LED devices account for about 40% to 70% of the cost of LED displays. The significant drop in the cost of LED displays is due to the cost reduction of LED devices. The quality of LED SMD Screen packaging has a greater impact on the quality of LED displays.
The key to packaging reliability includes chip material selection, packaging material selection and process control. In addition, strict reliability standards are also key to inspecting high-quality LED devices.
As LED displays gradually penetrate into the high-end market, the quality requirements for LED display devices are becoming higher and higher. This article discusses the key technologies for realizing high-quality LED display devices based on practical experience in packaging high-quality LED display devices.
Current status of LED display device packaging
SMD Screen (Surface Mounted Devices) refers to surface mount package structure LED, which mainly includes PCB board structure LED (Chip LED) and PLCC structure LED (TOP LED). This article mainly studies TOP LED. The SMD Screen LED mentioned in the following article refers to TOP LED.
There are eight main materials used in LED display device packaging: brackets, chips, die-bonding glue, bonding wires, packaging glue, and die-bonding glue. The following is an introduction to some basic domestic development status in terms of packaging materials.
1.1 LED bracket
(1) The role of the bracket
The PLCC (Plastic Leaded Chip Carrier) bracket is the carrier of LED SMD Screen 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 stent 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
Since 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 SMD Screen display devices, some packaging plants have improved the structural design of the bracket. For example, Mustang LED 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. In addition to saving packaging costs, this design improves the reliability of the product.
In outdoor LED displays, it is widely used. The SAM (Scanning Acoustic Microscope) was used to test the airtightness of the LED bracket with a bent structure design after packaging and the normal bracket. The results show that the airtightness of the product with a bent structure design is better.
1.2 Chip
The LED chip is the core of the LED SMD Screen device, and its reliability determines the life and luminous performance of the LED device and even the LED display. The cost of LED SMD Screen chips accounts for the largest share of the total cost of LED SMD Screen devices. As costs decrease, LED chip sizes are getting smaller and smaller, which also brings about a series of reliability issues.
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, which can easily cause the gold ball to detach and 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 increase excessively, and the current will accumulate locally 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.
1.3 Bonding wire
LED packaging relies heavily on bonding wire. 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. Wires made from gold, copper, palladium-plated copper, and alloys are commonly used.
(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 places 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.
1.4 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 the colloidal state. The thermal stress does not match the LED SMD Screen 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, and 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.