Tin Electroplating from Sulfate Electrolyte Containing Syntanol, Formalin, and Coumarin

Article abstract of DOI:10.1023/A:1025640415337

Tin electroplating from sulfate electrolyte containing organic additives was studied. The electrolyte formulation for preparing lustrous smooth tin coatings was developed.

Full text of DOI:10.1023/A:1025640415337

Russian Journal of Applied Chemistry, Vol. 76, No. 3, 2003, pp. 387 390. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 3,
2003, pp. 402 405.
Original Russian Text Copyright
2003 by Medvedev, Nekrasova, Makrushin.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Tin Electroplating from Sulfate Electrolyte Containing
Syntanol, Formalin, and Coumarin
G. I. Medvedev, L. A. Nekrasova, and N. A. Makrushin
Novomoskovsk Institute, Mendeleev Russian University of Chemical Engineering,
Novomoskovsk, Tula oblast, Russia
Received April 29, 2002
Abstract Tin electroplating from sulfate electrolyte containing organic additives was studied. The electro-
lyte formulation for preparing lustrous smooth tin coatings was developed.
Our previous study [1 4] on tin electroplating from
sulfate electrolyte containing organic additives show
that introduction of Syntanol, Formalin, butane-,
2-butene-, and 2-butyne-1,4-diols, and benzyl and pro-
pargyl alcohols favors formation of lustrous coatings.
P = 2.3a/[2 havlog(H0 /Hi)],
where a is the wave amplitude of the sinusoid micro-
profile ( m), h is the average thickness of the coat-
av
ing ( m), and H and H are the initial and final am-
0
i
plitude of the sinusoid microprofile, respectively ( m).
The capacitance of the electrical double layer in the
course of electrolysis was measured by the sequential
substitution scheme using a P-5021 ac bridge operat-
ing at 30 kHz.
In this work we studied tin electroplating from sul-
fate electrolyte containing Syntanol, Formalin, and
coumarin. Coumarin is a luster-producing and smooth-
ing additive to the nickel plating solution [5].
We studied tin electroplating in the electrolyte con-
Tin was plated at 18 25 C without stirring or with
stirring of the electrolyte with a petal stirrer.
1
taining from 5 to 50 g l SnSO and from 90 to
4
1
100 g l H SO . Synthanol DS-10, coumarin, and
2
4
We studied the appearance of the coating as in-
fluenced by the organic additives to an electrolyte
Formalin (37 % formaldehyde solution) were intro-
duced in amounts of 1 4 g l , 0.1 2 g l , and 1
1
1
1
1
containing, respectively, 30 and 100 g l of SnSO
10 ml l , respectively.
4
and H SO . Dull coatings were obtained in the pres-
ence of 1 4 g l Syntanol. Low-quality coatings
were plated from the electrolytes containing Formalin
(1 10 ml l ), coumarin (0.1 2 g l ), or both these
additives. Dull coating are formed in the electrolyte
containing Syntanol (1 4 g l ) and coumarin (0.1
2
4
1
The coumarin solubility in the electrolyte was
1
found to be 2 g l . The equilibrium concentration is
1
1
attained within 20 h. Tin films 6 15 m thick were
deposited on copper supports. The polarization curves
were recorded with a P-5828 potentiostat. The luster
of the samples was measured on an FB-2 photoelectric
luster meter.
1
1
1
2 g l ), or Syntanol and Formalin (6 8 ml l ).
The quality of the coatings is substantially im-
proved after addition of coumarin to the electrolyte
The degree of the surface filling with the organic
additives was estimated from the polarization data
by the equation [6]
1
containing Syntanol (2 3 g l ) and Formalin (6
1
8 ml l ). At the coumarin concentration of 0.5
1
1 g l , lustrous coatings with dark strips along the
= 1
exp( E /b),
edges are obtained. At the coumarin concentration
1
from 1.5 to 2 g l , the whole cathode surface is
where E is the cathodic polarization; b is the con-
stant of the Tafel equation, determined form the po-
lustrous.
When the Formalin concentration is out of the in-
dicated range, silvery coatings are formed. At the
larization curves in the E logi coordinates.
c
1
The smothing power of the electrolytes P was
determined by direct profilographic measurements of
the sample surface with a sinusoid microprofile. The
calculation was performed by the equation [7]
Syntanol concentration of 1 g l , the range of the
current densities providing formation of lustrous coat-
ings strongly narrows down. At the Syntanol concen-
1
tration of 4 g l , the range of the current densities
1070-4272/03/7603-0387$25.00 2003 MAIK Nauka/Interperiodica

388
MEDVEDEV et al.
1
concentration of 5, 10, and 20 g l , the lustrous coat-
CE, %
2
ings are plated at i = 1 5 A dm ; at [SnSO ] =
c
4
4
1
1
2
30 g l , at i = 2 8 A dm ; and at [SnSO ] =
c
2
50 g l , at i = 4 10 A dm .
c
The sulfuric acid concentration in the electrolyte
1
should be in the range 90 100 g l . At the concentra-
1
tions below 90 g l , the electrolyte is unstable in
the course of tin plating and the range of i providing
c
formation of lustrous coatings narrows down. At the
1
sulfuric acid concentration higher than 100 g l , the
range of current densities for obtaining lustrous coat-
ings does not change. It should be noted that high-
quality lustrous coatings are plated only in the stirred
electrolyte. Without stirring, dull coatings are formed.
2
ic, A dm
Fig. 1. Current efficiency (CE) of tin electroplating as
a function of the current density i . Electrolyte contains
100, 2, and 2 g l of H SO , Syntanol, and coumarin,
respectively, and 6 ml l of Formalin; mechanical stirring.
c
1
2
4
1
The dependence of the current efficiency (CE) on
the current density in the electrolyte with the organic
additives is shown in Fig. 1. The current efficiency
1
SnSO concentration, g l : (1) 5, (2) 10, (3) 20, (4) 30,
4
and (5) 50.
2
2
decreases with increasing i and grows with increasing
c
ic, A dm
C, F cm
the SnSO concentration. At the SnSO concentration
4
4
1
2
of 5 g l and i = 1 5 A dm , CE ranges from 96 to
c
1
71% and at the SnSO concentration of 50 g l and
i = 4 10 A dm , from 99.9 to 90.1 % (Fig. 1,
curves 1, 5).
4
2
c
To determine the effect of the organic additives on
tin plating, we recorded cathodic polarization curves
on a rotating disc electrode. As seen from Fig. 2, in
the presence of the organic compounds, the cathodic
polarization E increases and the cathodic polari-
c
zation curves flatten out at the limiting currents
(curves 1, 2). Curves 1 and 2 were recorded on the
stationary disc electrode. At the rotation rate of the
Ec, V
disc electrode increased from 200 to 500 rpm, i
lim
Fig. 2. (1 7) Cathodic polarization curves of tin, recorded
on a rotating disc electrode, and (8, 9) the capacitance of the
electrical double layer C as a function of cathodic potential
increases, and the cathodic polarization decreases
(curves 3, 4). When the rotation rate increases to
900 2000 rpm, the section of the limiting current
E
(vs. SHE) in the tin plating solutions. Electrolyte con-
c
1
tains 30 and 100 g l of SnSO and H SO , respectively.
disappears and E is 320 mV (curves 5 7).
4
2
4
c
(i ) Cathodic current density. (1, 8) Initial electrolyte; (2 7,
c
1
The plateau of the limiting current in the polariza-
tion curves is due to adsorption of the organic com-
pounds on the cathode surface, which is confirmed by
variation of the capacitance of the electrical double
layer. As seen from Fig. 2 (curves 8, 9), the capacit-
ance of the electrical double layer decreases after
introduction of the organic compounds. In the range
of potentials E from 0.3 to 0.4 V, the capacitance
9) the electrolyte containing Syntanol and coumarin (2 g l
each) and 6 ml l of Formalin. Cathode rotation rate (rpm):
(1, 2) 0, (3) 100, (4) 500, (5) 900, (6) 1500, and (7) 2000.
1
providing formation of lustrous coatings does not
change, but the electrolyte foams on stirring, which
complicates tin plating.
The range of working current densities providing
formation of lustrous coatings was also studied as
influenced by the SnSO concentration. We found that
the current density required to obtain lustrous coating
increased as the SnSO concentration was increased
from 5 to 50 g l in the electrolyte containing H SO
(90 100 g l ), Syntanol (2 3 g l ), Formalin (6
8 ml l ), and coumarin (1.5 2 g l ). At the SnSO
2
decreases from 32 to 18 F cm . At higher cathodic
potentials, the organic compounds are desorbed from
the electrode surface and the continuity of the adsorp-
tion layer gets broken, thus increasing the capacitance
of the double layer.
4
4
1
2
4
1
1
1
The dependences of the coverage of the cathode
surface (curve 1) and of the degree of the coating
1
4
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 3 2003

TIN ELECTROPLATING FROM SULFATE ELECTROLYTE
luster (curve 2) on the current density are shown in
Fig. 3. The degree of the coating luster increases and
the surface coverage decreases with increasing current
389
, %
L, %
density i . The surface coverage ranges from 13.8 to
c
6.8 % at i providing formation of lustrous coatings. It
c
should be noted that the lustrous coatings are plated
from the electrolyte with organic additives only in a
definite range of i and . Probably, at the optimal
c
values of these parameters the roughness of the grow-
ing films is minimal.
We measured the smoothing power of the sulfate
electrolyte as influenced by the cathodic current den-
2
ic, A dm
sity and the coumarin and SnSO concentrations.
4
Fig. 3. (1) Coverage of the cathode surface and (2) degree
of coating luster L as functions of the cathodic current
density i . Electrolyte contains, respectively, 30, 100, 2,
and 2 g l of SnSO , H SO , Syntanol, and coumarin and
6 ml l of Formalin; mechanical stirring.
The results are shown in Fig. 4. As seen from Fig. 4,
the electrolytes containing Syntanol, Formalin, and
c
1
coumarin smooth the coating surface. When i in-
4
2
4
c
1
2
creases from 1 to 5 A dm in the electrolyte con-
taining 10 g l SnSO , S decreases from 0.5 to 0.4
1
4
S
(Fig. 4, curve 1). At the same time, at the SnSO con-
4
1
2
centration of 30 g l and i = 1 8 A dm , the
smoothing power increases from 0.62 to 0.98 (Fig. 4,
c
curve 2). The dependences of the smoothing power of
1
the electrolytes containing 10 and 30 g l SnSO on
4
1
the coumarin concentration in the range 0.25 2 g l
differ. As seen from Fig. 4, the S c dependence at the
1
SnSO concentration of 10 g l
passes through
4
a minimum (curve 3), and at the SnSO concentra-
tion of 30 g l , through a maximum (curve 4). The
smoothing power of the electrolyte containing 30 g l
4
1
1
2
SnSO is higher.
4
ic, A dm
The mechanism of surface smoothing can be ex-
plained in the framework of the conventional adsorp-
tion diffusion theory [7] which suggests that the
polarization curves recorded on a rotating disc elec-
trode at various rotation rates qualitatively simulate
the cathodic process on profile micropeaks (high rota-
tion rate) and microvalleys (low rate). To estimate the
influence of various additives, the cathodic potential
in the electrolyte with an additive (or additives) was
measured at two rates of electrode rotation. If the
polarization increases with increasing rotation rate,
the additive(s) will have the smoothing effect.
1
c, g l
Fig. 4. Smoothing power S of tin sulfate electrolytes as
a function of the (1, 2) cathodic current density i and
(3, 4) coumarin concentration c. Electrolyte contains, re-
spectively, 100 and 2 g l of H SO and Syntanol and
c
1
2
4
1
6 ml l of Formalin; mechanical stirring. (1, 2) Electro-
1
1
lyte + 2 g l coumarin + 10 and 30 g l SnSO , respec-
4
1
tively; (3, 4) electrolyte + 10 and 30 g l SnSO , respec-
4
tively.
celerates, i.e., the change in the tin plating rate looks
like a manifestation of the antismoothing effect. It
should be also noted that this phenomenon was ob-
served during tin plating from the sulfate electrolyte
with other organic additives [1 4] and during electro-
deposition of an Sn Sb alloy [8].
Analysis of the cathodic polarization curves re-
corded in a tin sulfate electrolyte on a rotating disc
electrode (Fig. 2, curves 3 7) shows that these curves
do not simulate the distribution of tin plating rates
over the microprofile. This is probably due to adsorp-
tion of organic compounds on the electrode surface.
Intensification of the hydrodynamic conditions affects
formation and degradation of the adsorption layer. In
this case, tin plating on all microprofile sections ac-
Based on the above results, we propose the follow-
ing formulation of the sulfate electrolyte for plating
lustrous and smooth tin coatings: SnSO (5 50 g l ),
H SO (90 100 g l ), Syntanol DS-10 (2 3 g l ),
coumarin (1.5 2 g l ), and 37% formaldehyde solu-
1
4
1
1
2
4
1
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390
MEDVEDEV et al.
1
2
tion (6 8 ml l ); i = 1 10 A dm . The current ef-
plateau increases and the cathodic polarization de-
creases with increasing rotation rate.
c
ficiency of tin electroplating is 71 99.9%. The elec-
trolyte temperature is 18 25 C. At higher tempera-
ture, the electrolyte becomes turbid and low-quality
deposits are formed. The electrolyte should be mech-
anically stirred. To decrease contamination of the
electrolyte with the anodic slime, the tin anodes
should be placed in woven polypropylene bags.
(3) The coverage of the cathode surface decreases
and the degree of coating luster grows with increasing
i . Lustrous coatings are formed at the coverage from
c
6.8 to 13.8%.
(4) The smoothing power of the sulfate electrolyte
containing the organic additives was studied. These
electrolytes exhibit the positive smoothing effect
which depends on i and on the coumarin and SnSO
The electrolyte was highly stable during prolonged
lab trials. However, after prolonged operation, a light
yellow precipitate deteriorating the film quality was
accumulated on the bath bottom. The precipitate
should be filtered off at regular intervals.
c
4
concentrations.
(5) The distribution of tin electroplating rate over
the microprofile cannot be simulated by the polariza-
tion curves recorded at different rates of electrode
rotation, since the adsorption layer inhibiting the elec-
trodeposition is sensitive to the hydrodynamic con-
ditions.
The IR spectrum of the precipitate (KBr pellet)
contains a strong broad band of OH stretching vibra-
tions in the range 3400 3600 cm [9]. The band at
1
1
895 cm can be due to bending vibrations of the
Sn O H group [10]. In addition, weak coumarin
bands are observed at 3040, 1720, 1610, 1260, 1180,
1125, and 760 cm . We suggest that the precipitate is
(6) A self-adjusted electrolyte for electroplating of
smooth and lustrous tin coatings was developed.
1
a mixture of tin hydroxide and coumarin.
REFERENCES
The SnSO , H SO , Formalin, and coumarin con-
4
2
4
1. Medvedev, G.I. and Gorbunova, I.M., Zh. Prikl.
Khim., 1990, vol. 63, no. 4, pp. 807 812.
2. Medvedev, G.I. and Mashutina, G.G., Zh. Prikl.
Khim., 1992, vol. 65, no. 4, pp. 789 795.
3. Medvedev, G.I. and Makrushin, N.A., Zh. Prikl.
Khim., 2001, vol. 74, no. 11, pp. 1787 1790.
4. Medvedev, G.I., Makrushin, N.A., and Duben-
kov, A.N., Zh. Prikl. Khim., 2002, vol. 75, no. 2,
pp. 227 230.
5. Blestyashchie elektroliticheskie pokrytiya (Lustrous
Electroplated Coatings), Matulis, Yu.Yu., Ed., Vil-
nius: Mitis, 1969.
6. Valentelis, L.Yu., Kamuntavichene, I.Yu., and Matu-
lis, Yu.Yu., Tr. Akad. Nauk. Lit. SSR, Ser. B, 1970,
vol. 63, no. 4, pp. 129 136.
centrations in the electrolyte were adjusted on the
basis of chemical analysis data [11]. After passage of
1
1
100 A l of electricity, 1 g l of Syntanol should be
added to the bath. Coumarin can be added in the
amount exceeding its solubility. In this case, the
coumarin concentration equal to its solubility is main-
tained throughout the tin plating. Excess coumarin
is introduced in woven polypropylene bags in the
amount no less than the weekly consumption. The
presence of coumarin in the bags placed between
the electrodes is monitored visually. In this case,
the analysis of the electrolyte for coumarin is not
required, since its concentration is self-adjusted. The
1
1
coumarin consumption is 0.01 g A h .
7. Kruglikov, S.S., Itogi Nauki Tekh., Ser.: Khim. Elek-
trokhim., 1965, pp. 117 147.
8. Medvedev, G.I., Kruglikov, S.S., and Fursova, N.Yu.,
Zh. Prikl. Khim., 2001, vol. 74, no. 11, pp. 1763
1764.
9. Bellamy, L.J., The Infra-Red Spectra of Complex
Molecules, London: Methuen, 1954.
10. Nakamoto, K., Infrared Spectra of Inorganic and
Coordination Compounds, New York: Wiley, 1963.
CONCLUSIONS
(1) Lustrous tin coatings are plated from the sul-
fate electrolyte containing Syntanol, Formalin, and
2
coumarin at i = 1 10 A dm depending on the
c
SnSO concentration.
4
(2) The organic additives inhibit tin electroplating.
The cathodic polarization curves recorded on a rotat-
ing disc electrode in the presence of these additives
flatten out at limiting current. The length of this
11. Vyacheslavov, P.M. and Shmeleva, N.M., Kontrol’
elektrolitov i pokrytii (Monitoring of Electrolytes and
Coatings), Leningrad: Mashinostroenie, 1965.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 3 2003