What is the reliability of LED packaging?

LED devices account for about 40% to 70% of LED display costs, and the significant drop in LED display costs is due to the lower cost of LED devices. The quality of LED package has a great influence on the quality of LED display. The key to package reliability includes the choice of chip materials, the choice of packaging materials and process control. As LED displays gradually penetrate into the high-end market, the quality requirements for LED display devices are also increasing.

The main material components used in LED display device packaging include brackets, chips, die attach adhesives, bond wires and encapsulants. SMD (Surface Mounted Devices) refers to surface mount package LEDs, mainly LEDs with PCB structure and LEDs with PLCC structure. This article focuses on TOP LEDs, and the SMD LEDs mentioned below all refer to TOP LEDs. The following is a brief introduction of some basic developments in China from the aspects of packaging materials.

LED bracket

(1) The role of the stent. The PLCC (Plastic Leaded Chip Carrier) bracket is the carrier of the SMD LED device and plays a key role in the reliability and light output of the LED.

(2) The production process of the bracket. PLCC bracket production process mainly includes metal strip punching, electroplating, PPA (polyphthalamide) injection molding, bending, five-sided three-dimensional inkjet and other processes. Among them, electroplating, metal substrates, plastic materials, etc. occupy the main cost of the stent.

(3) Improved structural design of the bracket. Since the PLCC bracket is physically combined with PPA and metal, the gap will become larger after the over-temperature reflow furnace, which will cause water vapor to easily enter the inside of the device along the metal channel and affect reliability.

In order to improve product reliability to meet the high-end market demand for high-quality LED display devices, some packaged factories have improved the structural design of the brackets, such as Foshan City, a photovoltaic company using advanced waterproof structural design, bending and stretching methods To extend the water vapor entry path of the bracket, and to increase the waterproofing measures such as waterproof grooves, waterproof steps, and water discharge holes inside the bracket. This design not only saves packaging costs, but also improves product reliability. It has been widely used in outdoor LED display products. The SAM (Scanning Acoustic Microscope) was used to test the airtightness of the LED bracket package and the normal bracket after the bending structure design. As a result, it was found that the product designed by the bending structure was more airtight.

chip

The LED chip is the core of the LED device, and its reliability determines the life and luminescence performance of the LED device and even the LED display. The cost of LED chips is also large in terms of the total cost of LED devices. As the cost decreases, LED chip size is getting smaller and smaller, and it also brings a series of reliability problems.

As the size shrinks, the pad of the P electrode and the N electrode also shrinks. The reduction of the electrode pad directly affects the quality of the wire, and it is easy to cause the gold ball to detach or even the electrode itself to detach during the packaging process and the use process, and eventually fails. At the same time, the distance a between the two pads will also shrink, which will cause the current density at the electrode to increase excessively, and the current will locally accumulate at the electrode, and the unevenly distributed current seriously affects the performance of the chip, causing the chip to have a local temperature. Excessively high, uneven brightness, easy leakage, off-electrode, and even low luminous efficiency ultimately lead to reduced reliability of led displays.

Bond wire

The bonding wire is one of the key materials of the LED package. Its function is to realize the electrical connection between the chip and the pin, and to function as the current and the current of the chip and the outside. Common bonding wires for LED device packages include gold wires, copper wires, palladium-plated copper wires, and alloy wires.

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

(2) Copper wire. The copper wire replaces the gold wire and has the advantages of low cost, good heat dissipation effect, and slow growth of intermetallic compounds during the wire bonding process. The disadvantage is that copper is easily oxidized, has high hardness and high strain strength. Especially in the heating environment of the bonded copper ball burning process, the copper surface is easily oxidized, and the formed oxide film reduces the bonding property of the copper wire, which puts higher requirements on the process control in the actual production process.

(3) Palladium-plated copper wire. In order to prevent oxidation of copper wires, palladium-bonded copper wires are gradually attracting attention from the packaging industry. Palladium-bonded copper wire has the advantages of high mechanical strength, moderate hardness, good soldering and ball bonding, and is very suitable for high-density, multi-pin integrated circuit packages.

glue

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

(1) Epoxy resin. Epoxy resin is easy to age, easy to be wet, and has poor heat resistance. It is easy to change color under short-wave illumination and high temperature. It has certain toxicity in colloidal state. Thermal stress and LED do not match very well, which will affect the reliability and life of LED. . Therefore, the epoxy resin is usually attacked.

(2) Silicone. Compared with epoxy resin, silicone has high cost performance, excellent insulation, dielectric properties and adhesion. But the disadvantage is that the air tightness is poor and it is easy to absorb moisture. Therefore, it is rarely used in the packaging application of LED display devices. ,

In addition, high-quality LED display also puts special requirements on the display effect. Some packaging plants use additives to improve the stress of the glue while achieving a matte matte finish.