INFLUENCE OF VARYING H2S CONCENTRATIONS AND HUMIDITY LEVELS ON ImAg AND OSP SURFACE FINISHES

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International Journal of Mechanical Engineering and Technology (IJMET)
Volume 6, Issue 12, Dec 2015, pp. 18-29, Article ID: IJMET_06_12_003
Available online at
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=6&IType=12
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication
INFLUENCE OF VARYING H2S
CONCENTRATIONS AND HUMIDITY
LEVELS ON ImAg AND OSP SURFACE
FINISHES
Amer Charbaji, Michael Osterman, and Michael Pecht
Center for Advanced Life Cycle Engineering,
Department of Mechanical Engineering,
University of Maryland, College Park, MD 20742
ABSTRACT
Corrosion impacts electronic systems by attacking boards or individual
components. Of particular concern is corrosion of the metallization on printed
wiring board assemblies due to attack from sulfur-containing species, most
notably sulfurous gases. Sulfurous gases are emitted by a diverse range of
processes, ranging from paper and pulp bleaching to the warming of clay used
in industrial modeling facilities. However, the impact of varying sulfur
concentrations and humidity levels on corrosion needs further examination. In
this study, corrosion induced by exposure to H2S gas is examined for copper
printed wiring board metallizations coated with Immersion Silver (ImAg) and
Organic Solderability Preservative (OSP) surface finishes at different levels of
humidity and H2S concentration. Optical images of the boards revealed that
boards with the OSP surface finish had more signs of copper corrosion than
boards with the ImAg surface finish. These images also revealed that
corrosion on the boards did not stop after 3 days of testing since boards
exposed for 10 days had more signs of corrosion than boards exposed for only
3 days. Optical images indicate that ImAg is more sensitive to sulfur
concentration than to relative humidity, while OSP is more sensitive to
humidity. Uniform corrosion of the ImAg surface was observed with no sign of
creep corrosion or dendrite formation. Pure copper coupons were also
subjected to the corrosive tests; the weight gain of the copper coupons
indicated a constant rate of corrosion over the test duration.
Cite this Article: Amer Charbaji, Michael Osterman, and Michael Pecht.
Influence of Varying H2s Concentrations and Humidity Levels on Imag and
OSP Surface Finishes, International Journal of Mechanical Engineering and
Technology, 6(12), 2015, pp. 18-29.
http://www.iaeme.com/currentissue.asp?JType=IJMET&VType=6&IType=12

Influence of Varying H2s Concentrations and Humidity Levels on Imag and OSP Surface
Finishes
1. INTRODUCTION
Electronic products are being used in a broad range of applications. They are
increasingly replacing traditionally used mechanical components, especially in the
fields of control and actuation, and they are finding greater demand in expanding
markets around the world. Many of these markets have different atmospheric
conditions, including higher temperatures, humidity levels, or corrosive gas levels
than the conditions found in North America and Western Europe [1]. Furthermore, in
an effort to reduce energy consumption, controls over temperature, relative humidity,
and contaminants from the environment to which the electronics are exposed are
being relaxed, such as in the cases of data centers with free air cooling [2] [3]. Finally,
the materials used to fabricate electronic products are changing due to restrictions on
the use of certain materials through government regulations, such as the Restriction of
Hazardous Substances (RoHS) directive [4]. Different environmental conditions
combined with an elevated sulfur content and increased restrictions on the selection of
engineering materials are negatively affecting product reliability, as evidenced by the
increased number of reports on electronic product failures in the field due to attacks
from sulfur compounds such as H2S [5].
The corrosion of metallization in electronic equipment can destroy conductive
paths, resulting in electrical opens, or create unintended conductive paths between
electrically isolated metallization. The latter may result in unacceptable current
leakage or electrical shorting. Corrosion can also impact signal integrity in processor
and memory applications by dampening the signal’s amplitude and adding noise [6].
Corrosion may result in permanent failure of a product. It can also cause intermittent
failure, as corrosion can create a temporary open or short that may not be found
through further testing of the returned product [6]-[15]. Sulfur-driven corrosion has
been documented to take place in different industrial applications that emit sulfurous
species [5] [12].
Copper is widely used as a metallization material in electronics, but it oxidizes
rapidly upon exposure to the environment [16]. Surface finishes are applied to protect
the exposed copper on printed wiring boards (PWBs) from forming oxides and thus
preserve the solderability of the surface metallization during assembly [16] [17]. PWB
surface finishes include Hot Air Solder Leveling (HASL), Electroless
Nickel/Immersion Gold (ENIG), Immersion Silver (ImAg), Immersion Tin (ImSn),
and Organic Solderability Preservative (OSP). Prior to implementation of the RoHS
directive, SnPb HASL was the most commonly used surface finish [10], and corrosion
due to reaction with sulfurous gases in the atmosphere was not an issue because of the
thick coating layer of HASL and the inherent corrosion resistance of its SnPb build-up
[11] [13]. But as system manufacturers have converted to lead-free products to
comply with the requirements set forth in the RoHS directive, they have struggled to
find a suitable alternative to HASL, since each finish has its own set of advantages
and disadvantages.
ImAg and OSP are preferred for many applications [11]. Previous work [10] [12]
[18] has shown that early ImAg chemistries were weaker than OSP in terms of
protecting the underlying metallization from corrosion and were susceptible to sulfur
creep corrosion and electrochemical migration. Another study [11] found that both
ImAg and OSP provide comparable protection for the metallization against sulfur
attack. However, these studies were limited in scope and cannot be generalized to all
finish chemistries. Veale [12] tested one ImAg chemistry and admitted the possibility
that other ImAg chemistries may have different effects on corrosion. Schueller et al.

[10] [18] reported that ImAg suppliers are working on improving corrosion resistance.
Zhang et al. [19] ran single gas H2S exposure tests on boards with an ImAg finish at
different temperatures, relative humidities, gas concentrations, and exposure times to
see the effects of the different parameters on ImAg, but they did not attempt to
compare ImAg exposure to other surface finishes.
Several different tests have been developed and used to qualify the corrosion
resistance of PWB surface finishes [10]-[22], including mixed flowing gas (MFG)
chamber tests, clay tests [10] [21] [22], flowers of sulfur [23], sulfur chambers [10]
[19], and sulfur powder [10]. Of these corrosion-testing techniques, MFG testing
allows for continuous monitoring of test parameters and for modification of system
settings to allow for a consistent value, or a change within an acceptable tolerance
range, of these parameters. MFG testing is conducted in a chamber where gases of
different concentrations are mixed at different chamber temperature and humidity
conditions. In addition to surface finish characterization [10]-[16], MFG test setups
have also been used to study the corrosion of electrical components [1] [24] [25],
electrical connectors [26]-[29], and pure and plated copper [30]-[32]. Many variables,
such as temperature, relative humidity, and gas concentration determine how
corrosive the MFG testing is. For a more in-depth analysis and description of the
variables affecting MFG testing, the reader is referred to [33].
The majority of MFG studies have used multiple corrosive gases inside the
chamber, but concerns over the adequacy of the acceleration of these tests have been
rising [10] [25] [34], and experience has shown that some resistors passed the Battelle
MFG qualification tests but failed in the field [25]. One way to address this concern is
to use higher concentrations of H2S than called are for in standards on MFG testing
[1] [6] [9] [11] [19] [20] [25] [29] [35] [36]. Clean copper coupons are placed inside
MFG chambers and are used as verification tools for identifying the environmental
corrosion class. The thickness of the corrosion layer on copper coupons is a
commonly used metric for classifying the environmental class. The use of silver
coupons in addition to copper coupons in corrosion monitoring is gaining popularity
because silver is more readily affected by sulfur and less affected by moisture [30]
[37]-[39]. All coupons that go into the MFG chamber are cleaned prior to the test to
remove oil, hydrocarbons, and oxides from the surface [40]-[42].
This paper compares the corrosion response of two commonly used surface
finishes, OSP and ImAg, with exposure to different humidity levels and sulfur
concentrations. First, the MFG testing procedure is introduced. Then, results from
several single H2S gas tests that were run using an MFG test setup are shown. Finally,
the test results are discussed and compared to results from previous corrosion studies.
2. EXPERIMENT
In order to examine the impact of H2S concentration and humidity on the corrosion of
the metallization on printed wiring boards with ImAg and OSP surface finishes,
unpopulated printed wiring boards were exposed to three separate corrosive
environmental conditions. Table 1 documents the three test conditions. The first test
examined the effect of a low concentration of gas (H2S at 250 ppb) combined with
high humidity (75% relative humidity (RH)). The second test studied the effect of a
high concentration of gas (H2S at 1800 ppb) combined with low humidity (20% RH).
The third test looked at the effect of a high concentration of gas (H2S at 1800 ppb)
combined with high humidity (75% RH). The term “low” is added before gas
concentration to signify that this concentration is considered low as compared to the

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1800 ppb concentration also used in this study. All test conditions used H2S
concentrations higher than those used in 10 out of the 11 MFG test methods
mentioned in [44]. The tests we conducted lasted for 10 days at a temperature of 40°C
with interruptions on days three and six to pull out some of the samples for
documentation. The interruptions included shutting off the H2S gas supply into the
chamber while maintaining the flow of filtered air until the H2S gas concentration
became zero. The samples were then pulled out of the chamber, the chamber door was
sealed, and the H2S gas was pumped back into the chamber. Flushing the chamber,
removing the samples, sealing the chamber, and bringing the gas concentration back
to test conditions took somewhere around 2 to 3 hours.
Table 1 Test Conditions for 10 days at 40°C
Test number H2S Gas Concentration Relative Humidity
I 250 ppb 75%
II 1800 ppb 20%
III 1800 ppb 75%
Each test involved subjecting a set of unpopulated printed wiring boards and
copper coupons to a specific corrosive environment. The surface finishes of the test
boards were either immersion silver (ImAg) or organic solder preservative (OSP) and
all test boards underwent a lead-free reflow process. The thickness of the ImAg finish
ranged from 0.201 to 0.377 μm with a mean of 0.304 μm and a standard deviation of
0.056 μm as detected by X-ray fluorescence spectroscopy. The copper coupons were
cut from an ultra pure Oxygen-Free High Conductivity Copper (Alloy 101/ 99.99%
pure) sheet into 1.4 × 1.4 × 0.4 cm square coupons using a wire electrical discharge
machine.
Prior to exposing the test boards to the corrosive environment, select features of
each board, such as mounting pads and printed through-holes, were documented
under a high magnification optical microscope (up to 200×) for post exposure
comparisons. Prior to being placed inside the chamber, the copper coupons were
abraded sequentially using 400X, 600X, and 1200X grit abrasive paper to remove
surface oxides. Then the coupons were rinsed with isopropyl alcohol and deionized
water, and then they were dried using filtered air. After the initial surface preparation
and after each exposure, the coupons were placed next to a calibrated balance to allow
them to equilibrate with the environment before being weighed as recommended in
[41]. The temperature, relative humidity, and gas concentration were monitored
several times during the day to ensure the stability of these parameters inside the
chamber.
Each test was initiated with two ImAg test boards, two OSP test boards, and a
minimum of six copper coupons being placed inside the MFG chamber. After three
days under the corrosive environment test conditions, one ImAg and one OSP board
were removed. The remaining two boards were removed after being exposed to ten
days under the assigned corrosive test conditions. Ten copper coupons were placed
under the first test conditions; four were removed on day three, and three were
removed on days six and ten. Six copper coupons were placed in each of the second
and third test conditions, and two coupons were removed on days three, six, and ten.
After removal from the corrosive gas chamber, the copper coupons were reweighed,
and the surfaces of the boards were documented under high magnification.

3. RESULTS
Figure 1 and Figure 2 show the conditions of copper pads on the boards before and
after exposure to the different test conditions for three and ten days, respectively.
Examination of the test boards revealed increased corrosion of the metal surfaces on
all boards subjected to ten days of exposure compared to boards subjected to three
days of exposure. For the OSP boards, elevated humidity was more detrimental than
increased corrosive gas concentration in producing surface corrosion. The OSP-
finished surfaces are also more susceptible to uniform corrosion than the ImAg-
finished surfaces for high humidity (75% RH) test conditions. In contrast, in the
second test condition (20% RH), corrosion on the ImAg board was uniform and
spread over a larger area of the copper pads than on the board with the OSP surface
finish.
Pre-Exposure
Test I
250ppb H2S 75%
RH
Test II
1800ppb H2S 20%
RH
Test III
1800ppb H2S 75%
RH
OSP
ImA
g
Figure 1 After a 3-day exposure in MFG chamber.
Pre-Exposure
Test I
250ppb H2S 75%
RH
Test II
1800ppb H2S 20%
RH
Test III
1800ppb H2S 75%
RH
OSP
ImA
g
Figure 2 After a 10-day exposure in MFG chamber.
In the 250ppb H2S 75% RH test, uniform corrosion of the copper pads was
evident on the board with an OSP surface finish after three days of exposure in the
MFG chamber, while a random set of corrosion sites was observed on the board with
the ImAg surface finish. Silver in the ImAg finish is believed to corrode and give the

Finishes
tarnish a bluish color due to sulfur exposure in the tests [43]. After ten days of
exposure under the same condition, uniform corrosion was seen on the surface of the
copper pads, was rough and textured on the OSP board, and was smooth and uniform
on the ImAg board.
In the 1800ppb H2S 20% RH test, a sporadic set of corrosion sites was observed
on the copper pads on the board with the OSP surface finish after three days of
exposure, while corrosion of ImAg was observed on a large portion of the pads. After
ten days of exposure, a larger area of copper pads was corroded on the OSP boards
compared to boards exposed for three days, as can be seen in Figure 2. In contrast,
the surface of the ImAg board at three days was nearly uniformly corroded with only
a slightly more uniform coverage after ten days. For both the three and ten day
exposures, the corrosion of the pads with OSP finish was significantly less severe and
spread over a smaller area than the corrosion of OSP-finished pads in the first test.
ImAg tarnish was spread over a larger area in the 3-day exposure of the second test
than in the first test. Figure 3 reveals the conditions of some copper pads on the ImAg
and OSP finished boards under optical microscopy at a magnification of 25×.
In the 1800ppb H2S 75% RH test, the spread and color of corrosion products on
the copper pads with OSP surface finish was comparable to that of the copper pads
with OSP surface finish that underwent the first test conditions (250ppb H2S 75%
RH). On the other hand, the corrosion of ImAg after three days of exposure at
1800ppb H2S 75% RH was similar to the three-day exposure in the 1800ppb H2S 20%
RH test and was spread over a larger area of copper pads than in the 250ppb H2S 75%
RH test. From these observations, it appears that corrosion on boards with OSP is
sensitive to high humidity while ImAg is sensitive to the high sulfur concentration.
(a) (b)
Figure 3 After 10-day MFG exposure under 2nd
test conditions: (a) OSP finish, (b)
ImAg finish (magnification of 25×).
The weight of copper coupons increased due to the formation of corrosion
byproducts on the surface as a result of reaction with the corrosive environments. The
copper coupons’ weight gain was normalized by the initial weight of each coupon,
and the corresponding weight increase is plotted in Figure 4 for the three test
conditions. The plots show a linear dependence on time for all test conditions with a
coefficient of determination (R2
-value) greater than 0.9 for all test conditions with the
inclusion of a non-zero y-intercept. Figure 5 shows the corrosion class of the
environment based on the ISA [40] classification. The weight gain method was used
to retrieve the thickness of corrosion products by normalizing the data to a one-day
gain, assuming a Cu2S corrosion product with a density of 5.6g/cc [38]. ISA
classification is based on the thickness of the corrosion product on the copper coupons

after 1 month of exposure. In Figure 5, each mark corresponds to the thickness of the
corrosion layer of one copper coupon subjected to the test normalized with respect to
time. As can be seen from the figure, each one-day exposure in the MFG chamber
simulates a 30-day exposure to G3 conditions for the 1800ppb H2S 20% RH test and a
30-day GX exposure for the 250ppb H2S 75% RH and 1800 H2S 75% RH tests.
(a)
(b)
(c)
Figure 4 Average copper coupon weight gain (normalized by the initial weight) for
(a) 250ppb H2S 75% RH test, (b) 1800ppb H2S 20% RH test, and (c) 1800ppb H2S
75% RH test. Error bars show range of weight gain.

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Figure 5 Corrosion product thickness distribution based on normalized weight gain of
copper coupons and assuming Cu2S as the corrosion product. G1, G2, G3 and GX are
based on ISA corrosion classes for a one-month exposure. Refer to [40] for more
information.
4. DISCUSSION
None of the boards showed signs of creep corrosion. Corrosion of copper pads with
the OSP surface finish appeared to be more directly dependent on relative humidity
than on H2S concentration, since surface corrosion was nearly uniform for exposures
with relative humidity at 75% and spotted and less severe when the relative humidity
was 20%. OSP is porous and may expose underlying copper [20] [36], which will
then react with the environment. A higher relative humidity will result in a thicker
layer of adsorbed moisture on the board that will also cover more surface area of the
board. The water will thus penetrate more of the OSP pores and contact a larger
portion of the underlying copper. The moisture layer provides a vehicle for ionic
transport [45] and will accelerate the rate of copper corrosion if it has a larger contact
area with the copper. Possible corrosion reactions are given by equations 1 through 4
[46]:
4Cu ↔ 4Cu+
+ 4e-
[eq. 1]
O2 + 2H2O + 4e-
↔ 4OH-
[eq. 2]
H2S + OH-
↔ HS-
+ H2O [eq. 3]
4Cu+
+ 2HS-
+ 2H2O ↔ 2Cu2S + 2H3O+
[eq. 4]
From test observations, the rate of ImAg tarnish is less sensitive to relative
humidity and more sensitive to sulfur concentration. In the high H2S concentration
tests (second and third), the spread of tarnish was larger and more uniform after 3-
days compared to the lower H2S test condition at day three. Since silver is susceptible
to general corrosion (tarnish) in the presence of sulfur [47], these results are in line
with the results given in [30] [37], which show that silver is more readily affected by
sulfur concentration than by relative humidity. Although the corroded area may be
similar or larger for ImAg than for OSP-finished boards, silver tarnish in ImAg boards
is only regarded as a cosmetic concern because it maintains its electrical conductivity
[10] [43]. However, corrosion of copper on OSP-finished boards results in a
resistance change and impacts signal integrity [6]. It is also important to note that
copper ions can diffuse through the ImAg silver layer to interact with the environment
and form corrosion products [36]. Kurella et al. [36] carried out depth profiling using
Time of Flight Secondary Ion Mass Spectroscopy (TOFSIMS) on the boards that
underwent the third test conditions (1800ppb H2S 75% RH) in this test and found that
corrosion products were thicker on the boards with OSP than on the boards with

ImAg. Possible reactions leading to the tarnished ImAg finish are given by the
sequence of equations 5 through 8:
H2S → 2H+
+ S2-
[eq. 5]
Ag → Ag+
+ e-
[eq. 6] [21]
2Ag+
+ S2-
→ Ag2S [eq. 7] [21]
2H+
+ 2e-
→ H2 [eq. 8]
Figure 4 shows two linear regression fits for each test, one with a zero-intercept
and one with a non-zero intercept. For tests 2 and 3, both regression fits yield a
coefficient of determination (R2
-value) greater than 0.9, meaning that both linear fits
are appropriate for modeling the data of each test. This means that for tests 2 and 3,
copper weight gain can be approximated as being linear for the entire duration of the
test. For test 1, the linear regression fit with a non-zero y-intercept gives an R2
-value
of around 0.9. The second (zero y-intercept) linear regression fit, however, gives a
negative value for R2
, meaning that the assumption of a zero y-intercept is not
appropriate. This may suggest that the rate of weight gain is different, and possibly
higher, before day 3 of the test. Tran et al. [46] show that there is a possibility for the
rate of copper weight gain to change during H2S corrosion testing and that the rate
may be composed of three parts that start with a linear rate, followed by a parabolic
rate, and ending with a second linear rate. Given that the R2
-value is greater than 0.9
for all three test conditions, the copper weight gain was linear from day three to day
ten for the first test condition, and over the entire ten-day exposure period for the
second and third test conditions. The copper coupons’ weight gain conforms to
previous results that show that copper corrosion is linear over a large range of H2S
concentrations [9].
5. CONCLUSIONS
It was observed that corrosion on the ImAg-finished boards was dependent on H2S
gas concentration and exposure duration and not on relative humidity. This is due to
the fact that silver is more readily affected by sulfur concentration rather than by
relative humidity.
Corrosion on the OSP-finished boards was more dependent on relative humidity
than on H2S gas concentration due to the inherent porosity of the OSP finish. An
adsorbed moisture layer provides a medium for the ionic transport of sulfur containing
ions to contact and react with the underlying copper. A high relative humidity will
result in a thicker adsorbed moisture layer that is spread over a larger area on the
surface, thus penetrating more OSP pores and resulting in greater copper corrosion.
Optical images suggest that ImAg is a more reliable finish for non-solder covered
metallization in high humidity applications (~75% RH) than OSP. In situations with
high humidity (~75% RH) and high sulfurous gas contaminant concentrations (ISA
G3 and GX conditions), it is recommended to take additional protective measures to
guard against corrosion, such as providing filtered air to the space were electronics are
placed or placing the electronic system in a protective NEMA type enclosure [5].
ACKNOWLEDGMENTS
The authors would like to thank the more than 100 companies and organizations that
support research activities at the Center for Advanced Life Cycle Engineering
(CALCE) at the University of Maryland annually, specifically the CALCE Electronic
Products and Systems Consortium. The authors would also like to thank Sungwon

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Han for running the second and third tests, Preeti Chauhan for her technical support,
Mark Zimmerman and Kelly Smith for copyediting, and the students and research
scientists at CALCE for their help and support.
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