Journal of The Electrochemical Society, 158 (6) D335-D341 (2011)
0013-4651/2011/158(6)/D335/7/$28.00 The Electrochemical Society
D335
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Preparation of Cu-Sn Layers on Polymer Substrate by
Reduction-Diffusion Method Using Ionic Liquid Baths
Kuniaki Murase, ,z Akira Ito, Takashi Ichii, and Hiroyuki Sugimura
*
Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
A novel metallization of non-conductive epoxy substrate with Cu-Sn “speculum alloy,” or “white bronze,” was performed through
successive electrochemical processes: (i) conventional electroless deposition of pure Cu layer and (ii) subsequent electrochemical
alloying of the resulting pure Cu layer with Sn using an ionic liquid bath at 150ꢀC, a medium-low temperature. Availability of the
Sn quasi-reference electrode for the alloying was verified, and the resulting compact and adhesive Cu-Sn layers, composed of
Cu6Sn5 and/or Cu3Sn intermetallic phases, were examined as an alternative to nickel plating. The abundance of the two intermetal-
lic phases was found to be dependent on the alloying potential and duration, and was discussed in terms of alloy formation thermo-
dynamics of the Cu-Sn system.
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2011 The Electrochemical Society. [DOI: 10.1149/1.3573984] All rights reserved.
Manuscript submitted February 4, 2011; revised manuscript received March 9, 2011. Published April 12, 2011.
Electrodeposition, or electroplating, of metals using cathodic
reduction of corresponding metal ions in a solution is a basic tech-
nology for thin-layer processing to add desired properties to materi-
als. While the phenomenon is nowadays applied for microfabrica-
tion in the fields of electronics and micromachining, the preparation
of decorative and/or protective coatings is still the major purpose
for which electrodeposition technology is used. Not only single met-
als but also alloy coatings are prepared by electrodeposition because
electrodeposited alloys usually have enhanced properties for a given
application compared with metals in the pure state.1 However, there
are sometimes difficulties associated with stable process operation
of alloy electrodeposition compared to the case of pure metal elec-
trodeposition because the deposition baths contain more constitu-
ents: two or more metal salts, corresponding complex formers, buf-
fering agents, and additives (e.g. brightener and leveler). This
complexity of alloy electrodeposition baths can shorten the lifespan
of the baths, and at the same time makes waste bath treatments com-
plicated and energy-consuming even though electrodeposition is
considered an environmentally-friendly technique for thin-layer
processes.
As an alternative, we have been developing a reduction-diffusion
(RD), or electrochemical alloying, method to form alloy layers
using baths that each contain a single metal component.2,3 By using
this method, silver-white Cu-Sn alloy layers with composition 40–
60 wt % Sn (i.e. 26–45 atom % Sn) called “speculum metal” or
“white bronze” are obtained through two successive steps:3 (i) con-
ventional electrodeposition of a pure Cu layer from an aqueous
CuSO4–H2SO4 bath containing only Cu2þ ions as a metal compo-
nent, followed by (ii) the RD alloying using an ionic liquid-based
another bath containing Sn2þ ions. Here, the use of nonvolatile and
nonflammable ionic liquid (IL) as a solvent renders it possible to
raise the process temperature of RD alloying up to 150–190ꢀC,
medium-low temperatures, and to speed up the alloy layer growth
compared to that in aqueous baths, which cannot be operated at tem-
peratures above 100ꢀC in an ambient atmosphere. Although this
method consists of two steps, the use of two separate baths each
containing a single-metal component will extend the lifespan of the
baths and also make waste bath treatment simpler and easier.
More recently, we demonstrated and briefly reported, as a short
communication,4 that the Cu-Sn alloy metallization of an epoxy
substrate was also feasible by using electroless Cu deposition for
step (i) above. Since the Cu-Sn alloy layer is a promising alternative
to an allergenic nickel underplating for decorative gold or chromium
electroplating, the Cu-Sn metallization of such non-conductive
polymer substrates also needs to be developed. In the present paper,
we report a more detailed analysis of resulting Cu-Sn layers in order
to thoroughly discuss the mechanism and thermodynamics of the
Cu-Sn alloy formation, and to investigate some materials properties
as an alternative to nickel coating. In addition, the electrochemical
stability of pure Sn electrode was also examined to verify the valid-
ity of this electrode as a quasi-reference when employed in RD
alloying.
Experimental
Preparation of electrolytic bath for alloying.— Ready-made
hydrophobic IL, 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sul-
fonyl]amide (EMI-Tf2N; note that the Tf2Nꢁ anion where Tf ¼ SO2CF3
is sometimes described as TFSAꢁ or TFSIꢁ), was purchased from
Kanto Kagaku or Merck and used without further purification.
Although another IL, trimethyl-n-hexylammonium bis[(trifluorome-
thyl)sulfonyl]amide, was tried previously,2,3 we used the thermally
more stable EMI-Tf2N in the present study.5 Tin(II) salt, Sn(Tf2N)2,
having the same anion as the IL was prepared by the acid-base reac-
tion of SnO (Nacalai Tesque) with bis[(trifluoromethyl)sulfonyl]-
amine (HNTf2; Fluka): SnO þ 2HNTf2 ! Sn(Tf2N)2 þ H2O. Here,
powdery SnO was added to 1 mol dmꢁ3 aqueous solution of HNTf2
and allowed to react at 70ꢀC for more than 2 h under a nitrogen
purge. Unreacted SnO was then filtered off and the water was vapor-
ized away at 120ꢀC under a nitrogen atmosphere, yielding a white
hydrated Sn(Tf2N)2, which was further dried at 120ꢀC for 3 days in a
vacuum dryer. Note that the resulting Sn(Tf2N)2 salt may contain re-
sidual water and/or HNTf2. In the present RD process, however,
these residues, if present, have an insignificant effect on the reduc-
tion of Sn2þ to Sn0, since the potential of Sn2þ/Sn0 redox couple
(ESn2þ/Sn0) is not that negative. We also expected that the most part of
these volatile residues would spontaneously vaporize away when the
bath for alloying (hereafter referred to as the “alloying bath”) is oper-
ated at medium-low temperatures. To yield the alloying bath, the
resulting Sn(Tf2N)2 salt was weighed and dissolved at 80ꢀC into
the EMI-Tf2N in an open dry chamber (Daikin HRG-50A), in which
the dew point was kept lower than ꢁ 50ꢀC; the Sn2þ ion concentra-
tion of the alloying bath was 0.05 mol dmꢁ3
.
Cu substrate for alloying.— Unless otherwise noted, a thin layer
of Cu (thickness, ca. 0.5 lm) electrolessly deposited on a heat-
resistant high-Tg glass epoxy substrate using a conventional aqueous
electroless plating bath (Okuno Chemical Ind., ATS Addcopper IW)
was used as the Cu substrate for the RD alloying (see Figs. 3a and
3b). As a plastic substrate for decorative electroless plating, ABS
(acrylonitrile-butadiene-styrene) resin is generally used; however,
ABS, a thermoplastic resin, cannot be used for the RD process at
medium-low temperatures. Since the glass epoxy substrate was pro-
vided as a copper foil-clad laminate (Hitachi Chemical MCL-E-
679) for printed-circuit board manufacturing, the copper foil was
first completely removed by dissolving it into an aqueous etchant
*
Electrochemical Society Active Member.
z E-mail: murase.kuniaki.2n@kyoto-u.ac.jp
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