Today, with the rapid development of science and technology, the replacement of electronic products is getting faster and faster, and the technology of LED lights is also constantly developing, which is colorfully decorated for our cities. LED devices account for about 40% to 70% of the cost of LED display screens, and the significant reduction in the cost of LED display screens is due to the reduction in the cost of LED devices. The quality of the LED package has a greater impact on the quality of the LED display. The key to packaging reliability includes chip material selection, packaging material selection, and process control. As the LED display screen gradually penetrates into the high-end market, the quality requirements of the LED display device are becoming higher and higher.
The main material components used for LED display device packaging include brackets, chips, die bonding glue, bonding wires and packaging glue. SMD refers to LEDs with surface-mount packaging structure, mainly LEDs with PCB structure and LEDs with PLCC structure. This article mainly studies TOPLED. SMDLED mentioned below refers to TOPLED. The following is a brief introduction of some basic domestic developments in terms of packaging materials.
The role of the bracket. The PLCC bracket is the carrier of the SMDLED device, which plays a key role in the reliability and light output of the LED.
The production process of the stent. The production process of PLCC bracket mainly includes metal strip punching, electroplating, PPA injection, bending, five-sided three-dimensional inkjet and other processes. Among them, electroplating, metal substrates, plastic materials, etc. occupy the main cost of the bracket.
The structure of the bracket improves the design. The PLCC bracket is a physical combination of PPA and metal, and the gap will become larger after the high temperature reflow furnace, resulting in water vapor easily entering the device along the metal channel and affecting reliability.
In order to improve product reliability and meet high-end market demand for high-quality LED display devices, part of the packaged into a factory to improve the structural design of the bracket, such as Foshan City, a photovoltaic company uses advanced waterproof structure design, bending and stretching methods To extend the water vapor entry path of the bracket, and add multiple waterproof measures such as waterproof grooves, waterproof steps, and water 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. After testing the airtightness of the LED bracket designed by the bending structure and the normal bracket through SAM, it can be found that the product with the bending structure design has better airtightness.
The LED chip is the core of the LED device, and its reliability determines the life and light-emitting performance of the LED device and even the LED display screen. The cost of LED chips also accounts for the total cost of LED devices. As the cost decreases, the size of LED chips is getting smaller and smaller, and it also brings a series of reliability problems. As the size shrinks, the pads of the P electrode and the N electrode also shrink. The shrinkage of the electrode pad directly affects the quality of the bonding wire, which may easily lead to the separation of the gold ball or even the electrode itself during the packaging process and the use process, and eventually fail.
At the same time, the distance a between the two pads will also decrease, which will cause the current density at the electrode to increase excessively, and the current locally gathers at the electrode, and the unevenly distributed current seriously affects the performance of the chip, causing the chip to have local temperature Problems such as too high, uneven brightness, easy leakage, electrode drop, and even low luminous efficiency will eventually reduce the reliability of the led display.
The bonding wire is one of the key materials for LED packaging. Its function is to realize the electrical connection between the chip and the pins, and it plays the role of the current import and export of the chip and the outside world. Common bonding wires for LED device packaging include gold wires, copper wires, palladium-plated copper wires, and alloy wires.
Gold Line. Gold wire is widely used and the process is mature, but the price is expensive, which leads to the high cost of LED packaging.
Copper wire. Copper wire instead of gold wire has the advantages of low cost, good heat dissipation, and slow growth of intermetallic compounds during the wire bonding process. The disadvantage is that copper exists easy oxidation, 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 performance of the copper wire, which places higher requirements on process control in the actual production process.
Palladium-plated copper wire. In order to prevent the copper wire from oxidizing, palladium-plated bonded copper wire has gradually attracted the attention of the packaging industry. The palladium-plated bonded copper wire has the advantages of high mechanical strength, moderate hardness, and good solderability to form a ball. It is very suitable for high-density, multi-lead integrated circuit packaging.
At present, the glue for LED display device packaging mainly includes epoxy resin and silicone.
Epoxy resin. Epoxy resin is easy to aging, susceptible to moisture, poor heat resistance, and easy to change color under short wave light and high temperature. It has certain toxicity in the state of colloid. The thermal stress does not match the LED very well, which will affect the reliability and life of the LED . Therefore, the epoxy resin is usually attacked.
Silicone. Compared with epoxy resin, silicone has higher cost performance, excellent insulation, dielectricity 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 packaging applications of LED display devices.
In addition, high-quality LED display screens also put forward special requirements for the display effect. Some packaging factories use additives to improve the stress of the glue, while achieving the effect of matte matte. No appearance damage and mark damage Soak in IPA solvent for 5 ± 0.5 minutes, dry at room temperature for 5 minutes, then wipe 10 times. I believe that in the future when science and technology are more developed, LED will bring greater convenience to our lives in a variety of ways, which requires our scientific researchers to work harder to learn knowledge, so as to contribute to the development of science and technology the power of.