What is Immersion Silver
Immersion silver, or IM silver, silver plating, or ENIAg, is a surface treatment material. It is a chemical process that involves depositing a thin layer of silver onto the copper surface of a PCB. This silver layer, with a thickness ranging from 0.1 to 0.5 micrometers, serves multiple purposes.
Immersion silver protects the copper from oxidation. By forming a barrier between the copper and the environment, it helps prevent the copper from corroding over time. Additionally, the silver layer enhances the solderability of the PCB, making it easier to create reliable solder joints during the assembly process.
The process of immersion silver is based on the Galvanic effect, which takes advantage of the difference in electromotive force and redox potential between copper and silver. Silver ions in the solution receive electrons from the copper, causing the copper to dissolve and the silver to precipitate onto the surface. This displacement reaction results in the formation of the thin silver layer.
Immersion silver offers several advantages in PCB manufacturing. It is considered environmentally friendly and compliant with regulations such as the RoHS (Restriction of Hazardous Substances) directive. It provides excellent surface planarity, making it suitable for high-density circuits and small spacing surface mounting. Finally, it exhibits low signal loss in high-frequency applications due to the skin effect.
However, PCBs with this surface finish are sensitive to chlorine and sulfur compounds, requiring careful storage and handling. Additionally, the silver coating on immersion silver PCBs can degrade over time when exposed to air for extended periods. Therefore, handling these PCBs within 48 hours is recommended to maintain their quality. Furthermore, the physical strength of immersion silver is not as robust as that of ENIG due to the absence of a nickel layer underneath the silver coating.