44
2 Electrochemistry and Electrodeposition
in which the metal deposition is accompanied with a side process like hydrogen
evolution. Therefore, no wonder that the diagram parameters are always given rather
qualitatively than in a strictly quantitative manner.
The ordinate in the Winand diagram indicates the inhibition efficiency, which is
even more ill-defined than the parameter related to the current density. Although the
impact of various additives is difficult to compare, it is plausible that the increase in the
adsorption bond strength to the surface is related to a larger inhibition effect. Also, the
increase in the additive concentration (at least, below the saturation concentration)
leads to a larger coverage of the surface, and hence, it increases the deposition
efficiency. All these trends can be phrased also with alternative terms: a larger surface
coverage with additives and a larger residence time of an additive molecule at the
surface is related to an enhanced inhibition effect.
In spite of the lack of the fully quantitative treatment, the Winand diagram plays
a very important role in the formation of our contemplation on the simultaneous
treatment of the bath operation parameters in relation to the deposit properties. One
of the major messages of the diagram is that the bath operation at high current
density may drive the planar metal coating growth to a regime where dendrites are
developed, especially when the process is accompanied with gas evolution, too.
Another important piece of information to remember is that by using an additiverich bath, there is more chance to obtain a fine-grained deposit than with a noninhibited bath. It is to be noted that complexing agents play a very similar role to
other additive types. The excess free molecules with complexing capability can also
attach to the surface and block the growth. Therefore, it is very typical that baths with
complexed metal salts yield a finer grain structure under otherwise similar conditions
than non-complexed baths of the same metal.
It is indispensable to mention that the modulation of the deposition current may
have an immense impact on the grain structure. Pulse plating usually leads to a very
fine grain structure. The principle of pulse plating is that a current pulse is applied
for a short time, often with much larger current density than the mass transportlimited current density corresponding to the steady-state deposition. Due to the high
current density, the surface concentration of the adatoms will be high that favours the
nucleation as opposed to the growth of the already existing crystals. The on-time is
fairly short (typically falls in the 1–100 ms range) and the pulsating depletion layer
is much thinner than the depleted solution layer during d.c plating. After the on-time,
an off-time follows the deposition period when the concentration of the precursor
ions near the substrate relaxes and approaches the original (bulk) concentration. The
repetition of this period, the on-time and the upcoming off-time, leads to a deposit in
which the nucleation zone is not restricted to the close vicinity of the substrate but
is extended to the entire deposition time, which results in a fine-grained structure.
Another advantage of the pulse plating is that the parts of the surface with different
accessibility can be plated in a much more even manner than with d.c. plating, simply
because the thickness of the depleted solution layer follows the shape of the work
piece. The theory and practice of pulse plating are available in various excellent
reviews [18, 19].
2 Electrochemistry and Electrodeposition
in which the metal deposition is accompanied with a side process like hydrogen
evolution. Therefore, no wonder that the diagram parameters are always given rather
qualitatively than in a strictly quantitative manner.
The ordinate in the Winand diagram indicates the inhibition efficiency, which is
even more ill-defined than the parameter related to the current density. Although the
impact of various additives is difficult to compare, it is plausible that the increase in the
adsorption bond strength to the surface is related to a larger inhibition effect. Also, the
increase in the additive concentration (at least, below the saturation concentration)
leads to a larger coverage of the surface, and hence, it increases the deposition
efficiency. All these trends can be phrased also with alternative terms: a larger surface
coverage with additives and a larger residence time of an additive molecule at the
surface is related to an enhanced inhibition effect.
In spite of the lack of the fully quantitative treatment, the Winand diagram plays
a very important role in the formation of our contemplation on the simultaneous
treatment of the bath operation parameters in relation to the deposit properties. One
of the major messages of the diagram is that the bath operation at high current
density may drive the planar metal coating growth to a regime where dendrites are
developed, especially when the process is accompanied with gas evolution, too.
Another important piece of information to remember is that by using an additiverich bath, there is more chance to obtain a fine-grained deposit than with a noninhibited bath. It is to be noted that complexing agents play a very similar role to
other additive types. The excess free molecules with complexing capability can also
attach to the surface and block the growth. Therefore, it is very typical that baths with
complexed metal salts yield a finer grain structure under otherwise similar conditions
than non-complexed baths of the same metal.
It is indispensable to mention that the modulation of the deposition current may
have an immense impact on the grain structure. Pulse plating usually leads to a very
fine grain structure. The principle of pulse plating is that a current pulse is applied
for a short time, often with much larger current density than the mass transportlimited current density corresponding to the steady-state deposition. Due to the high
current density, the surface concentration of the adatoms will be high that favours the
nucleation as opposed to the growth of the already existing crystals. The on-time is
fairly short (typically falls in the 1–100 ms range) and the pulsating depletion layer
is much thinner than the depleted solution layer during d.c plating. After the on-time,
an off-time follows the deposition period when the concentration of the precursor
ions near the substrate relaxes and approaches the original (bulk) concentration. The
repetition of this period, the on-time and the upcoming off-time, leads to a deposit in
which the nucleation zone is not restricted to the close vicinity of the substrate but
is extended to the entire deposition time, which results in a fine-grained structure.
Another advantage of the pulse plating is that the parts of the surface with different
accessibility can be plated in a much more even manner than with d.c. plating, simply
because the thickness of the depleted solution layer follows the shape of the work
piece. The theory and practice of pulse plating are available in various excellent
reviews [18, 19].