In the manufacturing process of multi-layer high-frequency printed circuit boards, the surface characteristics of polytetrafluoroethylene materials bring significant challenges to the metallized hole process. Due to the inherent low surface energy characteristics of PTFE, it is difficult to effectively improve the surface microstructure of PTFE by traditional electroless copper plating process or conventional potassium permanganate surface treatment, which directly affects the quality of metallized holes. Therefore, the surface modification of PTFE substrate before pore metallization is the key. The surface modification methods of PTFE high frequency microwave plate include sodium naphthalene solution treatment, melting treatment at high temperature, laser irradiation and plasma surface treatment.

Sodium naphthalene solution treatment

Sodium naphthalene solution treatment is widely used in PTFE surface modification. Sodium naphthalene solution can easily and quickly destroy the groups on the surface of PTFE and change the wettability of PTFE surface. The mechanism of action is to use a unique corrosion solution to react with the PTFE substrate, thereby stripping the fluorine atoms at both ends of the surface substrate, thereby forming a carbonized structure with active sites on the surface. Fourier transform infrared spectroscopy ( FTIR ) characterization results showed that the surface of the treated material successfully introduced polar functional groups such as hydroxyl ( -OH ), carbonyl ( C = O ), and carbon-carbon unsaturated bonds. The introduction of these chemical groups significantly increases the surface free energy of the material, significantly reduces the contact angle, and greatly improves the affinity of the surface to liquid wetting behavior. Sodium naphthalene solution treatment has the advantages of general efficiency and simple operation, and the modification effect is obvious. However, the sodium naphthalene solution needs to use toxic tetrahydrofuran as a solvent, and it will emit a strong irritating odor when used. In addition, the sodium naphthalene solution is expensive, the modified waste liquid is not easy to handle, and it will change the original color of the substrate.

Plasma surface treatment

Plasma surface treatment technology is a dry modification method based on the principle of gas discharge. The technology ionizes the working gas by applying an electric field ( glow discharge, corona discharge, high frequency or microwave excitation ) in a low-pressure environment to form a plasma containing high-energy electrons, excited molecules, active radicals and various ions. Among them, the inelastic collision process of high-energy electrons ( energy is usually in the range of 5-20 eV ) with gas molecules will produce chemically active particles, whose energy level is sufficient to break the chemical bonds of most organic materials ( such as the bond energy of C-C bond is about 3.6 eV ). When these highly active particles interact with the surface of the material, the surface roughness can be increased and the wettability of PTFE can be improved by physical impact of plasma on the surface of PTFE substrate. The reactive plasma used in the chemical reaction can introduce oxygen-containing groups on the surface of the substrate to improve the surface energy and wettability, while the non-chemical reaction Ar plasma can improve the contact angle and surface energy of the substrate surface with the liquid by surface crosslinking or etching to achieve the modification effect.

With the development of 5G communication, unmanned driving and high-frequency electronic equipment, the demand for high-frequency printed circuit boards has surged. PTFE has become the mainstream choice for high-frequency substrates due to its low dielectric constant and low loss factor. However, the chemical inertness, low surface energy and hydrophobicity of PTFE make it difficult to achieve effective hydrophilicity on its surface, which directly affects the reliability of the metallized hole process. The process realizes electrical interconnection between multilayer PCB layers by electroless copper plating, which is the core link to ensure signal transmission performance. Although the traditional sodium naphthalene solution etching method can activate the PTFE surface, there are problems such as surface discoloration, operational safety hazards and environmental pollution after treatment. The plasma surface treatment technology can optimize the surface wettability without changing the bulk properties of the substrate, which is both environmentally friendly and process controllable, and is regarded as a better solution.