Why Electroplating is Often Performed Using Constant Current Power Sources

Power sources are often designed to put out a particular voltage with current levels falling within a certain range. For instance, a power source may be rated 12 volts DC, with currents from 0 to 100 milliamperes, or perhaps 0 to 1.0 amperes, etc. Such sources may experience fluctuating current draw, whereas the voltage remains pretty much constant.

Electroplating

Most electroplating applications are not performed using constant-voltage power sources, with their fluctuating currents. Rather, most electroplating is performed using constant current, with the voltage being free to fluctuate. The plating thickness is evaluated in terms of time and current used. For instance, many gold-plating baths function best at approximately 3.0 amperes per square foot of surface area. The time of electroplating accurately determines the resulting thickness.

Example

You have a gold bath known to plate best at 3.0 amps per square foot, with fifty microinches (millionths of an inch) of gold plating onto a substrate in six minutes. If you wish to plate an object that has 0.17 square feet of surface area with a layer of gold 300 microinches thick, you will set the electrical current at:

0.17 square feet x 3.0 amps per square foot = 0.51 amperes of current

Thus he will set it for a half ampere of electrical current, rounding off. Since he wants 300 microinches, and six minutes equals 50 microinches, he will plate the piece:

(300 microinches) / (50 microinches) / (6 minutes) = (300 x 6) / 5 = 36 minutes

Thirty-six minutes of plating at one-half ampere of electrical current is what you will use.

Why are Current and Thickness Related?

Current and plating thickness are related, because current flowing through an object (such as a wire or a piece to be plated) is directly equivalent to the number of electrons that flow through it. Gold ions in a gold-plating solution are neutralized to atoms of metal by a specific number of electrons (in this case, probably two); thus the atoms of gold produced are directly related to the current used. The voltage-sometimes related to the force of the electrons-is nearly immaterial, and primarily depends upon the conductivity of the bath.

The Math

The mathematical relationship is not complex. The amount of charge that flows is related to electrical charge by the relationship:

Equation: I = Q / T

Where I is the current, Q is the charge, such as in Coulombs, and T is the time. This is a slight simplification. In fact, the total charge divided by the charge carried per electron gives the number of electrons that flow in a given time. Divide that number of electrons by the number required by each atom in order to electroplate out (say two electrons per atom), and you get the number of atoms of metal plated. Really, one needn't know much concerning the mathematics of plating, as most electroplating baths come complete with a data sheet.

References and Resources:

Case Western Reserve University - Electrochemistry Encyclopedia - Electroplating

David W. Brooks, University of Nebraska-Lincoln - How to Copper Plate Your Car Keys