Using Faraday's law to predict plating thickness
Electrodeposition follows Faraday's law: the amount of metal deposited is proportional to the charge passed. Thickness equals deposition rate Ć current density Ć time Ć current efficiency, and the deposition rate itself is fixed by the metal's atomic weight, ion valence, density and the Faraday constant of 96,485 C/mol. A familiar shop check falls straight out of it ā acid copper at 25 ASF lays down about 1 mil in 45 minutes.
Current efficiency is what separates the textbook number from the tank. Watts nickel and acid copper typically run 95% or better, but hexavalent hard chrome puts most of the current into hydrogen evolution and lands near 10 to 15%. Leaving efficiency at 100% would make a chrome cycle look roughly seven times faster than it really is, so use the measured efficiency for the bath you actually run.
The result is an average thickness over areas that see uniform current. Real parts build thicker on edges and high points and thinner inside recesses and deep holes. When a print calls out a minimum thickness, time the cycle for the thinnest area, and review thief anodes, racking and bath maintenance limits with your plating supplier before locking in a process.
Frequently asked questions
Usually the assumed efficiency is too high, or contact resistance and an aged bath mean less current actually flowed than the setting showed. Back-calculate efficiency from the rectifier reading and a measured thickness, then use that figure.
It scales proportionally, but only within limits. Every bath has a maximum working current density, and exceeding it causes burning, rough deposits and poor adhesion, so stay inside the supplier operating range.