Teach you fire eyes and golden eyes to distinguish right and wrong

- Jan 27, 2020-

LED devices account for about 40% to 70% of the cost of LED display screens. The significant reduction in the cost of LED display screens benefits from the cost reduction of LED devices. The quality of the LED package has a greater impact on the quality of the LED display. The key to package reliability includes the selection of chip materials, the selection of packaging materials, and process control. In addition, strict reliability standards are the key to testing high-quality LED devices.


As the LED display screen gradually penetrates into the high-end market, the quality requirements for LED display devices are becoming higher and higher. This article discusses the key technologies to achieve high-quality LED display devices based on the practical experience of high-quality LED display device packaging.


Status of LED display device packaging


SMD (Surface Mounted Devices) refers to surface-mount package structure LEDs, which mainly include PCB board structure LEDs (ChipLED) and PLCC structure LEDs (TOP LED). This article focuses on TOP LEDs. The SMD LEDs mentioned below refer to TOP LEDs.


The main materials used in LED display device packaging include brackets, chips, die-bonding glue, bonding wires, and packaging glue. Here are some basic developments in China from the perspective of packaging materials.


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 and light output of LEDs.


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


(3) Improved design of the structure of the bracket. The PLCC bracket is a physical combination of PPA and metal. After the high temperature reflow furnace, the gap will become larger, which will cause water vapor to easily enter the device along the metal channel and affect the reliability.


In order to improve the reliability of the product and meet the high-end market demand for high-quality LED display devices, some packaging factories have improved the structural design of the bracket. For example, Foshan Guoxing Optoelectronics Co., Ltd. uses advanced waterproof structure design, bending and stretching, etc. Methods 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, drain holes, etc., 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) was used to test the airtightness of the LED bracket after the packaging structure design and the normal bracket.


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 total cost of LED devices. With the cost reduction, LED chip size cutting is getting smaller and smaller, and it also brings a series of reliability problems.


As the size is reduced, the pads of the P electrode and the N electrode are also reduced. The reduction of the electrode pad directly affects the quality of the bonding wire, and it is easy to cause the gold balls to detach or even the electrodes to detach during the packaging process and use, and eventually fail. At the same time, the distance a between the two pads will also be reduced. This will cause the current density at the electrodes to increase excessively, and the current will locally accumulate at the electrodes. The unevenly distributed current will seriously affect the performance of the chip, making the chip appear local temperature Problems such as high brightness, uneven brightness, easy leakage, electrode loss, and even low luminous efficiency eventually reduce the reliability of the LED display.


Bonding wire


Bonding wire is one of the key materials of LED packaging. Its function is to realize the electrical connection between the chip and the pins, and it plays the role of the current introduction and export 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 thread. Gold wire is the most widely used and the most mature technology, but it is expensive, which leads to excessive packaging costs for LEDs.


(2) 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. Disadvantages are that copper is susceptible to oxidation, high hardness, and high strain strength. Especially in the heating environment of the copper-copper sintering 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 the process control in 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. The palladium-plated bonded copper wire has the advantages of high mechanical strength, moderate hardness, and good soldering properties, and 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, and has poor heat resistance. It is easy to change color under short-wave light and high temperature. It has certain toxicity in the gel state. The thermal stress does not match the LED, which will affect the reliability and life of the LED . Therefore, epoxy resin is usually attacked.


(2) Organic silicon. Compared with epoxy resin, silicone has higher cost performance, excellent insulation, dielectric properties and adhesion. However, the disadvantage is that the air tightness is poor and it is easy to absorb moisture. So it is rarely used in packaging applications of LED display devices.


In addition, high-quality LED displays have special requirements for display effects. Some packaging factories use additives to improve the stress of the glue, while achieving the effect of matte matte.