Process Principles

The theory behind pulse plating is simple (Ref 1, 2). The cathode film is kept as rich in metal ions as possible and as low in impurities as possible. During the period when the current is on, the metal ions next to the cathode are depleted and a layer rich in water molecules is left. During the portion of the cycle when the current is off, the metal ions from the bulk of the plating solution diffuse into the layer next to the cathode. Then the process is repeated again. Also during the time the current is off, gas bubbles and impurities that have been adsorbed on the cathode have a chance to desorb.

Typical on-time range from 0.1 to 9.9 ms, and typical off-times range from 1 to 99 ms. If an ammeter is inserted into a pulsed-current plating circuit, it responds to the average current. In order to have the same plating rate using pulsed current as with conventional continuous current, the average current must be the same. This can be achieved by adjusting the peak current, the on-time, or the off-time. The physical and chemical properties of deposits can be very precisely controlled through the careful selection of pulse-plating parameters.

Concepts and Terminology. Selected terms that have special meaning when applied to pulse plating are defined as follows:

• Cathodic (forward) and anodic (reverse) are used to describe current direction; cathodic indicates flow is in the normal (plating) direction, anodic indicates flow is in the reverse (deplating) direction. In normal operation of a reversing pulse unit, current direction alternates in a controllable forward and reverse pattern.

• Envelope is the time span during which current may flow in only one direction. The time spans of the forward envelope and the reverse envelope are set individually.

• Pulse train is a regularly interrupted current flow in either the cathodic or anodic direction. A pulse train exists within the envelope.

• Pulse is the individual interval in a pulse train, consisting of one "on and off" period.

• Pulse rate is the number of times the current is switched on in a given period of time (usually 1 s).

• Duty cycle is the ratio of time an individual pulse is on compared to the total (on and off) pulse time. For example, 5 ms on and 5 ms off is a 50% duty cycle, 4 ms on and 1 ms off is an 80% duty cycle, and so on. (Note: if the duty cycle is 100%, there is no off time; the current is on for the duration of the envelope and there is no pulse or frequency.)

• Frequency is the pulse rate expressed as hertz units (e.g., 100 Hz = 100 pulses/s).

• Pulse width is the time span of the on portion of a pulse. Pulse width is a function of both frequency and duty cycle. For example, a 1000 Hz pulse with a duty cycle of 50% has a pulse width of 0.5 ms.

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