ENIG is a process of surface plating commonly used for finishing of printed circuit boards. It is the abbreviation of Electroless Nickel Immersion Gold. The basic process is the electroless nickel plating over bare copper of PCB that appears after etching and then immersed in gold to form a thin layer of gold. The gold immersion protects the nickel from oxidation. If the PCB is left without any coating over the copper tracks, vias and pads they will be oxidized (will corroded) over the time. This makes the tracks broken and open circuit. ENIG is a two layer process in which the PCB surface is coated with 0.12 0.24 mils thick nickel coating which is again coated with 0.002 0.008 mils thick layer of gold. Moreover, the finishing makes an interface between the PCB pad surface and opponent pin and helps the solder flow easily. In this regards, the surface finishing has two main objectives, first it protects all the exposed copper elements from oxidation and secondly it provides a corrosionless surface for soldering the components. The corroded pad surface tends to produce an improper solder joint that results into broken joint over the time.
ENIG process is more costly but has advantages over the common HASL method of PCB surface finishing. The HASL stands for hot air solder levelling. The ENIG has more surface planarity and excellent nickel oxidation resistance. It is specifically useful for fine pitch and thin pin contacts SMD components like TQFP, QFN and BGA packaged ICs. ENIG is most widely used in finishing process in the industry now due to good interface for solder joint, process growth and RoHs regulations.
Fig. 1:- ENIG finishing process for PCB
Fig. 2:- Finished PCB after ENIG process
Although ENIG has many advantages but it has disadvantages as well. It is an expensive and complicated process particularly due to materials used and way of usage. It is not a good for environment where frequent re-work is required over components i.e., repetitive soldering and desoldering of components. Because during the re-work or desoldering a component the pad is ripped off. It is damaged during the ET (electrical Test) inspection of the PCB also.