Immersion Silver for PCB
Pure silver is a very ideal surface treatment material, as it has the features of softness, good ductility, thermal conductivity, electrical conductivity; and can hardly react with water or oxygen.
These properties make Immersion silver another common surface finish in PCB manufacturing. It chemically generates a thin layer of silver with a thickness of 0.1~0.5um on the copper surface, to protect the copper from oxidation and improve solderability.
Immersion silver process is based on Galvanic effect. Driven by the difference in electromotive force and redox potential between copper and silver, silver ions receive electrons from the copper metal while the copper dissolves and the silver precipitates. Namely, a displacement reaction occurs between the silver ion and Copper atom in the solution.
Immersion silver is an environment friendly surface treatment and totally RoHS compliant.
It has excellent surface planarity,which is suitable for high density circuit and small spacing surface mounting. Due to the skin effect, its signal loss in high frequency applications is also very low.
At the same time, it maintains the stability of silver,and it can keep the good solderability even if being exposed in hot and wet environment.
Unexceptionally, it has the disadvantages too. The PCB with immersion silver is sensitive to chlorine and sulfur compound, and requires more care in storage and handling than immersion tin. Improper handling of Immersion silver circuit boards is prone to scratches.
The silver coatings will degrade over time if it’s exposed in the air for long time, so it is usually suggested to handle it within 48 hours. Meanwhile, the physical strength is not as good as ENIG since there is no nickel underneath.