Mechanism of the Plating Process - The 8th William Blum Lecture
This paper is a re-publication of the 8th William Blum Lecture, presented at the 54th AES Annual Convention in Dallas, Texas, on June 19, 1967. Dr. Henry B. Linford discussed the transition of electroplating from an art to a science as the mechanism of the plating process became better understood.
Dr. Henry B. Linford
Recipient of the 1966 William Blum
AES Scientific Achievement Award
Originally published as H.B. Linford, Plating, 55 (1), 35-39 (1968).
Editor’s Note: This paper is a re-publication of the 8th William Blum Lecture, presented at the 54th AES Annual Convention in Dallas, Texas, on June 19, 1967. A printable PDF version is available by clicking HERE.
Since 1947, the literature has contained a considerable volume of material dealing with the mechanism of metal deposition. These studies have been made using very pure metal electrodes and highly purified solutions with oxygen exclusion being the general rule. These are necessary precautions in order that the reaction can be studied on a purely theoretical basis. Platers, however, do not take these precautions; thus, conclusions drawn from such theoretical, mechanistic studies cannot be safely applied to a practical plating process. The plater needs to know more about the deposition process in the presence of the soils and contaminants which he is forced to accept. Some of these conditions have been studied and we are gradually gaining a better understanding of the mechanism of the plating process.
It is well known that, in order to control a process properly and make improvements in a scientific manner, the mechanism of the reactions involved must be understood. The plating process developed as an art in the 1800s when very little of the chemistry of the processes was understood. The artisan or tank man knew from experience that certain ills could be cured by making changes in solution composition or mode of operation. These adjustments were usually moderately successful; however, common to all such procedures, serious difficulties could not always be avoided. The development of a fundamental understanding of the chemistry of aqueous solutions through the work of Faraday, Gibbs, Helmholtz, Arrhenius and others in the 1800s and early 1900s gave us a better understanding of plating.
Professor W.D. Bancroft of Cornell in 1913l published what he called the axioms of plating. They are as follows:
- Bad deposits are due to excessive admixture of some compound or to excessively large crystals.
- Excessive admixture of any compound can be eliminated by changing the conditions so that the compound cannot precipitate.
- Increasing the current density, increasing the potential difference at the cathode, or lowering the temperature, decreases the size of the crystals.
- The crystal size is decreased when there are present, at the cathode surface, substances which are adsorbed by the deposited metal.
- If a given solution will give a good deposit at any current density, it will give a good deposit at any higher current density, provided the conditions at the cathode surface are kept constant.
- Treeing is facilitated by a high potential drop through the solution and by conditions favorable to the formation of large crystals.
Professor Bancroft had these axioms printed on cards and handed them out during the discussion following a symposium on electroplating, held by the American Electrochemical Society in Atlantic City in 1913. The discussion that followed showed what a "can of worms" he had opened, pointing out the difference in thinking between the electrochemists and the practical platers. Such old-time platers as Charles H. Proctor and George Hogaboom were present. Mr. Hogaboom2 foretold the AES Research Program during this same discussion period when he said, "This day marks an epoch in the history of electroplating in this country, and the gentlemen who have compiled this collection of formulas should have the thanks of the American Electroplaters' Society for the very great service they have performed to the practical electroplater.
"One of the greatest needs of the electroplating industry today is co-operation between the practical man and the electrochemist. One of the ways I think this can be accomplished is by having the universities that have courses in electrochemistry cooperate directly with the plater."
As a result of improved methods of analysis, the introduction of the pH meter allowing accurate pH control, and an increased interest in the plating process by people with backgrounds in physical chemistry, rather successful electroplating was being accomplished by the 1930s. However, it was still too strongly dependent upon the platers' "know-how" to be classified strictly as a science. In the latter 1940s, research on the kinetics of the reactions was initiated. Through these studies, it should be possible eventually to gain an understanding of the process sufficient to put it on a sound scientific basis.
The background of kinetic studies is deeply rooted in overvoltage researches. In 1905, Tafel3 gave us empirical relationships between overvoltage and current density. The constants of the Tafel equation were not given any theoretical interpretation until the development of the theory of the rate process, so ably summarized in the book, "The Theory of Rate Processes," by Gladstone, Laidler and Eyring in 1941.4 Up to this time, about the only overvoltage measurements that were meaningful were those made on the hydrogen deposition reaction, and the most valuable of these studies were conducted on mercury surfaces. There are many reasons for this limitation; among them is the fact that on mercury surfaces the condition of the electrode is extremely reproducible. Also, the overvoltage was fairly large, making corrections for ohmic overvoltage and concentration overvoltage relatively unimportant.
The overvoltage, often called overpotential of a reaction, is defined as the excess voltage over the reversible potential that must be applied to a reaction to cause it to proceed at a particular rate. The researches of Tafel show that, at least at higher current densities, the overvoltage varies as the logarithm of the current density. The overvoltage is commonly broken into three component parts; ohmic, concentration and activation. It is the activation overvoltage which is of theoretical interest in rate studies. As in the case of metal deposition where ohmic overvoltage and concentration overvoltage may be of significant size, some method must be used to measure the activation portion of the overvoltage. The ohmic overvoltage is a direct result of the IR loss through the electrolytes since the reference electrode must be placed a finite distance from the working electrode; some fraction of this IR loss is normally included in overvoltage measurements and, thus, proper correction must be made. Concentration overvoltage results from the removal of reactants or the addition of products to the cathode film as a result of the electrode reaction. Using the methods of Rosebrugh and Miller,5 concentration polarizations may be calculated. When galvanostatic techniques are used,6 the three overvoltages are separated on a time scale (Fig. 1) thus making it possible to read directly the activation overvoltage. The distance on the y axis from the zero point to the beginning of the trace gives the ohmic overvoltage. The vertical distance from the beginning of the trace to the plateau is the activation overvoltage. At approximately 5 × 10-4 sec insufficient time has elapsed to develop any measurable concentration overvoltage.