The corrosion behavior of PCB-ImAg in industry polluted marine atmosphere environment

Materials and Design 115 (2017) 404–414

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The corrosion behavior of PCB-ImAg in industry polluted marine atmosphere environment Lidan Yan a,b, Kui Xiao a,b,⁎, Pan Yi a,b, Chaofang Dong a,b, Junsheng Wu a,b, Ziheng Bai a,b, Chengliang Mao a,b, Li Jiang a,b, Xiaogang Li a,b,c a b c

Corrosion and Protection Center, University of Science and Technology Beijing, Beijing 100083, PR China Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, PR China Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, Zhejiang, PR China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• The effects of SO2 and Cl- on the printed circuit boards with an immersion Ag surface finish (PCB-ImAg) are studied. • The results of outdoor atmosphere experiments are compared to those of indoor simulated experiments. • The results show that PCB-ImAg cannot be used in industrially polluted marine environment without any protective measures.

a r t i c l e

i n f o

Article history: Received 23 July 2016 Received in revised form 17 November 2016 Accepted 19 November 2016 Available online 22 November 2016 Keywords: Silver-plated circuit boards Industry polluted marine atmosphere environment Failure mechanism Corrosion products

a b s t r a c t This experiment was carried out in Wheat Island Qingdao which is characterized with a typical industrial polluted marine atmosphere environment. The atmosphere contains a large amount of Cl− and SO2. The corrosion mechanism of printed circuit boards (PCBs) with an immersion silver surface finish (PCB-ImAg) in this phenomenon is still unknown. The corrosion behavior of PCB-ImAg is different under the influences of Cl− and SO2. Silver has strong resistance to Cl−, which mainly causes micro-hole corrosion. However, silver is sensitive to SO2. The corrosion degree can be accelerated in the environment containing Cl− and SO2. In this environment, the main corrosion products are oxides, sulfate and carbonate of silver and copper. The results summarized in this paper indicated that PCB-ImAg cannot be used in industrially polluted marine environment. Protective measures such as conformal coating are necessary to protect PCB-ImAg. © 2016 Published by Elsevier Ltd.

1. Introduction

⁎ Corresponding author at: Corrosion and Protection Center, University of Science and Technology Beijing, Beijing 100083, PR China. E-mail address: [email protected] (K. Xiao).

http://dx.doi.org/10.1016/j.matdes.2016.11.074 0264-1275/© 2016 Published by Elsevier Ltd.

Electronic systems are widely used in various fields of society, and the printed circuit boards (PCBs) act as the supporting body of various electronics. PCBs have a structure that contains multilayer metals. The commonly used surface treatments include electonicless Ni/immerse Au (PCB-ENIG), immersion Ag (PCB-ImAg), lead-free HASL (PCB-

L. Yan et al. / Materials and Design 115 (2017) 404–414

HASL) and organic solderable preservative (OSP) [1]. PCBs are vulnerable to corrosion by polluted medium. With the constant innovation of electronic, PCBs are further developed in the direction of integration and miniaturization, which leads to even greater electrochemical instability of PCBs [2]. More than 90% of PCBs are used in the semi-closed atmosphere environment and are inevitably affected by the negative environmental factors. The main factors influencing the corrosion of PCBs are temperature, humidity, pollutants and dust particles. With the exploitation and utilization of marine resources, more and more electronic devices are used in the marine environment. The concentration of Cl− in marine environment is higher, while Cl− is easy to penetrate into basement as the diameter of Cl− is very small, and then the metal substrate is corroded heavily [3–5]. With the development of industry, the normal operation of PCBs has been seriously affected by industrial polluted gas. PCBs which were placed in marine polluted atmospheric environment for long term service may be subjected to various damages from Cl−, high temperature, high humidity and pollutants. So the influences on the corrosion failures of PCBs in marine polluted atmospheric environment cannot be ignored. Recently, many researchers have studied the corrosion behavior of PCBs in environments with a single pollutant through indoor simulation experiment [6–8]. Cl− would destroy the silver oxide to form silver chloride (AgCl) [9,10], and then AgCl is dissolved to form AgCl− 2 complex with the continuous penetration of Cl− [11,12]. Cuprous oxide (Cu2O) would be destroyed by Cl− to form the less dense compounds such as CuCl and Cu2(OH)3Cl. In addition, cuprous chloride complex − [13– (CuCl− 2 ) may also be formed in the environment containing Cl 15]. SO2 could dissolve in the liquid film formed on the surface, and then the liquid film becomes acidified, thereby increasing the corrosion activity in this environment [16–19]. The corrosion products of silver and copper in the SO2-containing environment typically consist of Ag2SO3 Ag2SO4, and Cu4SO4(OH)6 [20]. Areas with micro holes were more prone to corrosion environment especially the environment containing SO2 and Cl−. For example, Zou had studied the corrosion behavior of immersion gold printed-circuit board (PCB-EING) in the environment containing H2S. The results indicated that micro holes on the surface induced galvanic corrosion and the expansion of corrosion products caused the gold coating to crack [21]. However, in practical applications, the factors impacting the corrosion failure of PCBs are often the synergistic action of several factors (temperature, humidity, electromagnetic field, SO2 and other pollutants). Thus, it was difficult to simulate the failure behavior of PCBs in the real application environment, which makes the outdoor exposure test under real environment particularly important. Furthermore, the outdoor exposure test was the foundation of indoor simulated test. This experiment in this study was carried out in Wheat Island Qingdao which has a typical industrial polluted marine atmosphere environment. The PCB-ImAg was exposed in this region for a long time and samples were taken periodically in the course of exposure time. The composition and structure of the corrosion products on the surface were analyzed by 3D laser scanning confocal microscopy (3D LSCM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The types of corrosion products were detected by confocal Raman microspectroscopy (CRM) and X-ray photoelectron spectroscopy (XPS). Scanning Kelvin probe (SKP), electrochemical impedance spectroscopy (EIS) and polarization curves were employed to analysis the corrosion tendency of the different areas on the surface. This experiment enriches the corrosion theory of PCB-ImAg in atmosphere. 2. Materials and methods

405

parameters of PCBs are shown in Table 1. The PCB-ImAg has a 0.8 mmthick substrate made of FR-4 epoxy glass cloth laminate and a 25– 30 μm-thick copper base. A 0.02 μm-thick chemical immersion silver coating was placed on the basis. Samples were retrieved after 1, 3, 6, 9 and 12 months (m) of exposure for analysis. The dust on the specimen surface was cleaned by hair drier. 2.2. Surface analysis methods After the corrosion tests, surface analysis methods were applied, such as 3D LSCM, SEM and EDS. CRM and XPS were employed to observe the chemical and morphological characteristics. The corrosion of each substance was examined by a Labram HR800 Raman spectroscope, with 532 nm as the wavelength exciting light to analyze PCB-ImAg, and with 50× as the objective. The spectra were obtained from 100 cm−1 to 2000 cm−1 to detect the corrosion products. The XPS measurement was performed using ESCALAB 250 X-ray spectrometer. The photoelectrons were excited with Al Kα (the photoelectron energy hv = 1486.6 eV). The power is 150 W and the diameter of X-ray spot is 500 μm. The Thermo Avantage software was applied for fitting of XPS spectra. 2.3. Macro-electrochemical measurements EIS and polarization curves were conducted by PARSTAT 2273 electrochemical workshop. EIS experiment on the specimen was conducted in a 0.1 M/L Na2SO4 solution, which was a mixture of analytically pure Na2SO4 and deionized water. Each set of tests was performed three times, and then ZSimp Win was used to process the EIS data. Electrochemical measurements were performed with a three-electrode cell. PCBs samples acted as the working electrode. Platinum gauze was used as the counter electrode, and a saturated calomel electrode was employed as the reference electrode. The electrolyte was saturated KCl. The scanning frequency was 1 × 105 Hz to 0.01 Hz. The available working area was 1cm2. Polarization curve analysis of the PCB-ImAg in 0.1 M/L Na2SO4 solution was performed. The polarization curve was measured from −0.5 V (vs. open circuit potential) with a scan rate of 0.333 mV/s. The test samples were cut from the PCBs and then the wires were welded to the welding spot. The soldered points were encapsulated with silicone, leaving an exposed test area of 1 × 1 cm2. The welded specimen was shown in Fig. 1. 2.4. Micro-electrochemical measurements M370 scanning Kelvin probe was used to measure the volta potentials of the PCBs. The working distance was 100 ± 2 μm. The vibration frequency was 80 Hz and amplitude was 30 μm. The scanning mode was step scan sweep and the scanned area was 2000 μm × 1000 μm. 3. Results 3.1. Environmental monitoring The experiment station is located in Wheat Island, Qingdao city, Shandong province, China. The geographic coordinate are latitude 36°03′ and longitude 120°25′. Moist air, abundant rainfall and moderate temperature are the climate characteristics of this region. The industry Table 1 Basic parameters of PCB-ImAg.

2.1. Design of the experimentation and materials

Type of PCBs

Basement (FR-4)

The thickness of copper basement

The thickness of silver coating

PCB-ImAg specimen are placed on horizontal racking in several home-made cases which are fixed on the specimen rack. The structural

Silver plating circuit board (PCB-ImAg)

0.8 mm

25– 30 μm

0.02 μm

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Wire

Silicone Testing area Basement (FR-4) Fig. 1. the PCB-ImAg specimen which is used for electrochemical test.

of this district is flourishing. Industrial and heating from coal burning result in polluting gases which contain a large amount of SO2. The atmosphere environment of this region is the typical industry polluted marine atmosphere environment. The atmosphere parameters in Qingdao, Wanning and Beijing are shown in Table 2. The two regions (Wanning and Beijing) were chosen to compare with Qingdao. The atmosphere in Wanning (latitude 18°58′ and longitude 110°30′) is characterized with high relative humidity but very low pollution especially the sulfating rate. The Beijing (latitude 39°59′ and longitude 116°16′) experiment site has a rural atmosphere and the relative humidity in this region is lower than others. The Qingdao test site is characterized by higher relative humidity, higher chlorine levels (mainly NaCl) and higher sulfur levels. Atmosphere in Qingdao combines environment characteristics in Wanning and Beijing. Atmospheric corrosion generally occurs under thin electrolyte layer or adsorbed thin electrolyte layer [22]. The thin electrolyte layer is more easily formed under this high relative humidity and with high concentration of polluted ions. 3.2. Morphology and elements distribution of PCB-ImAg specimen Surface morphologies of PCB-ImAg after 1, 3, 6 and 12 m are showed in Fig. 2. The surface of PCB-ImAg as shown in Fig. 2(a) is covered by a large amount of particles and the color has changed considerably, while the original silver color can still be seen. As the test time went on, the color of the surface changed to yellow-brownish with time as shown in Fig. 2(b)–(d). The phenomenon is in accordance with other studies. The change of the color may be attributed to the effects of SO2 [23]. With time went on, the amount of corrosion products increased and presented hill shaped distribution. A large amount of corrosion products had formed, but there were still more complete silver coating on the surface at the end time of test. The micro-morphologies of PCB-ImAg specimen after different exposure time are shown in Fig. 3. The morphology of the blank sample is displayed in Fig. 3(a) which shows some micro holes on the surface. EDS results are shown in Table 3. EDS results of points A and B show that the ratio of Ag/Cu is lower in point A than point B, which indicates that silver coating in the defect regions cannot provide good protection for the copper basement and this areas will be corroded first in the process of experiment. Fig. 3(b)–(f) show that the amount of corrosion products increased and the species of corrosion products changed as the exposure time went on. Corrosion behavior happened in some

areas which may be the defects on the surface. Then the corrosion products were once joined together and silver coating was broken heavily. EDS results of area C on the PCB-ImAg sample after 1 month show that the corrosion products mainly consist of copper corrosion products, which indicates serious corrosion had occurred on some areas in a short time. The species of corrosion products have changed after 9 month. Some loose needle-shaped corrosion products appeared on the surface. This type of products may not provide good protection for copper basement. EDS results of area D show that elemental composition of the needle-shaped corrosion products contains Cl and S. This phenomenon also indicates a small amount of Cl and S can heavily damage PCB-ImAg specimen. The EDS results from Fig. 4(a) are summarized in Table 4. S and Cl elements in area A are higher than area B which indicates the corrosion products in bulge area mainly consists of O, S and Cl containing products of copper and silver. The elemental mapping results of Fig. 4(a) show Cu, O, S and Cl elements are higher in A area, which are similar to the EDS results in this area. However, the area B in which the amount of corrosion products is lower had a higher amount of Ag, which presents products in this area mainly consist of corrosion products of silver. Fig. 5 shows that area which contains less Ag element has more Cl element. But S element distributes on the whole surface. This phenomenon also indicates S element has better combination of silver. 3.3. Electrochemical analysis Fig. 6 shows the EIS results of PCB-ImAg exposed in Qingdao after different periods. All the impedance spectra exhibit two capacitive semicircles. One capacitive semicircle is located at high frequency and the other is located at low frequency. Fig. 6 (b) shows the corresponding Bode plots and two time constants are observed. So, to quantify the electrochemical parameters, the equivalent circuit shown in Fig. 6(c) is used, where Rs is the solution resistance, CPE-f is corrosion products film capacitance, Rf is the corrosion products resistance, CPE-dl is double layer capacitance, Rct is charge transfer resistance. The fitting results of EIS are shown in Fig. 6(a). Rct is frequently used to represent corrosion rate. The value of Rct is higher, then the corrosion rate is lower. The trends of Rct of PCB-ImAg display in Fig. 7. The Rct increases to a much higher value after 3 month, and then decreases with further increase of exposure times. This indicates the corrosion rate decreases in the beginning, and then increases. 3.4. XPS analysis The corrosion products of PCB-ImAg were analyzed by XPS. The XPS survey spectra after 12 months is shown in Fig. 8. The main components of PCB-ImAg are O, Cu, Na, C and Cl elements, as well as the microelements including S and N. But Ag element is not detected that due to the content of Ag is less in the corrosion products in the micro holes. The higher intensity of O element indicates that the PCB-ImAg was seriously destroyed. The appearance of Na element shows that sodium salt appeared. The sodium salt is mainly derived from the marine environments. The sodium salt crystallized after the sample surface drying. The elements content (at%) as shown in Table 5 was analyzed through Thermo Avantage software. From the results shown in Fig. 9, we can make the conclusion that the precipitates are composed of a small amount of Ag, AgO, Ag2O,

Table 2 The 2013–2014 annual average of atmosphere parameters in the three experiment stations. Regions

Temperature (°C)

Relative humidity (RH%)

Sulfating rate (mg/10 m2·day)

Sea salt particles (mg/10 m2·day)

pH

SO2− 4 in rain water (mg/m3)

Cl− in rain water (mg/m3)

Qingdao Wanning Beijing

13.5680 25.0917 13.4480

73.9600 84.9167 45.1539

0.2576 0.0103 0.2371

0.1647 0.0083 0.0274

5.0617 5.1344 6.9529

14,400 4521.0520 9144.2860

17,065 6571 4585

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407

Fig. 2. Morphology of PCB-ImAg after different exposure times: (a) 1 m, (b) 3 m, (c) 6 m, (d) 12 m.

and AgCl. As the peaks position of AgO, Ag2O and AgCl are very close, the left peak in the Fig. 9(a) may belong to AgO, Ag2O and AgCl [24,25]. The right peak in the Fig. 9(a) can be attributed to Ag [26]. But Ag element was not detected in the survey spectra, the reason of which may be the content in corrosion products was less. There has been a lot of researches have shown that copper are easy to be oxidized to form Cu2O. With the extension of time, the corrosion products of copper gradually transform to green basic cupric carbonate [27,28]. The Cu2p spectrum of the corrosion products is displayed in Fig. 9(b). Two valences exist in corrosion products of copper. Cu (I) state is the compounds of Cu+, such as Cu2O. Cu (II) state in the compounds of Cu2+, such as chlorine-containing compounds and sulfur-containing compounds. Combine Fig. 9(c), (d) and (e), the existence of S is in the and SO2− and the existence of C is in the form of CO2− form of SO2− 3 4 3 . Combining all the results obtained above, sulfate and carbonate are the main forms of the corrosion products. There is small amounts of Cl

(a)

elements in the product and according the previous studies the conclusion that Cu2Cl(OH)3 is part of corrosion products can be made [29,30]. 3.5. CRM analysis The Raman spectra analysis was carried out on the PCB-ImAg species exposed for 1, 6 and 12 months. The results are shown in Fig. 10 and Table 6. The recorded spectra show that the intensity increases with extended exposure, which may be attributed to the fact that the content of vibration group is higher as time increases. However, the peak positions are basically the same. Several same peaks appeared on the different spectra, such as 117, 146, 217, 420, 521 and 623 cm−1. The peaks at 117, 146, 217, 420 and 521 cm−1relate to the OH deformation modes in Cu4(SO4)(OH)6 (Brochantite) and Cu4(SO4)(OH)6·2H2O (Langite) [31]. The bands at 117, 217 and 420 cm− 1 may be attributed to Cu2Cl(OH)3 [32,33]. Peaks located at 217, 521 and 623 cm−1 may be

(b)

(c)

C

(d)

(f)

(e)

D

Fig. 3. SEM morphology of PCB-ImAg after different exposure times: (a) 0 m, (b) 1 m, (c) 3 m, (d) 6 m, (e) 9 m, (f) 12 m.

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Table 3 EDS results of A, B, C and D areas in Fig. 3 (at%).

Table 4 EDS results of A and B areas in Fig. 4 (at%).

Chemical elements

Ag

Cu

O

Cl

S

C

Chemical elements

Ag

Cu

O

S

Cl

C

A B C D

27.05 57.87 7.31 3.94

72.95 42.13 60.38 35.61

– – 21.57 35.72

– – 1.67 9.34

– – – 3.45

– – 9.07 11.93

A B

8.78 19.16

26.49 26.81

47.78 51.68

3.18 2.34

3.88 –

9.89 –

assigned to Cu2O. As shown in Fig. 10(b), there are two peaks which are marked as ① and ②. The position of peak ① is 117 cm−1. However, there is a peak at 122 cm−1 which is adjacent to the peak position at ①. The peak at 122 cm−1 is attributed to AgCl. The band at 146 cm−1 appears and might be related to the lattice vibration of Ag in Ag2O and Ag2SO3 [34]. The band with the peak at 623 cm−1 can be attributed to vibration of O\\S\\O and S\\O in Ag2SO4 [35,36]. Also, 147, 217 and 623 cm−1 may be assigned to the existence of CuCO3·Cu(OH)2 [37– 39]. All of these results obtained by CRM are in conformity with the results of XPS. 4. Discussions (1) The surface morphology and corrosion products Three different test environments were carried out in laboratory. The parameters of the first laboratory experiment containing Cl− only included a temperature of 35 °C and a salt fog with a concentration of 5% NaCl. The second laboratory experiment contains SO2 only and the temperature was set at 35 °C. SO2 was continuously bubbled into the

chambers during the experiment. The parameters of the third laboratory experiment integrated the experiment containing Cl− and the experiment containing SO2. After testing and before further observation, the samples were washed with de-ionised water and were dried using compressed air. Firstly, the color of the sample in the test containing Cl− remained the original silver color. The only difference is that there are several corrosion parts on the silver surface after 16 h, as shown in Fig. 11(b). However, Fig. 11(c) shows that the color of silver coating has changed seriously after 16 h of experiment. The original silver color has completely lost. The morphology is quite different to the one in the test containing Cl− only. However, the morphology of PCBImAg in the third experiment as shown in Fig. 11(d) integrates the corrosion characteristics of the other two experiments. Both the color of silver coating changed and local serious corrosion appeared. All of these phenomena indicate that silver has strong resistance to Cl−, so the color of silver has no significant changes in a short period. Although silver is not sensitive to Cl−, silver will combined with Cl− to form ion complex AgCl. In the defect areas, the corrosion effects of Cl− will be stronger. Cl− in marine water typically cause pitting corrosion. Cl− dissolves into the film to form electrolytes and gathers in the defects areas. Then the micro-environment changes. These defects became active spots, through which Cl− penetrated to the Cu base. And Cl− will preferentially react with bare copper in the defects. In this experiment,

(a)

A B

(b)

(c)

(d)

(e)

(f)

(g)

Fig. 4. (a) SEM of the sample after 1 month; (b–g) Elemental mapping of the enlarged map of Fig. 3(a) on the plate after the 1 month test: (b) Ag, (c) C, (d) Cl, (e) Cu, (f) O, (g) S.

L. Yan et al. / Materials and Design 115 (2017) 404–414

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Fig. 5. Elemental mapping of the enlarged map of Fig. 4(a) morphology of PCB-ImAg sample after the 9 month, (b) Ag, (c) Cl, (d) Cu, (e) O, (f) S.

the results show that silver coating is quiet sensitive to SO2, even if there are only tiny amounts of SO2. To sum up all of these descriptions above in a sentence: as the PCB-ImAg has a special structure, Cl− mainly induces the micro-hole corrosion. SO2 causes the corrosion failure of the

whole silver coating. The corrosion rate and the degree of corrosion increase with the sustained dissolving of SO2. Some studies have shown that corrosion rate of copper and silver increased in the effects of SO2 and Cl−, when the humidity reached a 80

9k

1m 6m 12m

(a) 8k

3m 9m fitting curves

70

7k

3m 9m

12m

60

6k

-Phase (Ang.)

-lm (Z) (Ohm .cm -2

1m 6m

(b)

5k 4k 3k

50 40 30 20

2k 10 1k 0 0 0

1k

2k

3k

4k

5k

6k -2

Re(Z) (Ohm.cm )

7k

8k

9k

0.01

0.1

1

10

100

log(freq.) (Hz)

(c)

Fig. 6. the EIS results of PCB-ImAg after different periods: (a) Nyquist plots; (b) Bode plots; (c) equivalent circuit for samples.

1000

10000

100000

410

L. Yan et al. / Materials and Design 115 (2017) 404–414

14.0k

PCB-ImAg 12.0k

Table 5 Elemental composition obtained from the XPS results for the corrosion products on the PCB-ImAg after 12 month of exposure. Elements

O

Cu

C

Na

S

N

Cl

at%

45.66

9.2

35.48

3.69

4.76

0.68

0.52

-2

10.0k

Rct/k

8.0k

6.0k

4.0k

2.0k 1m

3m

6m

9m

12m

Exposure Time/months Fig. 7. The change trend of Rct of PCB-ImAg.

certain value [40,41]. The attachments may be assigned to the deposition of salt particles. Through the indoor experiments, we can make the conclusion that the SO2 may corrode silver and then the color of silver changes considerably. The corrosion mechanism of PCB-ImAg in Qingdao can be roughly deduced by the corrosion morphologies of PCB-ImAg in indoor experiments. According to the analysis of environment parameters which are shown in Table 1, the atmosphere in Qingdao belongs to the synergistic effects of pollutant, especially of SO2 and Cl−. The morphology of PCB-ImAg in Qingdao is similar to the third test in indoor experiment. Qingdao marine atmospheric environment contains a large number of sea salt particles, and then the marine aerosol formed and settled on the surface of PCB-ImAg. The sedimentation of NaCl particles accelerates the formation of thin electrolyte film. And studies have shown that because the surface coating was thin, the surface has micro holes defects which are the corrosion active sites [42]. As a result, the local corrosion environment has changed. These particles enhanced the formation of liquid film and support better condition for the dissolving of SO2. Silver coating will react with Cl− to form AgCl. Also, Cl− will directly contact

with copper and result in the formation of copper chlorine compounds in the areas which have micro holes. Previous studies also have shown that SO2 can induce a color change on silver [43,44]. SO2 dissolves in the thin electrolyte film to acidify the chemical environment and to 2− form HSO− 3 and SO4 . This phenomenon results in the formation of silver sulfur compounds, such as Ag2SO4 or Ag2SO3 [45]. The dissolved SO2 also results in the copper sulfur compounds, such as Cu4(SO4)(OH)6 and Cu4(SO4)(OH)6·2H2O. Then the corrosion degree of PCB-ImAg surface increased in the end [46]. After anodic dissolution of the silver coating, the copper basement was exposed to be contacted with corrosion ions to form chemical compound of copper. The basement contacts with the corrosion ions directly and then was corroded. In the areas which have some defects, copper basement directly contact with corrosion ions and then corrosion ions reacts with cupric ions to form compounds of copper in the micro holes area. The SEM and elemental mapping of PCB-ImAg after 1 month of exposure as shown in Fig. 4 indicate that the corrosion products in raised areas mainly consist of copper compounds, which also verify the occurring of micro-hole corrosion. Then the corrosion products travel through defects to the surface and piled up on the surface. Also, the cracks of silver coating will appear around the micro holes. Then the ions channels increased. According to the results of CRM analysis and XPS analysis, the main corrosion products are oxides, sulfate and carbonate of Ag and copper. O2, SO2, Cl− and CO2 dissolved into the thin electrolyte film and then and CO2− OH−, SO2− 4 3 formed. Ag and Cu were easily oxidized to Ag2O, AgO, Cu2O and CuO. And then the silver oxide reacts with SO2 in the humidity environment. Also, in the acidic conditions, the silver reacts with SO2 and O2 to form Ag2SO4 [47]. Moreover, the silver oxides may transform into AgCl in the chloride containing environment [48]. The Cu2O reacts with SO2, Cl− and CO2– 3 which are dissolved in the liquid film to form chlorine containing, sulfur containing and carbon containing compounds [16,49]. The main reaction equations were shown in following Eqs. (1)–(15). Dissolution of SO2 in the thin water film on the specimen surface [50, 51]: SO2 þ H2 O→Hþ þ HSO3 −

O1s 400k

HSO3 − →Hþ þ SO3 2

Na1s Cu2p

2SO3 2

Intensity/a.u.

300k

−

ð1Þ

−

þ O2 →2SO4 2

ð2Þ −

ð3Þ

Anodic reactions [52]: Ag→Agþ þ e−

Eθ ¼ 0:7991V

ð4Þ

Cu→Cuþ þ e−

Eθ ¼ 0:521V

ð5Þ

200k

C1s S2p Cl2p N1s

100k

Cuþ →Cu2þ þ e−

Eθ ¼ 0:167V

Cathodic reaction: 2H2 O→4Hþ þ O2 þ 4e−

0

0.0

200.0

400.0

ð6Þ

600.0

800.0

1.0k

1.2k

1.4k

Binding energy/eV Fig. 8. XPS survey spectra of the corrosion product film formed on PCB-ImAg after 12 month of exposure.

Eθ ¼ 1:229V

ð7Þ

The formation of silver compounds [53–55]: −

Ag þ Cl →AgCl þ e− −

Eθ ¼ 0:222V

2Ag þ SO4 2 →Ag2 SO4 þ 2e−

Eθ ¼ 0:653V

ð8Þ ð9Þ

L. Yan et al. / Materials and Design 115 (2017) 404–414

3k

30.0k

Ag 3d

(a)

411

(b)

Cu2p

27.0k 3k

Ag

Cu( )

Cu( ) Intensity/a.u.

Intensity/a.u.

AgO/Ag2O /AgCl 3k

3k

24.0k

21.0k

18.0k 3k 15.0k 3k 366

367

368

369

370

371

372

926.0

928.0

930.0

Binding energy/eV

932.0

934.0

936.0

938.0

Binding energy/eV

3.6k 40.0k

(d)

O2p

(c)

S2p

10k

35.0k

C1s

3.2k 8k

30.0k

C-C

25.0k 20.0k 15.0k

Cu2O

2.8k

SO4

SO3

2-

7k

2-

Intensity/a.u.

SO4 /SO3

2-

Intensity/a.u.

2-

Intensity/a.u.

(e)

9k

2.4k

6k

C=O

5k

CO3

2-

4k

10.0k 2.0k

3k

5.0k 0.0 527

2k 528

529

530

531

532

533

534

1.6k 164

535

166

168

Binding energy/eV

170

172

174

282

284

286

Binding energy/eV

288

290

292

Binding energy/eV

Fig. 9. XPS high-resolution spectra of the corrosion products film formed on the PCB-ImAg after 12 month: (a) Ag, (b) Cu, (c) O, (d) S, (e) C.

−

2Agþ þ SO4 2 →Ag2 SO4

ð10Þ

The formation of copper compounds [16,56]: −

Cu þ Cl →CuCl þ e− 2Agþ þ SO3 − →Ag2 SO3

ð11Þ

Eθ ¼ 0:124V

2Cu þ H2 O→Cu2 O þ 2Hþ þ 2e−

ð12Þ

Eθ ¼ 0:472V

ð13Þ

(b)

Intensity

Intensity

(a)

200

400

600

800

1000

1200 -1

Raman Shift cm

1400

1600

1800

2000

100

120

140

160

180

200 -1

Raman Shift cm

Fig. 10. Raman spectra of PCB-ImAg after 1, 6 and 12 month of exposure; Fig. (b) is the enlarged drawing of Fig. (a).

220

240

412

L. Yan et al. / Materials and Design 115 (2017) 404–414

Table 6 Assignment and chemical species of the Raman bands in the spectrum.

PCB-ImAg 0.4

Species

117 146 217 420 521

OH deformation modes in Cu4(SO4)(OH)6 or Cu4(SO4)(OH)6·2H2O Cu2Cl(OH)3 Cu2O AgCl Lattice vibration of Ag2O or Ag2SO3 Vibration of O\ \S\ \O and S\ \O in Ag2SO4 CuCO3·Cu(OH)2

117 217 420 217 521 623 122 146 623 147 217 623

0.3

Xc/V

Raman shift (wavenumbers/cm−1)

0.2

0.1

−

Cu2þ þ SO4 2 →CuSO4

ð14Þ

Cu2þ þ HCO3 – þ OH– →CuCO3 þ H2 O

ð15Þ

0.0

0

2

4

6

8

10

12

exposure/months Fig. 12. The surface voltaic potential trends of PCB-ImAg with different periods.

(2) The change of corrosion potential 5. Conclusions To study the corrosion mechanism behavior of PCB-ImAg after different periods in Qingdao atmospheric environment, the SKP analysis was applied to determine the surface Kelvin potential and Gaussian fitting analysis was also used to analyze the data. Volta potential was detected by measuring the metal electric work function in the air. The volta potential linear relationship with the corrosion potential of PCBImAg [57]. The Gaussian fitting results of PCB-ImAg are shown in Fig. 12 from which we can see that the surface Kelvin potential decreases as the exposed time goes on. Also, the potentiodynamic polarization behavior of PCB-ImAg specimens after different periods is shown in Fig. 13. The corrosion potential of the sample gradually decreases with the passage of time. The results of potentiodynamic and SKP have a rather nice corresponding relationship showing a decreased corrosion potential with extended exposure. The concentration of SO2 is higher in Qingdao and silver is heavily damaged by SO2. Moreover, the copper basement can be badly damaged by Cl−. Then the protection of silver coating is poor. So the corrosion potential gradually decreases.

(1) Combined with surface morphology of PCB-ImAg in indoor and outdoor experiments, the conclusion can be made that silver is sensitive to SO2 and then the color changes considerably after short time of experiments. The bulge-shaped corrosion products formed in the micro holes areas. S element content is higher than Cl element in the corrosion products. (2) The settlements of dusts and salt particles on the surface of PCBImAg act as the corrosion active points. Corrosion medium in micro holes come into directly contact with the basement. Corrosion products increase and travel through micro holes to the surface of PCB-ImAg. (3) As silver is sensitive to SO2 and the Cl− has strong corrosiveness, PCB-ImAg is continuous corroded. Then the results of SKP and potentialdynamic polarization represent the corrosion tendency increases as time goes on. (4) The results summarized in this paper indicated that PCB-ImAg

(a)

(b)

(c)

(d)

Fig. 11. Morphology of PCB-ImAg in indoor simulated environment: (a) 0 h, (b) PCB-ImAg after 16 h in the experiment with Cl− only, (c) PCB-ImAg after 16 h in the experiment with SO2 only, (d) PCB-ImAg after 16 h in the experiment with SO2 and Cl−.

L. Yan et al. / Materials and Design 115 (2017) 404–414

0.4

1m 6m

(a) 0.3

3m 9m

0.1

12m

413

1m 6m

(b)

3m 9m

12m

0.2 0.1

E/V vs.SCE

E/V vs.SCE

0.0 0.0 -0.1 -0.2 -0.3

-0.1

-0.4 -0.5 -8

-7

-6

-5

-4

-3

-2

log|i|/A.cm-2

-8

-7

-6

log|i|/A.cm-2

Fig. 13. Polarization curves of PCB-ImAg after different periods: (a) the polarization curves of PCB-ImAg; (b) the enlarged map of the circled area in Fig. 12(a). This test was conducted by PARSTAT 2273 electrochemical workstation. The test was performed in 0.1 M/L Na2SO4 and measured from −0.5 V (vs. open circuit potential) at a scan rate of 0.333 mV/s.

without any protective coating cannot be used in industrially polluted marine environment.

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