Two or three things that must be said about SMD packaging of LED display

LED devices account for about 40% to 70% of the cost of LED displays, and the substantial decline in the cost of LED displays is due to the reduction in the cost of LED devices. The quality of LED packaging has a great influence on the quality of LED display. The key to packaging reliability includes the selection of chip materials, packaging materials and process control. In addition, strict reliability standards are also the key to verifying high-quality LED devices.

With the gradual penetration of LED displays into the high-end market, the quality requirements for LED display devices are also getting higher and higher. Based on the actual experience of high-quality LED display device packaging, this article discusses the key technologies for realizing high-quality LED display devices.

1 Current status of LED display device packaging

SMD (Surface Mounted Devices) refers to surface-mounted package structure LEDs, mainly including PCB board structure LEDs (ChipLEDs) and PLCC structure LEDs (TOP LEDs). This article mainly studies TOP LEDs, and the SMD LEDs mentioned in the following text all refer to TOP LEDs.

The main materials used in the packaging of LED display displays devices include brackets, chips, die-bonding adhesives, bonding wires, and packaging adhesives. The following introduces some basic development status in China from the aspect of packaging materials.

1.1 LED bracket
(1) The role of the bracket. The PLCC (Plastic Leaded Chip Carrier) bracket is the carrier of the SMD LED device, which plays a key role in the reliability and light output of the LED.
(2) The production process of the bracket. 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. occupy the main cost of the bracket.
(3) Structural improvement design of the bracket. Because the combination of PPA and metal in the PLCC bracket is a physical combination, 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 the high-quality LED display devices demanded by the high-end market, some packaging factories have improved the structure design of the bracket. For example, Foshan Guoxing Optoelectronics Co., Ltd. adopts advanced waterproof structure 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 water holes inside the bracket, as shown in Figure 1. This design not only saves packaging costs, but also improves product reliability. It has been widely used in outdoor LED display products. SAM (Scanning Acoustic Microscope) was used to test the airtightness of the bent structure designed LED bracket after encapsulation and the normal bracket, and it can be found that the product with the bent structure design has better air tightness.

1.2 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 reduction of cost, the cutting of LED chip size is getting smaller and smaller, which also brings a series of reliability problems. The structure of the LED blue-green chip.

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 it is easy to cause the gold ball to detach during the packaging process and the use process, and even the electrode itself to detach, eventually failing. At the same time, the distance a between the two pads will also be reduced, which will cause the excessive increase of the current density at the electrode, the current will gather locally at the electrode, and the uneven distribution of current will seriously affect the performance of the chip, causing the chip to appear local temperature Too high, uneven brightness, easy leakage, electrode drop, and even low luminous efficiency will eventually lead to a decrease in the reliability of the LED display.

1.3 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 to play the role of current import and export 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 WIRE. Gold wire is the most widely used and 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 low cost, good heat dissipation effect, and slow growth of intermetallic compounds 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 firing ball process, the copper surface is easily oxidized, and the formed oxide film reduces the bonding performance of the 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 the oxidation of copper wires, palladium-plated bonding copper wires have gradually attracted the attention of the packaging industry. Palladium-plated bonding copper wire has the advantages of high mechanical strength, moderate hardness, and good solderability into balls, which 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, easy to get wet, poor heat resistance, and easy to change color under short-wave light and high temperature. It has certain 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. . So the 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 poor air tightness, easy to absorb moisture. So it is rarely used in the packaging application 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 at the same time achieve a matte matte effect.