to prevent the chip from long-term exposure to the air or mechanical damage and failure, to improve the stability of the chip; For LED packaging, it is also necessary to have good light extraction efficiency and good heat dissipation, and a good package can make the LED have better luminous efficiency and heat dissipation environment, thereby improving the life of the LED.

The role of the package: to achieve the input electrical signal, protect the chip normal operation, output visible light function.

3. Commonly used LED chip packaging form

1) Pin-type package

2) Flat package

3) Surface mount encapsulation

4) Piranha encapsulation

5) Power type package

4-pin package construction

LED pin-type package using lead frame for a variety of package shape pins, is the first successful development of the packaging structure on the market, a wide number of varieties, high technical maturity, the package structure and reflection layer is still continuously improved.

The LED chip is glued to the lead bracket. The positive electrode is linked to a bracket with gold wire, and the negative electrode is connected to the bracket reflector cup with gold wire or glued directly to the bracket reflector cup with silver paste. The top is coated with epoxy resin and made into a cylindrical + hemispherical shape. The shape of the epoxy resin acts as a lens, controlling the beam divergence angle. At the same time, the refractive index of epoxy resin plays a transition between the chip and air, improves the luminous efficiency of the chip, the refractive index of the chip is high, and the total reflection angle of the air is small, while for the epoxy resin, the total reflection angle is increased, which can make more light output.

Generally, the size of this LED chip is 0.25mmX0.25mm, the radius of the component after the package is 5mm, the luminous power is 1 to 2 lumens, and the working current is 20 to 30mA. Traditional small chip LEDs limit their application because of their low luminous power. Most of them are used for display or indication. For example, the indicator light of the instrument.

4.1 Process for pin-on packages

5. Flat package

Flat package is a structural package that combines multiple LED chips into a flat structure package. Through the appropriate connection of LEDs (series and parallel) and a suitable optical structure, the luminous segment and luminous point of the luminous display may be constituted, and then the various luminous displays are composed of these luminous segments and luminous points, such as digital tubes, “meter” tubes and matrix tubes.

Taking the Xlamp XP-E product of Cree in the United States as an example, it adopts a more productive packaging manufacturing process, with ceramics as the packaging substrate, and the product units are on a ceramic substrate, which is flat and arranged together in an array. The chip is mounted on each substrate unit, and then a layer of silicone is encapsulated on the ceramic substrate and the desired optical lens structure is formed. Its structure includes ceramic substrates, new products, Au wires and plastic silicone, etc.

6 Surface Mount Package (SMD)

Surface Mount Package (SMD) is a new type of surface mount package. It has the advantages of small size, large scattering angle, good luminous uniformity and high reliability. Its luminous color can be white light including a variety of colors, very suitable for mobile phone keyboard display lighting, BACKlighting of television sets, and electronic products that need illumination or indication, in recent years, the patch package has developed in the direction of large size and high power, a patch package of three or four Led chips, can be used to assemble lighting products.

7 Piranha Encapsulation

The piranha package is square and uses a transparent resin package because it is shaped much like piranha piranha in the Amazon River, so it is called a piranha package. It has four pins and a missing pin at the negative electrode. Piranhas are astigmatism-type LEDs with a luminous angle greater than 120 degrees, a high luminous intensity, and can withstand greater power. Since the bracket used in the piranha LED is made of copper and has a large area, heat transfer and heat dissipation are fast. After the LEDs are lit, the heat generated by the PN junction can quickly go from the four feet of the bracket to the copper strip of the PCB.

Piranha encapsulation step

1) Select the stand of the piranha LED

2) Clean the bracket

3) Determine the shape and size and depth of the bowl in the bracket

4) The LED chip is fixed in the bracket bowl

5) Drying

6) Solder LED chip two levels

7) Select mold particles

8) Filling in the mold

9) The bracket of the welded LED chip is placed upside down in the mold

10) Drying, demolding, die cutting

11) Testing and sorting

8. power package

With the improvement of the input power of LED chips, it brings large heat generation and high light efficiency, which puts forward newer and higher requirements for LED packaging technology, making the packaging technology of power LED a hot spot in recent years, and power LED is also the core of future semiconductor lighting

High-power LED package design optical, electrical, thermal, structural and process aspects specifically, the key technologies of the package include:

8.1 Geothermal resistance encapsulation process

It mainly includes chip layout, packaging material selection and process, heat sink design, etc. Led package thermal resistance mainly includes internal thermal resistance and interfacial thermal resistance of materials. The role of the heat dissipation substrate is to absorb the heat generated by the chip and conduct it to the heat sink to achieve heat exchange with the outside world. Commonly used heat dissipation substrate materials include silicon, metals (aluminum, copper), ceramics (such as Al2O3, AlN, SiC) and conforming materials. The encapsulation interface also has a great impact on thermal resistance, and it is difficult to obtain a good heat dissipation effect if the interface is not handled correctly. Therefore, the choice of thermal interface material between the chip and the thermal substrate is very important. LED packaging using low temperature or eutectic solder, solder paste or nanoparticle-dosed conductive silver glue as interface materials can greatly reduce the interface thermal resistance.

8.2 High light absorption package structure and process

The loss of photons generated by the emitting radiation of the chip when emitted outward mainly includes three aspects: 1) the internal structural defects of the chip and the absorption of the material; 2) the reflection loss caused by the refractive index difference of the photon at the outlet interface; 3) the total reflection loss caused by the angle of incidence is greater than the critical angle of total reflection. By coating a transparent adhesive layer (potting glue) with a relatively high refractive index on the surface of the chip, because the rubber layer is between the chip and the air, the loss of photons at the interface is effectively reduced and the light extraction efficiency is improved.