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28 PCB007 MAGAZINE I JUNE 2022 the cathode which is rendered positive. e deposited weight depends on the current and the time, and is governed by Faraday's law. Electroplating requires connectivity. It is used to plate metal alloys, and in dispersion plat- ing. e latter involves the uniform dispersion of a non-conductive entity in the electroplated metal. Electroless Plating In electroless plating, the negative electron is supplied by a reducing agent present in the electrolyte. e reducing agent is oxidized, and the metallic target ion is reduced to the metal. Plating continues as long as the reduc- ing agent and the target ion concentration are maintained through replenishment. Elec- troless plating is independent of connectiv- ity. Examples are electroless copper and elec- troless nickel, electroless gold, and electroless palladium. Immersion Plating In immersion plating, the reducing electron is supplied by the substrate where the depo- sition is occurring. e substrate metal is oxi- dized to the metal ion giving up an electron. e electron reduces the target ion to the metal for deposition. Only metals higher in the EMF series can replace metals below them. Immer- sion deposition is a displacement reaction and is limited to the availability of the substrate being plated. Examples include immersion sil- ver, immersion tin, immersion gold, and oth- ers. e general principles of all three methods are well understood and have been used for decades in electronics manufacturing. Refin- ing the plating process to meet specific elec- tronics requirements is the challenge faced by chemical manufacturers as complexity and miniaturization continue to increase. e fol- lowing examples demonstrate how plating processes are modified and adapted to meet specific design requirements. Copper Plating Copper is a highly conductive non-precious metal. It is the conduit that carries the flow of current through printed wiring boards and semiconductors (integrated circuits) devices. Electroplated copper is the dominant method for plating traces and through-holes. e challenge for electroplated copper in electronics is achieving consistent quality and thickness distribution throughout the part. Controlling the quality of the deposited cop- per (tensile strength and elongation) ensures that the deposit will not crack or fracture with the thermal excursion the parts are exposed to during manufacture and throughout their life cycle. ickness distribution is critical for con- trolling impedance throughout the device. e use of additives (brightener, suppressor, and leveler) in the electrolyte, plating current density, and plating cell geometry are designed and manipulated to achieve a specific out- come, such as via filling, plating high aspect- ratio holes, and plating fine lines and spaces. Surface Finishes Surface finishing of electronic products is an integral part of manufacturing. It serves many functions, the most prominent of which is to create a solderable surface. It is also used as a corrosion barrier and an electrical contact- ing surface, as well as a wire bonding surface. One or more of the three methods of plating are used to create the desired surface. e layer may be a single component such as immer- sion silver, tin, gold, or palladium, or multi- ple consequential layers such as electroless nickel/immersion gold (ENIG), electroless nickel/electroless palladium/immersion gold (ENEPIG) and electroplated nickel/gold. e choice of what thickness of metal to plate and what plating method to use is always based on meeting specific design require- ments with the lowest cost. For example, sol- derable and gold wire-bondable finishes limit the choice of deposit. At the moment, the lead-