Reference cited in this section

2. J.C. Norris, Brush Plating: Part I, Metal Finishing, July 1988, p 44-48 Key Process Elements

Anode-Cathode Motion. Controlling continuous movement between the anode and the workpiece, or cathode, is a key element in obtaining high-quality brush-plated deposits. However, quality also depends on plating within a specific current density range, so both variables affect ultimate deposit quality. This relationship is illustrated in Fig. 2. Solution suppliers routinely recommend ranges of anode-cathode speeds and current density values for each solution; a representative list is given in Table 5.

Table 5 Anode-cathode motion and current density for selective plating solutions

Table 5 Anode-cathode motion and current density for selective plating solutions

Selective plating solution

Anode-cathode motion

Current density

m/s

ft/min

A/dm2

A/ft2

Cadmium (acid)

0.26-0.561

50-110

86.4

864

Cadmium LHE (low hydrogen embrittlement formula)

0.20-0.41

40-80

86.4

864

Chromium

0.02-0.03

4-6

86.4

864

Cobalt (machinable)

0.13-0.26

25-50

115.2

1152

Copper (high speed, acid)

0.20-0.51

40-100

144.0

1440

Gold

0.15-0.31

30-60

28.8

288

Lead

0.15-0.26

30-50

86.4

864

Nickel (acid)

0.10-0.26

20-50

86.4

864

Nickel (high speed)

0.20-0.41

40-80

144.0

1440

Nickel-tungsten alloy

0.10-0.15

20-30

72.0

720

Rhodium

0.03-0.05

5-10

43.2

432

Silver (heavy build)

0.10-0.31

20-60

72.0

720

Tin (alkaline)

0.10-0.41

20-80

86.4

864

Zinc (alkaline)

0.15-0.612

30-120

115.2

1152

Data courtesy of SIFCO Selective Plating

Data courtesy of SIFCO Selective Plating

Fig. 2 Relationship between current density and anode-to-cathode speed. Source: Ref 3

The visual appearance of the electroplate is also an indicator of quality. A dark gray or black color usually corresponds to a burnt deposit, which results from too high a current density or insufficient movement. In contrast, inadequate current density or too much movement produces a generally shiny surface.

Anode-to-cathode movement may be achieved manually or mechanically, such as by using turning equipment to provide a constant rotational speed for cylindrical parts (Fig. 3) or by using specially designed tilting turntables to rotate large parts at controlled speeds. Another option is the rotostylus (Fig. 4), which rotates the anode instead of the workpiece.

Fig. 3 Turning head. Courtesy of SIFCO Selective Plating
Fig. 4 Rotostylus. Courtesy of SIFCO Selective Plating

Anodes and Flowthrough. For the plating process to be efficient, the plating solution must flow between the anode and the area being plated. Solution can be supplied by periodically dipping the plating tool into the electrolyte. However, the most efficient method is to pump the solution through the block anode and into the interface between the anode and the workpiece (Fig. 5). Plating of large areas at high currents requires the use of a pump to recirculate the solution. This keeps the solution from overheating, results in thicker buildups on large areas, and allows the use of higher current densities. In addition, the entire process is faster. Various types of pumps can be used, depending on the amperage and on whether preheating and/or filtering is necessary for the solution used.

Fig. 5 Flow-through deposition. Courtesy of Vanguard Pacific, Inc.

Solutions. Three basic types of solutions are used in selective plating: preparatory, plating, and special-purpose. Table 6 shows the most common solutions and their uses. The base metal and the type of plating generally dictate which solutions are appropriate.

Table 6 Typical selective plating solutions

Preparatory solutions

Activating (for industrial hard chromium) Cadmium activator

Chromium activator (for decorative applications) _Cleaning (for most materials)_

Desmutting (for cast iron, carbon and alloy steels, copper alloys) Etching (for aluminum alloys, steels, cast iron)

Etching and activating (for high-temperature nickel-base alloys and stainless)

Plating solutions for ferrous and nonferrous metals

Nickel (acid strike) Nickel (alkaline) Nickel (dense)

Nickel (ductile, for corrosion protection)

Nickel (neutral, for heavy buildup)

Nickel (sulfamate, hard, low stress)

Nickel (sulfamate, moderate hardness)

Nickel (sulfamate, soft, low stress)

Antimony

Bismuth

Cadmium (acid)

Cadmium (alkaline)

Cadmium (no-bake)

Chromium (hexavalent)

Chromium (trivalent)

Cobalt (for heavy buildup)

Copper (acid)

Copper (alkaline)

Copper (high-speed acid)

Copper (high-speed alkaline for heavy buildup)

Copper (neutral)

Iron

Lead (alkaline) Lead (for alloying) Tin (alkaline) Zinc (acid) Zinc (alkaline) Zinc (bright) Zinc (neutral)

Plating solutions for precious metals

Gallium Gold (acid) Gold (alkaline) Gold (neutral) Indium Palladium Platinum Rhenium Rhodium Silver (hard) Silver (noncyanide) Silver (soft)

Plating solutions for alloys

Babbitt Navy Grade 2 Babbitt SAE 11 Brass

Cadmium-tin Cobalt-tungsten Nickel-cobalt _Nickel-tungsten_

Tin-indium Tin-lead-nickel

Special-purpose solutions

Anodizing (chromic) Anodizing (hard coat) Anodizing (sulfuric) Chromate treatment Electropolishing solution

Source: Ref 2

Preparatory solutions clean the substrate surface so that it can effect a better bond with the electroplate. Preparation typically involves precleaning, electrocleaning, and electroetching; some base materials also require desmutting, activation, and preplate operations. Parts with heavy corrosion, lubricants, oil, and so on ordinarily require more aggressive cleaning, such as vapor/solvent degreasing or grit blasting, prior to precleaning.

Plating solutions used for selective plating have a much higher concentration of metal, usually as organometallic salts, than do solutions used for tank plating. This higher metal content permits the use of higher current densities, which results in faster deposition, better bond strength, and less porosity than in tank plating.

Two basic kinds of plating solutions are used. One deposits a thin preplate ("strike") that boosts adhesion on certain metals and alloys, and the other builds up the coating to its functional thickness. Suppliers usually offer a choice of solution for each type of electroplate, because different properties are required for different applications (e.g., high hardness and wear resistance for one, ductility for another).

Special-purpose solutions include those used for post-treatment, anodizing, and electropolishing.

Thickness Control. The thickness of a deposit can be controlled by monitoring the ampere-hour meter. Each solution has a prescribed energy factor, which indicates how many ampere-hours are required to deposit a given metal thickness on a given area:

where A • h is the ampere-hours required, F is the energy factor, A is the area to be plated (in square centimeters), and T is the thickness of the deposit (in microns). The calculation yields a fixed value that can be monitored on the meter (or set on the ampere-hour counter, if the power pack is so equipped). If needed, simple additional calculations can also be performed to determine the optimum current, plating time, volume of plating solution, and even rotational speed (Ref 4).

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