Copper coatings porosity

On substrates of brass and copper alloys, determines coatings porosity and resistance to handling (perspiration). Consists of immersion in 85 % lactic acid solution, drying and incubating above acetic acid vapors for 20 h to reveal discoloration spots at failure points or delaminations. 

Porosity is particularly important in thin cathodic coatings, such as gold contacts on copper used in the electronics industry, because corrosion will occur on the more reactive substrate and result in either perforation of the base, or the formation of corrosion products on the surface, which in turn have a detrimental effect. 

Nickel coatings are usually prepared by electroplating. The metal is plated either directly on steel or sometimes over an intermediate coating of copper. The copper underlayer is used to facilitate buffing of the surface on which nickel is plated, because copper is softer than steel, and also to reduce the required thickness of nickel (which costs more than copper) for obtaining a coating of minimum porosity. The automotive industry uses nickel as an underlayer for microcracked chromium to protect steel [9]. 

Porous coatings of copper onto aluminum are shown by the SEM images in Fig. 9.17. These samples were obtained by an immersion of aluminum powders into alkaline Cu(II) solutions [3]. Due to porosity of the deposited copper, the electrolyte penetrates through pores and reacts with aluminum. In this way, simultaneous reactions of aluminum dissolution, hydrogen evolution, and copper deposition occur. A complete dissolution of aluminum and a production of hollow copper particles are shown in Fig. 9.17. 

The likelihood of an unacceptable porosity increases markedly for thin coatings, and this may be catastrophic for noble coatings (as noted in Chapter 10), such as copper on steel The upper limit on deposit thickness is influenced by such factors as an excessive process time, too high an internal stress or development of irregular deposit growth, e.g. nodules or dendrites (Figs. 8.8 8JO),... 

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