Electroplating surface roughing

Pulse plating of zinc, cadmium, nickel, chromium and precious metals in aqueous media and molybdenum, chromium, tungsten, niobium and titanium in fused salts improves the properties of the deposits [1,15]. Dense, not dendritic coatings can be obtained because concentration polarization is minimized by the use of pulse current (PC) [16]. Moreover, other pulse plating effect can improve the surface roughness and morphology of the electroplated coatings [17, 18],... 

Roughness has important implications in wetting applications. While the eutectic solder, SnPb, normally forms a contact angle of 15-20° with copper, it completely wets the surface of rough electroplated copper and forms a fractal spreading front [69]. 

Poor preparation of the substrate can result in loss of adhesion, pitting, roughness, lower corrosion resistance, smears, and stains. Because electroplating takes place at the exact molecular surface of a work, it is important that the substrate surface be absolutely clean and receptive to the plating. In the effort to get the substrate into this condition, several separate steps may be required, and it is in these cleaning steps that most of the problems associated with plating arise. 

Figure 6.13 Scanning electron microscopy image of an Au SSV structure. Note the smooth electroplated metal walls and top surface. The rough circular areas at the bottom of each cavity are the evaporated Au substrate.

This raises some important possibilities, which have not escaped the attention of the electroplating community. For example, while metal deposition is conducted in fairly concentrated solutions of the metal being plated, and at current densities well below the mass-transport limit, additives acting as inhibitors for metal deposition are often introduced at concentrations that are several orders of magnitude lower, to ensure that their supply to the surface will be mass-transport limited. In this way, the tendency for increased rate of metal deposition on certain features on the surface, such as protrusions, will be moderated by the faster diffusion of the inhibitor to the very same areas. Furthermore, if deposition occurs in the region of mixed control, which is usually the case, it must be remembered that the relevant roughness factor is quite different for the charge-transfer and the mass-transport processes, and this may well be a function of current density, since the Faradaic resistance is inherently potential dependent. 

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