LED display manufacturers tell you that led devices account for about 40% to 70% of the cost of LED electronic screen. The sharp reduction of the cost of LED electronic screen is due to the reduction of the cost of LED devices. The quality of LED packaging has a great influence on the quality of LED electronic screen. The core of package reliability is chip material selection, package material selection and process control. With the LED electronic screen gradually entering the high-end market, the quality requirements of LED electronic screen equipment are higher and higher.
The main material components used in the packaging of LED display devices include bracket, chip, solid adhesive, bonding wire and packaging adhesive. Surface mount device (SMD) refers to the surface mount package structure led, which mainly includes the chip LED with PCB structure and the top led with PLCC structure.
This paper mainly studies the top led, the SMD LED mentioned below refers to the top led. The present situation of basic development in China is introduced from the aspect of packaging materials.
01 LED bracket
(1) The function of stent. PLCC bracket is the bracket of SMD LED device, which plays an important role in the reliability and luminous equivalent energy of LED.
(2) Support production process. PLCC stent production process mainly includes punching, electroplating, PPA injection molding, bending, five dimensional ink jet and other processes. Among them, electroplating, metal substrate and plastic materials account for the main cost of the bracket.
(3) The structure of the bracket is improved. PLCC support is the physical combination of PPA and metal, so after high temperature reflux furnace, the gap becomes larger, so that water vapor is easy to flow into the equipment along the metal channel, thus affecting the reliability.
In order to improve the reliability of products and meet the requirements of advanced market, high quality LED display equipment extends the water vapor entry path of the bracket through advanced waterproof structure design, bending and stretching, and adds waterproof tank, waterproof steps, waterproof holes and other waterproof measures inside the bracket to improve the structural design of the bracket.
The design not only improves the packaging cost, but also improves the reliability of the product, which is widely used in outdoor LED electronic screen products.
At the same time, the distance a between the two pads is also reduced, which will lead to the current density on the electrode is too large, the current is gathered locally on the electrode, the uneven distribution of current, the performance of the chip is greatly affected, the local temperature on the chip is too high, the brightness is not uniform, the leakage is easy, the electrode falls off, and even the luminous efficiency is low, thus reducing the reliability of the LED electronic screen.
03 bonding wire
Bonding wire is one of the core materials of LED packaging, which enables the electrical connection between chip and pin, and imports and exports chip and external current. The bonding wires commonly used in LED device packaging include gold wire, copper wire, copper wire and gold wire.
(1) Gold thread. Gold wire is widely used and the process is mature, but the price is expensive, which leads to the high packaging cost of LED.
(2) Copper wire. Copper wire instead of gold wire has the advantages of low cost, good heat dissipation and slow growth of intermetallic compounds in the welding process. The disadvantages are high oxidation, hardness and deformation strength of copper. Especially in the heating environment of copper bonding ball process, the copper surface is easy to oxidize. As a result, the oxide film reduces the bonding performance of copper wire, which puts forward higher requirements for the process control of actual production process.
(3) Palladium plated copper wire. In order to prevent copper wire from oxidation, palladium plated copper wire has gradually attracted the attention of packaging industry. Palladium plated copper wire has the advantages of mechanical strength, appropriate hardness and good soldering configuration, which is suitable for high-density, multi pin integrated circuit packaging.
At present, the adhesive for LED display device packaging is mainly composed of epoxy resin and silicon.
(1) Epoxy resin. Epoxy resin is easy to aging, easy to damp, low heat resistance, easy to change color under short wave illumination or high temperature, toxic in gum state, thermal stress does not match with LED, which will affect the reliability and life of LED. So it usually attacks epoxy.
(2) Silicon. Compared with epoxy resin, silicon has high cost performance, good insulation, good heredity and adhesion. But the disadvantage is low confidentiality, easy to absorb moisture. Software package applications for LED display devices are rarely used.
In addition, high quality LED electronic screen has special requirements for display effect