LED devices account for about 40% to 70% of the cost of LED display screens. The significant drop in the cost of LED display screens is due to the cost reduction of LED devices. The quality of the LED packaging has a greater impact on the quality of the LED display. The key to packaging reliability includes the selection of chip materials, the selection of packaging materials, and process control. As LED displays gradually penetrate into the high-end market, the quality requirements for LED display devices are getting higher and higher
The main materials used in LED display device packaging include brackets, chips, die-bonding glue, bonding wires and packaging glue. SMD (Surface Mounted Devices) refers to a surface-mounted package structure LED, mainly including PCB board structure LED (ChipLED) and PLCC structure LED (TOP LED).
What factors affect the packaging reliability of LED lamp beads
One LED bracket
(1) The role of the stent. The PLCC (Plastic Leaded Chip Carrier) bracket is the carrier of SMD LED devices, which plays a key role in the reliability and light output of the LED.
(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. occupy the main cost of the bracket.
(3) Improved design of the support structure. Because the PLCC bracket is a physical combination of PPA and metal, the gap will become larger after the high temperature reflow oven, which will cause water vapor to easily enter the device along the metal channel and affect 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, an Optoelectronic Co., Ltd. uses advanced waterproof structure design, bending and stretching methods to extend The water vapor enters the path of the bracket, and at the same time, multiple waterproof measures such as waterproof grooves, waterproof steps, and water drainage holes are added to the inside of the bracket.
This design not only saves packaging costs, but also improves product reliability. It has been widely used in outdoor LED display products. Through SAM (Scanning AcousTIc Microscope) to test the airtightness of the LED stent with the bending structure design after packaging and the normal stent, it can be found that the product with the bending structure design has better airtightness.
Chip LED chips are the core of LED devices, and their reliability determines the life and luminous performance of LED devices and even LED displays. The cost of the LED chip accounts for a large amount of the total cost of the LED device. 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 and N electrodes also shrink. The shrinking of the electrode pads directly affects the quality of the bonding wire, which easily causes the gold ball to detach and even the electrode itself detaches during the packaging process and use, and eventually fails.
At the same time, the distance a between the two pads will also be reduced, which will cause an excessive increase in the current density at the electrode, and the current will be locally concentrated at the electrode, and the unevenly distributed current will seriously affect the performance of the chip, causing the chip to have a local temperature. Excessively high, uneven brightness, easy leakage, electrode drop, and even low luminous efficiency, * eventually lead to reduced reliability of the led display.
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 pin, and play the role of introducing and deriving the 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.
What factors affect the packaging reliability of LED lamp beads
(1) Gold thread. The gold wire is widely used and the technology is mature, but the price is expensive, which leads to the high cost of LED packaging.
(2) Copper wire. Copper wire instead of gold wire has the advantages of low cost, good heat dissipation effect, and slow growth of intermetallic compound during wire bonding. The disadvantage is that copper is easy to oxidize, has high hardness and high strain strength. Especially in the heating environment of the bonding copper ball firing process, the copper surface is extremely easy to oxidize, and the formed oxide film reduces the bonding performance of the copper wire, which puts forward higher requirements for process control in the actual production process.
(3) Palladium-plated copper wire. In order to prevent oxidation of the copper wire, palladium-plated bonding copper wire has gradually attracted attention from the packaging industry. Palladium-plated bonded copper wire has the advantages of high mechanical strength, moderate hardness, and good ball-forming properties. It is very suitable for high-density, multi-pin integrated circuit packaging.
Glue At present, the glue for LED display device packaging mainly includes epoxy resin and silicone:
(1) Epoxy resin. Epoxy resin is easy to age, easy to be wet, poor in heat resistance, and easy to change color under short-wave light and high temperature. It has a certain degree of toxicity in the colloidal state. The thermal stress does not match the LED very well, which will affect the reliability and life of the LED. . Therefore, epoxy resin is usually attacked.
(2) Silicone. Compared with epoxy resin, silicone has higher cost performance, excellent insulation, dielectric and adhesion. But the disadvantage is that the air tightness is poor, and it is easy to absorb moisture. So it is rarely used in the packaging applications of LED display devices.
In addition, high-quality LED displays also have special requirements for display effects. Some packaging factories use additives to improve the stress of the glue and achieve the effect of matte matte surface at the same time.
