Rigid-flex Printed Circuit Board (R-FPCB) refers to a printed circuit board (PCB) fabricated by selectively combining rigid substrates and flexible substrates in different regions. In rigid-flex bonding areas, conductive patterns on various rigid and flexible substrates can usually be interconnected. Among them, the R-FPCB contains one or more bendable regions to realize circuit interconnection. As one of the most complex electronic interconnection structures at present, this circuit structure integrates traditional PCBs with smaller-sized and easy-to-install Flexible Printed Circuits (FPC), solving the interconnection reliability issues of various electronic systems and achieving successful applications in the industrial sector. R-FPCB offers the advantages of reducing overall equipment weight and cost, shortening assembly time, enhancing reliability, and saving rework and repair time, making it an excellent electronic interconnection solution. Therefore, rigid-flex PCBs are widely used in computers, communication equipment, automobiles, military and aerospace applications, as well as other electronic devices.

The hole metallization process of Printed Circuit Boards (PCBs) directly affects the electrical connection performance of electronic products. The quality of desmearing directly impacts the plating quality inside the holes, which in turn determines the product yield. There are many desmearing methods available today, divided into wet and dry processes. Wet processing uses chemical solutions for reactions, including potassium permanganate, concentrated sulfuric acid, concentrated chromic acid, and PI adjustment treatments. Dry processing removes smear by reacting plasma with the smear on hole walls in a vacuum environment. Materials of FPC and R-FPCB may swell and delaminate in wet processing solutions, accompanied by environmental pollution problems. Thus, plasma cleaning is preferred for processing flexible multilayer boards and rigid-flex PCBs.

Plasma Cleaning Process

Plasma is the fourth state of matter, composed of four forms: electrons, ions, reactive free radicals, and molecules. Under low pressure, reaction gases gain energy under the action of an electric field, ionizing gas molecules and generating glow discharge. The ionized gas interacts with or reacts with material molecules in the chamber, breaking down small-molecule substrate structures into smaller molecules that turn into volatile gases, which are then exhausted by a vacuum pump. The plasma cleaning process is a combined physical and chemical reaction.

Plasma cleaning equipment mainly consists of four components: electrodes, RF generator, vacuum chamber, and vacuum pump. Figure 1 shows a schematic diagram of typical plasma cleaning equipment, and Figure 2 shows a physical view of the plasma cleaner chamber.

Figure 1 Schematic Diagram of Typical Plasma Cleaning Equipment

Figure 2 Physical View of Plasma Cleaning Chamber

Reactive gases used in plasma cleaning and etching of circuit boards include Ar, He, H₂, N₂, O₂, CF₄, etc. Mixtures such as N₂/O₂/CF₄ or O₂/He can be used to remove smear generated by FR‑4 materials, BT materials, polyimide, and acrylic materials. For processing PTFE fiber‑filled and PTFE ceramic‑filled materials, H₂ is additionally required in the final stage to reduce certain substances.

The main components of plasma gas for treating rigid-flex PCBs are N₂, CF₄, and O₂. The entire process can be divided into three stages. First, N₂ is used for physical cleaning of hole walls to improve wall cleanliness while heating the substrate. Then, CF₄ and O₂ are employed for chemical cleaning. Under the high-frequency and high-voltage action of the plasma generator, CF₄ and O₂ are ionized to produce a plasma atmosphere containing free radicals, atoms, molecules, and electrons, as shown in reaction formula (1).

Finally, the plasma generated in (1) reacts with dielectric layer materials such as smear on hole walls to form gases that are exhausted, achieving hole wall cleaning, as shown in reaction formula (2).

When the hole wall dielectric layer contains glass fibers, the following additional reactions occur:

The above describes the basic principle of plasma cleaning. The entire process takes place in gaseous and particle forms, enabling convenient and rapid reactions. Plasma desmearing eliminates the need for an entire wet process line, reducing chemical treatment costs and water consumption. During plasma desmearing, panels are placed in a vacuum chamber, gases are introduced and converted into plasma via a power supply, the plasma reacts on the panel surfaces, and volatile resin smear is removed by the vacuum pump.