SMD (Surface Mounted Devices) refers to surface mount package structure LED, mainly including PCB board structure LED (ChipLED) and PLCC structure LED (TOP LED)The main materials used in LED display device packaging include brackets, chips, die-bonding adhesives, bonding wires and packaging adhesives, etc.
LED bracket
(1) The role of the bracket. The PLCC (Plastic Leaded Chip Carrier) bracket is the carrier of SMD LED devices and plays a key role in the reliability, light emission and other performance of LEDs.
(2) The production process of the stent. The 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. account for the main costs of the bracket.
(3) Structural improvement design of the bracket. Because the PPA and metal are physically combined in the PLCC bracket, the gap will become larger after being exposed to a high-temperature reflow oven, causing water vapor to easily enter the device along the metal channel, thus affecting reliability.
In order to improve product reliability and meet high-end market demand for high-quality LED display devices, some packaging plants have improved the structural design of the bracket. For example, Foshan Guoxing Optoelectronics Co., Ltd. has adopted advanced waterproof structural design, bending and stretching, etc. The method is to extend the water vapor entry path of the bracket, and at the same time add multiple waterproof measures such as waterproof grooves, waterproof steps, and drain holes inside the bracket, as shown in the figure. 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) test was used to test the airtightness of the LED bracket with a bent structure design after packaging and the normal bracket. The results showed that the airtightness of the product with a bent structure design was better.
chip
The LED chip is the core of the LED device, and its reliability determines the life and luminous performance of the LED device and even the LED display. The cost of LED chips accounts for the largest share of the total cost of LED devices. As costs decrease, LED chip sizes are getting smaller and smaller, which also brings about a series of reliability issues. The structure of the LED blue-green chip is shown in the figure.
As can be seen from the above figure, as the size shrinks, the pads of the P electrode and N electrode also shrink. The shrinkage of the electrode pad directly affects the quality of the bonding wire, and can easily cause the gold ball to detach or even the electrode itself to detach during the packaging process and use. eventually failed. At the same time, the distance a between the two pads will also shrink, which will cause the current density at the electrode to excessively increase, and the current will locally accumulate at the electrode. The unevenly distributed current will seriously affect the performance of the chip, causing local temperatures to appear on the chip. Problems such as excessive brightness, uneven brightness, easy leakage, electrode drop, and even low luminous efficiency will ultimately lead to reduced reliability of LED displays.
bonding wire
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 plays the role of introducing and exporting current between the chip and the outside world. Commonly used bonding wires for LED device packaging include gold wires, copper wires, palladium-plated copper wires, and alloy wires.
(1) Gold thread. Gold wire is the most widely used and has the most mature technology, but it is expensive, resulting in high LED packaging costs.
(2) Copper wire. Copper wire instead of gold wire has the advantages of cheapness, good heat dissipation effect, and slow growth of intermetallic compounds during the wire bonding process. The disadvantages are that copper is easily oxidized, has high hardness and high strain strength. Especially in the heating environment of the copper ball bonding process, the copper surface is easily oxidized, and the oxide film formed reduces the bonding performance of the copper wire, which puts higher requirements on process control during the actual production process.
(3) Palladium-plated copper wire. In order to prevent oxidation of copper wires, palladium-plated bonded copper wires have gradually attracted attention from the packaging industry. Palladium-plated bonding copper wire has the advantages of high mechanical strength, moderate hardness, and good welding properties, and is very suitable for high-density, multi-pin integrated circuit packaging.
glue
At present, the glue used for LED display device packaging mainly includes epoxy resin and silicone.
(1) Epoxy resin. Epoxy resin is easy to age, susceptible to moisture, and has poor heat resistance. It is also easy to change color under short-wave light and high temperatures. It has a certain degree of toxicity in its colloidal state. The thermal stress does not match the LED very well, which will affect the reliability and life of the LED. . So epoxy resins are usually attacked.
(2) Silicone. Compared with epoxy resin, silicone has higher cost performance, excellent insulation, dielectric and adhesion properties. But the disadvantage is that it has poor air tightness and easily absorbs moisture. Therefore, it is rarely used in packaging applications of LED display devices.
In addition, high-quality LED displays also put forward special requirements for display effects. Some packaging factories use additives to improve the stress of the glue and achieve a matte finish.
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