Surface finishing is one of the critical steps in PCB manufacturing. Its primary function is to apply a protective metallic or organic coating over the exposed copper pads on the PCB. The core objective is to prevent oxidation of the copper surface, ensure excellent solderability of the pads during storage and subsequent assembly processes, and ultimately influence the reliability and electrical performance of the final product.
Common surface finishing processes primarily include HASL (Hot Air Solder Leveling), ENIG (Electroless Nickel Immersion Gold), OSP (Organic Solderability Preservative), Electroless Tin, and Electroless Silver. HASL is low-cost and highly compatible, though it results in relatively thick pads with moderate flatness; it is suitable for standard PCBs. ENIG offers excellent surface flatness and superior oxidation resistance, making it ideal for BGA packages, soldering of precision components, and applications requiring contact points for keypads; however, it entails higher costs [6]. OSP is the most cost-effective option; it is environmentally friendly and offers good solderability, but its protective layer is extremely thin, resulting in a short shelf life that necessitates prompt use prior to soldering. Electroless Tin and Electroless Silver processes provide flat surfaces and good solderability, making them suitable for high-frequency and high-speed signal boards; notably, the Electroless Silver process requires precautions to prevent discoloration caused by sulfidation.
The basic workflow for surface finishing typically involves a pre-treatment phase to clean the pads-removing oxides and contaminants-followed by the deposition of the appropriate metallic layer or organic protective film onto the cleaned copper surface, based on the selected process. The final step involves a post-treatment cleaning to remove any residual chemical solutions.
The selection of a specific surface finishing process requires a comprehensive evaluation of factors such as cost, solderability requirements, surface flatness, shelf life, environmental regulations, and the intended end-use application. For instance, ENIG is frequently chosen for BGA packages, whereas cost-sensitive, high-volume products may opt for HASL or OSP.
