Effect of low concentrations of Pb2+ on Sn electrodeposition in methyl sulphonic acid solutions

Article abstract of DOI:10.1016/j.electacta.2007.10.023

The influence of Pb²⁺ on the initial stages of Sn electrodeposition onto copper from methyl sulphonic acid solutions in the absence of organic additives has been investigated. Particular attention is placed on the effect of Pb²⁺ at concentrations much lower than previously reported and where no lead is detected in the resulting deposit. Linear sweep scans and i-t transients show that Pb²⁺ catalyzes Sn plating at Pb²⁺:Sn²⁺ molar concentration ratios between about 1:1000 and 20:1000. This effect can be significant, with as much as a 30% increase in the amount of tin deposited over that obtained in the absence of Pb²⁺ when the concentration ratio is 2:1000 or 5:1000. At concentration ratios above 20:1000, the effect disappears and, in fact, Pb²⁺ has an inhibitory effect on Sn²⁺ reduction. Under conditions where Pb²⁺ enhances Sn electrodeposition, it also begins to affect electrocrystallization by promoting faster nucleation and a more 2-dimensional coating that more rapidly covers the substrate. This trend continues as the Pb²⁺ concentration is further increased and the catalytic effect disappears.

Full text of DOI:10.1016/j.electacta.2007.10.023

Available online at www.sciencedirect.com
Electrochimica Acta 53 (2008) 2430–2440
Effect of low concentrations of Pb2+ on Sn electrodeposition
in methyl sulphonic acid solutions
Haiyan Wanga, Mark Pritzkerb,∗
a Department of Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China
b Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Received 14 June 2007; received in revised form 3 October 2007; accepted 4 October 2007
Available online 22 October 2007
Abstract
The influence of Pb2+ on the initial stages of Sn electrodeposition onto copper from methyl sulphonic acid solutions in the absence of organic
additives has been investigated. Particular attention is placed on the effect of Pb2+ at concentrations much lower than previously reported and
where no lead is detected in the resulting deposit. Linear sweep scans and i-t transients show that Pb2+ catalyzes Sn plating at Pb2+:Sn2+ molar
concentration ratios between about 1:1000 and 20:1000. This effect can be significant, with as much as a 30% increase in the amount of tin
deposited over that obtained in the absence of Pb2+ when the concentration ratio is 2:1000 or 5:1000. At concentration ratios above 20:1000, the
effect disappears and, in fact, Pb2+ has an inhibitory effect on Sn2+ reduction. Under conditions where Pb2+ enhances Sn electrodeposition, it also
begins to affect electrocrystallization by promoting faster nucleation and a more 2-dimensional coating that more rapidly covers the substrate. This
trend continues as the Pb2+ concentration is further increased and the catalytic effect disappears.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: Sn; Pb; Electrodeposition; Linear sweep voltammetry; Morphology
1. Introduction
responding single metal systems [5,7,9,19]. This effect has been
attributed to the fact that Sn2+ and Pb2+ reduction kinetics are
both so rapid that the two processes occur largely independent
of each other [7].
Duetotheirexcellentsolderability, lowmeltingpointandcor-
rosion resistance, Sn-Pb alloys are widely used in applications
such as the electronics and steel industries. Most Sn-Pb alloy
coatings are now produced electrochemically due to advances
in electroplating methods and advantages over hot dipping pro-
cesses. Thisinteresthasledtostudiesontheeffectsofplatingand
solution conditions on coating composition, morphology and
properties [1–18]. Several of these studies have shown that the
interaction between the two metals depends on whether organic
additives are also present. In the absence of organic additives,
the dependence of Sn-Pb alloy content on solution composi-
tion tends to follow an S-shaped curve, with no detection of the
minor component in the deposit if its atomic percent in solution
is below a certain level [1,7,8]. The potential dependence of the
partial currents for Sn2+ and Pb2+ reduction during alloy plating
has been found to be the same as that of the currents in the cor-
On the other hand, studies have shown that when an organic
additive such as peptone or Triton X-100 is present in the binary
system, significant interactions occur between Sn, Pb and the
adsorbed organic on the electrode surface and the electrode
kinetics for Sn2+ and Pb2+ reduction now differ from the kinet-
ics of the single metal systems [2,3,5,7,14,18]. Whether or not
organic additives are present, it generally has been found that the
addition of Pb(II) tends to inhibit Sn2+ reduction [7–11,14,18].
Also, the deposit morphology changes to resemble that char-
acteristic of Pb coatings even when the solution contains more
Sn2+ than Pb2+ [10,11,18].
Another important application for this system is the addition
of relatively small amounts of Pb2+ to Sn electroplating baths
to modify the nature of the resulting deposits. Sn coatings
are widely used in the food, shipping, container, automotive
and electronics industries due to their excellent mechanical
properties, corrosion resistance, solderability and non-toxicity.
Unfortunately, pure Sn tends to form dendrites when electrode-
∗
Corresponding author. Tel.: +1 519 888 4567; fax: +1 519 746 4979.
E-mail address: pritzker@uwaterloo.ca (M. Pritzker).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.10.023

H. Wang, M. Pritzker / Electrochimica Acta 53 (2008) 2430–2440
2431
posited and compressive stresses can cause metallic whiskers
to form [20–23]. This last effect is particularly troublesome in
electronic applications and devices with closely spaced compo-
nents. However, the presence of as little as 1 or 2 wt% Pb in the
deposit has been shown to alleviate these problems and suppress
whisker formation [22–24] and so has often been employed for
those applications such as electronic fabrication where its tox-
icity is not an overriding consideration. Britton [22] proposed
that foreign adatoms impede the movement of Sn atoms along
the surface toward growing dendrites or whiskers. Kakeshita et
al. [23] observed that coatings formed in the presence of Pb and
organic additives were whisker-free and had a well-polygonized
crystal grain structure with a relatively uniform size of 1–3 ␮m.
On the other hand, whisker formation in the absence of Pb was
associated with smaller (<1 ␮m) and more angular and irregular-
shaped grains. Recently, Zhang and Schwager [18] proposed that
Pb suppresses whisker formation on a copper electrode by caus-
ing the CuSn intermetallic compound at the coating/substrate
interface to form more uniformly over the copper surface and
be in a slightly tensile state rather than a compressive state.
In most studies, the Pb2+ and Sn2+ concentrations have been
maintained at roughly the same levels. Thus, both metals code-
posit during cathodic polarization, making it more difficult to
determine whether the effect on Sn deposition is due to bulk
deposition of Pb or other effects. It will therefore be useful to
focus on much lower Pb2+ concentrations than considered in
previous studies. Also, given the interest to eliminate or reduce
the use of Pb in industrial processes for environmental and tox-
icity reasons, it would be beneficial to determine the minimum
amount required to positively affect Sn electrodeposition. Con-
sequently, the objective of this study is to examine the influence
of Pb2+ on the electrode response and coating morphology dur-
ing the initial stages of deposition onto a copper substrate from
methyl sulphonic acid solutions with Pb2+:Sn2+ molar concen-
tration ratios as low as 1:1000. These experiments are carried
out in the absence of organic additives to isolate the influence
of Pb2+.
Sn(CH3SO3)2 and Pb(CH3SO3)2 (both reagent grade) were
added in the desired amounts. No organic additives were
included in the solutions in this study.
Linear sweep voltammetry and chronoamperometry were
used to investigate deposition kinetics. Voltammetry was car-
ried out a rate of 2 mV s−1 at various solution compositions
by scanning from an initial potential of −0.1 V in the negative
direction to −0.6 V before reversing the direction back in the
positive direction. The effect of Pb2+ on the i-t transients over the
first few seconds of polarization was also obtained under stirred
conditions. In some initial experiments, precautions were taken
to eliminate oxygen from the system by bubbling high purity
nitrogen through the solution prior to the experiments and main-
taining a nitrogen blanket over the solutions during the tests.
However, the electrode responses obtained during these experi-
ments were found to be unchanged from those observed when no
efforts were made to eliminate oxygen from the system. Conse-
quently, no efforts were made to eliminate oxygen in all subse-
quent experiments and those reported herein. Scanning electron
microscopy (LEO field-emission model 1530) was used to char-
acterize the morphology of the deposits, while their Sn and Pb
contents (accurate to within 1–2 mass%) were determined by
meansof energy dispersivespectroscopy(EDS) usingtheEDAX
Pegasus 1200 analysis system integrated within the SEM.
3. Results
3.1. Voltammetry
Voltammograms obtained at 500 rpm in solutions containing
0.1 M Sn2+ and Pb2+ concentrations from 0 to 50 mM are shown
in Figs. 1–3. The onset of metal deposition during the forward
scan and dissolution during the reverse scan remain at approxi-
mately −0.46 and −0.44 V, respectively, regardless of the Pb2+
concentration. In each case, the CV exhibits a nucleation loop
in which more cathodic current flows after the scan is reversed
in the anodic direction than before the reversal, very typical for
nucleation and growth processes and reported previously for
Sn-Pb deposition on substrates other than copper [3,10,25].
2. Experimental
Electroplating experiments were conducted using a rotating
disc electrode (Pine Instruments) in a conventional 3-electrode
cell. The working electrode was a polycrystalline copper disc
with exposed area of 0.317 cm2. Before each experiment, the
copper disc was prepared as follows: wet-grinding with SiC-
type abrasive paper, polishing with 0.3 and 0.05 ␮m ␣-Al2O3
to a mirror finish, ultrasonic cleaning for 10 min in ethanol and
final rinsing in deionized water. The counter electrode was a
2 cm × 2 cm platinum plate at the bottom of the cell about 2 cm
from the working electrode. The working electrode potentials
reported herein were measured against a saturated calomel refer-
ence electrode and correspond to this scale. The electrochemical
experiments were carried out using an Autolab PGSTAT 10
potentiostat (Eco Chemie).
All experiments were conducted at room temperature
in a methyl sulphonate-based electrolyte containing 1.0 M
CH3SO3H (MSA) to which 50 wt% aqueous solutions of both
Fig. 1. Voltammograms obtained at 2 mV s−1 scan rate and 500 rpm rotation
speed in solutions containing 1.0 M MSA, 0.1 M Sn2+ and 0 (curve 1), 0.1
(curve 2), 0.2 (curve 3) and 0.5 (curve 4) mM Pb2+
.

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H. Wang, M. Pritzker / Electrochimica Acta 53 (2008) 2430–2440
Fig. 4. Voltammograms obtained at 2 mV s−1 scan rate and 500 rpm rotation
speed in solutions containing 1.0 M MSA and 1 (curve 1), 10 (curve 2), 50
Fig. 2. Voltammograms obtained at 2 mV s−1 scan rate and 500 rpm rotation
speed in solutions containing 1.0 M MSA, 0.1 M Sn2+ and 1 (curve 1), 2 (curve
(curve 3) and 100 (curve 4) mM Pb2+
.
2), 4 (curve 3), 6 (curve 4) and 8 (curve 5) mM Pb2+
.
Another feature of the voltammogram for the Sn2+-only case
inFig. 1istheappearanceofasignificantnucleationovervoltage.
The magnitude of this effect generally depends strongly on the
nature of the substrate. It has been shown to be large in the
case of platinum [2,3] and glassy carbon [10], but much smaller
on polycrystalline gold [11]. Comparison of Figs. 1 and 4 for
the single metal systems shows that the nucleation overvoltage
for Pb deposition is much smaller than that associated with Sn
deposition. Also the nucleation loop in which the current during
the reverse scan exceeds that during the forward scan is much
smaller in the case of Pb deposition than Sn deposition.
Another interesting difference of the single metal voltam-
mograms is that the current for Pb2+ reduction has very nearly
levelled off to a plateau when the lower potential limit of −0.6 V
is reached, but not that for Sn2+ reduction. The current never
reaches a plateau over the potential range studied in any of the
voltammograms for the binary system even when the Pb2+ con-
centration is half that of Sn2+ (Fig. 3). This result contrasts with
that of Goodenough and Whitlaw [25] who observed a plateau
for a 0.092 M Sn2+–0.048 M Pb2+ solution. From the responses
in Figs. 1–4, the rates of Sn2+ and Pb2+ reduction appear to
have a similar potential dependence in the kinetically controlled
region. Since Sn2+ and Pb2+ have similar diffusion coefficients,
the appearance of a current plateau in the Pb-only case (Fig. 4),
but not when Sn2+ is present (Figs. 1–3), is not likely due to
differences in mass transfer to the disk surface. On the basis of
the reported diffusion coefficient for Sn2+ [26], the limiting cur-
rent density for Sn deposition onto a disk electrode rotating at
500 rpm in a 0.1 M Sn2+ solution is determined from the Levich
equation to be −6.9 A dm−2. Comparison with curve 1 of Fig. 1
shows that the limiting current density is lower than the mag-
nitude reached during the cathodic scan. Thus, both Sn2+ and
Pb2+ reduction are likely mass transfer limited when a potential
of −0.6 V is reached although a current plateau does not appear
in the former case. As shown later, the most likely explanation
that the current density for Sn2+ reduction can exceed the lim-
iting value obtained from the Levich equation is the extensive
roughening of the electrode surface that occurs in this case. This
effect becomes important since deposition has been underway
for approximately 75 s when the cathodic limit of −0.6 V is
reached during the scans.
As noted above, the cross-over potential of −0.44 V in the
Sn-only system remains virtually unchanged when Pb2+ is
added, even when as much as 0.1 M is present. This value com-
pares well with the reversible potentials between −0.423 and
−0.421 V for the Sn/Sn2+ reaction accounting for non-ideal
behaviour in 1.0 M CH3SO3H–0.1 M Sn(CH3SO3)2 solutions
with Pb(CH3SO3)2 concentrations ranging from 0.1 to 100 mM.
The measured cross-over potential compares less favourably
with the reversible potentials for the Pb/Pb2+ reaction of −0.501,
−0.472, −0.442, −0.421 and −0.411 V in the same solutions
at Pb2+ concentrations of 0.1, 1.0, 10, 50 and 100 mM, respec-
tively. This result is not surprising since Sn oxidation likely
occurs before Pb oxidation during the reverse scan. It should
also be noted that the cross-over potential can be affected by the
electrodeposition conditions, particularly on a substrate which
does not have a perfectly homogeneous surface.
Certainly the most significant influence of Pb2+ evident from
Figs. 1–3 is on the magnitude of the cathodic current and cor-
responding anodic current. Table 1 presents the cathodic charge
Fig. 3. Voltammograms obtained at 2 mV s−1 scan rate and 500 rpm rotation
speed in solutions containing 1.0 M MSA, 0.1 M Sn2+ and 10 (curve 1), 20
(curve 2), 30 (curve 3), 40 (curve 4) and 50 (curve 5) mM Pb2+
.

H. Wang, M. Pritzker / Electrochimica Acta 53 (2008) 2430–2440
2433
Table 1
Effect of Pb2+ on charge passed during cathodic and anodic scans in Figs. 1–3
Pb2+ concentration (mM)a
Qc (C dm−2
)
Qa (C dm−2
)
Qa/Qc
0
0.1
0.2
0.5
1.0
2.0
4.0
6.0
947.1
1068.1
1212.3
1171.1
1074.2
966.7
877.3
848.7
833.9
915.7
904.8
952.8
999.7
975.2
855.3
975.7
1109.9
1109.9
981.6
888.4
833.6
818.4
806.2
879.4
878.9
928.9
966.6
943.9
0.90
0.91
0.92
0.95
0.91
0.93
0.95
0.96
0.97
0.96
0.97
0.97
0.97
0.97
8.0
10.0
20.0
30.0
40.0
50.0
Fig. 5. i-t transients obtained after a potential step to −0.6 V under stirred con-
ditions at 500 rpm rotation speed in solutions containing 1.0 MSA and 0.1 M
Sn2+ (curve 1), 0.1 M Sn2+ + 0.2 mM Pb2+ (curve 2), 0.1 M Sn2+ + 1.0 mM Pb2+
(curve 3), 0.1 M Sn2+ + 10.0 mM Pb2+ (curve 4), 50 mM Pb2+ (curve 5) and
100 mM Pb2+ (curve 6). t0 corresponds to the induction time.
a
Each solution contains 1.0 M MSA and 0.1 M Sn(CH3SO3)2 in addition to
the indicated Pb2+ concentration. Pb2+ is added as Pb(CH3SO3)2.
Qc, anodic charge Qa and their ratio obtained from these scans.
With the assumption that all of the deposit produced during the
cathodic portion of the scan is subsequently oxidized during
the anodic portion, the ratio Qa/Qc is equivalent to the cur-
rent efficiency with respect to metal deposition. Several trends
are apparent from these results. First, Qa/Qc is always higher
in the presence of Pb2+ than in its absence and rises as the
Pb2+ concentration increases. This is consistent with the well-
known suppression of H2 evolution due to the displacement of H
adatoms by Pb adatoms and the inability of H adatoms to adsorb
onto Pb adatoms [27–29]. A particularly interesting trend is the
effect of Pb2+ concentration on the amount of metal deposited as
reflected by Qa. As the Pb2+ concentration is increased to 0.2 or
0.5 mM, the anodic charge increases substantially to a level that
is 30% higher than that observed in the Sn-only case. This effect
has not been reported in previous studies since these were con-
cerned with higher Pb2+:Sn2+ concentration ratios. However, a
further increase in Pb2+ concentration causes the catalytic effect
to decline and metal deposition to be eventually inhibited rela-
tive to the Sn-only case when the solution contains more than
2.0 mM Pb2+. This trend continues until the Pb2+ concentration
rises above 8.0 mM, whereupon the charge again increases and
rises above that observed in the Sn-only case. The inhibitory
effect at Pb2+ concentrations between 2.0 and 8.0 mM is con-
sistent with that reported in previous studies on deposition onto
polycrystalline copper [7,14,18]. It should be noted that no Pb
is detected in the coating until the Pb2+ concentration reaches
10.0 mM, as will be discussed later. The onset of Pb2+ reduction
is therefore the likely cause for the rise in total charge associ-
ated with metal deposition that begins as the Pb2+ concentrations
increases above 8.0 mM.
circuit value to −0.6 V in the Sn2+-only solution are similar to
the corresponding transients obtained in 0.1 M Sn2+ with Pb2+
concentrations up to 10 mM. In each case, current first rises
sharply immediately after the potential step, but then begins to
decrease over an induction period of a few seconds due to dou-
ble layer charging, transient diffusion-controlled processes and
any metal deposition not involving grain nucleation [30]. At the
end of this induction period, the current begins to rise, signi-
fying the onset of deposition via nucleation and growth. Fig. 5
shows the responses over the first 10 s after the end of the induc-
tion period. The curves shown here correspond to the average
of several repeated experiments. Steady state is still not reached
at the end of the plating period when the Pb2+ concentration
is 10 mM or less (curves 1–4). Most importantly, the effect of a
small amount of Pb2+ is similar to that observed previously in the
LSV experiments. The current is larger over the entire 10 s when
the solution contains 0.2 mM Pb2+ than in the Sn-only case, but
this enhancement disappears at a Pb2+ concentration of 1 mM.
Although not shown here, additional experiments were carried
out in solutions containing 0.1 M Sn2+ and 0.1 M Sn2+ + 0.2 mM
Pb2+ for longer durations than 10 s. In both cases, the current
increased gradually over the time period after 10 s, but the cur-
rent in the presence of 0.2 mM Pb2+ remained approximately
10 A dm−2 greater in magnitude than that obtained in the Pb-
free system for the remainder of the plating periods, indicating
that the enhancement is not restricted to the initial stages of
deposition.
The presence of Pb2+ tends to flatten out the transient
responses, particularly when it reaches 50 or 100 mM (not shown
here). In these cases, the current drops very little at the outset
of polarization and quickly reaches a plateau that is main-
tained thereafter. The responses no longer show the slow gradual
increase in current at the end of the plating time as observed at
lower Pb2+ concentrations. To determine whether these changes
are characteristic of the influence of Pb2+, we obtained transients
for Pb2+ reduction in the single metal system. Curves 5 and 6 in
Fig. 5 show the responses obtained in 50 and 100 mM Pb2+ solu-
3.2. Chronoamperometry
Chronoamperometry experiments were carried out to deter-
mine the effect of Pb2+ on the i-t transients under stirred
conditions. The features of the transient obtained after stepping
the potential of an electrode rotating at 500 rpm from the open

2434
H. Wang, M. Pritzker / Electrochimica Acta 53 (2008) 2430–2440
tions, respectively. Theyarequalitativelysimilartothetransients
for the binary system containing 50 and 100 mM Pb2+ in that the
current quickly drops to a plateau and no longer changes. This
supports the conclusion that the binary system adopts features
characteristic of Pb-only deposition at these higher Pb2+ levels.
It is also interesting to observe that the current plateaus for the
Pb2+-only system in curves 5 and 6 closely match the limiting
current plateaus observed in the corresponding LSV curves in
Fig. 4. The plateau of about −3.9 A dm−2 obtained in the 50 mM
Pb2+ solution (curve 5) is half that obtained in the 100 mM Pb2+
solution (curve 6). These results indicate that Pb2+ reduction in
the single metal system is mass transfer controlled at −0.6 V,
but does not significantly increase the deposit surface area dur-
ing the first 10 s of plating. The diffusion coefficient of Pb2+ is
estimated from the Levich equation to be 8.6 × 10−6 cm2 s−1
using these current plateau values and agrees well with pre-
viously reported values between 9.2 and 9.8 × 10−6 cm2 s−1
[31,32].
Fig. 6. SEM images of deposits obtained after plating for 10 s at −0.6 V and 500 rpm rotation speed in solutions containing 1.0 M MSA and (a) 0.1 M Sn2+, (b) 0.1 M
Pb2+, (c) 0.1 M Sn2+ + 0.2 mM Pb2+, (d) 0.1 M Sn2+ + 0.5 mM Pb2+, (e) 0.1 M Sn2+ + 1.0 mM Pb2+ and (f) 0.1 M Sn2+ + 10.0 mM Pb2+. Each image has dimensions
41.34 ␮m × 28.18 ␮m.

H. Wang, M. Pritzker / Electrochimica Acta 53 (2008) 2430–2440
2435
3.3. SEM images
particles tend to be slightly shorter. It should be noted that the
current over the course of the 10 s of plating at −0.6 V has also
increased.
Petersson and Ahlberg [11] showed that the addition of Pb2+
to tin fluoroborate plating solutions at Pb2+:Sn2+ molar ratios
greater than 0.1 causes the resulting deposits to adopt some of the
features characteristic of pure Pb deposits. However, given the
results of the previous sections, it would be useful to examine the
morphology of deposits obtained at the lower ratios considered
in the current study.
The effect of Pb2+ becomes noticeable when its concentra-
tion reaches 0.5 mM (Fig. 6d). The nucleation density increases
significantly and coalescence and bridging of grains is clearly
evident. The resulting aggregates have very unusual shapes with
no evidence of the geometry obtained from Sn2+-only solutions.
Examination of the image in Fig. 6d shows that much less of the
copper substrate is uncovered after 10 s of plating than is the case
when the solution contains 0.2 mM Pb2+ (Fig. 6c). At the same
time, the corresponding i-t transients indicate that very similar
charge has been passed over this time at these two concentra-
tions. Thus, the difference in coverage reflects the ability of Pb2+
to promote a more 2-dimensional type of growth. This effect
becomes more apparent as the Pb2+ concentration is increased
to 1 mM (Fig. 6e) and 10 mM (Fig. 6f). This trend is consistent
with the flattening of the transients as the Pb2+ concentration is
increased above 0.2 mM (Fig. 5). Petersson and Ahlberg [10,11]
also reported a similar effect, but our results reveal that this effect
manifests itself at Pb2+:Sn2+ molar ratios as low as 5:1000. At
still higher Pb2+ concentrations, the trends continue so that the
deposit forms a honey-combed structure with isolated pockets
of exposed substrate (Fig. 7a–c). It is important to note that these
deposits are still considerably different than the Pb coatings pro-
duced in solutions containing the same Pb2+ concentrations in
the absence of Sn2+ (Fig. 6b).
Once the i-t transients in the previous section were deter-
mined, the resulting deposits were examined using scanning
electron microscopy. Fig. 6 shows the coatings obtained after
plating at −0.6 V and 500 rpm rotation speed for 10 s in single
metal solutions containing 0.1 M Sn2+ and 0.1 M Pb2+ and in
binary solutions containing 0.1 M Sn2+ and various Pb2+ con-
centrations below 10 mM. The deposit obtained in a Sn2+-only
solution (Fig. 6a) is similar to that reported previously by Rinne
et al. [14] who also carried out deposition onto polycrystalline
copper at a similar potential under stirred conditions, but for a
longer duration. The deposit contains aggregates ranging widely
in size from small sub-micron particles distributed evenly over
the surface to a few very large dendritic-like clusters that are
tens of microns long. The larger are the particles, the higher
is their aspect ratio. The number density of the smaller parti-
cles distributed evenly over the surface is estimated to be about
1–2 × 107 aggregates per cm2. Significant patches of uncovered
copper substrate are still evident. This structure is consistent
with previous findings that Sn deposition in additive-free acidic
solutions proceeds by a diffusion-controlled 3-D nucleation
and growth mechanism [10,11,33]. The wide range of crystal-
lite sizes and the increasing acicularity of particles with size
reflect that surface diffusion of adatoms to growth centers occurs
readily, leading to unstable growth, dendrite formation and a
continual increase in available area for deposition. This ten-
dency is consistent with the absence of a limiting current plateau
in Fig. 1 and the current increase in the transient in Fig. 5 after
the induction period.
As evident from Fig. 6b, deposition in the Pb-only system
at −0.6 V from a 0.1 M Pb2+ solution occurs quite differently
than does the corresponding Sn-only system. The copper sub-
strate is completely covered within 10 s of plating despite the
fact that considerably less charge has passed than during the
corresponding period of Sn-only deposition (Fig. 5). Several
types of features appear in the lead coating. Since Pb crystal-
lizes according to the hexoctahedral class, it is not surprising
that the dominant structure is made of small (primarily 1–5 ␮m
size) octahedral grains covering the substrate. In addition, a few
large flat platelets, a common crystal habit of Pb, form. Also
dispersed over the surface are needles lying flat along their long
axis.
EDS measurements of the deposits after 10 s of plating were
carried out to determine the effect of Pb2+ concentration in
solution on deposit composition. Examination of curves 1–4
in Fig. 5 shows that at least 1 C cm−2 charge is passed during
10 s of plating in all cases. Based on the atomic radii of the
two metals, the charge density associated with the electrode-
position of a completely filled close-packed monolayer of each
metal is approximately 302 ␮C cm−2 for Pb and 370 ␮C cm−2
for Sn. Thus, the total charge passed over 10 s is equivalent to
2000–3000 monolayers, which should be sufficient to detect Pb
by EDS if any significant amount is present.
The resulting dependence of deposit composition on Pb2+
concentration in solution (not included here) is similar to that
reported previously for the case of longer plating times [1,7,8].
No Pb is detected in the deposit unless the solution contains
more than about 10 mM Pb2+ (9.1 at%). However, above this
minimum amount, the Pb content in the deposit rises steeply
with Pb2+ concentration and reaches 50% when only 40 mM
Pb2+ is present. An important implication of these results is that
the changes in deposit structure obtained at Pb2+ concentrations
less than 10 mM (Fig. 6a and c–e) occur without any Pb detected
in the deposit by EDS. Furthermore, the enhancement in current
in the LSV curves and i-t transients over that observed in the Sn-
only system (Figs. 1 and 5) also occurs under conditions where
no Pb is found in the deposit.
When Sn plating is carried out in the presence of 0.2 mM
Pb2+, the features of the deposit are very similar to those in
the Pb2+-free case (Fig. 6c). The types of aggregates still vary
widely from sub-micron rounded grains to dendritic-like aggre-
gates. A few trends are worth noting upon comparison with the
deposit produced in the absence of Pb2+. The number density of
aggregates increases to about 4 × 107 cm−2, while the dendritic
The backscatter images in Fig. 8 show the distribution of Sn
(gray) and Pb (white) on deposits produced after 10 s of plat-
ing from binary solutions containing 0.1 M Sn2+ and 20, 25,
30, 40 and 50 mM Pb2+. As shown in previous research, the
deposits obtained in the presence of 20, 25 and 30 mM Pb2+

2436
H. Wang, M. Pritzker / Electrochimica Acta 53 (2008) 2430–2440
on the copper substrate in any of the deposits in Fig. 8 (or oth-
ers not included here) obtained in binary solutions containing
0.1 M Sn2+ and Pb2+ concentrations of 50 mM and below. On
the other hand, Pb easily deposits directly on and covers the cop-
per substrate within 10 s of plating at −0.6 V in a Sn-free system
(Fig. 6b). Also, as shown in Fig. 4, Pb deposition onto copper
in the absence of Sn2+ requires very little nucleation overvolt-
age. Clearly, Pb deposition is affected significantly if Sn is also
present in the system.
4. Discussion
A particularly noteworthy aspect of the previous section is
the large effect that the addition of small amounts of Pb2+ to the
solution has on the electrode response during metal reduction
and the resulting deposit structure. From the analysis in Table 1
of the CV scans for the charge ratio Qa/Qc, H2 evolution cannot
be responsible for the increase of the reduction current relative
to that observed in the Sn-only system. In fact, the results show
that this enhancement is likely accompanied by a decrease in the
amount of H2 evolution. The presence of 0.2 or 0.5 mM Pb2+ in
solution has a substantial effect on the total charge associated
with metal deposition during the cathodic scan by increasing
it by 30% over that observed in its absence. Thus, one should
have detected a considerable amount of Pb in the coating if this
enhancement in current is caused by Pb2+ reduction as part of
the formation of a pure phase or an alloy. Since no Pb is detected
in the deposit under these conditions, it is reasonable to conclude
that the additional current is due to Sn2+ reduction and that alloy
formation is not a significant factor. This is supported by previ-
ous studies indicating that a Sn-Pb alloy does not form when the
Pb2+:Sn2+ concentration ratio in solution is as low as 2:1000 or
5:1000 molar, but only when the ratio exceeds about 1:10 [1,7,8],
a result similar to that found in the current study. Given the sim-
ilar potentials at which the two metals tend to deposit, it would
be surprising to obtain an alloy with any significant amount of
Pb at a Pb2+:Sn2+ concentration ratio of 2:1000 or 5:1000 par-
ticularly when no organic additive is present. Of course, one
cannot rule out the possibility that a Sn-Pb alloy plays a role as
an intermediate in the deposition process.
To explain the effect of the small amounts of Pb2+ on Sn
deposition and deposit structure, one may be able to draw upon
previous studies showing the ability of small amounts of metal
cations such as Pb(II), Cd(II), Hg(II), Tl(IV), Sb(III) or Bi(III) to
catalyze several electrochemical processes [29,34,35], e.g., O2
reduction [36–38], oxidation of organic compounds [39–41],
H2 evolution [42–45], Fe2+/Fe3+ redox couple [46], oxidative
leaching and/or dissolution of Au in cyanide solutions [47–50]
and metal deposition [36,51–53]. As noted in Section 3.3, no
Pb was detected in EDS measurements of deposits after 10 s
of plating if the solution contained 0.1 M Sn2+ and less than
10 mM Pb2+. Although not included here, X-ray diffractograms
of coatings produced in solutions containing 0.1 M Sn2+ and
various amounts of Pb2+ do not show any Pb peaks at Pb2+ con-
centrations of 0.5 and 6.0 mM. The inability of EDS to detect
any Pb in some deposits does not preclude that some may actu-
ally be present since a sample must contain an equivalent of
Fig. 7. SEM images of deposits obtained after plating for 10 s at −0.6 V
and 500 rpm rotation speed in solutions containing 1.0 M MSA and (a) 0.1 M
Sn2+ + 20 mM Pb2+, (b) 0.1 M Sn2+ + 50 mM Pb2+ and (c) 0.1 M Sn2+ + 100 mM
Pb2+. Image (a) has dimensions 40.64 ␮m × 27.72 ␮m; images (b) and (c) have
dimensions 41.34 ␮m × 28.18 ␮m.
contain much more Sn than Pb, but the situation is reversed
when the Pb2+ concentration is raised to 50 mM. The Sn and
Pb areas in the backscatter image are roughly equal at 40 mM
Pb2+ in agreement with the EDS data (Fig. 8d). The images
show that Pb is always located on top of Sn regardless of the
solution composition. Petersson and Ahlberg [10,11] observed
similar behaviour for Sn-Pb deposition onto glassy carbon and
polycrystalline gold. Pb grains are never observed to lie directly

H. Wang, M. Pritzker / Electrochimica Acta 53 (2008) 2430–2440
2437
Fig. 8. SEM backscatter images showing tin (gray) and lead (white) on surfaces of deposits obtained after plating for 10 s at −0.6 V and 500 rpm rotation speed in
solutions containing 1.0 M MSA and (a) 0.1 M Sn2+ + 20 mM Pb2+, (b) 0.1 M Sn2+ + 25 mM Pb2+, (c) 0.1 M Sn2+ + 30 mM Pb2+, (d) 0.1 M Sn2+ + 40 mM Pb2+ and
(e) 0.1 M Sn2+ + 50 mM Pb2+. Images (a)–(d) have dimensions 20.32 ␮m × 13.86 ␮m; image (e) has dimensions 27.06 ␮m × 18.46 ␮m.
about five monolayers of the metal in order to be measurable.
It is important to note from the CV scans and i-t transients that
the enhancement in Sn2+ reduction in the presence of 0.2 and
0.5 mM Pb2+ begins immediately at the start of deposition and
is maintained thereafter. For this to be possible, a Pb-bearing
species must remain on or rapidly segregate to the surface dur-
ing the process and be readily regenerated by each reduction
event. The segregation of Pb on the deposit surface is sup-
ported by the results of our SEM analysis. Pb always appears
to lie on top of the Sn components in the backscatter images of
the deposits (Fig. 8). Similar behaviour was observed in Pb-Sn
coatings on glassy carbon and polycrystalline gold substrates
´
ˇ ´
[10,11]. McIntyre and Peck [36,51] and Davidovic and Adzic
[52] obtained evidence of the segregation of Pb and Sb on gol−d
deposit surfaces when Pb(II) and Sb(III) catalyzed Au(CN)2
electroreduction.

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H. Wang, M. Pritzker / Electrochimica Acta 53 (2008) 2430–2440
Petersson and Ahlberg [10,11] concluded from their results
containing species on the substrate. Furuya and Motoo [44]
devised a method to compare the amount of oxygen-containing
species that adsorb onto Pb adatoms to that on Sn adatoms and
foundthattheamountadsorbingonPbisrelativelylowandmuch
less than that on Sn. Studies of Pb underpotential deposition on
Cu(III) single crystal electrodes showed that Pb deposition is
accompanied by the desorption of oxygen-containing species
[54,55]. Thus, the addition of Pb2+ may destabilize the oxygen-
containing species present on the substrate in a Sn-only system
and facilitate Sn2+ reduction.
that the codeposition of Pb inhibits the electrodeposition of Sn
on pre-existing Sn sites. However, this explanation is not con-
sistent with our findings that the presence of Pb2+ enhances Sn
deposition at concentrations of 2.0 mM and below. Furthermore,
this enhancement persists throughout the cathodic scans in Fig. 1
and over extended periods of plating at constant potential when
coverage extends beyond the first monolayer and deposition of
Sn onto Sn predominates. Finally, it does not address the obser-
vation that Pb is never located directly on top of the copper
substrate in any of the deposits produced in the binary system at
Pb2+ concentrations of 50 mM and below, although underpoten-
tial and overpotential Pb deposition directly onto copper readily
occur in Sn-free systems [27,53–56]. A more likely explanation
may be the opposite to that proposed by Petersson and Ahlberg,
at least at lower concentrations—deposition of Pb on either Sn
or Cu sites induces the reduction of Sn2+ to occur at these or
nearby sites. Thus, a Pb cluster will always be surrounded by
Sn in the deposit structure, as observed in the SEM images.
Pb accumulates on top of Sn continually throughout deposi-
tion due to segregation or a place-exchange reaction, similar
to that proposed for the electrocatalysis of Au deposition by
Pb(II) and Sb(III) [36,51,52]. This segregation could be facil-
itated by the very low solubility of Pb in Sn and vice versa at
room temperature [14].
Sn deposition may also be enhanced through the effect of
Pb2+ on electrocrystallization. Sn electrodeposits formed in the
absence of additives are characterized by 3-dimensional nucle-
ation and growth of grains. Surface diffusion of adatoms is a
major contributor to grain growth and yields crystallites with a
wide range of sizes and increasing acicularity as they grow. A
significant number of Pb adatoms likely become distributed over
the electrode surface due to a strong Pb-Cu interaction. Due to
the aforementioned effects of chemical displacement by Sn2+
and destabilization of oxygen-containing surface species, these
Pb adatoms become centers for nucleation of Sn crystallites. The
net effect is an increase in the nucleation rate of Sn centers and
less Sn2+ and energy available for growth of existing grains. This
idea is supported by our observation of an increase in the num-
ber density and a more uniform size distribution of crystallites.
Regeneration of Pb2+ by chemical displacement and segrega-
tion of Pb to the deposit surface will sustain the enhancement
of the nucleation rate. This change in electrocrystallization and
increase in nucleation rate can enhance the rate of Sn electrode-
position in several ways. In the presence of relatively low Pb2+
concentrations, Sn electrodeposition is still likely dominated by
surface diffusion of adatoms to existing crystallites. The short-
ening of the surface diffusion length of Sn adatoms due to a
larger number of nuclei on the electrode surface will therefore
enhance the Sn deposition rate. The coalescence and bridging of
grains clearly evident in the SEM images for deposits formed in
the presence of 0.5, 1.0 and 10.0 mM Pb2+ (Fig. 6d–f) suggests
that grain growth occurs along the periphery of the crystallites.
Obviously, more surface area is available for deposition as the
number of nuclei increases. Another factor may be the reduction
of the energy of formation of 2-dimensional nuclei and decrease
in crystallization overvoltage that has been shown theoretically
to occur in the presence of foreign metal adatoms [36].
The catalytic effect of Pb2+ on Sn deposition and the segrega-
tion of Pb on top of Sn may arise due to several processes. One
possibility is the combination of Pb deposition and chemical
displacement by Sn2+:
Pb2+ + 2e− → Pb
Pb + Sn2+ → Pb2+ + Sn
(1)
(2)
Based on the thermodynamics of the bulk forms of the species
above, reaction (2) proceeds spontaneously to the right when the
[Pb2+]/[Sn2+] ratio is less than 0.43, a condition met when Pb2+
is observed to enhance Sn deposition. An important aspect of
reaction (2) is the regeneration of Pb2+ which allows the pro-
cess to repeat itself continuously. As noted by McIntyre and
Peck [36,51] in regard to the depolarization of Au deposition by
Pb(II), Tl(I), Bi(III) and Hg(II), the regenerated ions may remain
“contact adsorbed” on the substrate and be readily accessible
for further reaction. The process above may serve as a place-
exchange reaction by which Pb segregates to the deposit surface.
Another requirement of this mechanism is that Pb2+ reduction
mustproceedmorerapidlythandoesSn2+ reductioninthebinary
system. Previous studies of this system have indicated that this
is indeed the case [5,9].
Thus, Pb2+ not only enhances the rate at which Sn is elec-
trodeposited but also helps to guide where Sn deposition occurs.
This may explain our earlier observation that the surface mor-
phology of deposits in the mixed Sn-Pb system are affected even
when only trace amounts of Pb2+ are present in solution and little
or no Pb is detected in the deposit.
Another contributing factor may be the effect of Pb2+ on
oxygen-containing species likely present on the substrate. Sev-
eral studies on Sn deposition have provided evidence of the
interaction between Sn and oxygen-containing anions or the
presence of ad-ions that are coordinated with oxygen-containing
species depending on the substrate, solution conditions and
electrode potential [57–63]. If present, these oxygen-containing
species could impede Sn deposition. When Pb2+ is added to such
a system, it may be able to lower the stability of the oxygen-
Another important aspect to explain is why Pb2+ loses its
catalytic effect and begins to inhibit Sn2+ reduction at higher
concentrations. Several factors may contribute to the loss of
the catalytic effect. As the Pb2+ concentration rises, the ther-
modynamic driving force for chemical displacement decreases.
An excessive number of Pb adatoms on the surface may make
it more difficult for them to segregate to the electrode surface
along grain boundaries and move along the surface, similar to

H. Wang, M. Pritzker / Electrochimica Acta 53 (2008) 2430–2440
[3] T.M. Tam, J. Electrochem. Soc. 133 (1986) 1792.
2439
the situation with surfactants. As discussed previously, the rise in
the density of nuclei may increase the area for electrodeposition
along the periphery of the growth centers as the Pb2+ concen-
tration is increased to about 2.0 mM. However, with a further
increase in the Pb2+ concentration, the nucleation density may
become so high that the area begins to decline due to the overlap
of the diffusion boundary layers surrounding the growth centers
and coalescence of the centers themselves. Associated with this
factor is the ability of Pb2+ to suppress vertical deposit growth
characteristic of the Sn-only system. This effect which begins
at lower Pb2+ concentrations when Sn2+ reduction is catalyzed
becomes much more dominant at higher Pb2+ levels and pro-
duces a coating with lower roughness than the one obtained in
the Sn-only case. These trends are complemented by the increas-
ing tendency of Pb adatoms to form their own metallic phase in
the coating and compete with Sn2+ for current as the Pb2+ con-
centration increases. Ultimately, the inhibition of Sn deposition
begins to occur when the Pb2+:Sn2+ molar concentration ratio
reaches 20:1000.
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5. Conclusions
This study focused on the influence of Pb2+ on the electrode
responseofSn2+ reductionandtheresultingcoatingmorphology
during the initial stages of electrodeposition at concentrations
much lower than previously investigated. Pb2+ begins to have
a significant effect at Pb2+:Sn2+ molar ratios as low as 1:1000,
below where Pb was detected by EDS analysis in the deposit.
The first effect observed involves the electrode response dur-
ing linear sweep voltammetry scans and i-t transients, with a
noticeable increase in the current for Sn2+ reduction beginning
at a Pb2+:Sn2+ concentration ratio of only 1:1000 and contin-
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charge during the LSV scans reaches a maximum of about 30%
over that obtained in the absence of Pb2+ at Pb2+:Sn2+ con-
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ratio of about 20:1000, Pb2+ begins to have the opposite effect
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reported previously. The effect on deposit morphology begins
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Acknowledgments
Gratitude is expressed to the China Scholarship Council for
providingoneoftheauthors(H. Wang)withafellowshiptostudy
abroad and to the Natural Sciences and Engineering Research
Council of Canada (NSERC) for financial support of the project.
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