In the manufacturing process of printed circuit boards (PCBs), the quality of surface treatment directly determines the reliability and performance of final products. However, the surfaces of PCB substrates (such as FR-4 and high-frequency board materials) contain various adverse factors that impair the adhesion of subsequent coatings. During processing, contaminants including fingerprints, dust, and release agents tend to remain on the substrate surface. Meanwhile, polymer substrates like FR-4 feature low surface energy, which hinders the sufficient wetting and spreading of liquid coating materials. These intertwined surface defects act as a critical bottleneck restricting the coating adhesion quality of PCBs.

Traditional PCB surface treatment methods mainly include mechanical grinding and chemical cleaning. Mechanical grinding may damage fine circuits on PCBs and is incompatible with high-density interconnect boards. Chemical cleaning relies on organic solvents, which poses environmental pollution risks and may leave chemical residues on the board surface. Furthermore, the above methods have limited effects on improving substrate surface energy and cannot fundamentally enhance coating adhesion. Therefore, improving the surface energy of PCB substrates to strengthen coating adhesion has become an urgent technical problem to be solved.

Plasma Cleaning Technology

Plasma cleaning is an advanced surface treatment technology that realizes surface cleaning, activation and modification of materials through plasma action. Compared with traditional wet cleaning, it features dry processing, environmental friendliness, high precision, controllable performance and zero obvious chemical residues.

Plasma is commonly known as the fourth state of matter. Gas ionized under specific energy conditions forms plasma containing active components such as electrons, ions, free radicals and excited-state particles. These highly active particles exert synergistic physical bombardment and chemical reaction effects on material surfaces, achieving efficient cleaning and activation of PCB surfaces.

Physical bombardment effect: High-energy active particles (ions and electrons) in plasma are accelerated by an electric field and bombard the PCB surface at high speed. The high-energy particle flow effectively removes the surface weak boundary layer and strips micro dust particles and micro contaminants with a thickness of approximately 3–5 nm. Meanwhile, the selective etching of plasma creates nano-scale concave-convex microstructures on the surface, increasing surface roughness and specific surface area by 15%–30%. This provides abundant physical anchoring points for subsequent coatings such as solder mask ink and protective paint, thereby strengthening the mechanical interlocking force.

Chemical reaction effect: Plasma is rich in highly active free radicals generated by the ionization of process gases, including oxygen radicals (O·), hydroxyl radicals (·OH) and nitrogen radicals (N·). These active free radicals react with molecular chains on the PCB surface through oxidation and grafting reactions, introducing polar functional groups such as hydroxyl (-OH), carboxyl (-COOH) and amino (-NH₂) on the cleaned surface. This chemical modification transforms the PCB surface from a low-energy hydrophobic state to a high-energy hydrophilic state, with the surface energy increasing to over 38–42 dyne/cm and the water contact angle decreasing significantly. Accordingly, the wettability and spreadability of coating materials on the substrate surface are greatly improved.

Benefiting from the synergistic effect of physical bombardment and surface chemical reactions, plasma-cleaned PCBs exhibit higher dyne values and lower water contact angles. In particular, for complex PCBs with fine circuits, blind holes and through holes, the gas-phase penetration characteristic of plasma enables dead-angle-free cleaning and activation of the inner walls of three-dimensional structures, breaking the limitations of traditional planar treatment methods. The plasma-treated PCB surfaces provide ideal interfacial conditions for the firm adhesion of subsequent solder mask ink, protective paint and other coatings. This significantly enhances coating bonding strength and effectively prevents coating failures such as peeling and blistering under thermal stress and environmental aging conditions.