Study on Immersion Tin Process by Electrochemical Methods and Molecular Orbital Theory

Article abstract of DOI:10.1149/1.2358119

Electroless deposition of tin onto copper surface has been studied by means of electrochemistry, X-ray fluorescence spectrometry, and theoretical Hartree-Fock methods. There are two processes in series involved in the hypophosphite-type immersion tin reaction, based on the analysis of the immersion tin potential curves and the structure of the deposits. The first displacement process is significantly affected by the complexing agents of cuprous and the concentration of SnSO4, mainly due to the formation of Cu-Sn alloy and a thinner tin layer. The second process is attributed to the stannous ion reduction by hypophosphite at higher temperatures.The Electrochemical Society

Full text of DOI:10.1149/1.2358119

C848
Journal of The Electrochemical Society, 153 ͑12͒ C848-C853 ͑2006͒
0013-4651/2006/153͑12͒/C848/6/$20.00 © The Electrochemical Society
Study on Immersion Tin Process by Electrochemical Methods
and Molecular Orbital Theory
,z
Jie Zhao,a Ning Li,a Guofeng Cui,b, and Jianwei Zhaoc
*
aDepartment of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, China
bSchool of Chemistry and Chemical Engineering, Sun-Yat Sen University, Guangzhou 510275, China
cDepartment of Chemistry, Nanjing University, Nanjing 210093, China
Electroless deposition of tin onto copper surface has been studied by means of electrochemistry, X-ray fluorescence spectrometry,
and theoretical Hartree–Fock methods. There are two processes in series involved in the hypophosphite-type immersion tin
reaction, based on the analysis of the immersion tin potential curves and the structure of the deposits. The first displacement
process is significantly affected by the complexing agents of cuprous and the concentration of SnSO4, mainly due to the formation
of Cu–Sn alloy and a thinner tin layer. The second process is attributed to the stannous ion reduction by hypophosphite at higher
temperatures.
© 2006 The Electrochemical Society. ͓DOI: 10.1149/1.2358119͔ All rights reserved.
Manuscript submitted December 30, 2005; revised manuscript received August 1, 2006. Available electronically October 20, 2006.
electrochemical workstation in a conventional three-compartment
Pyrex cell. A defined sample area of 2.0 cm2 was exposed to the
electrolyte. The Luggin capillary was placed close to the working
electrode to minimize the solution resistance. A Pt foil was used as
the auxiliary electrode and a saturated Hg2SO4 electrode was used
as a reference. All electrode potentials were referred to saturated
Hg2SO4 potential if not stated especially. In immersion tin potential
measurements, the substrates were first mechanically polished with
various grades of sand paper ͑up to 600 grit͒ and then degreased in
an acidic cleaning solution. Then they were microetched in sodium
persulfate and sulfuric acid solution and thoroughly rinsed with
deionized water.
Theoretical calculations were performed with the Gaussian
03 rev. C 02 program package.14 Following the pioneering work
made by Homma et al.,15-17 the geometries of the reactants, prod-
ucts, and transition states were optimized using second-order
Møller–Plesset ͑MP2͒ method and at the MP2/6-311G͑d,p͒ level.
Harmonic vibrational frequency calculations at the same level were
used to make sure that the transition state was located. In order to
get the simulations close to the real system, we adopted the Tomasi’s
polarizable continuum ͑PCM͒ solvation model18-21 to evaluate the
effect of temperature and solvent on the reaction in the whole cal-
culation. The dielectric constants of water at various temperatures
were obtained from Lange’s handbook of chemistry.22
Conventional solderable materials, Sn–Pb alloys, used as final
coatings on printed circuit boards do not meet the environmental
requirements because of the usage of toxic lead. The electroless tin
deposition is an alternative approach leveling for the flat mounting
of electronic components instead of the Sn–Pb1,2 because it is low-
cost, free of lead, compatible with all solder alloys, and applicable in
a wide variety of surface technologies.3-6
Until now, two kinds of electroless tin deposition techniques
have been well known as disproportionation of Sn͑II͒ compounds in
alkaline bath and displacement of copper by tin in Sn͑II͒–thiourea
acidic bath.7,8 The latter has attracted much attention because it
provides a much more uniform and solderable surface. In addition,
the operation is much easier under the lower temperature
conditions.9-11
There are many factors affecting the solderability of the immer-
sion tin coatings, such as tin whisker, contamination of impurity,
composition of copper–tin intermetallic compound, and thickness of
coatings.8,12,13 Among them, the thickness of the coatings is crucial.
Increasing the thickness of the tin coatings can improve solderability
at multiple reflow cycles, enhance the surface flatness, fine the pitch
compatibility, and restrict the tin whisker growth. Therefore, it is
desired to study how to increase the thickness of immersion tin
coatings.
The purpose of this work is to study the mechanism of the
hypophosphite-type immersion tin process. The correlation between
the deposition layer and the conditions such as Sn͑II͒-salt, concen-
trations of thiourea and hypophosphite, plating time, and tempera-
ture of bath are given as well.
Results and Discussion
Displacement deposition process in the immersion tin.— Effect
of thiourea on the displacement process.— Figure 1 shows the
open-circuit potential ͑OCP͒ of the immersion tin process, i.e., im-
mersion tin potential ͑ITP͒ as a function of the immersion time. It
was observed that the immersion tin potential sharply increases from
0 to 50 s and then stablizes. This evolution indicates that there are
two intrinsically different processes in the whole immersion tin re-
action.
Experimental
Copper sheets ͑20 ϫ 50 mm and 0.5 mm thick͒ were used as the
substrate for the deposition of tin coatings. The substrate was first
degreased in acid solution, followed by microetching in a sodium
persulfate ͑80 g/L͒ and sulfuric acid ͑4 g/L͒ solution. After rinsing
with deionized water, the substrate was transferred into the plating
bath. Typical compositions of immersion bath are listed in Table I.
All solutions were freshly prepared before use. The deposition was
carried out for 15 min for every sample. All reagents used were of
analytical grade.
Table I. Constitution and operation condition of immersion tin
bath.
The thickness of the tin coating was measured by an S4-Explorer
X-ray fluorescence spectrometer ͑XRF͒ ͑Bruker AXS Inc.͒. The
X-ray diffraction ͑XRD͒ measurements of the coatings were per-
formed by a Rigaku D/MAX-3B X-ray diffractometer using Cu K␣
radiation ͑k = 1.5406 Å͒.
Parameter
Quantity
Stannous sulfate ͑SnSO4͒
Sodium hypophosphite ͑NaH2PO2͒
Thiourea ͑Tu͒
Sulfuric acid ͑H2SO4͒
Betaine ͑C5H11NO2͒
pH
28.0 g/L
80.0 g/L
80.0 g/L
77.5 g/L
1.5 g/L
Electrochemical experiments were performed on a Gamry Inc.
0.8
pH adjuster
Temperature
Immersion time
2 mol/L H2SO4
45 2°C
15 min
*
Electrochemical Society Student Member.
z E-mail: cuiguofeng@hit.edu.cn
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Journal of The Electrochemical Society, 153 ͑12͒ C848-C853 ͑2006͒
C849
␪
␾ ͑Cu+/Cu͒ = 0.5120 V
͓1͔
͓2͔
␾ ͑Sn2+/Sn͒ = − 0.1375 V
␪
However, the potential of Cu+/Cu can decrease with the addition
of thiourea, because thiourea is a very strong complexing agent to
Cu+ cation. When the potential of Cu+/Cu is negative than that of
Sn2+/Sn, the displacement of copper by stannous ions occurs. Espe-
cially, a considerable amount of ͓Cu͑Tu͒ ͔+ complex can be formed
4
while the Cu͑I͒ concentration is 0.01 mol/L or less and, alterna-
tively, the concentration of Tu is 0.1 mol/L or higher26
+
Cu+ + 4Tu ꢀ ͓Cu͑Tu͒ ͔
͓3͔
4
where Tu denotes the thiourea complexing group. The complex for-
mation constant Kf keeps the Cu+ ions in solution while the immer-
sion tin deposits according to Eq. 3. For equilibrium Cu+–Tu com-
26,27
Figure 1. Immersion potential of immersion tin at various thiourea concen-
trations for 1000 s. Other bath compositions and conditions are shown in
Table I.
plex at 25°C, Kf = 1015.4
.
Knowing Kf, the nonstandard potential of ͓Cu͑Tu͒ ͔+/Cu can be
4
obtained according to the Nernst equation
RT
␪
␪
␾ ͕͓Cu͑Tu͒ ͔+/Cu͖ = ␾ ͑Cu+/Cu͒ −
ln Kf
It is unambiguous that the first step is a displacement reaction.
Moreover, the changes of cuprous amount and stannous amount ex-
actly follow the stoichiometry of the presented displacement reac-
tion, i.e., Cu + Sn2+ → Cu2+ + Sn.23,24 In this process, the ITP has
an apparent change, as shown by the sharp increase of the curves in
Fig. 1. Another important feature of Fig. 1 is that the ITP curves
gradually decrease with increasing thiourea ͓SC͑NH2͒2͔ concentra-
tion, indicating the addition of thiourea can reduce the average value
of ITP. In this paper, the thiourea is a complexing agent of copper
ion, which can lead to improvement of the potential difference of
␾͑Sn2+/Sn͒ and ITP, making the immersion tin reaction proceed
smoothly.
4
zF
= 0.5120 − 0.05917 log Kf
= − 0.3992
͓4͔
͓5͔
+
RT ͕͓Cu͑Tu͒ ͔ ͖
4
␾͕͓Cu͑Tu͒ ͔+/Cu͖ = ␾ ͕͓Cu͑Tu͒ ͔+/Cu͖ +
ln
␪
4
4
4
zF
͓Tu͔
+
͕͓Cu͑Tu͒ ͔ ͖
4
=− 0.3992 + 0.05917lg
4
͓Tu͔
In addition
XRD patterns of the deposited tin coatings are given in Fig. 2,
from which one can observe that the major components of the coat-
ings are of copper and tin. Those weak extra peaks can be attributed
to the formation of copper–tin intermetallic phase, Cu6Sn5, for ex-
ample, during the displacement process.25 Through the observation
in this figure, it is reasonable to draw a profile of the components in
the tin-deposited structure. In the bottom layer, there is a pure cop-
per substrate. Then, a thin intermediate layer exists due to the for-
mation of the alloy. On the top, there should be a pure tin layer,
which is formed plentifully soon after the alloy formation.
Mechanism of displacement process in immersion tin process.—
Based on the above observations, the mechanism and the effect fac-
tors of the displacement process have been further investigated in
detail. According to the thermodynamic principles, the displacement
reaction cannot occur, because copper is more electrochemically in-
ert than tin in terms of the standard electrochemical potential
RT
␪
␾͑Sn2+/Sn͒ = ␾ ͑Sn2+/Sn͒ +
ln͓Sn2+͔
zF
0.05917
= − 0.1375 +
lg͓Sn2+͔
͓6͔
2
The displacement of copper by stannous ions can occur only when
␾͑Sn2+/Sn͒ − ␾͕͓Cu͑Tu͒ ͔+/Cu͖ Ͼ 0
͓7͔
4
That is
␾͑Sn2+/Sn͒ − ␾͕͓Cu͑Tu͒ ͔+/Cu͖ = 0.2617
4
0.05917 ͓Tu͔ ͓Sn2+͔
8
+
lg
Ͼ 0
͓8͔
+ 2
2
͕͓Cu͑Tu͒ ͔ ͖
4
Considering the particular operation conditions, i.e., high con-
centrations of stannous ion and thiourea, the concentration of
+
͓Cu͑Tu͒ ͔ should be rather low because a small amount of copper
4
is dissolved. Therefore, the displacement reaction can continue
smoothly. With increasing concentrations of thiourea, the electrode
potential of ͓Cu͑Tu͒ ͔+/Cu decreased, resulting in remarkable ac-
4
celeration of the displacement reaction. The ITP increases from
−0.927 to −0.950 V when the concentration of Tu increases from 20
to 140 g/L in bath. This discussion is consistent with the observa-
tion in Fig. 1.
In the displacement process, copper atoms are dissolved as a
result of the formation of ͓Cu͑Tu͒ ͔+, and tin atoms are deposited
4
on the copper substrate. In the experiments, we usually measure the
deposition layer as thick as over 1 ␮m, indicating that tin deposition
continues even when the copper substrate is completely covered by
a layer of tin atoms. This result indicates that the immersion tin
reaction cannot be a simple surface displacement process. Addition-
ally, there is a pure tin layer on the Cu–Sn alloy, according to analy-
sis of the XRD pattern in Fig. 2. Therefore, we considered the pure
Figure 2. XRD of immersion tin coatings deposited on copper substrate.
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Journal of The Electrochemical Society, 153 ͑12͒ C848-C853 ͑2006͒
Figure 3. Immersion tin potential in 1000 s at various SnSO4 concentra-
tions. Other bath compositions and conditions are shown in Table I.
Figure 5. Effect of cuprous complexing agent concentrations on OCP and
␾͑Sn2+/Sn͒ obtained in basic bath in Table I, except no addition of SnSO4. A
copper sheet is used as a working electrode.
tin layer formed in another process, which should be chemical re-
duction, due to the addition of hypophosphite in bath. This reduction
process is discussed later.
change to negative when more proper cuprous complexing agent is
added, as described in Eq. 8. The effect of copper complexing agent
on OCP is distinctly different, as illustrated in Fig. 5. The variation
of OCP is not obvious when imidazole is added in the bath, but
Effect factors of displacement process.— Concentration of
SnSO4.— The immersion tin potential at various concentrations of
SnSO4 in 1000 s is presented in Fig. 3. We increased the concentra-
tion of SnSO4 gradually from 10 to 42 g/L in basis bath as shown in
Table I. The increase of the concentration of SnSO4 facilitates the
positive shift of the chemical reaction of Eq. 8 as well as the positive
shift of the ITP curves in Eq. 6. However, the tin coating thickness
does not increase with the increase of concentration of SnSO4.
The thickness of the immersion tin coatings as a function of the
SnSO4 concentration is given in Fig. 4, from which one can observe
that the thickness decreases almost linearly with increasing SnSO4
concentration. We also noted that there are many tin crystal whiskers
on the substrate when the concentration of SnSO4 is under 10 g/L.
In addition, the immersion time can be a good indicator for the
termination of the displacement process. In this example, the immer-
sion time reduced from 200 s for 10 g/L to 30 s for 42 g/L. There-
fore, under the high concentration of SnSO4, the copper substrate
can be quickly covered by a tin layer, and consequently, the dis-
placement process is inhibited.
thiourea and thiosemicarbazide can make ␾͕͓Cu͑X͒ ͔+/Cu͖ signifi-
N
cantly reduced. The effect of thiosemicarbazide on OCP is appar-
ently smaller than thiourea.
The thickness of immersion tin coatings obtained at various con-
centrations of the complexing reagents in a basic bath by XRF is
given in Fig. 6. When we use imidazole as the complexing agent, no
tin coatings can be observed during the reaction. On the contrary, an
increase in the concentrate of both thiourea and thiosemicarbazide
may significantly increase the thickness of the coatings. Moreover,
the effect from thiourea is more obvious. These results are also
consistent with the analysis of Fig. 5, in which there is no OCP
change for imidazole complexing reagent but dramatic decrease for
both thiourea and thiosemicarbazide. Therefore, the OCP measure-
ment can be used as an effective and convenient method for select-
ing the good complexing agents as well as for rationalizing the
working conditions.
Reduction process in immersion tin reaction.— Effect of im-
mersion time and temperature on thickness of tin coatings.— The
thickness of tin coatings as functions of deposition time and bath
temperature are presented in Fig. 7. The thickness of coatings is
continuously increased, indicating that another reaction is involved
Effects of other complexing agents.— The complexing agent of
copper is a crucial factor dominating the displacement process dur-
ing immersion tin. The displacement reaction of copper by tin can
occur only when ␾͕͓Cu͑X͒ ͔+/Cu͖ is more negative than
N
␾͑Sn2+/Sn͒, according to the thermodynamic principles. Here, X
represents the copper complexing agent. The ␾͕͓Cu͑X͒ ͔+/Cu͖ can
N
Figure 4. Effect of SnSO4 concentration on thickness of immersion tin ob-
tained in basis bath as shown in Table I. The period of deposit time is
15 min.
Figure 6. Effect of concentration of cuprous complexing agents on thickness
of immersion tin coatings. Other bath compositions and conditions are
shown in Table I.
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Figure 8. ͑Color online͒ Relationship of ͑immersion time͒ and coating
thickness at various bath temperatures. Other bath compositions and condi-
tions are shown in Table I.
Figure 7. Relationship of immersion time and coating thickness at different
temperatures. Other bath compositions and conditions are shown in Table I.
Molecular orbital (MO) theory study on oxidation process of hypo-
phosphite.— According to proposal by Touhami et al.,28-30 the re-
duction process is generated through the preceding chemical reac-
tion of hypophosphite oxidation. This reaction is a deprotonation
reaction of hypophosphite and can be written as
in the process. Based on the above discussion and the particular
working conditions, the chemical reaction of hypophosphite is most
likely involved to reduce stannous ion to tin atom.
A dynamic tin deposition has been studied in this work. It is
found that the deposition persistently continues at least for 7000 s,
according to a linear relationship with the square root of the immer-
sion time ͑t1/2͒, as shown in Fig. 8. This observation indicates that
the growth kinetics of immersion tin obeys a parabolic law as char-
H2PO−2 → HPO−2 + H͑ads͒
͓9͔
The hypophosphite ion possesses a C2v symmetry in water solution,
in which the two P–O bonds and P–H bonds are equivalent, as
shown in Fig. 9a and d. The detail vibrational spectroscopy of the
hypophosphite ion was analyzed in the reference.31 The metallic
ions can be reduced by hydrogen-free radicals, originated from the
breakage of the P–H bond in hypophosphite ion. Thus, the energy
barrier of the whole reduction process is the breakage energy of the
3
acterized by the ͱt-growth formula. The growth rate is found to be
governed by diffusion-controlled kinetics in tin deposition.8 Higher
temperature is more favorable to the coating growth, but a too-high
bath temperature is not proper for the tin deposition in the practice.
Table II. Harmonic vibrational frequencies calculated at various temperatures in water solution at the calculated MP2/6-311G(d,p) level for the
optimized geometries of transition states.
Temperature
͑°C͒
Vibration frequency ͑cm−1
͒
25
30
40
60
80
169.9i, 68.9, 156.5, 457.9, 630.4, 1029.6, 1136.3, 1443.9, 2457.3
175.4i, 70.2, 158.6, 458.0, 630.6, 1029.4, 1136.3, 1443.8, 2457.3
168.9i, 65.6, 140.3, 457.9, 630.2, 1029.7, 1136.3, 1443.9, 2457.6
168.8i, 76.3, 122.2, 457.8, 630.8, 1030.3, 1136.5, 1444.3, 2458.8
175.9i, 72.7, 154.9, 457.9, 631.1, 1030.4, 1136.3, 1444.4, 2457.6
Table III. Total energies and zero-point energies (ZPEs) for the reactant, product, and transition state for the breakage P–H bond process at
various temperatures in water solution calculated at the MP2/6-311G(d,p) level.
Species
25°C
30°C
40°C
60°C
80° C
H2PO−2
ZPE
0.026544
0.026544
0.026543
0.026543
0.026543
Total energy
H2PO−2 loss H
ZPE
−492.331428
−492.331394
−492.331321
−492.331164
−492.330992
0.016398
0.016458
0.016393
0.016354
0.016306
Total energy
H
−492.220033
−492.224914
−492.234852
−492.244630
−492.250327
ZPE
0.000000
0.000000
0.000000
0.000000
0.000000
Total energy
−0.498399
−0.498399
−0.498399
−0.498399
−0.498399
HPO−2
ZPE
0.015113
0.015113
0.015112
0.015112
0.015112
Total energy
−491.762869
−491.762834
−491.762762
−491.762608
−491.762439
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Journal of The Electrochemical Society, 153 ͑12͒ C848-C853 ͑2006͒
Figure 11. Effect of NaH2PO2 concentration and bath temperature on the
thickness of immersion tin deposits obtained in basis bath as shown in Table
I. The period of deposit time is 15 min.
Figure 9. ͑Color online͒ Optimized geometric parameters of the ͑a͒ reactant,
͑b͒ product, and ͑c͒ transition state for the breakage P–H bond process and
͑d͒ charge distribution of hypophosphite ion were calculated at the MP2/6-
311G͑d,p͒ level in water solution. Bond lengths and angles are given in
angstroms and degrees, respectively.
process. These results agree well with the calculation results by MO
theory. The thickness of tin deposits cannot continuously increase
after the displacement process without hypophosphite added in bath,
as shown in Fig. 7. Obviously, hypophosphite can play the role of
reducing agent in the reduction process. However, the reduction pro-
cess is not severe like the electroless plating nickel process, because
the tin species has weak catalytic ability and the bath temperature is
lower.
P–H bond. We calculated the energy change of the breaking bond
process in water solution at various temperatures by ab initio meth-
ods.
There is a single imaginary frequency of the transition state at
various temperatures in water solution, as shown in Table II. Mean-
while, the vibration direction of the imaginary frequency is in agree-
ment with the P–H bond breakage direction, as shown in Fig. 9c.
These results indicate a correct transition state is located in the con-
cerned system. The total energy of all reaction species and potential
energy surfaces are shown in Table III and Fig. 10, respectively. The
energy barrier of P–H bond breakage decreases from 64.9, 61.8,
55.6, and 49.5 to 45.9 kcal/mol, with the temperature increasing
from 25 to 80°C. The bond of P–H can break spontaneously in
hypophosphite water solution at higher temperatures. Thus, stannous
ions can be reduced by the hydrogen radical in the hypophosphite
solution after the displacement process. The deposition of tin atoms
takes place on the previous tin layer, eventually forming the pure tin
layer.
Conclusions
Immersion tin process was studied by electrochemical methods
and molecular orbital theory. The structure of immersion tin coat-
ings was analyzed by XRD. There are two main reaction processes
in the whole immersion tin reaction, which are displacement and
reduction processes, respectively.
In the displacement reaction, copper atoms are dissolved and
produce a stable complex ͓Cu͑Tu͒ ͔+, which can displace stannous
4
ions. This leads to deposition of tin atoms on the copper substrate.
Moreover, the tin and copper alloy layer can be formed in the pro-
cess. The displacement reaction is significantly affected by the cu-
prous complexing agents and concentration of SnSO4.
Effect of hypophosphite concentration and temperature on reduction
process
.— Figure 11 depicts the effect of NaH2PO2 concentration
When the surface is covered by the alloy, probably not fully, a
chemical reduction of hypophosphite takes place, leading to the for-
mation of a pure tin layer as demonstrated by experimental measure-
ments and ab initio calculations.
and bath temperature on the thickness of tin coatings. With
NaH2PO2 concentration increasing, the thickness can be improved
under various bath temperatures. The improvement becomes more
significant at higher temperature. Additionally, high temperature is a
favorable condition to immersion tin process, especially to reduction
Acknowledgments
The authors thank the Chinese Doctoral Foundation
͑20030213007͒ and the National Natural Science Foundation of
China ͑NSFC͒ ͑20435010, 20503012͒ for financial support.
Harbin Institute of Technology assisted in meeting the publication costs
of this article.
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