Electroless Pb-Sn alloy plating bath composition

An electroless Pb-Sn alloy plating bath composition is obtained by mixing at least one of methane sulfonic acid, 1-hydroxyethane-1, 1-diphosphoric acid and a salt of 1-hydroxyethane-1, 1-diposphoric acid, compounds for forming Pb-Sn alloy and a reducing agent.

BACKGROUND OF THE INVENTION 
Field of the Invention 
This invention relates to an electroless Pb-Sn alloy plating bath 
composition. 
Solder plating is used with electronic components, printed circuit boards 
and the like for corrosion prevention and component attachment. 
The steady increase in the integration density of such electronic 
components and printed circuits has been accompanied by a tendency toward 
discrete conductor portions and finer circuit line features. As methods 
for conducting solder plating there are known the electrolytic solder 
plating method and the molten solder plating method. Both have drawbacks. 
Specifically, depending on the shape of the component etc. to be plated, 
the former method is not able to produce a uniform plating. Moreover, 
since it is not possible to secure electrical contact at portions that are 
not in electrical continuity, plating of these portions is impossible. On 
the other hand, the latter method is incapable of controlling, or ensuring 
the uniformity of, the plating thickness and is thus likely to result in 
the formation of electrical shorts, particularly in the case of fine 
patterns. It also entails heat-related problems such as heat-induced 
deformation of the substrate of the article being plated. 
Prior Art Statement 
As a result of research conducted for overcoming these shortcomings of the 
conventional methods, a number of electroless plating methods have been 
announced. The first plating bath compositions developed for these methods 
typically consisted of a mixture of stannous chloride, lead chloride, 
thiourea and hydrochloric acid, having a reducing agent etc. blended 
therewith. In a bath of such composition, reaction between the sparingly 
soluble metal chlorides and the thiourea produces addition compound 
precipitates. For ensuring that the precipitates dissolve completely so 
that the liquid remains transparent, the bath has to be maintained at a 
temperature of at least 70.degree. C. Even then, the aforesaid addition 
compounds tend to precipitate and separate from the liquid when the 
article to be plated is immersed therein and causes the temperature of the 
liquid surrounding it to decrease. Moreover, when electroless plating is 
conducted at the ordinary temperature using a bath of the foregoing 
temperature, the plating rate is on the order of only 1 micron per 10 
minutes. 
While the plating rate can be increased by raising the temperature of the 
bath composition, when this is done the surface of the plating film tends 
to become uneven and to be degraded by the deposition of coarse crystal 
grains. What is more, the plating film obtained reaches a thickness of no 
more than 3 microns. 
For overcoming the defects of the aforementioned chloride bath, it has been 
proposed to use a borofluoride bath such as the bath comprising tin 
borofluoride, lead borofluoride, thiourea, borofluoric acid and a reducing 
agent disclosed in Japanese Patent Publication No. 62(1987)-2630 or, as 
disclosed in Japanese Patent Publication No. 62(1987)-17202, to use a bath 
comprising tin borofluoride, lead borofluoride, thiourea and a reducing 
agent whose concentration is varied between at least two levels during the 
electroless plating process. Being composed of borofluorides, the baths 
used in these methods are themselves toxic and pose a danger to those who 
work with them. They also require special treatment of the waste liquid 
and involve other problems regarding practical application. 
In either the method using a chloride bath or that using borofluorides, the 
thickness of the film obtained when plating is conducted at 80.degree. C. 
for 15 minutes is on the order of only about 3 microns. The percentage of 
tin in the Pb-Sn alloy film obtained is too low to meet any of the S . A 
and B standards prescribed by JIS-Z3282. Nor does the film do well in 
fusing tests. 
The inventors therefore conducted research for developing a plating bath 
composition that is free of the aforesaid defects of the prior art, 
specifically for developing a bath composition that does not require any 
special waste liquid treatment and which is capable of forming an 
electroless solder plating film on a copper surface with ease. As a 
result, they accomplished this invention. 
SUMMARY OF THE INVENTION 
This invention provides an electroless plating bath composition obtained 
from a liquid which contains as its essential main components at least one 
member selected from among the group consisting of methane sulfonic acid, 
ester of 1-hydroxyethane-1, 1-diphosphoric acid and salts, ester of of 
1-hydroxyethane-1, 1-diphosphoric acid, a tin compound for solder plating 
film formation, a lead compound for solder plating film formation, and a 
reducing agent. 
In addition to the aforesaid main components, the liquid is required to 
contain thiourea and a surface active agent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As the tin compound, it is preferable to use, for example, tin (II) oxide. 
The lead compound is preferably lead oxide or lead acetate. 
Differently from the case where a chloride is used, the use of tin oxide 
results in virtually no generation of low-solubility products. As a 
result, no precipitation is caused by the temperature drop occurring when 
the article to be plated is immersed in the bath, the likelihood of an 
inferior plating being formed is eliminated and the metal concentration of 
the liquid can easily be maintained at a high level. The area of the 
plating film formed per unit area of the plating bath is therefore 
increased and the life and film forming rate of the bath are greatly 
improved. 
The concentration of the tin compound is preferably 18 g/l-36 g/l as tin, 
while the concentration of the lead compound is preferably 15 g/l-35 g/l 
as lead. 
For preparing the bath composition of this invention there is required at 
least one member selected from among methane sulfonic acid, ester of 
1-hydroxyethane-1, 1-diphosphoric acid and salts of ester of 
1-hydroxyethane-1, 1-diphosphoric acid (hereinafter referred to as HEPA 
and/or salts thereof). The methane sulfonic acid content is preferably 130 
g/l-240 g/l and the content of HEPA and/or salts thereof is preferably 4 
g/l-20 g/l. 
When the methane sulfonic acid content is too low, the bath becomes milky 
and hydrolysis occurs. When it is too high, the once formed alloy film 
dissolves again, which slows the rate of film formation and increases the 
percentage of the film accounted for by tin. 
When the tin oxide concentration is lower than that mentioned above, the 
tin content of the alloy becomes low, the plated surface becomes coarse, 
and the effect of preventing corrosion of the copper substrate, which is 
one of the aims in providing the solder alloy plating, cannot be realized. 
On the other hand, when the concentration is too high, the tin content of 
the coating film becomes excessive. 
The lead salt is preferably lead oxide or lead acetate and, as mentioned 
earlier, is preferably contained at 15 g/l-35 g/l as lead. When its 
concentration is too low, the tin content of the alloy film becomes large. 
On the other hand, when its concentration is too high, the lead content of 
the alloy film becomes large, which is undesirable because it causes the 
coating film surface to become coarse, reduces the adherence of the film 
to the substrate and, by raising the melting point of the alloy film, 
markedly degrades its solder attachment property thus making it impossible 
for the film to achieve one of its main purposes. 
The thiourea concentration is preferably 70 g/l-140 g/l. When its 
concentration is too low, the plating rate becomes low and when too high, 
the tin content of the alloy film becomes high and the plating rate 
decreases sharply. 
As stated above, the concentration of HEPA and/or salts thereof is 4 g/l-20 
g/l. At a concentration within this range, the surface of the deposited 
film is fine grained and exhibits a dull luster. In short, the results are 
excellent. Lower concentrations do not contribute to obtaining a 
fine-grained texture nor to realizing a finish with a dull luster. Above 
this range, deposition of tin is does not increase in proportion to the 
increased amount of tin present, with the result that the tin content of 
the alloy film decreases. 
When HEPA and/or salts thereof and methane sulfonic acid are both present 
in the plating composition, the concentration of the methane sulfonic acid 
is preferably 125 g/l-225 g/l. 
The concentration of the reducing agent is preferably 44 g/l-150 g/l. At 
lower concentrations, a large amount of metal remains in the bath after 
completion of the reduction reaction, while at high concentrations, gas is 
generated as the temperature increases and causes pits to form in the 
deposited film. 
An anionic surface active agent is used. For example, it is possible to use 
lignosulfonic acid at a concentration of 0.1 g/l-0.2 g/l or polyacrylic 
acid at a concentration of 5 ml/l-8 ml/l. The amount of either of these 
added is determined in light of the concentration of the composition. 
In the solder plating composition according to the invention, the tin and 
lead are present as salts of methane sulfonic acid and HEPA, 
The effect of the invention will now be explained with respect to examples 
of the invention and comparative examples. The substrates for plating and 
the processing steps used in the examples and comparative examples were as 
follows: 
______________________________________ 
1) Substrates for plating 
a) 50 .times. 50 .times. 0.4 mm copper sheet 
b) 60 mm .times. 10 mm .phi. copper tube 
c) Double-sided printed circuit having a discrete 
copper circuit constituted of 50 [500 mm (L) .times. 
0.1 mm (W)] lines with a line spacing of 0.05 mm 
2) Processing steps 
Each of the substrates of 1) were treated as 
follows: 
a) Pretreatment 
Acid degreasing (40.degree. C.) ----&gt; water rinsing 
----&gt; soft etching ----&gt; water rinsing ----&gt; acid 
soaking ----&gt; water rinsing 
b) Posttreatment 
Cold water rinsing ----&gt; hot water rinsing ----&gt; 
drying in hot air stream 
3) The resulting substrates were subjected to 
electroless plating in the baths indicated as 
examples of the invention in Tables 1(a) and (b) 
or in the baths indicated as comparative examples 
in Tables 2 and 3, under the conditions shown in 
the respective tables. 
______________________________________ 
The plating film obtained in each of the examples and comparative examples 
was then subjected to the following tests. 
______________________________________ 
4) Plating film test 
a) Measurement of film thickness and alloying 
ratio by the fluorescent x-ray method 
b) Measurement of state of fusing following 
holding at 190.degree. C.-195.degree. C. for 40 sec in an 
infrared furnace. 
G (Good) = total fusing F (Fair) = 40- 
60 % fusing NG (No Good) = total lack of 
fusing 
c) Testing for short-circuiting of printed 
circuit board. 
G = No shorting NG = Shorting 
d) Bending test (copper sheet and copper tube) 
Observation of abnormalities after bending 
at 180.degree. C. 
G = No abnormalities NG = Abnormalities 
found 
e) Visual observation of plating film surface 
G = Luster F = Rough NG = Very 
uneven 
f) Plating film-to-substrate adhesion test (by 
adhesive tape) 
G = Pass 
______________________________________ 
The results of the plating film tests with respect to the examples and 
comparative examples are shown in the tables. Table 1 relates to examples 
according to the invention. Table 2 relates to comparative examples using 
baths containing the same materials as the baths in the examples but in 
amounts outside the range of the invention. FIG. 3 relates to comparative 
examples using conventional electroless plating baths. 
TABLE 1a 
__________________________________________________________________________ 
Example No. 1 2 3 4 5 6 7 8 9 10 
__________________________________________________________________________ 
Bath Methane sulfonic acid g/l 
130 
130 
130 
150 
150 
150 
170 
170 
170 
200 
composition 
HEPA g/l 
HEPA Na salt g/l 
HEPA K salt g/l 
Sn (II) oxide (as Sn) g/l 
18 18 18 25 25 25 28 28 28 32 
Sn (III) chloride (as Sn) g/l 
Lead oxide (as Pb) g/l 
15 15 15 23 23 23 
Lead acetate (as Pb) g/l 21 21 21 27 
Hydrochloric acid 35% ml/l 
Lead chloride (as Pb) g/l 
Thiourea g/l 70 70 70 100 
100 
100 
120 
120 
120 
130 
Hypophosphorous acid Na salt g/l 
44 44 44 60 60 60 80 80 80 75 
Dodecylsulfuric acid Na salt g/l 
Polyacrylic acid ml/l 
5 5 5 
Polyacrylic acid Na salt ml/l 
6 6 6 7 
Ligunosulfonic acid g/l 0.1 
0.1 
0.1 
Ligunosulfonic acid Na salt g/l 
Plating 
pH 0.8 
0.8 
0.8 
0.7 
0.7 
0.7 
0.7 
0.7 
0.7 
0.8 
conditions 
Temp. (.degree.C.) 
70 70 70 70 70 70 70 70 70 70 
Plating time (min) 
5 15 30 5 15 30 5 15 30 5 
Film Plating 
Thickness (.mu.m) 
3.2 
7.0 
13.5 
3.4 
7.2 
14.0 
3.0 
6.9 
12.8 
2.9 
test film Sn ratio (wt %) 
63.6 
64.5 
65.1 
62.2 
62.8 
63.5 
61.5 
61.9 
62.6 
62.4 
results Fusing test 
G G G G G G G G G G 
Shorting test G G G G G G G G G G 
Bending test G G G G G G G G G G 
Surface state G G G G G G G G G G 
Adhesion test G G G G G G G G G G 
__________________________________________________________________________ 
Example No. 11 12 13 14 15 16 17 18 19 20 
__________________________________________________________________________ 
Bath Methane sulfonic acid g/l 
200 
200 
220 
220 
220 
240 
240 
240 
200 
200 
composition 
HEPA g/l 4 4 4 
HEPA Na salt g/l 
HEPA K salt g/l 
Sn (II) oxide (as Sn) g/l 
32 32 30 30 30 36 36 36 30 30 
Sn (III) chloride (as Sn) g/l 
Lead oxide (as Pb) g/l 25 25 25 35 35 35 25 25 
Lead acetate (as Pb) g/l 
27 27 
Hydrochloric acid 35% ml/l 
Lead chloride (as Pb) g/l 
Thiourea g/l 130 
130 
125 
125 
125 
140 
140 
140 
130 
130 
Hypophosphorous acid Na salt g/l 
75 75 110 
110 
110 
150 
150 
150 
100 
100 
Dodecylsulfuric acid Na salt g/l 
Polyacrylic acid ml/l 8 8 8 6 6 
Polyacrylic acid Na salt ml/l 
7 7 
Ligunosulfonic acid g/l 
Ligunosulfonic acid Na salt g/l 
0.2 
0.2 
0.2 
Plating 
pH 0.8 
0.8 
0.8 
0.8 
0.8 
0.7 
0.7 
0.7 
0.7 
0.7 
conditions 
Temp. (.degree.C.) 
70 70 75 75 75 75 75 75 70 70 
Plating time (min) 
15 30 5 15 30 5 15 30 5 15 
Film Plating 
Thickness (.mu.m) 
6.5 
13.0 
3.1 
5.9 
13.1 
2.8 
6.2 
14.1 
3.6 
7.0 
test film Sn ratio (wt %) 
63.0 
63.8 
64.1 
64.8 
65.2 
62.7 
63.2 
64.0 
66.5 
67.0 
results Fusing test 
G G G G G G G G G G 
Shorting test G G G G G G G G G G 
Bending test G G G G G G G G G G 
Surface state G G G G G G G G G G 
Adhesion test G G G G G G G G G G 
__________________________________________________________________________ 
Example No. 21 22 23 24 25 26 27 28 29 30 
__________________________________________________________________________ 
Bath Methane sulfonic acid g/l 
200 
125 
125 
125 
155 
155 
155 
220 
220 
220 
composition 
HEPA g/l 20 20 20 15 15 15 10 10 10 
HEPA Na salt g/l 
HEPA K salt g/l 
Sn (II) oxide (as Sn) g/l 
30 18 18 18 20 20 20 30 30 30 
Sn (III) chloride (as Sn) g/l 
Lead oxide (as Pb) g/l 
25 16 16 16 25 25 25 
Lead acetate (as Pb) g/l 13 13 13 
Hydrochloric acid 35% ml/l 
Lead chloride (as Pb) g/l 
Thiourea g/l 130 
70 70 70 100 
100 
100 
120 
120 
120 
Hypophosphorous acid Na salt g/l 
100 
45 45 45 75 75 75 100 
100 
100 
Dodecylsulfuric acid Na salt g/l 
Polyacrylic acid ml/l 
6 5 5 5 6 6 6 
Polyacrylic acid Na salt ml/l 
7 7 7 
Ligunosulfonic acid g/l 
Ligunosulfonic acid Na salt g/l 
Plating 
pH 0.7 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
0.9 
0.9 
0.9 
conditions 
Temp. (.degree.C.) 
70 75 75 75 70 70 70 70 70 70 
Plating time (min) 
30 5 15 30 5 15 30 5 15 30 
Film Plating 
Thickness (.mu.m) 
13.5 
3.2 
5.8 
13.6 
3.4 
5.8 
12.9 
3.3 
6.1 
14.4 
test film Sn ratio (wt %) 
67.4 
61.9 
62.5 
63.2 
61.6 
62.4 
63.7 
60.6 
61.2 
63.4 
results Fusing test 
G G G G G G G G G G 
Shorting test G G G G G G G G G G 
Bending test G G G G G G G G G G 
Surface state G G G G G G G G G G 
Adhesion test G G G G G G G G G G 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
C. Example No. 1 2 3 4 5 6 7 8 9 10 11 12 
__________________________________________________________________________ 
Bath Methane sulfonic acid g/l 
80 80 80 270 
270 
270 240 
240 240 
200 200 
200 
composition 
HEPA g/l 
HEPA Na salt g/l 
HEPA K salt g/l 
Sn (II) oxide (as Sn) g/l 
18 18 18 36 36 36 45 45 45 32 32 32 
Sn (III) chloride (as Sn) g/l 
Lead oxide (as Pb) g/l 
15 15 15 35 35 35 35 35 35 
Lead acetate (as Pb) g/l 35 35 35 
Hydrochloric acid 35% ml/l 
Lead chloride (as Pb) g/l 
Thiourea g/l 70 70 70 140 
140 
140 140 
140 140 
140 140 
140 
Hypophosphorous acid Na salt g/l 
44 44 44 150 
150 
150 150 
150 150 
150 150 
150 
Dodecylsulfuric acid Na salt g/l 
Polyacrylic acid ml/l 
5 5 5 8 8 8 8 8 8 8 8 8 
Polyacrylic acid Na salt ml/l 
7 7 
Ligunosulfonic acid g/l 
Ligunosulfonic acid Na salt g/l 
Conditions 
pH 0.8 
0.8 
0.8 
0.7 
0.7 
0.7 0.7 
0.7 0.7 
0.7 0.7 
0.7 
Temp. (.degree.C.) 
70 70 70 75 75 75 70 70 70 75 75 75 
Plating time (min) 
5 15 30 5 15 30 5 15 30 5 15 30 
Film Film Thickness (.mu.m) 
1.3 
2.0 
2.4 
1.8 
2.2 
6.5 2.2 
3.6 7.1 
3.2 6.8 
15.5 
test Sn ratio (wt %) 
Hydrolysis 
74.6 
75.0 
77.2 
84.6 
86.2 
88.5 
42.2 
49.5 
50.1 
results Fusing test 
Testing F F F F F F NG NG NG 
Shorting test impossible 
G G G G G G -- -- -- 
Bending test G G G G G G NG NG NG 
Surface state NG NG NG NG NG NG NG NG NG 
__________________________________________________________________________ 
C. Example No. 13 14 15 16 17 18 19 20 21 22 23 24 
__________________________________________________________________________ 
Bath Methane sulfonic acid g/l 
composition 
HEPA g/l 20 20 20 
HEPA Na salt g/l 15 15 15 
HEPA K salt g/l 
Sn (II) oxide (as Sn) g/l 
Sn (III) chloride (as Sn) g/l 
48 48 48 48 48 48 10 10 10 10 10 10 
Lead oxide (as Pb) g/l 
Lead acetate (as Pb) g/l 
20 20 20 20 20 20 
Hydrochloric acid 35% ml/l 
86 86 86 90 90 90 
Lead chloride (as Pb) g/l 6 6 6 6 6 
Thiourea g/l 100 
100 
100 
100 
100 
100 120 
120 120 
120 120 
120 
Hypophosphorous acid Na salt g/l 
110 
110 
110 
110 
110 
110 30 30 30 30 30 30 
Dodecylsulfuric acid Na salt g/l 
0.5 
0.5 
0.5 
0.5 
0.5 
0.5 0.2 
0.2 0.2 
0.2 0.2 
0.2 
Polyacrylic acid ml/l 
Polyacrylic acid Na salt ml/l 
Ligunosulfonic acid g/l 
Ligunosulfonic acid Na salt g/l 
Conditions 
pH 0.7 
0.7 
0.7 
0.7 
0.7 
0.7 1.2 
1.2 1.2 
1.2 1.2 
1.2 
Temp. (.degree.C.) 
80 80 80 85 85 85 90 90 75 75 70 70 
Plating time (min) 
5 15 30 5 15 30 5 15 5 15 5 15 
Film Film Thickness (.mu.m) 
3.8 
7.0 
13.5 
3.0 
6.2 
13.3 
0.9 
1.2 0.5 
0.7 Precipi- 
test tation 
results Sn ratio (wt %) 
67.6 
69.4 
70.9 
66.6 
67.8 
69.2 
80.2 
82.5 
81.9 
83.3 
Plating 
Fusing test 
G G F G G G F F F F not 
Shorting test G G G G G G G G G G possible 
Bending test G G G G G G G G G G 
Surface state NG NG NG NG NG NG 
__________________________________________________________________________ 
TABLE 3 
__________________________________________________________________________ 
Comparative Example No. 25 26 27 28 
__________________________________________________________________________ 
Composition 
Borofluoric acid 
ml/l 
45 45 90 90 
Tin (II) borofluoride 
g/l 30 30 30 30 
Lead borofluoride 
g/l 15 15 10 10 
Thiourea g/l 130 
130 
130 
130 
Sodium hypophosphate 
g/l 50 50 50 50 
Sodium g/l 15 15 10 10 
ethylenediaminetetraacetate 
Surface active agent 
g/l 0.5 
0.5 
0.5 
0.5 
Conditions 
pH 1.2 
1.2 
1.0 
1.0 
Temperature .degree.C. 
85 85 80 80 
Plating time min 5 15 5 10 
Film Film Thickness 
.mu.m 
1.9 
2.8 
2.3 
2.7 
test Tin ratio 
wt % 
76.2 
77.6 
82.2 
83.4 
results Fusing test F F F F 
Shorting test G G G G 
Bending test G G G G 
Surface test F NG F F 
__________________________________________________________________________ 
From the results indicated in Tables 1, 2 and 3 it is clear that the 
electroless Pb--Sn alloy plating bath composition according to the present 
invention produces excellent effect in practical applications.