Plating current density

FIGURE 26.26 Plating current efficiency for NiFe film deposition on a rotating disk electrode vs. the total plating current density, for different disk rotation speeds. Plating bath composition was the same as that in Figure 26.25. Data from [106]. (Reproduced by permission of ECS—The Electrochemical Society.)... 

Measurements of Rs transients were conducted in order to assess the effect of dopants / plating parameters on the kinetics of the transformation of electroplated copper [8]. Results are shown in Figure 6. For a constant bath temperature, the parameters that affect dopant incorporation the most are current density, rotation speed, and additive concentration. It is seen that an increase in additive concentration and rotation speed leads to a delay in the resistance transformation and to an increase in dopant content. Similarly, an increase in plating current density causes an acceleration of the resistance transformation and a decrease in dopant incorporation. It is thus concluded that dopant content increase causes delays in the resistance transformation of plated copper in accordance with the observations of Harper et al [8]. Results shown in Figs. 7 and 8 corresponding to different bath temperatures as well as plating from three different commercial chemistries are consistent with this correlation. 

A tation A filter pump circulates the solution without allowing air to be introduced. Cathode rod agitation is also useful for a wider range of plating current densities. 

The Fe, Co, and Ni deposits are extremely fine grained at high current density and pH. Electroless nickel, cobalt, and nickel—cobalt alloy plating from fluoroborate-containing baths yields a deposit of superior corrosion resistance, low stress, and excellent hardenabiUty (114). Lead is plated alone or ia combination with tin, iadium, and antimony (115). Sound iasulators are made as lead—plastic laminates by electrolyticaHy coating Pb from a fluoroborate bath to 0.5 mm on a copper-coated nylon or polypropylene film (116) (see Insulation, acoustic). Steel plates can be simultaneously electrocoated with lead and poly(tetrafluoroethylene) (117). Solder is plated ia solutioas containing Pb(Bp4)2 and Sn(Bp4)2 thus the lustrous solder-plated object is coated with a Pb—Sn alloy (118). 

Cadmium is usually plated from a cyanide bath that consists of an aqueous solution of cadmium oxide (35 g/L) and sodium cyanide (75 g/L). An additive and a brightener are used to produce smooth, fine-grain deposits. Current density ranges from 1.4 to 3.7 A/dm, depending on the concentration of cadmium cations in the electrolyte. 

Plating variables for this process maybe summarized as higher (87°C) operating temperatures enable the oxygen content of the metal to be reduced to 0.01% the CrO iSO ratio should be below 100 to obtain low oxygen metal current efficiencies >8% are associated with high oxygen contents and better current efficiencies are obtained at low current densities. 

Hard plating is noted for its excellent hardness, wear resistance, and low coefficient of friction. Decorative plating retains its brilliance because air exposure immediately forms a thin, invisible protective oxide film. The chromium is not appHed directiy to the surface of the base metal but rather over a nickel (see Nickel and nickel alloys) plate, which in turn is laid over a copper (qv) plate. Because the chromium plate is not free of cracks, pores, and similar imperfections, the intermediate nickel layer must provide the basic protection. Indeed, optimum performance is obtained when a controlled but high density (40—80 microcrack intersections per linear millimeter) of microcracks is achieved in the chromium lea ding to reduced local galvanic current density at the imperfections and increased cathode polarization. A duplex nickel layer containing small amounts of sulfur is generally used. In addition to... 

Mass Transport. Probably the most iavestigated physical phenomenon ia an electrode process is mass transfer ia the form of a limiting current. A limiting current density is that which is controlled by reactant supply to the electrode surface and not the appHed electrode potential (42). For a simple analysis usiag the limiting current characteristics of various correlations for flow conditions ia a parallel plate cell, see Reference 43. 

Electrolytic plating rates ate controUed by the current density at the metal—solution interface. The current distribution on a complex part is never uniform, and this can lead to large differences in plating rate and deposit thickness over the part surface. Uniform plating of blind holes, re-entrant cavities, and long projections is especiaUy difficult. 

Electroless plating rates ate affected by the rate of reduction of the dissolved reducing agent and the dissolved metal ion which diffuse to the catalytic surface of the object being plated. When an initial continuous metal film is deposited, the whole surface is at one potential determined by the mixed potential of the system (17). The current density is the same everywhere on the surface as long as flow and diffusion are unrestricted so the metal... 

Plate Thickness. In plating processes, plate thickness can be predicted knowing the cathode efficiency of a particular plating solution, the current density, and time of plating. 

A problem that affects the accuracy of the prediction of plating thickness is in estimating the actual current density. Current is not evenly distributed over the surface of the part being plated, rather, it takes the path of least resistance. Current also concentrates on sharper points, corners, and edges even the shape of the plating tank can have an influence on the current distribution. The difference in current and, subsequendy, the plate thickness distribution, is minimal when geometrically conforming anodes are part of the system, but this condition is not often achieved. 

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