PCB Wet Process: Core Processes and Equipment

Wet processing is a critical stage in the manufacturing of printed circuit boards (PCBs). Centered on chemical treatment, it covers substrate pretreatment, through-hole metallization, circuit patterning, solder masking, and surface finishing. These steps directly determine a PCB’s electrical conductivity, adhesion, corrosion resistance, and reliability. The entire process relies heavily on specialized equipment to enable automated and precision manufacturing.

Pretreatment forms the foundation of wet processing. It primarily removes surface oils, oxides, and contaminants to improve plating adhesion. For inner-layer boards, a browning treatment is applied to form a uniform surface film, ensuring lamination strength in multi-layer PCBs. Key equipment includes horizontal degreasing cleaners, ultrasonic degreasers, horizontal acid cleaning lines, and browning production lines, laying a solid quality base for subsequent processes.

Through-hole metallization (PTH) is essential for establishing interlayer conductivity in double-sided and multi-layer PCBs. After drilling, insulating hole walls undergo resin smear removal followed by electroless copper deposition to form a thin conductive layer, preparing for electroplating. Primary equipment consists of potassium permanganate desmear machines, vertical lift PTH lines, and gantry-type PTH lines, ensuring uniform and dense copper deposition on hole walls to prevent open circuits.

Pattern plating builds the conductive circuitry of a PCB by selectively thickening copper layers and depositing a tin etch resist according to the designed pattern. Core equipment includes vertical gantry plating lines and annular plating lines, supported by rectifiers, filters, and air agitation systems. These precisely control plating thickness and uniformity, enhancing circuit conductivity and corrosion resistance.

Developing, etching, and stripping (DES) define the final circuit pattern. Developers remove unexposed photoresist outside the circuit area; etchers precisely remove exposed copper foil; and strippers clean the anti-plating layer from the circuit surface. Horizontal integrated DES lines combine these three stages, greatly improving production efficiency and circuit accuracy.

Solder masking provides PCBs with a protective insulating layer, or “protective coat,” through coating, exposure, developing, and curing. It prevents oxidation and short circuits while marking component positions. Key equipment includes pumice grinding machines, solder mask developers, UV exposure machines, and curing ovens, ensuring strong adhesion of the protective layer and extending service life.

Surface finishing serves as the final protection for finished PCBs, optimizing solder pad performance. Electroless nickel immersion gold (ENIG), immersion silver, and electroplated nickel-gold lines enhance solderability and corrosion resistance. OSP coaters form an ultra-thin anti-oxidation film suitable for high-precision soldering, while hot air solder leveling (HASL) machines support traditional soldering to meet diverse application requirements.