2RJ Tedeschi Met Finish Nov 1971 p 49 Process Control

Microprocessor-controlled modulation of applied direct current to improve the electrodeposition process has found use in reel-to-reel selective plating, automatic tab plating, barrel line plating, still plating, electroforming, anodizing, electrocleaning, electropolishing, and electromachining. It recently has been adapted for use with semiconductor bump and wafer plating technologies.

The use of this sophisticated power control can result in greatly increased plating speeds, improved deposit distribution, lower deposit stress, finer grain structure, increased ductility, improved adhesion, increased micro-throwing power, reduced susceptibility to hydrogen embrittlement, and a markedly decreased need for additives.

The use of modulated dc power supplies with advanced electronic circuitry enables the user to control output patterns with extreme precision. A high-quality unit will superimpose periodic reverse on a high-frequency pulse. The power pattern that results is quite complex, with a wide range of profiles. The output-- a series of pulses with controllable amplitude, frequency, duration, and polarity--influences the deposition characteristics of any solution. The characteristics obtained with modulated power supplies are quite different from those obtained with conventional pulse or periodic-reverse equipment. By "tuning" or shaping the output power pattern to a given plating application, the operator can dramatically enhance the rate of deposition and the character of the deposit.

In periodic-reverse plating, the polarity of a constant dc output is switched back and forth in a regular pattern. Figure 4(a) depicts the ideal output; Figure 4(b) shows the actual output from a slow-response control unit.

Fig. 4 Current patterns in periodic-reverse pulsed-current plating. (a) Ideal pattern. (b) Pattern of a slow-response control unit as seen using an oscilloscope

The sharpness of the output current pattern as revealed by an oscilloscope depends on the degree of ripple in the rectifier output and the quickness of response in the internal switching circuitry of the controller. High-quality units produce extremely sharp square-wave patterns (Fig. 3 and 4). Figure 5 illustrates the wave form of the forward (cathodic) and reverse (anodic) output of a high-quality unit.

Fig. 5 Square-wave current pattern of the forward and reverse envelopes in periodic-reverse pulse plating

The duration of the current in each direction (the forward and reverse envelopes) can be individually controlled from 0.1 ms to 99.99 s. (A zero current delay of less than 0.1 ms between forward and reverse is a design feature of high-quality units that serves to prevent transistor failure due to "shoot through.") The simple, square-wave pattern shown in Fig. 5 is the result of a precisely controlled periodic-reverse output; pulse frequencies can then be superimposed upon this output pattern. Within each envelope, a square-wave pulse is generated (Fig. 6). The frequency and the duration of the pulses are set independently for the forward and reverse envelopes; frequencies range from 10 to 9999 Hz. Duty cycle settings in percentages determine the on and off times for each pulse.

Fig. 6 Current pattern for periodic-reverse pulse plating in which pulse frequencies have been superimposed on the forward and reverse envelopes

On some models, forward and reverse amplitude can be controlled individually (Fig. 7). This permits, for example, a higher current density in the reverse (deplating) stage than in the forward (plating) stage, which is highly desirable for some applications. (More complete explanation of output control with specific units is available in the operation manuals supplied by their manufacturers.)

Fig. 7 Current pattern for periodic-reverse pulse plating in which pulse frequencies of different current densities have been superimposed on the forward and reverse envelopes
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