PERFORMANCE CHARACTERISTICS OF AGITATED ELECTROLESS PLATING BATHS
4.3 Summary and conclusions
4.2.2.2.4 Film thickness
Table 4.8 presents the variation in average thickness of the nickel layer with stirrer speed for various values of initial metal solution concentration and loading ratios evaluated using Eq. (2.12). The porous nickel film thickness varied from 2.34–55.17 µm.
The porous nickel film thickness is observed to be more strongly dependent on the solution concentration as well as loading ratio. A near inversely proportional dependence of thickness is observed with the loading ratio at higher metal solution concentration thereby indicating that higher amount of metal in the solution provides nickel films with higher thickness. Stirring contributes to about 20–30% additional enhancement in the metal film thickness, thereby indicating greater metal transport to the membrane surface.
The observed data trends in thickness indicate that electroless plating is capable to provide thicker nickel films as intermediate diffusion barriers that are required for better mechanical strength and permeation characteristics.
combinatorial performance characteristics are evaluated in terms of conversion, plating efficiency, metal film thickness, average pore diameter and PPD.
A comparative assessment of the performance of the two reducing agents in agitated electroless plating baths is presented in Table 4.9. From this table (Table 4.9), it can be noticed that the hydrazine based electroless plating baths result in higher values of conversion, efficiency, PPD, metal film thickness and average mass transfer coefficient indicating that these are superior to hypophosphite baths. Lack of competence of hypophosphite based electroless plating baths with hydrazine based baths even under agitation is due to the basic problem of hydrogen generation in the prior case. However, the optimal values of the process parameters namely concentration, loading ratio and stirrer speed are found to be the same (0.08 mol/L, 393 cm2/L and 100 rpm respectively) for both the cases.
The mean pore diameter of the nickel–ceramic composite membrane as obtained from the gas permeation experiments is also confirmed by the surface FESEM micrographs (Figure 4.16). Further, it can be observed from the Figure 4.16 that agitation
Table 4.9: Comparison between agitated hypophosphite and hydrazine baths.
Parameter Agitated
hypophosphite baths
Agitated hydrazine
baths
Conversion (%) 12.5–44.0 17–58.5
Efficiency (%) 96.9–41.9 99.2–74.1
Pore diameter (nm) 100–59 81–14
PPD (%) 86.7–95.5 91.3–99.7
Porosity of metal layer 0.001–0.028 0.002–0.092
Thickness (µm) 1.8–20.5 2.7–41.9
Average mass transfer coefficient, k (s–1) 60–154 104–573
Optimal concentration (mol/L) 0.08 0.08
Optimal loading ratio (cm2/L) 393 393
Optimal stirrer speed (rpm) 100 100
results in smoother surface of the metal film than the base case (Figure 3.17) and the surface smoothness increases with increasing the stirrer speed from 50 to 200 rpm. A
Hypophosphite baths Hydrazine baths
50 rpm 50 rpm
100 rpm 100 rpm
200 rpm 200 rpm
Figure 4.16: Surface FESEM micrographs of composite membranes prepared in agitated electroless plating baths. (Ci = 0.16 mol/L and θ = 196 cm2/L).
100 nm 200 nm
100 nm 100 nm
200 nm 200 nm
number of pin holes are observed on the surfaces of the nickel films deposited in hypophosphite baths, which is not the case with hydrazine baths.
Based on thorough experimental investigations, it is found that the bath agitation by means of membrane stirring improves the performance of electroless plating baths but results in lower plating efficiencies compared to the base case. The following observations have been concluded from the experimental investigations.
a) Based on conversion, plating efficiency, PPD, metal thickness, average reaction rates, the optimal conditions of electroless plating for both the reducing agents are initial nickel solution concentration of 0.08 mol/L, stirrer speed of 100 rpm and a loading ratio of 393 cm2/L.
b) The influence of loading ratio is significant on the parameters such as metal film thickness and effective porosity but not with respect to the other parameters such as average metal film pore size, conversion, plating efficiency and PPD.
c) Mass transfer enhancement in the form of membrane stirring brings about 20–56%
enhancement in the average nickel deposition rate (efficient plating). Stirred electroless hypophosphite baths do not function efficiently at higher stirrer speeds and higher solution concentrations. This is attributed to the enhanced nucleation under these conditions in the solution, which contributes significantly to inefficient metal plating.
d) Even with the well known versatility of nickel–hypophosphite plating baths in metal finishing industries, these baths are characterized with low conversions (10–
44%), moderately good plating efficiencies (42–99%) and PPD values (78–96%).
Therefore, further improvement to the plating process that target doubling
conversions while maintaining similar trends in plating efficiencies and PPD values is desired.
e) Data analysis based on air permeation experiments is a novel approach that can effectively capture the quality of metal plating on membrane surfaces. The approach can eventually replace the usually followed convention of physical examination using scanning electron and atomic force microscopy methods.
f) The approach presented in this work can be used as a new methodology for the assessment of nickel electroless plating baths with variant process parameters and conditions of operations.
The effect of process parameters such as concentration and loading ratio on the performance characteristics of the nickel–ceramic composite membranes is presented in the previous chapter (Chapter 3) while studies on the effect of mass transfer enhancement through membrane rotation is presented in this chapter. The next chapter (Chapter 5) presents the effect of sonication on the performance of electroless plating baths. Finally, the performance of electroless plating baths under hydrothermal conditions is presented in Chapter 6.