Equipment

Typical selective plating systems include a power pack, plating tools (called styli or anodes), anode covers, specially formulated plating solutions, and any auxiliary equipment required for the particular application. To achieve optimum deposits, equipment should be designed expressly for selective plating. Although tank plating solutions and rectifiers are occasionally used, they are not usually recommended because the resultant deposits are thin and quality is typically below par.

Power packs (rectifiers) supply the direct current and are specially designed with the features and/or controls required by the process. Output voltage can typically be varied from 0 to between 25 and 30 V, compared to the usual 6 to 12 V for tank-plating power packs. Power packs are available in a variety of ratings to suit specific applications (Table 4).

Table 4 Commercially available power packs

Alternating current input, V

Phase

Maximum output current, A(a)

115 or 230

1

30

115 or 230

1 or 3

60

230 or 460

1 or 3

100

230 or 460

1 or 3

150

230 or 460

1 or 3

200

230 or 460

3

Source: Ref 2

Voltage control is extremely important because it regulates the current supplied to the process. In turn, the amount of current consumed over time, measured in ampere-hours (A ■ h), determines the deposit thickness. Ampere-hour requirements vary widely for different types of electroplates, as indicated by the energy factors in Table 1.

Stepless voltage control is a typical feature of selective plating power packs; most can be adjusted from 0 to 100%. The magnitude of power required depends on the part size, the deposit thickness required, and the plating type. For example, for delicate electronics parts, an output of 5 A and 12 V should be ample, whereas plating of large areas with thick buildups requires a much larger source.

A voltmeter and an ammeter should be available and should show 0 to 100% ranges. The voltmeter permits monitoring of the voltage required to plate a given part. The ammeter displays the amount of current flowing between the anode and the cathode, allowing calculation of the current density.

A polarity-reversing switch allows the operator to automatically change current flow direction, which is necessary in preparatory operations (e.g., etching and desmutting) and in stripping, when the current must run opposite to the direction used in electroplating. Otherwise, the connections to the anode and cathode must be changed manually, slowing down the entire process. Indicator lights to show polarity direction may also be incorporated.

Safety circuit breakers instantly shut off the current should a short circuit occur between the anode and the workpiece. Typically, shutdown is within one-half cycle (1/120 s), preventing workpiece damage and injury to the operator.

Energy counters (ampere-hour meters) are required by many industrial and government specifications. Besides keeping track of the energy being used, these meters make it possible to control the thickness of the deposit. Counters may incorporate set points with visual and/or audio warnings to indicate when the desired thickness has been reached.

Optional microprocessor-controlled systems reduce the chance of miscalculation and allow continuous monitoring of the process via an alphanumeric readout. The amount of energy required for an area to be plated to the desired thickness and the applicable parameters are determined by a microprocessor, not the operator. Such equipment is also of merit for plating multiple parts to the same specifications. In some systems, the software prompts the operator for data entry for a particular operation, then displays the correct process parameters.

The plating tool (stylus) must have an insulating handle and an anode material that is inert, insoluble in plating solutions, and able to carry high current. Graphite is by far the most practical choice for anode material. High purity is preferred; any additive used to harden the graphite or give it a different physical characteristic interferes with its beneficial qualities. Graphite can be machined or shaped to fit the contour of the part being processed. Stainless steel is much more durable, but it dissolves in some plating solutions, and after a short time it changes the characteristics of the electrolyte being used. Platinum-iridium, platinum-clad niobium, and platinum-clad titanium are inert to all of the electrolytes used in selective plating operations, and they are used for smaller-diameter anodes. Their disadvantages are increased cost and high hardness, which makes shaping difficult.

The anode covers (wrapping materials) serve as an insulator between the anode and cathode and help ensure smooth deposits at high current densities. Because they hold the electrolyte, they must be absolutely free of oil and foreign materials. Any substance that contaminates the electrolyte has a detrimental effect on the plating. For example, any oil contamination whatsoever results in poor adhesion. Consequently, in a machine shop or other environment where oil is widely used, the work area should be carefully selected. When necessary, applicable part surfaces can be solvent or vapor degreased or go through a separate cleaning cycle.

Various fiberlike materials make suitable covers. Cotton works very well if it is sterile, long fiber. Synthetic fibers, such as polyester and nylon, do not wet or hold electrolytes as well as cotton, but this does not preclude their use. Polyester felt is typically selected when the same anode will be used for numerous parts or for heavy deposits. Most of these materials work well in the form of tube gauze ("bandage covers") as covers over cotton. Scotch-Brite has been used when heavy/hard deposits are required, and it can also function as a burnishing tool, improving the surface as plating continues. If a surface is soft and easily scratched, a different wrap should be chosen.

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