ELECTROCHEMICAL CORROSION BEHAVIOR OF PCB-HASL IN NaHSO3/Na2SO3 SOLUTION
DING Kangkang, XIAO Kui(), ZOU Shiwen, DONG Chaofang, ZHAO Ruitao, LI Xiaogang
Corrosion and Protection Center, University of Science and Technology Beijing, Beijing 100083
Cite this article:
DING Kangkang, XIAO Kui, ZOU Shiwen, DONG Chaofang, ZHAO Ruitao, LI Xiaogang. ELECTROCHEMICAL CORROSION BEHAVIOR OF PCB-HASL IN NaHSO3/Na2SO3 SOLUTION. Acta Metall Sin, 2014, 50(10): 1269-1278.
With the innovation of electronic technology, integration and miniaturization become the future developing direction of printed circuit board (PCB). Meanwhile, the corrosion problems of PCB also stand out more clearly, and even trace amounts of corrosion products will have a serious impact on the reliability of PCB. Under the actual condition for use, like sulfur-containing industrial environment, due to the diurnal temperature variations or/and the temperature field fluctuations for PCB itself, condensation phenomenon is likely to occur. Furthermore, as a result of the moisture absorption effect of granular deposit or supersaturated humidity, a layer of electrolyte solution will be formed on the surface of PCB, causing electrochemical corrosion. In this work, electrochemical impedance spectroscopy (EIS) and scanning Kelvin probe (SKP) techniques were used to study the corrosion behavior and mechanism of hot air solder leveling printed circuit boards (PCB-HASL) in a simulated electrolyte 0.1 mol/L NaHSO3 and 0.1 mol/L NaHSO3/Na2SO3 solutions with different pH values, and the influences of immersion time and pH value on the change of corrosion mechanism were discussed. Meanwhile, with the aids of OM, SEM combined with EDS, the nucleation and propagation processes of corrosion products on the surface of PCB-HASL were observed and analyzed. SEM and EDS results showed that the corrosion behavior of PCB-HASL in acid simulation solution was similar to pitting corrosion, and the corrosion pits were in a state of accelerated expansion at the early immersion stage. The corrosion products mainly consisted of oxides and sulfates of Sn. EIS and SKP analysis indicated that the PCB-HASL surface could be activated by NaHSO3 solution and pitting nucleation process only occurred at the early immersion stage. In the neutral or alkaline solution system of NaHSO3/Na2SO3, pitting corrosion couldn't occur, and the transmission of the electrolyte to the electrode interface through the oxide film was the control step of the corrosion reaction.
Fig.1 OM images of hot air solder leveling printed circuit boards (PCB-HASL) immersed in 0.1 mol/L NaHSO3 solution for 0 h (a), 0.5 h (b), 3 h (c), 12 h (d), 36 h (e) and 120 h (f)
Fig.2 Variations of coverage ratio of corrosion products on PCB-HASL surface with immersion time (R—coverage ratios of corrosion products, t—immersion time)
Fig.3 SEM images of PCB-HASL after immersion times of 0.5 h (a), 3 h (b), 12 h (c), 36 h (d) and EDS analysis of area A (e), area B (f) in Fig.3c and area C in Fig.3d (g)
Fig.4 Surface Kelvin potentials distribution of PCB-HASL after immersed in 0.1 mol/L NaHSO3 solution for 0 h (a), 0.5 h (b), 3 h (c), 12 h (d), 36 h (e), 120 h (f) (Ekp—surface Kelvin potential)
Fig.5 Column diagram of Ekp distribution for PCB-HASL before immersion (a) and Gauss fitting curves after different immersion times (b)
Time / h
m / V
s
0
-0.5558
0.0228
0.5
-0.6312
0.0276
3
-0.6024
0.0316
12
-0.5393
0.0349
36
-0.4498
0.0320
120
-0.3197
0.0355
Table 1 Gauss fitting results of surface Ekp distribution
Fig.6 EIS and fitting curves of PCB-HASL immersed in 0.1 mol/L NaHSO3 for 0.5~6 h (a) and 12~120 h (b) (ZRe—real part of impedance, ZIm—imaginative part of impedance)
Fig.7 EIS equivalent circuits of PCB-HASL immersed in 0.1 mol/L NaHSO3 solution for 0.5~1 h (a), 2~12 h (b) and 24~120 h (c) (Rs—solution resistance, CPEf—film capacitor of surface oxides or/and corrosion product film, Rf—resistance of surface oxides or/and corrosion product film, RL—resistance related to the generating and dissolving process of surface film at pitting nuclear site, L—equivalent inductance related to thickness change of surface film at pitting nuclear site, CPEdl—electric double layer capacitance, Rct—charge transfer resistance, W—Warburg impedance)
Time h
Rs Ω·cm2
CPEf
n1
Rf Ω·cm2
L H·cm2
RL Ω·cm2
CPEdl
n2
Rct Ω·cm2
W S·s5·cm-2
0.5
13.45
6.96×10-6
0.8687
1617
5.48×104
1.30×104
0.000078
0.8639
1014
0.004347
1
14.80
7.62×10-6
0.8798
1291
7.79×104
1.41×104
0.000147
0.8144
627.6
0.007080
2
14.09
1.23×10-5
0.8629
1057
-
-
0.000538
0.8778
265.2
0.012620
3
14.33
1.57×10-5
0.8642
856.4
-
-
0.001199
0.9109
190.8
0.012550
6
14.35
2.49×10-5
0.8748
769.4
-
-
0.001360
1.0000
198.1
0.012980
12
15.02
3.90×10-5
0.8871
4231
-
-
0.000966
0.9675
721.2
0.005176
24
11.83
1.07×10-4
0.9312
3229
-
-
0.000162
0.8475
7817
-
36
19.26
1.02×10-4
0.9429
3435
-
-
0.000141
0.8429
9405
-
72
14.15
1.30×10-4
0.9074
6213
-
-
0.000251
0.8375
8179
-
120
18.86
7.82×10-4
0.7614
6418
-
-
0.000155
0.9063
10000
-
Table 2 EIS fitting results of PCB-HASL immersed in 0.1 mol/L NaHSO3 solution for different times
Fig.8 OM images of PCB-HASL in 0.1 mol/L NaHSO3/Na2SO3 solution with different pH values
Fig.9 EIS results and fitting curves of PCB-HASL immersed in 0.1 mol/L NaHSO3/Na2SO3 with different pH values
Fig.10 EIS equivalent circuits of PCB-HASL in neutral or alkaline NaHSO3/Na2SO3 solution system (O—finite-layer diffusion impedance element)
Condition
Rs Ω·cm2
CPEf
n1
Rf Ω·cm2
CPEdl
n2
Rct Ω·cm2
W S·s5·cm-2
O S·s5·cm-2
B s5
pH=6
15.83
1.35×10-5
0.8881
1914
6.6×10-4
1.0000
230
0.00958
-
-
pH=7
8.95
1.87×10-5
0.9146
2532
2.4×10-4
1.0000
702
-
2.89×10-3
3.166
pH=8
12.53
1.49×10-5
0.9270
2332
2.0×10-4
0.7269
3951
-
1.48×10-3
5.851
Na2SO3
9.97
4.41×10-6
0.8467
1409
1.2×10-5
0.8207
64400
-
4.79×10-5
6.190
Table 3 EIS fitting results of PCB-HASL in 0.1 mol/L NaHSO3/Na2SO3 solution with different pH values
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