The reliability of LED packaging has a lot to do with the choice of LED display beads

Time:2018-10-13 Views:256

LED devices account for about 40% to 70% of the cost of LED displays. The sharp decline of LED display costs is attributed to the cost reduction of LED devices.The quality of LED packaging has great influence on the quality of LED display.The key of packaging reliability includes selection of chip materials, selection of packaging materials and process control.As the LED display screen gradually permeates to the high-end market, the quality requirements for LED display device are increasingly higher.

The main materials used in the packaging of LED display devices include supports, chips, solid-crystal glue, bonding wire and packaging glue.SMD(Surface Mounted Devices) refers to the Surface Mounted encapsulation structure LED, which is mainly the PCB structure LED(chiplete) and PLCC structure LED(TOP LED).This paper mainly studies TOP LED. The SMD LED mentioned below refers to TOP LED.The following from packaging materials to introduce the current domestic some basic development status.

1. LED bracket

(1) function of scaffold.PLCC (Plastic Leaded Chip Carrier) stent is the Carrier of SMD LED device, on the reliability of the leds and the light performance play a key role.

(2) production technology of supports.PLCC bracket production process mainly includes metal strip punching and cutting, electroplating, PPA(poly-o-phenylenediamide) injection molding, bending, five - sided three - dimensional inkjet and other processes.Among them, electroplating, metal substrate, plastic material and so on occupied the main cost of the support.

(3) structural improvement design of supports.PLCC bracket due to the physical combination of PPA and metal, the gap will become larger after the high temperature reflux furnace, thus water vapor can easily enter into the device along the metal channel, thus affecting reliability.

In order to improve the product reliability to meet the high-end market demand of high quality LED display devices, part of the encapsulated into a factory improved bracket structure design, such as a photoelectric co., LTD., foshan city, USES the advanced waterproof structure design, such as tensile bending method to extend the water vapor into the path of scaffold, at the same time increase anti tank in internal stent, waterproof steps, multiple waterproof measures such as relief hole.The design not only saves the packaging cost, but also improves the product reliability, which has been widely used in outdoor led display products.The air tightness of the LED scaffold designed with bending structure was tested by SAM(Scanning Acoustic Microscope) after encapsulation and normal scaffold. The results showed that the product designed with bending structure had better air tightness.

2, chip

LED chip is the core of LED devices. Its reliability determines the life and luminescence performance of LED devices and even LED display.The cost of LED chips accounts for the largest total cost of LED components.As the cost decreases, LED chip size cutting becomes smaller and smaller, which also brings a series of reliability problems.

As the size decreases, the pad of P electrode and N electrode will also shrink. The shrinking of the pad will directly affect the quality of welding line, which will easily lead to gold ball disengaging and even the electrode itself disengaging in the process of encapsulation and use.At the same time, the distance between two pad will also be a narrow, this makes the electrode in the excessive increase of current density, current local concentrations at the electrode, and the uneven distribution of the current serious impact on the performance of the chip, the chip in the local high temperature, uneven brightness, easy leakage, electrode, and even low luminous efficiency, eventually led to the led display lower reliability.

3. Bonding lines

Bonding wire is one of the key materials of LED packaging. Its function is to realize the electrical connection between chip and pin, which plays the role of current import and export between chip and the outside world.Common bond lines used in LED device packaging include gold wire, copper wire, palladium plated copper wire and alloy wire.

(1) the gold thread.The gold wire is the most widely used and the process is the most mature, but the price is expensive, resulting in the high packaging cost of LED.

(2) copper wire.Copper wire instead of gold wire has the advantages of low cost, good heat dissipation and slow growth of intermetallic compounds in the welding process.The disadvantages are that copper is easy to oxidize, high hardness and high strain strength.Especially under the heating environment of the bonding copper ball firing process, the copper surface is easy to be oxidized, and the oxide film formed reduces the bonding performance of copper wire, which puts forward higher requirements for the process control in the actual production process.

(3) palladium - coated copper wire.In order to prevent the oxidation of copper wire, palladium - bonded copper wire has been paid more and more attention.Palladium-plated bonded copper wire has the advantages of high mechanical strength, moderate hardness and good soldering properties.

4, glue

At present, the glue of LED display device mainly includes epoxy resin and organosilicon.

(1) epoxy resin.Epoxy resin is prone to aging, humidity, poor heat resistance, and easy to change color under short wave light and high temperature. It is toxic in colloidal state, and the thermal stress does not match well with LED, which will affect the reliability and life of LED.So it‘s usually going to attack the epoxy.

(2) organosilicon.Compared with epoxy resin, organosilicon has high cost performance, excellent insulation, dielectric and compactness.But the drawback is the air tightness is poor, easy to absorb moisture.Therefore, it is rarely used in the packaging application of LED display device.

In addition, high quality LED display also requires special display effect.Some sealing plants use additives to improve the stress of the glue and achieve the effect of matte fog surface.