XV International PhD Workshop
OWD 2013, 19–22 October 2013
ELECTROLESS TIN DEPOSITION ON COPPER FROM
THIOUREA TYPE BATHS
Aneta Araźna, Tele- and Radio Research Institute
Abstract
Immersion tin coating can be deposited on the
copper surface in two ways: by disproportionation
process of Sn(II) or by displacement reaction
between copper and tin. This process of tin
deposition on copper is widely used in the printed
circuit board technology [1-5].
Displacement reaction between copper and tin is
running from acid solution. For many years baths
based on chloride acid were applied. In the last years
this solutions are replaced with a solution basic on
the compounds friendly to the environment, for
example so as choline chlorice or methanesulphonic
acid (MSA) [6-12].
In the present work, the correlation between the
immersion Sn coatings deposition rate and the
deposition conditions, such as concentrations of
acid, Sn(II) salt, and thiourea, from immersion tin
plating solution based on hydrochloric (SnHCl) salt
or methanesulphonic (SnMSA) salt, is discussed.
The experiments results show the dominating
influence of the thiourea and HCl concentration on
the Sn deposition rate irrespective of the type of
solution. The deposition rate increases with
increased concentration of thiourea. The potential of
Cu decreases with the increase of thiourea
concentration and then deposition rate increased.
The deposition rate first increased and then
decreased with the increase in acid concentration.
The reason of this situation is depassivation of Cu
surface at the beginning process of tin deposition
and then the dissolution of Sn layer in the strongly
acid environment. The concentration of tin(II) has a
smaller influence on the tin deposition rate.
The solderability of as-deposited coatings was
good or very good. Only for the very thin coatings
(0.2 µm) spread area was lower than 90%. The
wettability of thermally aging tin coatings by the
solder paste was worse than for as-deposited
coatings and depended on the thickness and type of
bath. The solderability of SnMSA coatings thicker
than 0.9 µm was worse than SnHCl coatings. It is
being assigned to the difference in the structure tin
layers and intermetallic grains.
1. Introduction
Immersion tin can be deposited on the copper
surface in two ways. The first way is deposition from
an alkaline type bath by the disproportionation
process of Sn(II) [1-3]. This kind of deposition
technology is not used in electronic applications
because it has destructive influence on photoresists
[4,5].
The second way is deposition from acidic
solutions by a chemical displacement reaction
between copper and tin. This reaction occurs in
solutions with appropriate agents (thiourea),
complexing metal ions, which give rise to reversing
of deposition potentials for these metals. The tin
coating grows up to the moment, when the solution
is still in contact with copper substrate. When the
whole substrate is coated with tin, the reaction
ceases. Therefore, the generated coatings are very
thin, about 1.5 µm at most. However, they are
thought to be continuous, smooth, uniform and
non-porous [6-8].
For many years baths based on chloride acid were
applied. However, in the last years this solutions are
replaced with a solution basic on the compounds
friendly to the environment for example so as
choline
chlorice
(2-hydroxy-ethyl-trimethylammonium chloride, ChCl) [9] or methanesulphonic
acid (MSA) [10-12].
In the present work, the driving force of
immersion Sn process from traditional immersion tin
plating solution based on hydrochloric (SnHCl) salt
and from environment friendly methanesulphonic
based (SnMSA) salt solution is presented. The
correlation between the immersion Sn coatings
deposition rate and the deposition conditions such as
concentrations of acid, Sn(II) salt, and thiourea, is
discussed. The solderability of immersion Sn
coatings is also compared.
2. Experimental
Investigated tin coatings were deposited on the
copper foil M1E in the immersion process. The
copper surface was degreased and microetched
before tin layers deposition. The deposition process
was made at 70°C. The baths composition were
respectively:
stannous
chloride
or
tin(II)
352
methanesulphonate – concentration from 0.05 M to
0.02 M, thiourea (TU) - concentration from 0.3 M to
1.0 M, hydrochloric acid or methanesulphonic acid
(MSA) - concentration from 0.1 M to 0.5 M. The
tinning time was 30 min.
The thickness of Sn coatings was determined by
coulometry in 1 M HCl with controlled potential
change of Cu/Sn electrode relative to saturated
calomel electrode (SCE). This method is fast and not
expensive and gives the information about layer
thickness and also about possibility of intermetallic
phase existence. It is
used for accurate
determination of coatings thickness in PCB industry
[13].
For solderability measurement of Sn layers the
solder spread test was used. We used the lead free
solder paste SAC (95.5Sn/4.0Ag/0.5Cu). The solder
spread tests were made at 260°C for 15 s according
to the procedure described at the work [14]. There
were examined tin layers deposited from two type of
solution with the thickness in the range 0.2÷1.5 µm.
The tin coatings were investigated after deposition
and after aging in the condition of dry heat (4 h at
155ºC, it represents storage over 12 month [15]).
2
Fig. 1. Effect of thiourea concentration on standard
electrode potential of Cu in 0.3 M HCl aqueous solution,
at room temperature. Concentration of thiourea: 1- 1mM,
2- 10 mM, 3- 100 mM.
3. Results and discussion
Replacement reaction between Cu and Sn2+
cannot occur, because the standard electrode
potential of Cu+/Cu is 0.522 V/SCE, which are
much more higher than those of Sn2+/Sn = -0.136
V/SCE. It is necessary to add a complexing agent of
copper ions, such as thiourea, which decreases the
standard electrode potential of Cu to be lower than
that of Sn (Cu+/Cu[SC(NH2)2]4+ = -0.620 V/SCE)
[9,16-18]. In the chloride aqueous solutions contains
Cu+ and Sn2+ ions are creating complexes thiourea
with Cu+ ions as well as thiourea and Sn2+ ions
[19,20]. The effect of thiourea on standard electrode
potential of Cu and Sn is illustrated in figure 1 and 2.
As we can see the standard electrode potentials of
Cu and Sn have different trend in aqueous solution
with varied thiourea concentration. The standard
potential of Cu electrode is moving to the more
negative values with the increase of thiourea
concentration, and is achieving value about -0.520
V/SCE at 100 mM thiourea concentration (fig. 1).
However, the standard electrode potential of Sn
electrode is shifting to the more positive values with
the increase of thiourea concentration (fig. 2). This
effect enables the course of replacement reaction
between Cu and Sn2+.
Fig. 2. Effect of thiourea concentration on standard
electrode potential of Sn in 0.3 M HCl aqueous solution,
at room temperature. Concentration of thiourea: 1- 1mM,
2- 10 mM, 3- 100 mM.
In every series of experiments only one
component of solution was changed. The influence
of thiourea, Sn(II) ions, and acid concentration on
rate of tin deposition was determined. The results of
investigations were presented in figure 3, 4 and 6.
The experiments results show the dominating
influence of the TU and acid concentration on the
Sn deposition rate irrespective of the type of
solution.
The deposition rate increases rapidly with
increased concentration of thiourea in the range of
TU contentration from 0.3 M to 0.6 M (fig. 3).
Above the concentration of TU 0.6 M the rate of tin
deposition is growing more slowly.
353
(fig. 5b). In the very strongly acid condition the part
of anodic areas is bigger and the rate of tin
deposition is stopped.
a) immersion cell
b) corrosion cell
CuTM4+
Sn2+
Sn2+
TM
Sn
K+
Sn
Cu
e
Fig. 3. Effect of thiourea concentration on Sn deposition
rate. SnHCl - hydrochloric based bath, SnMSA –
methanesulphonic based bath.
As we discussed earlier, the potential of Cu
decreases with the increase of thiourea
concentration. Thus, the increased thiourea
concentration strengthened the complexing ability to
Cu ions and the deposition rate increased. However,
the thiourea can change the electrode potentials of
Sn too, and at too higher TU concentration, the
complex of TU with Sn2+ would be formed to
decrease free Sn2+ in the bath. As a result, the
deposition rate decreases. The second reason of the
lowered deposition rate may be TU decomposition.
The concentration of acid directly influenced the
deposition rate of immersion tin process irrespective
of the type of solution. As shown in figure 4
deposition rate first increased with the increase in
acid
concentration
until
0.15
M
for
methanesulphonic salt based solution and 0.35 M for
hydrochloric salt based solution, and then decreased
with increase of acid concentration. The reason of
the lowered deposition rate may be dissolution of Sn
finish in the strongly acid environment.
Sn
Cu
A
e
Cu
K+
Fig. 5. The schema of galvanic cells on the surface of
samples in the acid environment: a) immersion
cell, b) corrosion cell.
There were made same investigation of
dissolution of tin coatings in immersion tin plating
bath. The samples of tin coatings one the copper
(tinning time was 10, 20 or 30 min) were putting into
bath containing 0.1 M SnCl2 and 0.2 M HCl (without
thiourea) in the 70°C for the time equal of the
tinning time. The results of the experiment show
that the loss in weight of tin layers is reaching to
about 14% for the longest time of the etching. This
value is similar to the value of the decrease in the
thickness of Sn layers during plating in solution with
the highest concentration of HCl – about 19%. This
means that dissolution of tin coatings is a main
factor of reducing rate of the tinning process with
the increase of the HCl concentration in the
examined solutions.
As shown in figure 6, the deposition rate of tin
coatings first increased with the increase in Sn(II)
concentration
until
about
0.1
M
for
methanesulphonic salt based solution and 0.125 M
for hydrochloric salt based solution, and then
decreased with growth of concentration.
Fig. 4. Effect of acid concentration on Sn deposition rate.
SnHCl - hydrochloric based bath, SnMSA –
methanesulphonic based bath.
At the tinning surface exist by themselves two cells.
The immersion cell in which tin is cathode and is
depositing on the copper surface (fig. 5a) and the
corrosion cell in which same part of tin coating is
getting anode and is dissolution in acid environment
H2
Sn
-
A-
H+
Fig. 6. Effect of Sn(II) concentration on Sn deposition
rate. SnHCl - hydrochloric based bath, SnMSA –
methanesulphonic based bath.
The solderability of as-deposited coatings was
good or very good. Only for the very thin coatings
(0.2 µm) spread area was lower than 90% (Fig. 7). In
the case of thin tin layers (0.9 µm and below) better
354
wettability had layers deposited from the
methanesulphonic solution. However, coatings with
the thickness of 1.6 µm and 1.9 µm had comparable
solderability.
suppose that it caused formation of small IMC grains
which were very active during aging. The layer with
coarse crystallites of the intermetallics could be
formulated. The thick and uneven layer of IMC
aggravated solderability of SnMSA coating.
However, the SnHCl coatings have structure
with large and some small grains as well as some
surface irregularities [21,22], and this probably
caused formation of large copper-tin IMC grains.
The presence of small grains within the larger ones
could also cause that the surface morphology of the
IMC phase became flat. Such SnHCl coatings after
aging had better solderability.
4. Conclusion
Fig. 7. Solderability tests results for different thickness
of tin coatings as deposited.
The wettability of thermally aging tin coatings
by the solder paste was worse than for as-deposited
coatings and depended on the thickness and type of
bath. For SnHCl and SnMSA coatings of thickness
below 1.6 µm a significant decrease in the solder
paste spread area (below 60%) was stated (Fig. 8).
This result was classified as “dewetting” and such
coating were not solderable. The spread over 90%
was observed only for SnHCl coatings of thickness
1.6 µm and higher. This indicated that copper-tin
intermetallics did not reach surface of these coatings.
The solderability of SnMSA coatings thicker than 1.2
µm was worse than SnHCl coatings. The value of
spread area below 80% suggested that a great
fraction of these coatings was converted into
intermetallic phases (IMC).
Fig. 8. Solderability tests results for different thickness
of tin coatings after aging.
The other reason of worse solderability could be
the structure of intermetallic grains. The SnMSA
coatings have very small crystals structure with a lot
pits (cavities) on the surface [21,22] and we can
In the present study the influence of the
deposition conditions such as concentrations of acid,
Sn(II) salt, and thiourea on the immersion Sn
coatings deposition rate was examined. The tin layers
were deposited from traditional immersion tin
plating solution based on hydrochloric (SnHCl) salt
and from environment friendly methanesulphonic
(SnMSA) salt based solution. The coulometry
method were used to determined thickness of Sn
coatings, and the solder spread test was used to
measure solderability of Sn layers.
The results of experiments showed that
dominating influence on the Sn deposition rate had
the TU and HCl concentration. The process was
stopped at low TU and HCl concentration because
the unsatisfied Cu(I) complexation and Cu
passivation effects. For high concentration of HCl
and TU this process was stopped as result of Sn
dissolution and TU decomposition.
The solderability of as-deposited SnHCl and
SnMSA coatings was good in the whole range of
coatings thicknesses. That means the layer of pure
tin over copper-tin intermetallic layer was thick
enough and decided on coatings good wettability.
The poor wetting properties of IMC phase, which
reached surface of SnHCl and SnMSA coatings with
thickness below 1.6 µm, were implicated as the main
cause of degradation in solderability performance of
annealed coatings. The solderability of thicker SnHCl
coatings was better than for SnMSA coatings of the
similar thickness because of differences in the
surface morphology of the copper-tin intermetallic
phase. Large grains of IMC and flat surface were
probably formed for tin coatings generated from
hydrochloric salts based bath in the contrary to small
grains with coarse crystallites for tin coatings
obtained from methanesulphonic salts based bath.
The presence of uneven IMC layer cased that more
pure tin was needed (thicker layer of SnMSA) to
obtain the same solderability as SnHCl coatings.
355
The author gratefully acknowledge financial
support from Tele- and Radio Research Institute.
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Dr inż. Aneta Araźna
Tele- and Radio Research Institute
ul. Ratuszowa 11
03-450 Warszawa
tel. (022) 6192241 wew. 213
e-mail: aneta.arazna@itr.org.pl