Current Distribution on Microprofiles - The 14th William Blum Lecture - Part 1
This article is the first of four parts of a re-publication of the 14th William Blum Lecture, presented at the 60th AES Annual Convention in Cleveland, on June 18, 1973. Dr. Otto Kardos gives a comprehensive discussion of how the deposit forms on surface textures, and how leveling and brightness are achieved.
Recipient of the 1972 William Blum
AES Scientific Achievement Award
Editor’s Note: Originally published as Plating, 61 (2), 129-138 (1974), this article is the first of four parts of a re-publication of the 14th William Blum Lecture, presented at the 60th AES Annual Convention in Cleveland, on June 18, 1973.
A printable PDF version of Part 1 is available by clicking HERE.
The printable PDF version of the complete 52-page paper is available HERE
A survey of experimental facts and theory is given. The three types of cathodic metal distribution on microprofiles (recess/peak thickness ratio = 1, < 1, > 1) are explained. Diffusion controlled inhibition is shown to be the preponderant mechanism of the last type. Cathodic and anodic microthrowing power are compared. Roughness formation on initially smooth surfaces, and its prevention by addition agents or by modulated current, are discussed and possible brightening and leveling mechanisms other than diffusion controlled inhibition are suggested.
Current distribution on microprofiles was also the subject of two papers written by Dr. Foulke and myself for the 1956 Convention of the American Electroplaters' Society. These papers supplied proofs for a diffusion theory of leveling and "bad microthrow."1,2 At about the same time, Watson and Edwards3 developed independently a very similar diffusion theory of leveling and Leidheiser4 had published the outline of a somewhat different theory.
These papers aroused considerable interest, and many studies on cathodic current and metal distribution on microprofiles, especially on leveling, were made in this and other countries. They generally supplied new or more exact proofs of the diffusion theory of leveling, but some studies reported at the same AES Convention by Thomas,5 supported another theory which ascribed the increased adsorption of the leveling agent on micropeaks not to a greater local diffusion rate but to "shape-sensitive" adsorption related to the small radius of curvature of the micropeaks.
In view of the great practical importance of leveling, of the possible relevance of a correct theory of leveling to the understanding of brightening, and of the considerable international research effort in this field, it seems useful to summarize the theory, report on the significant contributions of others, and to look at where we are now.
For reviews of the field of microthrowing power see references 6-12.
Cathodic metal distribution on microprofiles, which may be defined as profiles which have a roughness depth or height of less than about 0.5 mm (500 μm, 0.02 in.), shows a surprising variety: on descent from a micropeak into a microdepression, the metal deposit thickness may decrease, remain constant, or increase. One speaks correspondingly of "bad microthrow," "good microthrow" and "true leveling" (excellent microthrow).
Good microthrow or the rather uniform metal deposition into small pores, crevices and scratches was first described by W. Meyer13 in 1935. All three types of microthrow were described by Gardam in 194714 and by Reinhard in 1950.15 Well-leveling nickel baths16,17 were available in the late forties.
But in the fall of 1955 when Dr. Foulke and I started to think more intensely about microthrowing power, no general theory was available. As is often the case in electroplating, and in many other technological areas, "art" preceded "science," or empirical technology preceded scientific understanding. However, one type of microthrowing power, namely practically uniform metal distribution ("good microthrow"), was already explained as a benefit from the well-developed theory of macro-throwing power.
2. The theory of macrothrowing power and its contribution to the understanding of microthrowing power
To understand microthrowing power it is necessary to first understand macrothrowing power, that is cathodic current and metal distribution on large-scale profiles, the so-called macroprofiles. The theory of macrothrowing power had been developed much earlier and the classical paper about this theory was published by Haring and Blum 50 years ago.18 Their paper defined "primary" and "secondary" current distribution, pointed out the influence of the variation of current efficiency with current density on metal distribution, and defined throwing power (which meant at that time, of course, macrothrowing power) as "the deviation (in per cent) of the metal distribution ratio from the primary current distribution ratio." Good reviews on macrothrowing power are found in references 9, 18-25.
Primary current distribution is the current distribution which would be obtained in absence of polarization, which increases with increasing local current density. It would be determined only by the resistance which the electrolyte opposes to the flow of electric current to the different electrode areas. Because of the much higher (often a million times) electrical conductivity of metals as compared to the conductivity of electrolytes, the surface of a metallic electrode can generally be considered to be at practically constant potential, unless the electrode is very long or thin. Finding the primary current distribution on a flat or profiled electrode in the presence of a counter-electrode and of the nonconducting boundaries formed by the walls of the plating cell consists thus in finding the potential distribution between two equipotential surfaces. Mathematical20,26,27,27A,29A as well as experimental methods23,28,28A,29,29A are available for the determination of primary current distribution. The latter depends on the shape, the relative sizes and positions of the electrodes and of the nonconducting walls of the electrolytic cell, but it is independent of the absolute size of the system, of the electrolyte properties and the conditions of electrolysis.
In primary current distribution the current concentrates on protruding areas not only because of their smaller distance from the counter electrode but, still more, because of their lateral accessibility to the current. Indeed primary current distribution would give infinite current density on ideally sharp peaks and zero current density on the sharp hollow corners of a triangular wave profile.