Introduction

CONVENTIONAL PULSED-CURRENT PLATING (commonly referred to as pulse plating) can be defined simply as metal deposition by pulsed electrolysis. In its simplest form, it involves using interrupted direct current to electroplate parts. This is accomplished with a series of pulses of direct current, of equal amplitude and duration in the same direction, separated by periods of zero current. The pulse rate (frequency) and "on" and "off' times (duty cycle) can be controlled to meet the needs of a given application. The ideal shape of the pulsed current is shown in Fig. 1(a). An oscilloscope should be used to reveal how well the equipment controls the output (Fig. 1b).

Fig. 1 Current patterns using an oscilloscope

in conventional pulsed-current plating. (a) Ideal pattern. (b) Typical pattern as seen

Pulse plating has gained acceptance in a number of metal finishing industries, especially the electronics industry. With the advent of solid state pulse plating power supplies, the imprecision has been taken out of the process. The amount of time the current is on and off is set directly on digital thumb-wheel switches or via programmable software. Two different modes of operation are used: constant current and constant voltage. Figure 2 illustrates the constant-current mode of operation. The tops of the current pulses are kept flat by allowing the voltage to vary during the pulse on-time. In the constant-voltage mode (Fig. 3), the tops of the voltage pulses are kept flat by varying the current. Because of the shape of the current pulse in the constant-voltage mode, the peak current is not useful for controlling the plating rate. An amp-minute controller is needed to accurately control the plating thickness.

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Fig. 2 Constant-current pulse plating with an on-time of 5 ms, an off-time of 45 ms, and a peak current of 75 A. Because the current is on one-tenth of the time, the average current is 7.5 A.

Fig. 3 Typical current pattern for constant-voltage pulse plating. The current falls during the on-time because of the increasing resistance of the cathode. On-time, 10 ms; off-time, 30 ms

Metals that are commonly deposited using pulsed current include gold and gold alloys, nickel, silver, chromium, tin-lead alloys, and palladium. Pulsed current is also used for anodizing and, in select cases, for etching, electrocleaning, and electroforming.

The advantages of pulse plating vary from one application to the next. The most common (relative to plating with conventional continuous current) include the following:

• Deposits are smooth, dense, fine-grained, and almost completely free of pinholes.

• Variation in plate thickness from one part to the next is considerably reduced.

• Plating speeds can normally be increased.

• Current efficiency generally is improved.

• Use of organic additives, in most cases, can be reduced by 50 to 60%. Deposits are free from dendritic growth even if additives are not used.

• For some electrodeposits, such as gold, less metal is required in the plating solution to meet end-use specifications.

The limitations of pulse plating include the fact that the cost of a pulsed-current rectifier is greater than that of a conventional dc unit. Also, optimization of results requires planning and experimentation, and plating equipment may need to be redesigned or upgraded.

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