Electroplating deposit uniformity

Some electroplating solutions produce more uniform deposits than others, and the ability of a solution to deposit uniform coatings is measured by its throwing power (see Section 12.1). 

Indium chemicals and electroplated metal deposits ate replacing mercury (qv) in the manufacture of alkaline batteries (qv). Indium, like mercury, functions to reduce outgassing within the battery and promotes the uniform corrosion of the anode and cathode while the battery is under electrical load. Indium inorganic chemicals also find use as catalysts in various chemical processes. 

In a similar procedure, the atomizer test, which depends on the behavior of an advancing rather than a receding contact angle, a fine mist of water is apphed to the metal surface and the spreading of water is observed. On a clean surface, water spreads to a uniform film. With oleic acid as the test soil, the atomizer test can detect the presence of 10 mg of soil per cm, less than a monomolecular layer (115). For steel that is to be electroplated, the copper dip test is often employed. Steel is dipped into a cupric salt solution and the eveimess of the resulting metallic copper deposit is noted. 

For large Wa, the current distribution is uniform. For example, when electroplating (qv) an object, usually a uniform deposit is desirable. Equation 35 suggests that a larger piece, ie, low Wa, would be more difficult to plate uniformly than a smaller one. 

Electroplating of one metal onto another is widely used for protection against corrosion and wear or for cosmetic purposes.16 Again, the source of metal for deposition could be anodic dissolution or a prepared solution with an inert anode. In contrast to electrolytic refining, only a very thin layer (typically on the order of 1 to 10 pm) of the plating metal is wanted, but usually this layer must be uniform, cohesive, and nonporous, and often a shiny appearance is desired. To understand the roles of some of the variables in electroplating, it is useful to consider the electrodeposition... 

In shielding for radio-frequency and electromagnetic interference an electroplated coating may -not be necessary. Highly effective performance can be obtained with deposits of thickness as low as 2 to 3 /on and these can be applied economically by electroless processes, which give very uniform thicknesses of deposits even in recessed areas. 

The type and nature of the current density distributions are very important when electroplating is used for uniform deposits, appropriate conditions e.g., electrolytes of high throwing power are needed. 

Consider the electroplating of copper, an established industrial process. It is well known to the expert in the field that fast plating of thick layers can be achieved in a so-called acid bath, which consists of CuSO in H SO (with some additives, which need not concern us at this point). If, on the other hand, one wishes to obtain a smooth and uniform deposit on an intricately shaped body, an alkaline cyanide bath is better. The alkaline bath consists of copper ions in an excess of KCN (kept at high pH, to prevent the formation of volatile and highly poisonous HCN). In this bath copper exists as the negatively charged complex ion [Cu(CN) ]. Now, we recall that to achieve uniform current... 

By the process described above, a plasma film could be obtained that had high enough electrical conductivity to allow direct electrodeposition of copper. The bulk resistivity of film measured by a four-point probe was 2.6 x 10 " ohm-cm for the copper-containing polymer film when deposition was stopped after 18 min at HOW. This value is critical if a uniform electrolytic deposit is to be obtained. For safety, deposition was carried out until a total film thickness of 150nm was obtained, giving a nearly pure metallic layer thick enough to allow subsequent electroplating. 

Cyanide solutions are used in the electroplating of gold, silver, zinc, cadmium, and other metals. In these solutions the concentrations of uncomplexed metal ions are very small, and this favors the production of a uniform fine-gTained deposit. Other complex-forming anions (tartrate, citrate, chloride, hydroxide) are also used in plating solutions. 

However, there are also drawbacks with the electroplating process. In practice, electroplating is not as simple as the description above suggests. It is difficult to get metal to deposit in depressions, so the metal layer often is not uniform. Sometimes, deposits are loose and powdery. Additives are used and conditions such as temperature and pH are carefully controlled to overcome these problems. Over time, impurities build up in the solutions used for electroplating. Eventually, the spent solutions must be discarded. They can contain high concentrations of such toxic metals as cadmium or chromium and require careful disposal to protect the environment. 

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