influence of varying h2s concentrations and … · influence of varying h2s concentrations and...

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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

http://www.iaeme.com/currentissue.asp?JType=IJMET&VType=6&IType=7

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


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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+ + S

2- [eq. 5]

Ag → Ag+ + e

- [eq. 6] [21]

2Ag+ + S

2- → 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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