Report 3
Evaluating lead-free plating using gas corrosion tests
Yuichi Aoki*, Takayuki Nagai**, Hiroko Katayanagi*, Hirokazu Tanaka*
as corrosion constitutes one aspect of environmental stress on electronic parts. The
gas corrosion tests carried out for evaluations in this report can be seen as one
weapon in the effort to increase reliability. The corrosiveness of gas varies according to
the temperature and humidity of the ambient environment, and testing involves other
factors as well, such as gas concentration, reproducibility of field conditions, and
accelerated investigation. A variety of public standards address test methods for gas
corrosiveness.
For this report, we have studied the plating of lead-free solder that industry has
been trying to incorporate. We have carried out gas corrosion tests, and we present our
findings on visible changes (discoloration), wettability and surface analysis.
1. Introduction
Electronic parts are subjected to a wide range of
environmental conditions resulting from their use in such
varied equipment as automobiles, household appliances,
and IT equipment. The usage environments contain
various types of stress such as temperature, humidity,
vibration, pressure, and corrosive gas.
The problem with corrosive gas is particularly acute in
such environments as seaside industrial belts, paper and
pulp factories, and in hot springs regions. Relocation of
manufacturing to overseas bases can also result in severe
environmental conditions, and require special consideration for such factors as the effect of outgas originating
from cardboard containers during transportation.
Corrosion is considered to be wear generated in
response to the environments to which materials are
exposed. Considerable economic loss can be traced to
corrosion, and preventing corrosion is also one aspect of
improving reliability. Furthermore, the lead eluting from
discarded products introduces toxic health effects into the
environment, and so we are now approaching the stage of
global employment of lead-free solder. In past reports, we
have pointed out the reliability problems encountered in
combining lead-free solder with conventional Sn-Pb
plating, and we are also calling for the use of totally leadfree solder. Several types of alternative plating are under
consideration, such as Sn-Ag, Sn-Bi, Sn-Cu, and Sn-Zn.
The metal additives in these alternatives include traces
of metals such as silver and copper, leading to
apprehension of gas corrosion. The gas corrosiveness of
individual metals is well-documented, but few available
reports evaluate the alloy gas corrosiveness stemming
from binary compounds such as those used in lead-free
solder. Because of this, we decided to study and compare
Sn-Ag and Sn-Cu used in lead-free plating, as well as Sn
and Sn-Pb used in conventional plating, in order to
examine the corrosion resistance exhibited by lead-free
plating.
2. Test methods and standards for gas
corrosion testing
2-1 The relationship between environmental
factors and corrosion
Gas corrosion appears as a variety of phenomena in
various forms, such as localized metal corrosion, or gas
entering solution in moisture-related humidity and
affecting the pH. Possible evaluation methods for this
type of complex contact area corrosion include tests
comparing such factors as the metals used and the
environmental conditions, as well as tests using electrical
resistance to make a quantitative comparison of the level
of corrosion. Up to now, the principle source of corrosive
gas has been free atmospheric gas (e.g., the SOx and NH3
produced in industrial areas, and the H2S emanating from
hot springs areas), but recently parts such as switches,
connectors, and sockets are experiencing contact reliability problems induced by gases originating from
packaging materials and materials used in the products
themselves. In addition, miniaturization of electronic parts
has contributed to situations in which sealed construction
traps corrosive gas, which then reacts with dew
condensation to produce corrosion.
* Reliability Engineering Department Technical Headquarters
** Environmental Test Technology Center
ESPEC TECHNOLOGY REPORT No. 12
23
Corrosion occurs when gas enters into solution in
moisture adsorbed onto the surface of metals, changing
the pH and causing localized corrosion. This means that
the quantity of adsorbed moisture affects the level of
corrosion, and so atmospheric humidity affects both
reproducibility and acceleration in testing.1) In particular,
since the moisture adsorption quantity rises abruptly in
the range from 60 to 70% RH, this range is generally
considered the corrosion enhancement range, and more
effective results in corrosion testing can be obtained using
humidity conditions above this range.
2-2 Test standards and test methods for gas
corrosion tests
Contact area corrosion leads to contact failure of
electronic circuits in electronic equipment parts such as
switches, connectors, and terminals. Various gas corrosion tests have been devised and standardized concerning
the corrosion of electronic parts being generated from the
presence of the following three combined factors: dust
particles, dew condensation, and corrosive gas. The four
most typical corrosive gases are H2S (hydrogen sulfide),
SO2 (sulfur dioxide), NO2 (nitrogen dioxide), and Cl2
(chlorine). Test conditions have been standardized not
only for individual gas corrosion tests, but also for mixed
gas corrosion forms as well. For items such as electronic
parts, there is an increasing tendency to employ mixed gas
tests at low concentration ppb (parts per billion) levels
that have higher field reproducibility. Table 1 shows the
principal test methods.
Table 1 Gas corrosion test standards
Test standard
Gas type
Gas
concentration
(ppm)
Temperature
(C)
Humidity
(% RH)
25 5
25 2
75 5
4, 10, 21
0.5 0.1
25 1
75 3
1, 2, 4, 10, 20,
30, 90
40 1
90 5
1, 2, 4, 10
Gas
concentration
(ppm)
Temperature
(C)
Humidity
(% RH)
10 to 15
25 2
75 5
4, 10, 21
0.1 0.02
25 1
75 3
1, 2, 4, 10, 20,
30, 90
40 1
90 5
4, 10, 21
IEC 60068-2-42
(JIS C 0090)
ISO 10062
Method A
SO2
25 5
JIS H 8502
1000 50
Test standard
Gas type
IEC 60068-2-43
(JIS C 0090)
ISO 10062
Method B
H2S
3 1
JIS H 8620
10 2
Test time
(days)
Gas type and concentration (ppb)
H2S
SO2
NO2
Method 1
100 20
500 100
Method 2
10 5
Method 3
100 20
Method 4
10 5
200 20
200 20
10 5
500 100
200 50
20 5
Method C
ISO 10062
Method D
100 20
200 50
Class I
Class II
EIA-364-65A1998
Class IIA
Class III
Class IIIA
Class IV
24
JIS also includes Temperature 40 2C,
Humidity 80 5% RH
Additional test times are 4 and 8 hours
Test time
(days)
Test standard
IEC 60068-2-60
(JIS C 0048)
Comments
Cl2
10 5
20 5
Comments
JIS also includes Temperature 40 2C,
Humidity 80 5% RH
Additional test times are 4 and 8 hours
Temperature
(C)
Humidity
(% RH)
25 1
75 3
30 1
25 1
70 3
Test time
(days)
4, 7, 10, 14, 20
75 3
1, 2, 4, 10, 20,
30, 90
Class I only abolished
10 5
100 20
200 20
10 3
100 20
200 50
200 50
20 5
30 5
30 2
30 1
70 2
30 2
75 2
30 1
70 2
40 2
75 2
20
ESPEC TECHNOLOGY REPORT No. 12
3. Test methods
4. Discussion of results
Table 2 shows plating conditions, Fig. 1 shows test
specimen shape, and Table 3 shows test conditions. To
compare corrosiveness, Sn and Sn-Pb plating were
exposed to identical environments, and evaluated using
gas corrosion tests.
4-1 Externally observable results
Table 2 Plating conditions
Plating
composition
(wt %)
Sn-3Ag
Sn-1Cu
Sn
Gloss
Semi-gloss
Semi-gloss
Gloss
Plating
thickness
Foundation
plating
4 mm
Nickel 1 to
2 mm
Sn-20Pb (Ref.)
Gloss
Material
Phosphor bronze
Dimensions 15 32 0.25 (mm)
Fig. 2 and 3 shows discoloration distribution ratios for
image processing and the discoloration process for Sn-Ag
and Sn-Cu. The discoloration distribution ratio was rated
numerically according to visual observation and scanner
images and showed trends that displayed correlation. In
this case, the quantification achieved through image
processing can be seen as a rough measure of corrosion.
In trends of change in plating, the Sn-Ag plating showed
heavy discoloration, and the higher the concentration, the
greater the discoloration. Among the gases, the H2S
showed the strongest tendency to cause discoloration. The
Sn and Sn-Pb platings showed very little change in
response to any of the gases.
Sn-Ag
H2S
Material: Phosphor bronze
Sn-Cu
SO2
H2S
SO2
32 mm
Initial
Thickness
0.25 mm
100 h
15 mm
Temperature
and humidity
Fig. 1 Test specimen shape
200 h
Table 3 Test conditions
300 h
40C, 80% RH
(a) Gas concentration: 1 ppm
Gas type
H2S (1 ppm, 5 ppm) SO2 (1 ppm, 5 ppm)
(concentration)
Test time
100, 200, 300 h
When setting test conditions, the high-temperature,
high-humidity Asian environment was assumed, and gas
that could be used for evaluating the corrosiveness of
silver and copper was selected. For evaluation, a scanner
was used to read the level of corrosiveness as determined
by external discoloration based on uniform conditions,
and corrosion was confirmed by visual observation.
Furthermore, to provide numerical values for corrosion
levels, the image was digitized in two gradations, and the
discoloration distribution ratio was calculated using the
initial black and white ratio as the standard. In addition,
using a uniform load (load 0.1N) the surface changes due
to corrosion film were confirmed using contact resistance
measurements, and the solder wettability was evaluated
using the meniscograph method.*1 In addition, changes in
granulation were confirmed using SEM (scanning
electron microscope) images, and elemental analysis was
used to identify the corrosive elements.
ESPEC TECHNOLOGY REPORT No. 12
Sn-Ag
H2S
Sn-Cu
SO2
H2S
SO2
Initial
100 h
200 h
300 h
(b) Gas concentration: 5 ppm
Fig. 2 Discoloration process of plating surface caused
by corrosive gas
25
50
Discoloration
distribution ratio (%)
Discoloration
distribution ratio (%)
50
H2S (1 ppm)
40
30
Sn-Ag
20
10
0
Sn-Cu
0
100
200
Test time (h)
300
SO2 (1 ppm)
40
30
Sn-Ag
20
10
0
Sn-Ag
30
20
Sn-Cu
10
0
100
200
Test time (h)
300
50
Discoloration
distribution ratio (%)
Discoloration
distribution ratio (%)
50
H2S (5 ppm)
40
Sn-Cu
0
100
200
Test time (h)
300
SO2 (5 ppm)
40
Sn-Ag
30
20
10
0
Sn-Cu
0
100
200
Test time (h)
300
Fig. 3 Changes in discoloration distribution ratio caused by corrosive gas
4-2 Contact resistance measurements
4-3 Wettability test results
Areas such as the silver contact points of electronic
parts often have performance problems due to increased
contact resistance caused by corrosion. Fig. 4 shows
measurement values for contact resistance. The results
showed marked discoloration, but almost no change is
seen in the contact resistance values. The lack of increase
in resistance values is thought to result from an extremely
thin corrosion film that does not reach beyond the surface,
and is broken by contact surface pressure.
Fig. 4 shows wettability test conditions and Fig. 5
shows test results using the meniscograph method. In an
H2S environment, the Sn-Cu showed worse deterioration
than the Sn-Ag, quite unlike the discoloration results. This
is thought to result from the affect of additives to the Cu
and Ag, and perhaps from differences in the form of
plating deposition.
Both discoloration and wettability changes were
minimal at over 200 hours. This is said to be due to the
barrier formed by the oxidation film and sulfuration film,
but in the case of the Sn-Ag and Sn-Cu, the possibility
can be considered that since the combined gases function
as oxidants, they accelerate the formation of oxidation
film, and so change is linear.
Zero cross time indicates the base line for the existence
of repellence between the test piece and the solder bath.
Therefore, we can judge that the longer the zero cross
time, the worse the wettability.
Resistance value
(mW)
50
H2S (5 ppm)
40
30
20
10
0
Resistance value
(mW)
50
100
200
Test time (h)
300
Table 4 Wettability test conditions using the
meniscograph method
SO2 (5 ppm)
40
30
Dip speed
20
10
0
100
200
Test time (h)
Sn-Ag
Sn-Cu
300
Sn
Sn-Pb
Fig. 4 Changes in contact resistance values due to gas
corrosion
26
10 mm/sec
Dip depth
5.0 mm
Dip time
10 sec
Solder bath
temperature
245C
Solder
Sn-2.5Ag-1Bi-0.5Cu
Flux
Rosin 30%
ESPEC TECHNOLOGY REPORT No. 12
H2S (5 ppm)
Zero cross time
(sec)
Zero cross time
(sec)
H2S (1 ppm)
4
2
0
100
200
Test time (h)
4
2
0
300
SO2 (5 ppm)
Zero cross time
(sec)
SO2 (1 ppm)
Zero cross time
(sec)
300
6
4
2
0
100
200
Test time (h)
100
200
Test time (h)
300
4
2
0
100
200
Test time (h)
Sn-Ag
Sn-Cu
300
Sn
Sn-Pb
Fig. 5 Effects of corrosive gas on wettability
As seen in Fig. 6, a corrosion film forms over all the
plating granules. A plating corrosion film of 1 nm to 1mm
due to gas corrosion is often reported, and the granular
diameter is roughly equivalent to the film thickness. The
factors affecting discoloration and wettability are thought
to result from this corrosion film.1), 2), 3)
Strength of luminescence
(arbitrary units)
4-4 Results of surface analysis
10
9
8
7
6
5
4
3
2
1
0
Sn
No S, O peak
Ni
.0250
25.00
Time (sec)
Sn-Ag initial surface
2mm
Strength of luminescence
(arbitrary units)
Pre-test
10
9
8
7
6
5
4
3
2
1
0
S peak
Sn
Cu
Ni
O peak
.0250
25.00
Time (sec)
Post-test
2mm
Fig. 6 SEM images of Sn-Cu plating surface
H2S: 5 ppm, 300 h
Fig. 7 GDS (Glow Discharge Optical Emission
Spectroscopy) analysis results
Fig. 7 shows the results of elemental analysis of
corrosion surfaces using GDS (Glow Discharge Optical
Emission Spectroscopy*2). Sulfur (S), which was not seen
on the initial surface, is seen on the surface layer only,
and is thought to lead to corrosion due to sulfuration.
ESPEC TECHNOLOGY REPORT No. 12
27
5. Summary
The results of analyzing the resistance of lead-free
solder to gas corrosiveness showed discoloration due to
corrosion in Sn-Ag and Sn-Cu plating in single gas
environments of H2S and SO2. However, the wettability
is inferior to conventional Sn-37Pb plating, though at
present, the results have not shown an extreme loss of
wettability. In the future, it will also be necessary to
evaluate the Cl2 and NO2 gases that function as
accelerating factors, from the standpoint of the presumed
field environment.
6. Acknowledgement
We would like to express our sincere gratitude to Mr.
Hiroyuki Moriuchi and Mr. Yoshiichi Nakano of the
Research and Development Department of the Dai-ichi
Denshi Kogyo K.K. for their opinions and cooperation
that played a valuable role in carrying out these tests.
Terminology
*1. Meniscograph method
When dipping the specimen in the molten solder bath,
this method detects the upward and downward forces
of the surface tension of the solder received by the
specimen, and by recording this over time, is able to
discover time-related changes in wettability. The
method is JIS C 0053, Environmental testing. Part 2:
Tests. Test ta; Soldering. Solderability testing by the
wetting balance method.
*2. Glow Discharge Optical Emission Spectroscopy
(GDS)
Glow discharge spattering is used to illuminate the
target for qualitative and quantitative analysis using
spectroscopy. The steel industry uses this method of
analysis because it is possible to measure the
elemental concentration in the direction of depth by
correlating basic properties of the materials.
[Bibliography]
1) Akihiro Nakayama, et al, The Investigation of Acceleration about
Sulphuration Gas Test of Contact, the 27th Symposium on
Reliability and Maintainability, 1997
2) Shoko Tanaka, et al, The Investigation of Mixed Gas Corrosion
Test Corresponded to Asian Environment, the 26th Symposium on
Reliability and Maintainability, 1996
28
3) Hiroshi Yamashita, Examination of Pb-free Lead Finish and Aging
Methods for Semiconductor Package, the 29th Symposium on
Reliability and Maintainability, 1999
ESPEC TECHNOLOGY REPORT No. 12
