Printed Circuit Boards (PCBs) are the backbone of modern electronics, enabling electrical connections between components. However, the exposed copper traces on a PCB’s surface are vulnerable to oxidation, corrosion, and mechanical wear, which can degrade performance over time. Surface finishing processes address these challenges by coating the copper with protective layers while ensuring reliable solderability. This article explores the key surface finishing techniques, their mechanisms, applications, and future trends.
Primary Purpose
Copper, while an excellent conductor, reacts readily with oxygen and moisture, forming non-conductive oxides (e.g., CuO or Cu₂O). These oxides hinder soldering and compromise electrical integrity. Surface finishes serve a three-fold role:
- Anti-Oxidation Protection: Copper material easily reacts with oxygen in the air to generate copper oxide, which will reduce the quality of soldering. Surface treatment can effectively prevent oxidation.
- Improve Solderability: By forming a uniform protective layer, the pads have better wettability and adhesion during the soldering process, which ensures solid soldering between components and PCB.
- Improve Corrosion Resistance: The surface coating can, to a certain extent, block moisture, acid, alkali, and other corrosive substances in the environment, prolonging the service life of the PCB.
Common Types
- HASL (Hot Air Solder Leveling): The PCB is immersed in molten solder (leaded or lead-free) and then blown flat with hot air to form a tin/lead or tin-based coating. HASL process is a low-cost, proven process with good solderability but may not be ideal for fine-pitch or high-density packages due to the possibility of a less-than-flat surface formed by the solder as it cools.
- ENIG (Electroless Nickel Immersion Gold): The process first deposits a layer of nickel on the surface of the PCB by an electroless method and then utilizes a displacement reaction to form a layer of gold on top of the nickel layer. This treatment not only provides a very flat surface suitable for fine pitch and BGA packages but also provides good oxidation resistance and solderability, but at a higher cost.
- OSP (Organic Solderability Preservative): The use of organic compounds to form a very thin layer of protective film on the surface of the PCB bare copper to protect the copper surface from oxidation. This process is environmentally friendly and low cost, but the protective layer is thin and not suitable for multiple soldering or long-term storage.
- Immersion Silver/Tin: Immersion silver and tin are the deposition of a silver or tin layer on the surface of copper through a chemical substitution reaction, respectively. Immersion silver process provides better flatness and solderability, but the silver layer is easily discolored by the effects of sulfides in the environment, while the immersion tin layer improves solderability but has tin whisker problems and a shorter shelf life.
- Electroless Nickel, Electroless Palladium, Immersion Gold (ENEPIG): This process adds a layer of palladium to ENIG, which further prevents oxidization problems of the nickel layer, provides better soldering performance and long-term reliability, and is particularly suitable for more demanding high-end applications. However, ENEPIG is also relatively costly.
Surface treatment is far more than a “final touch” in PCB manufacturing—it’s a critical determinant of a board’s performance, longevity, and compatibility with assembly processes. From cost-effective HASL to high-reliability ENEPIG, each technique addresses specific needs in the ever-evolving electronics landscape. As devices shrink and speeds increase, innovations in materials and processes will continue to redefine the boundaries of what’s possible, ensuring that PCBs remain at the heart of technological progress.
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