Electrodeposition of Sn-Bi lead-free solders: Effects of complex agents on the composition, adhesion, and dendrite formation

Article abstract of DOI:10.1016/j.electacta.2007.09.002

The influences of complex agents, including citric acid, ethylenediaminetetraacetic acid (EDTA), and polyethylene glycol (PEG) on the electrochemical co-deposition of Sn-Bi alloys were systematically investigated through the linear sweeps voltammetric (LSV) analyses. The onset deposition potential of Bi ions is obviously shifted to a negative value, close to that of Sn ions by the simultaneous addition of the above three compounds into the plating solution. Based on the LSV results, deposits plated from the typical solutions with the Sn⁴⁺/Bi³⁺ ratio of 1, pH of 6.0, and various combinations of complex agents were characterized by scanning electron microscopic (SEM), energy-dispersive spectroscopic (EDS), and X-ray diffraction (XRD) analyses. The 30Sn-70Bi alloy (in wt%) is successfully plated from the typical plating bath containing 0.4 M citric acid, 1.0 M EDTA, and 0.2 M PEG, suggesting the possibility in plating the Sn-Bi alloys with their composition around the eutectic point (42Sn-58Bi). The adhesion of deposits and the formation of dendrites are respectively improved and inhibited by the synergistic effects of these three complex agents. Contrary to this combination, nearly pure Bi deposits are obtained from the typical plating solutions containing complex agents in the other combinations.

Full text of DOI:10.1016/j.electacta.2007.09.002

Available online at www.sciencedirect.com
Electrochimica Acta 53 (2007) 2040–2047
Electrodeposition of Sn–Bi lead-free solders: Effects of complex
agents on the composition, adhesion, and dendrite formation
Yi-Da Tsaib, Chi-Chang Hua,b,∗,1, Chi-Cheng Linb
a Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu 30013, Taiwan
b Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan
Received 6 June 2007; received in revised form 1 August 2007; accepted 3 September 2007
Available online 7 September 2007
Abstract
The influences of complex agents, including citric acid, ethylenediaminetetraacetic acid (EDTA), and polyethylene glycol (PEG) on the elec-
trochemical co-deposition of Sn–Bi alloys were systematically investigated through the linear sweeps voltammetric (LSV) analyses. The onset
deposition potential of Bi ions is obviously shifted to a negative value, close to that of Sn ions by the simultaneous addition of the above three
compounds into the plating solution. Based on the LSV results, deposits plated from the typical solutions with the Sn4+/Bi3+ ratio of 1, pH of
6.0, and various combinations of complex agents were characterized by scanning electron microscopic (SEM), energy-dispersive spectroscopic
(EDS), and X-ray diffraction (XRD) analyses. The 30Sn–70Bi alloy (in wt%) is successfully plated from the typical plating bath containing 0.4 M
citric acid, 1.0 M EDTA, and 0.2 M PEG, suggesting the possibility in plating the Sn–Bi alloys with their composition around the eutectic point
(42Sn–58Bi). The adhesion of deposits and the formation of dendrites are respectively improved and inhibited by the synergistic effects of these
three complex agents. Contrary to this combination, nearly pure Bi deposits are obtained from the typical plating solutions containing complex
agents in the other combinations.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: Lead-free solder; Sn–Bi alloy; Electrodeposition; Complex agent; Linear sweep voltammetry
1. Introduction
ders (e.g., Sn–Cu, Sn–Bi, Sn–Ag, Sn–Ag–Cu, Sn–Zn, etc.)
becomes an urgent assignment in modern electronic industries
[1–11].
Tin–lead solders were considered as the most prominent
material for the interconnection and packaging of electronic
components [1–3], which was originally deposited by means
of a vacuum evaporation technique developed by IBM [4]. In
order to increase the density of interconnections, a variety of
new methods/processes were developed to form solder bumps,
especially the electroplating process [5,6]. However, on July 1st
2006, European Union (EU) mandated a switch to use lead-free
solders in many of the electronic assembly processes because of
theenvironmentalandhealthconsiderations[1,3,5,7,8]. Accord-
ingly, lead-free processes for electronic devices and components
are profoundly required and the development of lead-free sol-
ThephysicochemicalpropertiesofSn-basedlead-freesolders
were found to strongly dependent upon the type and composi-
tion of alloys [1,8]. For example, Sn–Ag alloys were shown
to have excellent mechanical properties while the manufacture
cost is high and the composition is hard to be controlled by elec-
troplating [1,3,5,6,8]. Sn–9Zn generally shows the advantages
of a low reflowing temperature (an eutectic point of 198 ◦C),
low cost, and good malleability while its wettability and joint
strength are relatively poor [1,8,11]. The 42Sn–58Bi deposit
was shown to have the merits of low reflowing temperatures (an
eutectic point of 139 ◦C), good joint strength, low surface ten-
sion, excellent creep resistance, high shear strength, and good
anticorrosive properties [1,3,8,12,13]. Unfortunately, the differ-
ence in equilibrium potentials between Bi and Sn deposition is
very large (ca. 0.454 V). Hence, how to effectively and reliably
electroplate Sn–Bi deposits with their composition approaching
the eutectic point is very important.
∗
Corresponding author at: Department of Chemical Engineering, National
Tsing Hua University, Hsinchu 30013, Taiwan. Tel.: +886 3 5715131x35725;
fax: +886 3 5715408.
E-mail address: cchu@che.nthu.edu.tw (C.-C. Hu).
ISE Member.
1
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.09.002

Y.-D. Tsai et al. / Electrochimica Acta 53 (2007) 2040–2047
2041
Table 1
In the past, few attempts have been made at the electroplating
The bath composition and plating conditions of Sn–Bi alloys
of Sn–Bi alloy [2,3], probably due to the large difference in the
deposition potential of Bi3+ (+0.317 V versus NHE) and Sn2+
(−0.137 V versus NHE) [7], which can be reduced from Sn4+.
Nevertheless, electrodeposition of this alloy in the solutions
containing various complex agents to reduce their deposition
potential difference [3,5,9,14], or using surfactants adsorbing
onto the surface of deposits to efficiently suppress the growth of
a certain metal [2,9,10] have been studied previously. Further-
more, Sn–Bi alloys with bad adhesion and dendrite formation
were found to electroplate from a chloride-based bath with citric
acidas the complex agent [15], or from an organic sulphonic bath
[3]. The formation of dendrites usually results from a deposition
process in the mass-transfer or mixed control regions [16–18].
This effect accelerates the growth of dendrites due to a faster
diffusion rate of metal ions at the tip of dendrites (similar to the
diffusion mechanism on a microelectrode [19,20]). Based on the
reason mentioned above, it may be possible to reduce the forma-
tion/growth of dendrites by adding additives (including complex
agents, wetting agents, or other additives) into the plating bath
to reduce the current density on the tip [21] or by using the pulse
plating method [22].
Sn4+ (SnCl4·nH2O)
Bi3+ (Bi(NO3)3·5H2O)
Citric acid (CA)
Ethylenediaminetetraacetic acid (EDTA)
polyethylene glycols (PEG, MW 400)
Current density
0.1
0.1
0.4
1.0
0.2
100
25
6.0
M
M
M
M
M
mA cm−2
◦C
Temperature
pH
The composition of the basic plating bath and electroplat-
ing conditions are shown in Table 1, which mainly consisted
of SnCl4·nH2O and Bi(NO3)3·5H2O with various complex
agents, including citric acid (CA), ethylenediaminetetraacetic
acid (EDTA), and polyethylene glycols (PEG, MW 400) in vari-
ous combinations. The pH of plating baths was adjusted to be 6.0
with concentrated NH4OH. The electrochemical analyses were
performed with an electrochemical analyzer system, CHI 633A
(CH Instruments, USA). All electrochemical analyses were car-
ried out in a three-compartment cell. An Ag/AgCl electrode
(Argenthal, 3 M KCl, 0.207 V versus a standard hydrogen elec-
trode (SHE) at 25 ◦C) was utilized as the reference electrode and
a piece of platinum gauze with an exposed area equal to 4 cm2
served as the counter electrode. A Luggin capillary, whose tip
was set at a distance of 1–2 mm from the surface of substrates,
was used to minimize errors from the surface due to iR drop in
the electrolytes.
Morphologies of all deposits were examined by a field-
emission scanning electron microscope (FE-SEM, Hitachi
S-4700). The mean composition of deposits was measured using
an energy-dispersive X-ray (EDX) spectroscope coupled with
this FE-SEM at three points. The mean error of this EDX analy-
sis is ca. 1.5 weight percents (wt%). X-ray diffraction patterns
were obtained from an X-ray diffractometer (Rigaku MiniFlex)
using a Cu target.
All solutions were prepared with pure water produced by
a reagent water system (Milli-Q SP, Japan) at 18 Mꢀ cm and
all reagents were Merck, GR. The solutions for linear sweep
voltammetric (LSV) analyses were degassed with purified nitro-
gen gas for 20 min before the electrochemical measurements and
nitrogen was passed over the solution during the measurements.
For the DC plating, all solutions were stirred by a magnetic bar
(2.5 cm in length and 0.6 cm in diameter) at a fixed stirring rate
(300 rpm) to promote the mass transfer of metallic ions. Solution
temperature was maintained at the specified temperatures with
an accuracy of 0.1 ◦C by means of a water thermostat (Haake
DC3 and K20).
In this preliminary work, a solution containing 0.1 M Bi3+
(i.e., Bi(NO3)3) and 0.1 M Sn4+ (i.e., SnCl4) is employed as the
basic plating bath, due to the fact that Sn2+ is easily oxidized
to Sn4+ under an air agitation at a relatively high temperature
[11,15]. In addition, citric acid, ethylenediaminetetraacetic acid
(EDTA), and polyethylene glycol (PEG) are tested as the com-
plex agents in the plating baths. In order to efficiently find a
suitable combination of complex agents, giving a synergistic
effect on suppressing the deposition rate of Bi (noble compo-
nent) [23,24] and improving the adhesion of resultant deposits
[3,25–27], linear sweep voltammetry (LSV) is introduced to
compare the influences of complex agents on the onset poten-
tial of Bi and Sn deposition. Based on the LSV results, some
deposits were plated from the basic plating bath containing com-
plex agents in various combinations to confirm their suitability.
Furthermore, the morphologies, compositions, and crystalline
structure of the resultant deposits are characterized by means of
scanning electron microscopic (SEM), energy-dispersive X-ray
(EDX) spectroscopic, and X-ray diffraction (XRD) analyses.
2. Experimental
The lead-free tin–bismuth deposits were electroplated from
a simple CN−-free solution onto copper (99.5%, 1 cm × 2 cm)
plates deposited with a nickel film (ca. 2 ␮m) used as the under
bump metallization (UBM) (denoted as Cu/Ni). The pretreat-
ment procedures of Cu/Ni substrates completely followed our
previous work [11,15]. These Cu/Ni substrates, rinsed with pure
water, were vertically placed in a 500-ml jacket cell surrounded
with an anode of DSA and electroplated with Sn–xBi deposits,
about 2 ␮m, under the plating conditions (see Table 1) with a
constant passed charge density (15.0 C cm−2). After the Sn–xBi
deposition, these electrodes were repeatedly rinsed with pure
water and finally dried in a vacuum oven at room temperature.
3. Results and discussion
3.1. The LSV study
The influences of complex agents on the onset deposition
potential of metal ions can be simply examined by using linear
sweep voltammetric (LSV) analyses. Curves 1–3 in Fig. 1 show
the effect of 0.4 M CA on the deposition behavior of Bi3+, Sn4+,

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Y.-D. Tsai et al. / Electrochimica Acta 53 (2007) 2040–2047
Fig. 1. LSV curves measured from the plating solution containing 0.4 M CA
with (1) 0.1 M Bi3+; (2) 0.1 M Sn4+; (3) 0.1 M Bi3+ + 0.1 M Sn4+. LSV curves
were measured at 8 mV s−1 under 25 ◦C.
and binary metal ions on the Cu/Ni substrates. From curve 1,
the deposition of Bi starts at approximately −390 mV with an
obvious peak centered at ca. −420 mV. A level-off diffusion lim-
iting current density of ca. 30 mA cm−2 is clearly found behind
this peak. The sharp increase in the current density at potentials
negative to −800 mV is attributed to the hydrogen evolution
reaction (HER). This result is similar to that measured from
a strong acidic Bi3+ solution without any additive (not shown
here), indicating that CA is not significantly coordinated with
Bi3+. Curve 2 shows that sensible currents become visible at
about −1050 mV, gradually increase with the negative shift in
electrode potentials, and a shoulder is found at ca. −1600 mV.
This result indicates that the deposition of Sn is significantly
inhibited by the addition of CA into the plating solution. From
curve 3, the i–E responses completely follow the trace of curve 1
when the electrode potentials are positive to ca. −750 mV, indi-
cating that the electrode kinetics is dominated by the reduction of
Bi3+. However, the plateau currents continue to ca. −1200 mV
and a gradual increase in cathodic currents is found between
−1200 and −1400 mV. The former result suggests the under-
potential deposition (UPD) or chemical adsorption of Sn ions
significantly depressing the HER. Since the UPD of Sn4+ is not
found on curve 2, chemical adsorption of Sn ions is believed
to depress the HER in this case. The latter result is reason-
ably attributed to the deposition of Sn and the sharp increase
in the cathodic currents at potentials negative to −1400 mV is
attributed to the HER. Based on the above results and discus-
sion, the onset deposition potential of Bi ions (−390 mV) is
much nobler than that of Sn ions (−1200 mV), revealing the
large potential gap (ca. 810 mV) between Bi and Sn deposition.
Curves 1–3 in Fig. 2a show the polarization behavior of Bi3+,
Sn4+, and binary metal ions in the plating baths containing 0.4 M
CA and 1.0 M EDTA. From curve 1, the deposition of Bi starts
at about −600 mV, reaches the limiting current density in the
potential region between −800 mV and −1200 mV, and an obvi-
ous enhancement in the current density is clearly found as the
potentials are negative to −1200 mV, due to the HER. From a
Fig. 2. LSV curves measured at 8 mV s−1 and 25 ◦C from the solution contain-
ing (a) 0.4 M CA and 1.0 M EDTA with (1) 0.1 M Bi3+; (2) 0.1 M Sn4+; (3)
0.1 M Bi3+ + 0.1 M Sn4+; (b) 0.4 M CA and 0.1 M Bi3+ with the concentration
of EDTA = (1) 0, (2) 0.02, (3) 0.08, (4) 0.3, (5) 0.5, and (6) 1.0 M; and (c) 0.4 M
CA and 0.1 M Sn4+ with the concentration of EDTA = (1) 0.02, (2) 0.08, (3)
0.15, (4) 0.2, (5) 0.3, and (6) 0.5 M.
comparison of curves 1 in Figs. 1 and 2a reveals that the addition
of EDTA into the electroplating bath not only causes a negative
shift in the onset potential of Bi deposition from −390 mV to
−600 mV but also reduces the current density (ca. 25 mA cm−2
)
of Bi deposition. The negative shift in the onset potential and

Y.-D. Tsai et al. / Electrochimica Acta 53 (2007) 2040–2047
2043
the reduction in the limiting current density indicate the sig-
nificant coordination between Bi3+ and EDTA. Actually, the
addition of Na2H2EDTA into an acidified Bi nitrate solution
was reported to promote the adhesion of Bi [27], probably due
to this polarized effect on Bi. On curve 2 in Fig. 2a, tin deposi-
tion commences at ca. −900 mV and a plateau with a limiting
current density of ca. 30 mA cm−2 is visible between −1220 and
−1500 mV. Accordingly, the addition of EDTA into the plating
solution containing CA significantly promotes the deposition
rate of Sn from a comparison of curves 2 in Figs. 1 and 2a.
Therefore, EDTA is considered as a depolarizer for the tin depo-
sition although it acts as a polarizer in the deposition of Bi from
curve 1 in Fig. 2a. On curve 3 in Fig. 2a, the current–potential
curve also follows the trace of curve 1 at potentials positive
to ca. −950 mV although the electroplating bath contains both
Bi3+ and Sn4+. Note the very broad wave between −950 and
−1250 mV, indicating the significant deposition of Sn in this
potential region. Again, the HER is depressed to occur at ca.
−1270 mV in this binary (CA + EDTA) complexing system, due
to the poor catalytic activity of Sn since this reaction commences
at ca. −1200 mV on curve 1 in Fig. 2a. The above results reveal
that the potential gap between the deposition of Sn and Bi is
significantly reduced by the simultaneous addition of CA and
EDTA as the complex agents.
From a careful examination of curves 1 and 3 in Fig. 2a, there
is a peak with a shoulder wave on both curves, suggesting that
the reduction of Bi3+ complex species should involve two (or
more) electrochemical steps or that the complex species may
exist as free complex ions and adsorbed state which have differ-
ent reduction potentials. Accordingly, the LSV curves measured
in a 0.1 M Bi3+ + 0.4 M CA solution with various concentra-
tions of EDTA are used to clarify the above speculation (see
Fig. 2b). Note that the onset potential of Bi3+ reduction is neg-
atively shifted from −0.39 to ca. −0.6 V by the addition of
EDTA with its concentration ≥0.08 M. In addition, two broad
waves respectively centered at ca. −0.6 and −1.05 V are clearly
found on curves 4–6. On the other hand, two reduction peaks
are clearly found between −0.4 and −0.6 V (with an unobvious
shoulder at ca. −0.85 V) when the concentration of EDTA added
in the test solution is only 0.02 M (see curve 2). On curve 3, two
shoulders (at ca. −0.6 and −0.9 V, respectively) and a peak cen-
tered at −0.75 V are clearly found. These results indicate that
significant amount of Bi3+ were coordinated with EDTA in a
relatively low concentration, resulting in the presence of two
reduction peaks or a peak with two shoulders. All the above
results and discussion support the viewpoint that the coordina-
tion between Bi3+ and EDTA is strong and significant and that
the two-electrochemical-step process is a reasonable explana-
tion responsible for the presence of a peak and a shoulder wave
on curves 1 and 3 in Fig. 2a.
Fig. 3. LSV curves measured from the plating solution containing 0.4 M CA and
0.2 M PEG with (1) 0.1 M Bi3+; (2) 0.1 M Sn4+; (3) 0.1 M Bi3+ + 0.1 M Sn4+
LSV curves were measured at 8 mV s−1 under 25 ◦C.
.
Second, the voltammetric current densities from ca. −0.8 to
−1.6 V are gradually increased with the continuous addition
of EDTA into the test solutions. Both results indicate that the
coordination of Sn4+ to EDTA favors the reduction of Sn4+ to
Sn2+ which is further reduced to deposit onto the Ni/Cu sub-
strate. Furthermore, Sn(IV)–EDTA complexes may adsorb onto
the substrate to form a bridge-type adsorbent (very similar to the
role of Cl− in the electroplating of Cu [28,29]) which favor the
reduction of Sn(IV) species. This phenomenon is also found for
the addition of tartrate into a Sn2+ solution [14], revealing the
above possibility.
The polarization behavior of Bi3+, Sn4+, and binary metallic
ions measured from the basic plating solution containing 0.4 M
CA and 0.2 M PEG is shown in Fig. 3 as curves 1–3, respec-
tively. On curve 1, a plateau with a current density of 8 mA cm−2
is found from −400 to −1200 mV, suggesting a decrease in the
deposition rate when Bi3+ is coordinated to PEG and/or CA
although the onset potential for the Bi deposition in this solution
is not significantly changed by the addition of both compounds
(similar to the case with adding CA, see Fig. 1). Moreover, the
HER is negatively shifted from −800 to −1400 mV, presumably
due to adsorption of PEG [30–32]. The sharp increase in the
hydrogen evolving currents at potentials negative to −1400 mV
implies the leaving of PEG in this potential region. If this is
the case, the adsorption of PEG onto the Cu/Ni substrate is
potential-dependent [30,32]. The i–E responses on curve 2 in
Fig. 3 are very similar in both shape and size to that on curve 2
in Fig. 1, suggesting that the Sn deposition is not significantly
influenced by adding CA and PEG. On curve 3, the deposition
of Bi commences at −400 mV and the voltammetric behavior
almost completely follows the i–E responses on curve 1 at poten-
tials positive to ca. −1250 mV. On the other hand, the HER is
inhibited by the co-deposition of Sn in the more negative poten-
tial region, rendering the overlapping of curves 2 and 3 in this
region. The above results indicate that addition of PEG sup-
presses the Bi deposition and hydrogen evolution but possesses
insignificant influence on the Sn deposition.
The addition of EDTA promoting the deposition of Sn found
in Fig. 2a is further investigated in this work. Typical LSV curves
measured from the solutions containing 0.1 M Sn4+, 0.4 M CA,
and various concentrations of EDTA are shown in Fig. 2c. From
an examination of Fig. 2c, two features have to be mentioned.
First, there are two waves on all curves in this figure, indicating
the successive reduction steps involving Sn4+/Sn2+ and Sn2+/Sn.

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Y.-D. Tsai et al. / Electrochimica Acta 53 (2007) 2040–2047
Fig. 4. LSV curves measured from the plating solution containing 0.4 M CA,
1.0 M EDTA, and 0.2 M PEG with (1) 0.1 M Bi3+; (2) 0.1 M Sn4+; (3) 0.1 M
Bi3+ + 0.1 M Sn4+. LSV curves were measured at 8 mV s−1 under 25 ◦C.
Based on all the results and discussion, it is possible to obtain
a synergistic effect by combining complexes on depressing the
deposition of Bi (due to a negative shift in the deposition poten-
tial and a reduction in deposition rate) but insignificant influence
on the deposition of Sn. Fig. 4 demonstrates the above idea
through the simultaneous addition of 0.4 M CA, 1.0 M EDTA,
and 0.2 M PEG into the electroplating baths. From curves 1 and
2, both the deposition of Bi and Sn commences at approximately
−1000 mV. More importantly, the current density of Sn deposi-
tion is obviously higher than that of Bi deposition, suggesting a
higher deposition rate for the former metal. On the other hand,
curve 3 shows the simultaneous depression in the Bi and Sn
deposition although a common onset potential for the Sn and Bi
deposition is found at ca. −1000 mV on curves 1 and 2. Note the
sharp increase in the hydrogen evolving rate at ca. −1400 mV
on curve 1, supporting the statement that the adsorption of PEG
onto the Cu/Ni substrate is potential-dependent, which acts as an
inhibitor for the HER. On curve 3, hydrogen evolution also com-
mences at ca. −1400 mV (attributable to the leaving of adsorbed
PEG) while its current density is lower than that on curve 1, rea-
sonably attributed to the co-deposition of Sn. Based on the above
results and discussion, the Sn–Bi co-deposition and HER occur
simultaneously at potentials negative to −1400 mV.
Fig. 5. XRD patterns of Sn–Bi deposits plated from the bath corresponding to
(a) Fig. 1; (b) Fig. 2; (c) Fig. 3; and (d) Fig. 4 at 100 mA cm−2 under 25 ◦C with
pH of 6.0; where ꢁ, ␧, z, and ꢀ indicate the diffraction faces of Bi, Sn, Ni, and
Cu, respectively.
CA, CA + EDTA, CA + PEG, and CA + EDTA + PEG, respec-
tively. The presence of diffraction peaks corresponding to Bi on
all patterns indicates the formation of polycrystalline, metallic
Bi in these deposits. However, only the deposit prepared from
the solution containing 0.4 M CA, 1.0 M EDTA, and 0.2 M PEG
shows the diffraction peaks corresponding to Sn. These results
suggest the failure in co-deposition of Sn–Bi in the former three
solutions. This statement is supported by the EDS analyses (see
Table 2) which indicate the fact that only the deposit plated
from the solution containing the above three complex agents
shows significant amount of Sn (30Sn–70Bi) while the other
deposits mainly consist of Bi with minor amount of Sn. From
an examination of the results shown in Figs. 1–4, the above
results are understandable. Based on Fig. 1, the onset deposi-
tion potential of Bi and Sn is insignificantly changed by the
addition of 0.4 M CA, which shows a large potential gap of
810 mV for the co-deposition of Sn–Bi. For the case with CA
and EDTA, the potential difference between Sn and Bi deposi-
tion is reduced from 810 to 600 mV (see Fig. 2). Although it
is significant, the potential gap for the co-deposition of Sn–Bi
3.2. Textural analyses
Fig. 5a–d shows the typical XRD patterns of the deposits
plated at 100 mA cm−2 from the bimetallic solution containing
Table 2
Summary of electroplating conditions and the resultant properties of Sn–Bi deposits
Complex agents
Current density (mA cm−2
)
Adhesion
Dendrite growth
Composition (wt%)
CA
100
100
20
100
100
Poor
Poor
Excellent
Very good
Excellent
Yes
Yes
No
No
No
Sn0.5Bi99.5
Sn0.5Bi99.5
Sn0.5Bi99.5
Sn0.5Bi99.5
Sn30Bi70
CA + EDTA
CA + EDTA
CA + PEG
CA + EDTA + PEG
CA: citric acid; EDTA: ethylenediaminetetraacetic acid; PEG: polyethylene glycol (MW 400). The mean error of EDX analysis is ca. 1.5 weight percents (wt%).

Y.-D. Tsai et al. / Electrochimica Acta 53 (2007) 2040–2047
2045
is still too large to obtain a significant amount of Sn. For the
deposit plated from the solution containing CA and PEG, a
large potential gap of 800 mV for the co-deposition of Sn and
Bi is clearly found although the current density of Bi deposi-
tion in the plateau region is significantly reduced. Hence, the
co-deposition of Sn–Bi is not successful. On the other hand,
the success in co-deposition of the Sn–Bi deposit with 30 wt%
Sn reveals the effective depression on the deposition of Bi by
the simultaneous addition of CA, EDTA, and PEG into the
electroplating baths. This result is predictable from the LSV
results shown in Fig. 4, due to the common deposition poten-
tial and the reduction in deposition rates for both Bi and Sn
ions.
From the surface morphology of a nearly pure Bi deposit
plated from the bimetallic solution with CA (Fig. 6a and b),
the irregular grains show a corallite-like structure, which are
surface-enriched with whiskers in various forms (see Fig. 6b).
On the other hand, the nearly pure Bi deposit plated from the
solution with CA and EDTA shows the morphology similar to
a fern, stacked dendrite structure that is very similar in shape to
that of Bi crystallites prepared by Jiang et al. [33] (see Fig. 6c
and d). This result indicates the ineffective co-deposition of Sn
onto the substrate and a very high current density of Bi depo-
sition is obtained, causing the formation of fern-like dendrites.
It has to be mentioned that the adhesion of both deposits shown
in Fig. 6a–d is very poor, supporting the statement that the
Fig. 6. SEM morphologies of Sn–Bi deposits plated from the bath corresponding to (a and b) Fig. 1 at 100 mA cm−2; (c and d) Fig. 2 at 100 mA cm−2; (e and f)
Fig. 2 at 20 mA cm−2; (g and h) Fig. 3 at 100 mA cm−2; and (i andj) Fig. 4 at 100 mA cm−2
.

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Y.-D. Tsai et al. / Electrochimica Acta 53 (2007) 2040–2047
Fig. 6. (Continued )
deposition rate of Bi at 100 mA cm−2 is too high. Accordingly,
a reduced current density of 20 mA cm−2 is used to obtain a
deposit with good adhesion [17,26] (see Fig. 6e and f). This
nearly pure Bi deposit plated from the bimetallic solution with
0.4 M CA and 1.0 M EDTA shows a relatively compact morphol-
ogy. The polycrystalline structure consists of many grains with
an average diameter of ca. 5 ␮m. Under such a deposition cur-
rent density, the stacking of Bi atoms belongs to the pyramidal
growth (see Fig. 6f) [17]. From Fig. 6g, the deposit plated from
the solution with CA and PEG is piled with many grains with-
out obvious dendrites although it mainly consists of Bi. Under a
high magnification (see Fig. 6h), the grains can be composed of
short rods and screw dislocations, indicating the enrichment of
imperfections although this deposit shows good adhesion on the
substrate. On the other hand, the relatively compact, polycrys-
talline, andsmoothmorphologywithoutdendrites, verydifferent
from that shown in Fig. 6a–d, reveals the significant improve-
ment in the physical properties of resultant deposits with good
adhesion, due to the addition of PEG. The deposit plated from
the bimetallic solution containing CA, EDTA, and PEG shows a
porous morphology enriched with spherical grains and nodules.
This phenomenon is a common found for most deposits pre-
pared at the current densities in the mixed-control region [17].
Accordingly, the applied current density seems to be too high
although this deposit shows good adhesion in an ultrasonic bath
for 5 min. Actually, the morphologies of pure Sn and Bi deposits
electroplated under 100 mA cm−2 from the solutions used for
measuring curves 1 and 2 in Fig. 4 also show the absence of
dendrites (not shown here). Furthermore, in Fig. 6g and h, the
deposit plated from the solution containing CA and PEG does
not show the dendrite structure although it mainly consists of
Bi. These results strongly suggest the depression of dendrite
formation through the addition of PEG although the applied cur-
rent density, 100 mA cm−2, for the DC electroplating seems to
be larger than the limiting diffusion current densities estimated
from the LSV curves.
The thickness of deposits shown in Fig. 6a, c, e, g, and i is
about 0.7, 0.5, 3.1, 3.3, and 0.4 ␮m, respectively, which is esti-
mated from the weight difference of a Ni-coated Cu electrode
before and after the deposition. The corresponding current effi-
ciency (C.E.) is ca. 5.1, 3.7, 22.8, 24.6, and 3.8%, respectively.
The estimation of current efficiency is based on the following
equations:
qdeposit
C.E. (%) =
× 100%; qpassed = I × t
qpassed
Wdeposit × fSn × 4F
Wdeposit × (1 − fSn) × 3F
qdeposit
=
+
AWSn
AWBi
where qdeposit and qpassed indicate the charges corresponding
to the deposit and the passed charges during the deposition.
Wdeposit, fSn, AWSn, and AWBi are indicative of the deposit
weight, weight fraction of Sn determined by EDX, atomic
weight of Sn, and atomic weight of Bi, respectively. From the

Y.-D. Tsai et al. / Electrochimica Acta 53 (2007) 2040–2047
2047
above low C.E. data, the applied current density of 100 mA cm−2
for the DC electroplating seems to be too large. The relatively
high C.E. for the deposit plated from the solution containing
CA and PEG (i.e., the deposit shown in Fig. 6g) is probably
due to the adsorption of PEG, significantly depressing the HER
[30–32]. Actually, the complicated influences of electroplating
conditions and relative concentrations of the above three com-
plexagentsonthecompositionofSn–Bidepositsareunderstudy
in order to obtain the robust electroplating settings for the prepa-
ration of Sn–Bi deposits with compositions close to the eutectic
point.
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4. Conclusions
Based on the LSV analyses, the onset deposition potential of
Bi ions is obviously shifted to a negative value, close to that of Sn
ionsbythesimultaneousadditionof0.4 MCA, 1.0 MEDTA, and
0.2 M PEG into the plating solution. The potential gap between
the deposition of Sn and Bi is significantly reduced by a com-
bination of CA and EDTA. The addition of PEG suppresses the
Bi deposition and hydrogen evolution but possesses insignifi-
cant influence on the Sn deposition. From the textural analyses,
a 30Sn70Bi alloy (in wt%) is successfully plated from the bath
with the Sn4+/Bi3+ ratio of 1, pH of 6.0, and the simultaneous
addition of CA, EDTA, and PEG. This success demonstrates the
possibility for preparing the Sn–Bi alloys with their composition
close to the eutectic point (42Sn–58Bi) through electrodeposi-
tion. The adhesion of this deposit and the formation of dendrites
arerespectively improved and inhibited by thesynergisticeffects
provided by a combination of the above three complex agents.
However, nearly pure Bi deposits are obtained from the plating
solutions of the same composition with the other combinations
of complex agents.
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Acknowledgment
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The financial support of this work, by the National Science
Council of the Republic of China, is gratefully acknowledged.
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