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88 PCB007 MAGAZINE I SEPTEMBER 2020 Plating or metal deposition is a key compo- nent in the manufacturing of electronic pack- ages (circuit boards and integrated circuits). Plating occurs when a metal ion in solution (electrolyte) is reduced to the metal. The re- duction takes place when electrons are sup- plied to the ion: M ++ + 2e- g M 0 The source of electrons differs with the type of plating as follows: • Electrolytic plating: Electrons supplied by an external power supply (rectifier) • Electroless plating: Electrons are supplied by a reducing chemical agent that is present in the electrolyte • Immersion plating: Electrons are supplied by the oxidation of the (metal) substrate This column will be dedicated to the immer- sion reaction. Immersion reactions occur between metals according to their location in the electromo- tive (EMF) series. The EMF series is a listing of the elements according to their half potentials or tendency to lose electrons (get oxidized), or their tendency to gain an electron (get re- duced), measured in voltage. Elements that lose electrons (reducers) have negative voltage values, and elements that gain electrons (oxi- dizers) have positive E o voltage values. Table 1 shows the half potentials E o measured in volts of common elements, and metals are ranked with respect to their inherent reactivity. The metals located at the top of the series are considered the noblest with the highest level of positive electrochemical potential. The electromotive force that drives an im- mersion reaction is determined by the differ- ence in E o (half potential) between the reduced and oxidized elements. The reduced element is the element receiving electrons and is derived as follows: E cell = E Red – E oxd If the cell potential is positive, the reaction is spontaneous. A negative voltage indicates that no reaction will take place. In printed circuit fabrication, immersion plat- ing plays a key role in the following processes: • Immersion gold in electroless nickel/ immersion gold (ENIG) • Corrosion in ENIG • Immersion gold in electroless nickel/electroless palladium/immersion gold (ENEPIG) • Palladium catalyst on copper • Immersion silver • Immersion tin Immersion Plating Reaction in Electronics Manufacturing The Plating Forum by George Milad, UYEMURA INTERNATIONAL CORPORATION Table 1: A partial listing of metals according to their "electromotive series" ranking.