Deep analysis LED display SMD package
LED devices account for about 40% ~ 70% of the cost of LED display, LED display cost is greatly reduced due to the cost reduction of LED devices. 5 minutes to learn about SMD LED. LED encapsulation quality to influence the quality of LED display. The key of package reliability includes the choice of chip material, package material and process control. In addition, strict reliability standards are also the key to testing high-quality LED devices.
With the gradual penetration of LED display to the high-end market, the quality requirements of LED display devices are increasingly high. Based on the practical experience of high quality LED display device packaging, this paper discusses the key technologies to achieve high quality LED display device.
Current status of LED display device packaging
SMD(Surface Mounted Devices) refers to surface mounted packaging structure LED, which mainly includes PCB board LED(ChipLED) and PLCC LED(TOP LED). This paper mainly studies TOP LED, and the SMD LED mentioned below all refer to TOP LED. The main materials used in the packaging of LED display devices include bracket, chip, solid adhesive, bonding line and packaging adhesive. The following from the packaging material aspect to introduce the current domestic some basic development status.
1. LED support
(1) The role of the support. The PLCC(Plastic Leaded Chip Carrier) bracket is the carrier of SMD LED devices, which plays a key role in the reliability and optical performance of LED.
(2) The production process of the support. The production process of PLCC bracket mainly includes metal strip punching, electroplating, PPA(polyphthalamide) injection molding, bending, five-sided three-dimensional inkjet and other processes. Among them, electroplating, metal substrate, plastic materials and so on occupy the main cost of support.
(3) Structural improvement design of the support. Due to the physical combination of PPA and metal in PLCC bracket, the gap will become larger after the overhigh temperature reflux furnace, which leads to water vapor easily entering the device along the metal channel, thus affecting the reliability.
In order to improve the reliability of the products to meet the high-end market demand for high-quality LED display devices, some of them are packaged into the factory to improve the structural design of the bracket. Sostron adopts advanced waterproof structural design, bending and stretching methods to extend the water vapor entry path of the bracket, and at the same time adds waterproof measures such as water proof sink, waterproof step and water drain hole inside the bracket. See Figure 1. This design not only saves packaging cost, but also improves product reliability. It has been widely used in outdoor LED display products. SAM(Scanning Acoustic Microscope) tests the air tightness of the packaged LED support designed with a bending structure and the normal support, and the results show that the products designed with a bending structure have better air tightness, as shown in Figure 2.
2. Chip
LED chip is the core of LED device, and its reliability determines the life and luminescence performance of LED device and even LED display. LED chip cost accounts for the total cost of LED devices is also the largest. With the reduction of cost, LED chip size cutting is getting smaller and smaller, but also brings a series of reliability problems. The structure of LED blue-green chip is shown in Figure 3.
As can be seen from Figure 3, the pad of P electrode and N electrode also shrink with the reduction of size. The reduction of electrode pad directly affects the quality of welding wire, which is easy to lead to the detachment of the golden ball or even the electrode itself in the process of packaging and use, and eventually failure. At the same time, the distance a between the two pads will also shrink, which will increase the current density at the electrode excessively, and the current will gather locally at the electrode. The uneven distribution of current will seriously affect the performance of the chip, resulting in high local temperature, uneven brightness, easy leakage, electrode loss, and even low luminous efficiency. Eventually, LED display reliability decreases.
3. Bonding line
Bond wire is one of the key materials of LED packaging, its function is to realize the electrical connection between the chip and the pin, and plays the role of current import and export between the chip and the outside world. LED device packaging commonly used bonding wire including gold wire, copper wire, palladium plated copper wire and alloy wire.
(1) Gold thread. Gold wire is the most widely used, the most mature technology, but the price is expensive, resulting in high packaging cost of LED.
(2) Copper wire. Copper wire instead of gold wire has the advantages of cheap, good heat dissipation effect, intermetallic compound growth several degrees slow in the welding process. The disadvantage of copper is easy oxidation, high hardness and high strain strength. Especially in the heating environment of bonded copper ball process, the surface of copper is easy to oxidize, and the formed oxide film reduces the bonding property of copper wire, which puts forward higher requirements for the process control in the actual production process.
(3) palladium plated copper wire. In order to prevent copper wire oxidation palladium bonded copper wire has been paid more and more attention in packaging field. The palladium-plated copper wire has the advantages of high mechanical strength, moderate hardness, good sphericity and so on. It is very suitable for high density and multi-pin integrated circuit packaging.
4. Glue
At present, LED display device packaging glue mainly includes epoxy resin and silicone two types.
(1) epoxy resin. Epoxy resin is easy to aging, easy to be wet, poor heat resistance, and short-wave light and high temperature easy to change color, in the colloid state has a certain toxicity, thermal stress and LED is not very match, will affect the reliability and life of LED. So it’s common to attack epoxy resins.
(2) organosilicon. Compared with epoxy resin, silicone has higher cost performance, excellent insulation, dielectric and tightness. But the disadvantage is poor air tightness, easy to absorb moisture. Therefore, it is rarely used in the packaging application of LED display devices.
In addition, high quality LED display also put forward special requirements for the display effect. Some packaging factories use additives to improve the stress of glue, while achieving the effect of matte mist.
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