In the electronics manufacturing industry, the surface plating process of printed circuit boards (PCBs) is crucial to product performance. It not only prevents copper oxidation but also ensures good solderability and electrical connection stability. Currently, mainstream processes include hot air leveling, organic coating, electroless nickel/immersion gold plating, immersion silver plating, immersion tin plating, and electroplating nickel-gold, each with different characteristics and applicable scenarios.
Hot air leveling is a traditional process where the PCB is immersed in molten solder, and then heated compressed air is used to level the surface, forming an oxidation-resistant and solderable coating. Its advantages include good solderability, as the solder and copper bond to form intermetallic compounds. However, it presents challenges in controlling solder thickness and pad flatness, making it difficult to adapt to narrow-pitch components. This process is divided into vertical and horizontal types. The horizontal type is more widely used due to its uniform plating and ability to be automated. The process mainly includes micro-etching, preheating, flux application, tin spraying, and cleaning. Lead-free versions use non-lead alloys instead of traditional tin-lead solder.
Organic coating has become a common choice due to its low cost and simple process, forming an organic barrier layer on the copper surface to isolate it from air. Early methods relied on imidazole and benzotriazole molecules for rust prevention, but now benzimidazole is the primary method, capable of chemically bonding with copper. To handle multiple reflow soldering cycles, molten copper is added to the chemical bath, allowing organic molecules to repeatedly aggregate and form multi-layer coatings. The latest processes can meet lead-free soldering requirements. The process involves degreasing, micro-etching, acid pickling, pure water rinsing, organic coating, and cleaning, with relatively low process control difficulty.
Electrochemical nickel/immersion gold plating is a complex process, requiring acid cleaning, micro-etching, pre-immersion, activation, electroless nickel plating, and electroless immersion gold plating, involving nearly a hundred chemicals, making control more difficult. The nickel layer thickness needs to be ≥3μm, serving as an isolation and flux; the gold layer thickness needs to be ≤1μm to avoid the "gold brittleness" problem. However, this process is prone to "black pads," often due to an insufficiently dense nickel-gold plating layer, abnormal nickel-phosphorus content, incomplete post-plating cleaning, or nickel oxidation caused by gold diffusion during soldering. The quality is more stable when the nickel-phosphorus content is between 7% and 10%.
Immersion silver plating falls between organic coating and electroless nickel/immersion gold plating. It is a fast and simple process, providing excellent electrical properties, good solderability, and good storage life, but its physical strength is inferior, and organic additives are needed to prevent silver corrosion and migration. Immersion tin plating has broad prospects due to its compatibility with various solders. By adding organic additives, a granular tin layer can be formed, solving the tin whisker problem. It combines thermal stability and solderability, and also prevents intermetallic diffusion, but its storage time is short, and assembly must follow the immersion tin sequence.
Electroplating nickel-gold is the "founding father" of PCB surface treatment. Nickel is first plated to prevent gold-copper diffusion, followed by gold plating. It is divided into soft gold and hard gold plating. To reduce costs, selective electroplating is often used to decrease the amount of gold used. However, electroplated gold is prone to brittle soldering, so soldering on electroplated gold surfaces should be avoided.
In addition, electroless palladium plating and similar processes are less commonly used. These processes are similar to electroless nickel plating, using a reducing agent to reduce palladium ions, allowing for coatings of any thickness. They offer good solderability, thermal stability, and surface smoothness, and have some application value in specific high-precision applications.
