Bath Constituents

Although less constricted than the particle properties the electrolyte composition is also largely determined by the desired composite. The bath constituents and pH can be varied only within certain limits to ensure a metal matrix of sufficient quality. Additives present an effective way of regulating the particle composite content, but can have adverse effects on the deposit quality. Consequently, also the electrolyte composition poses restrictions on the attainable particle composite contents 

Since the particle surface composition is determined by the adsorption of electrolyte ions, changes in surface composition of the particles are expected to play a role in the effect of the bath constituents. Kariapper and Foster67 found that the amount of metal ions adsorbed on a particle increases with increasing metal ion concentration in the electrolyte. For SiC particles this was again related to the -potential of the particles, because it was found68 that the -potential increases with 

An adverse effect of adsorption of H2PO2, which is the reducing agent in the bath, is a decrease in plating speed in the presence of suspended particles. Additionally, adsorption of stabilizers results in a reduced stability of electroless bath containing suspended particles. Typically, the life-time of a Ni(P) bath is reduced from 10-15 metal turn-overs in a particle-free bath to around 5 in a particle-containing bath. Hence, through adsorption bath constituents do not only affect particle incorporation, but suspended particles also influence the metal deposition. 

The metal surface properties also change with the bath constituents and thereby affect the particle-electrode interaction. Metal deposition constitutes a multi-step reaction mechanism that depends on the bath composition. In quite a number of reaction mechanism adsorbed intermediates, e.g. the presence of chromium and catalyst polyoxides on the metal surface during chromium plating, are involved. Not the metal surface, but the adsorbed intermediates will determine the particle-electrode interaction and might even compete for adsorption sites on the electrode surface with the particle. Although the reverse, i.e., the change in metal deposition mechanism due to the presence of particles has been investigated (see Section 3.U), no studies on the effect of the deposition mechanisms on particle codeposition have been reported. 

Otherwise changes in metal deposition behavior with pH could be involved. Due to the competition between reduction of metal ions and hydrogen ions at the cathode the pH affects metal deposition. The current efficiency70 of nickel deposition was seen to decrease markedly below pH 2 in the presence of SiC particles. Unfortunately, it was not determined if this effect is accompanied by a decrease in particle content below pH 2. 

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Process Control. Some hot nickel and flash electroless copper solutions are plated to the point of exhaustion and then discarded. Most baths are formulated to give bath fives of >6 turnovers of the bath constituents some reach steady-state buildup of the by-products and can be used indefinitely. AU. regenerable solutions should be filtered to remove particulates that can cause deposit roughness and bath instability. 

Commercial processes Commercial electroless nickel plating stems from an accidental discovery by Brenner and Riddell made in 1944 during the electroplating of a tube, with sodium hypophosphite added to the solution to reduce anodic oxidation of other bath constituents. This led to a process available under licence from the National Bureau of Standards in the USA. Their solutions contain a nickel salt, sodium hypophosphite, a buffer and sometimes accelerators, inhibitors to limit random deposition and brighteners. The solutions are used as acid baths (pH 4-6) or, less commonly, as alkaline baths (pH 8-10). Some compositions and operating conditions are given in Table 13.17 . 

Lowering the Concentration of Plating Bath Constituents. A decrease in the concentration of metal salts and other components of the plating solution directly reduces the amount of hazardous substances dragged out of the bath. It also leads to lower solution... 

Typical bath constituents for alloy plating are given in Table 3, which also gives some impression of the variety of alloys which may be electrodeposited. 

Alloys Bath constituents Alloys Bath constituents... 

The mechanism of particle incorporation is treated extensively in the next section, but a generalized mechanism is given here to better comprehend the effects of the process parameters. Particle incorporation in a metal matrix is a two step process, involving particle mass transfer from the bulk of the suspension to the electrode surface followed by a particle-electrode interaction leading to particle incorporation. It can easily be understood that electrolyte agitation, viscosity, particle bath concentration, particle density etc affect particle mass transfer. The particle-electrode interaction depends on the particle surface properties, which are determined by the particle type and bath composition, pH etc., and the metal surface composition, which depends on the electroplating process parameters, like pH, current density and bath constituents. The particle-electrode interaction is in competition with particle removal from the electrode surface by the suspension hydrodynamics. 

Despite these successes, important process parameters, like bath agitation, bath constituents and particle type are disregarded. The constants k, 0 and B inherently account for these constants, but they have to be determined separately for every set of process parameters. Moreover, the postulated current density dependence of the particle deposition rate, that is Eq. (2), is not correct. A peak in the current density against the particle composite content curve, as often observed (Section III.3.H), can not be described. The fact that the peak is often accompanied by a kink in the polarization curve indicates that also the metal deposition behavior can not be accounted for by the Tafel equation (Eq. 4). Likewise, the (1-0 term in this equation signifies a polarization of the metal deposition reaction, whereas frequently the opposite is observed (Section 111.3,(0 It can be concluded that Guglielmi s mechanism... 

The main component in nickel sulfamate baths — sulfamate - can be determined in a single rim, in addition to the decomposition product sulfate and other bath constituents such as chloride and bromide (see Fig. 8-32). To achieve a sufficient separation between the sulfamate ions and chloride, both of which elute near the void volume, two identical anion exchangers were used in series, even though this increases the total analysis time to about 20 minutes. 

Metal-EDTA complexes may be separated with the aid of anion exchange chromatography (see Fig. 3-78 in Section 3.3.4.4). They are detected again via electrical conductivity measurements. In electroless copper baths, for example, it is thus possible to distinguish between free and complex-bound EDTA. Other bath constituents do not interfere with the determination of the copper-EDTA complex, Cu(EDTA)2-. 

Bath constituents Stannous sulfate and sulfuric acid are maintained by analysis, additives by spectrophotometry. Ah usage, Hull cell, and the percentage of sulfuric acid additions. 

Krohn C, Smlie M, 0ye H. Perretration of sodium and bath constituents into cathode carbort materials used in industrial cells. Light Met. 1982 111 311. 

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