Driven by the explosive surge in demand for AI computing power, the global optical communication module market is undergoing rapid expansion. High-speed optical modules such as 800G and 1.6T impose sub-micron-level requirements on manufacturing precision and long-term reliability. As the core packaging process that interconnects optical chips and PCBA substrates, wire bonding directly determines the transmission performance and service life of optical modules. Nevertheless, organic contaminants, oxides and particulate residues on PCBA bonding pads drastically degrade bonding strength, triggering failures such as poor bonding and bond lift-off. Featuring dry processing, low-temperature treatment and non-contact operation, plasma cleaning has become an indispensable surface pretreatment step prior to wire bonding for optical module packaging.

Chip-on-Board Packaging & Wire Bonding

(a) PCB substrate for COB packaging; (b) & (c) Microscopic images of wire bonding

Wire bonding is a process that electrically interconnects chips and external circuits by bonding fine wires between chips and PCB pads. Its working mechanism relies on pressure, thermal energy or ultrasonic vibration to break the oxide film and contaminants on metallic pads and induce plastic deformation. This brings the bonding wire and pad contact surface within atomic attraction distance, forming permanent interconnection via atomic diffusion.

A prevailing defect of conventional wire bonding stems from copper pads on PCBA substrates: copper exhibits poor chemical stability and readily oxidizes to form a surface oxide layer. This film drastically reduces bonding strength and frequently causes incomplete bonding. Therefore, an additional plasma cleaning step before wire bonding can significantly enhance wire bonding quality.

Mechanism of Plasma Cleaning

Unlike traditional wet cleaning using chemical solvents, plasma cleaning utilizes an RF power supply to excite process gases (typically argon, oxygen or argon-hydrogen mixed gas) inside a vacuum chamber, generating active plasma composed of high-energy electrons, ions, free radicals and ultraviolet photons. High-energy particles within the plasma deliver physical sputtering and chemical reactions to clean and activate material surfaces.

Physical Sputtering (Micro Sandblasting Effect)
Taking argon (Ar) as an example, high-energy argon ions accelerated by an electric field bombard pad surfaces. This effectively strips adsorbed micro-particles and breaks loose oxide layers, reshaping the surface roughness of gold pads and expanding the effective bonding area.

Chemical Cleaning (Reduction Reaction)
Hydrogen (H₂) is critical for oxide removal. Active hydrogen free radicals generated by hydrogen plasma possess strong reducibility, which reduces metal oxides on pad surfaces and restores the clean, active state of the underlying metal layer.

Surface Activation
Beyond surface purification, plasma treatment breaks surface chemical bonds to create dangling bonds and active sites, drastically raising surface energy. High surface energy greatly facilitates the spreading and wetting of molten gold balls, directly increasing the contact area and atomic diffusion depth of bonding joints.

Water contact angle tests conducted on PCBA pads before and after plasma cleaning demonstrate an obvious contact angle reduction from 90° to 10°. Plasma treatment thoroughly eliminates particulate matter and surface contaminants on PCBA pads, boosting wire bonding strength and suppressing void formation at bonding points. This greatly improves the long-term reliability of bonding joints during subsequent manufacturing processes and field operation.

Comparison of Hydrophilicity of Optical Module PCBA Pads Before and After Plasma Cleaning