PCBA (Printed Circuit Board Assembly) refers to the entire manufacturing process of a bare PCB board undergoing SMT component mounting followed by DIP through-hole insertion. Also known as a printed circuit board, it is a critical electronic component that serves as a support for electronic parts and a provider of circuit connections for electronic components.  

Contact with water will cause short circuits on the board and rust on pin terminals, which gradually corrode metal parts and lead to poor electrical contact. In severe cases, it may even result in short-circuit burnout. Specifically, metal components on the board (e.g., copper traces, solder joints, component pins) are prone to oxidative corrosion when exposed to water, compromising the long-term stability and reliability of the circuit. Moisture can also damage or degrade the performance of sensitive components on the board such as capacitors and resistors.  

Therefore, water ingress not only impairs the performance of insulating materials on the board and reduces electrical performance, but also promotes the growth of microorganisms on the board, which in turn causes further corrosion and damage.  

To prevent such water-induced issues in existing technologies, a protective coating (e.g., silica gel, polyurethane) is typically applied to the PCBA board. This coating isolates moisture and air, thereby protecting the board from the adverse effects of humid environments.  

An excessively low surface energy of the PCBA will prevent the conformal coating from wetting the substrate. As shown in Figure 1, an overly high surface tension of the conformal coating hinders the wetting ability of the PCBA board, leading to an increased contact angle between the coating and the board surface. The applied coating then retracts, resulting in uneven film thickness, discontinuous and irregular edges, and poor coverage. Thus, improving the surface tension of the PCBA substrate is the fundamental solution to the anti-wetting defect of conformal coatings.

Figure 1 Anti-wetting Defects of Conformal Coating on PCBA

Chemical surface treatment methods such as electrochemical treatment and chemical oxidation cause a certain degree of environmental pollution, and surface damage is also difficult to control. With the growing demand for ecological protection, it is necessary to reduce the excessive consumption of chemical reagents, water resources and energy through multiple approaches (e.g., process optimization, material preparation, equipment development), and conduct research and application of green surface treatment technologies.  

Plasma cleaning is a physical surface modification method that involves no chemical reagents and has achieved significant development in recent years. It is a microscopic surface treatment technology with an extremely short action time: it modifies the microscopic morphology of the surface layer without causing damage to the internal substrate, making it an environmentally friendly surface modification method.  

Plasma Cleaning

Plasma is an ionized gas composed of ions, electrons, free radicals, photons and neutral particles, with approximately equal densities of positive ions and electrons. Plasma cleaning of PCBA achieves the removal of surface contaminants primarily through the activation effect of plasma-state substances.  

During the cleaning process, a high-frequency and high-voltage electric field is applied between the workpiece and the electrode plate. In the working atmosphere (usually O₂), electrons are accelerated to bombard and generate oxygen ions. Contaminants on the PCBA are oxidized to carbon dioxide and water, which are then removed under vacuum.  

The principle of plasma cleaning involves two key aspects:  

1. The surface layer is bombarded by plasma, forming new chemical structures such as active atomic groups. This process cleans the substrate surface and achieves a hydrophilization effect, while the attached polar groups increase the substrate surface tension, thereby enhancing its bondability.  

2. Plasma removes surface impurities and performs a certain degree of etching on the surface, increasing surface roughness and the contact area between the surface and the coating, which significantly improves the interfacial bonding strength.  

As a new type of cleaning technology, plasma cleaning offers numerous advantages:  

1. No cleaning agents required, resulting in substantial cost savings;  

2. No risk of damage to the PCBA board;  

3. Strong penetration ability, capable of cleaning complex structures (e.g., special components and various holes), making it particularly suitable for precision electronic components and recognized as the most thorough stripping cleaning method.  

Unlike solids, liquids and gases, plasma consists of high-energy and highly reactive particles such as electrons and ions, which readily react with solid surfaces. Plasma cleaning can effectively remove molecular-level contaminants on the surface layer, significantly increase surface tension, and improve wetting and bonding ability. It thus has an increasingly broad application prospect and is widely adopted in the manufacturing of avionics and electrical products, as well as the fabrication of microwave integrated circuits and assembly of hybrid microcircuit products. Plasma cleaning can remove contaminants on the internal surfaces of substrates or hybrid microcircuit products, enhance surface activity, and significantly improve product quality and reliability.