Packaging is a very important intermediate step in realizing the transition of LED from chip to practical use. The main purpose of LED packaging is reflected in two aspects: firstly, to protect the mechanical structure of the light-emitting chip and LED, making its structure more stable; secondly, to achieve the optical characteristics of LED through packaging.

Application of plasma cleaning for LED packaging

LED packaging has gone through various forms such as pin type, patch type, bracket type, and chip on board structure (COB). The LED packaging process directly affects the yield of LED products, and 99% of the main culprits for problems in the packaging process come from particle pollutants, oxides, epoxy resins and other pollutants on the chip and substrate. How to remove these pollutants has always been a concern for people. Plasma cleaning, as a cleaning process developed in recent years, provides an economically effective and environmentally friendly solution to these problems.

Plasma cleaning before SMT (Die Bonding)

Usually, there are many oxides or organic pollutants on the surface of the product to be cleaned. If the chip is directly bonded, it will cause incomplete bonding of the chip, resulting in void defects (Voidfail), and the heat dissipation ability of the packaged product will also be reduced, seriously affecting the reliability of chip packaging. Therefore, before chip bonding, plasma cleaning is used to remove organic matter and other pollutants from the chip surface, which can effectively enhance the surface activity and bonding ability of the product, reduce void defects, and thus improve the reliability and stability of the bonding process.

Plasma cleaning before wire bonding

Plasma cleaning is an important method to improve welding reliability. After the chip is attached to the substrate and cured at high temperature, the pollutants present on it may include micro particles and oxides. These pollutants cause incomplete soldering or poor adhesion between the leads and the chip and substrate through physical and chemical reactions, resulting in insufficient bonding strength. Performing plasma cleaning before wire bonding can not only clean the surface of the chip and lead frame of dirt and oxides, but also activate the surface of the wire bonding area, making the bonding surface more firm.

Plasma cleaning before LED sealing

During the process of injecting epoxy glue into LED, pollutants can cause a high bubble formation rate, resulting in low product quality and service life. Therefore, avoiding the formation of bubbles during the sealing process is also a concern for people. After plasma cleaning, the chip and substrate will be more tightly combined with the gel, greatly reducing the formation of bubbles and significantly improving heat dissipation and light emission rate.

MiniLED package plasma cleaning application

Plasma cleaning before solder paste printing

The solder paste printing process is one of the most important processes in the miniLED production line, and ensuring the quality of the printing process is very important. Before printing the solder paste, the surface of the solder pad may still be contaminated, corroded, and affected by the process environment, which not only affects the melting and forming of the solder paste, but may also cause more voids.

In addition to removing dirt from the surface of the solder pad, plasma cleaning can better improve the surface free energy of the solder pad during the welding process, significantly improve the wetting ability of the solder pad surface, enhance the fluidity of solder paste melting, and enable the solder paste to better adhere to the solder pad, thereby facilitating the discharge of gas and avoiding the formation of voids. This provides a guarantee for the mechanical strength and quality reliability of the solder layer.

Plasma cleaning before dispensing and film pressing

At present, COB packaging is the best packaging process for MiniLED displays, but traditional dispensing processes are becoming increasingly difficult to apply in situations where the number of chips is large and the spacing is small. In order to improve the efficiency and reliability of COB packaging, researchers have proposed various processes such as the "glue film method". The "glue film method" mainly involves first making a silicone film, then using a film sticking machine to attach the silicone film to the substrate, and finally performing high-temperature curing to complete the packaging. This process is simple, efficient, and suitable for large-scale production. The prepared glue film has a smooth surface and good light consistency, making it an ideal dispensing process.

The surface of untreated MiniLED light source boards usually has poor hydrophilicity, which is not conducive to the wetting and spreading of the adhesive solution. Plasma cleaning before dispensing and laminating can introduce polar functional groups such as hydroxyl and carboxyl groups on the surface of the material, increase its hydrophilicity, and better bond with the adhesive in the later stage, ensuring the quality and performance of the adhesive layer.

Plasma cleaning has obvious advantages, simple operation, controllable accuracy, the entire process does not require heating, does not produce any pollution, is safe and reliable, and has large-scale applications in the field of LED packaging. In the entire LED packaging process, the role of plasma cleaning mainly includes preventing packaging delamination, improving wire quality, increasing bonding strength, improving reliability, and increasing yield to save costs.