Department of Industrial Chemistry, Pukyong National University, Busan 48547, Republic of Korea

Transcription

1 Met. Mater. Int., Vol. 22, No. 5 (2016), pp. 781~788 doi: /s Galvanic Corrosion of Cu Coupled to Au on a Print Circuit Board; Effects of Pretreatment Solution and Etchant Concentration in Organic Solderability Preservatives Soft Etching Solution SeKwon Oh 1, YoungJun Kim 1, MinYoung Shon 2,*, and HyukSang Kwon 1, * 1 Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon 14141, Republic of Korea 2 Department of Industrial Chemistry, Pukyong National University, Busan 48547, Republic of Korea (received date: 18 February 2016 / accepted date: 15 April 2016) In present study, we quantitatively define the galvanic corrosion phenomenon of Cu electrically coupled to Au on Print Circuit Board in Organic Solderability Preservatives (OSP) pretreatment (pickling and soft etching) solutions. As a result of polarization and ZRA test, galvanic corrosion rate of Cu in soft etching solution was about 3000 times higher than that of pickling solution. The oxone in OSP soft etching solution was acted as strong oxidant for Cu on PCB substrate. And the galvanic corrosion of Cu in OSP soft etching solution was examined with the change of etchants (oxone (KHSO 5), sulfuric acid (H 2SO 4)) concentration. The galvanic corrosion rate of Cu was increased by the increase of the oxone and sulfuric acid concentrations, which lead to the increase of cathodic reactant such as HSO 5 and H + ions. And the degree of galvanic corrosion rate of Cu ( isoft etching =i couple, (Cu-Au) -i corr, Cu) decreased with the decrease of the oxone and sulfuric acid concentrations. Keywords: corrosion, electronic materials, electrochemistry, PCB, Cu 1. INTRODUCTION A printed circuit board (PCB) is a module of interconnected electronic components found in electronic devices. Recently, packaging substrates for PCBs that connect the substrate and chip using a solder ball have been widely used as a result of the increased use of portable electronics [1,2]. One of the methods for fabricating PCBs is by replicating a pattern on a CCL (copper clad laminate) material through photoengraving followed by etching with a developer. After that, to connect the substrate and the solder ball of a chip successfully, gold bumps are formed by electrodeposition on the substrate. For a completely patterned PCB substrate, a surface protective coating is used to suppress the oxidation of the exposed Cu on the PCB [3]. The protective coating layer must be solderable and have good barrier protection performance against oxidation for the exposed Cu. Among the many protective treatments, treatment with organic solderability preservatives (OSP) has gained a great deal of attention due to its fast and detailed process, cost effectiveness and use of environment-friendly materials. During the OSP process, the exposed Cu is protected against oxidation by selectively adsorbed organic compounds such as Benzotriazole imidazole, Benz *Corresponding author: myshon@pknu.ac.kr, hskwon@kaist.ac.kr KIM and Springer imidazole, etc. [4,5]. Figure 1 shows the overall procedure of the OSP process. Before OSP, pretreatments such as acid cleaning and soft etching process were conducted to eliminate any contaminants on the exposed Cu and to improve the wettability of the exposed Cu for the OSP. The OSP pretreatment was usually conducted by oxidative acidic solutions such as H 2 SO 4 and oxone for few seconds or minutes in PCB industry. The criterion of the acid cleaning and etching process are based on the exposed single Cu, which does not electrically connect with any other metal parts on the PCB. However, PCB was composed of not only single Cu but also Cu electrically connected to Au. When the Cu electrically connected to Au was immersed in the OSP pretreatment solutions, Cu acts as anode and Au as a cathode based on the galvanic series, which lead to a subsequent over-etching problem caused by galvanic corrosion due to the electrochemical potential difference between Au and Cu. The electrical signal on the PCB was transferred by patterned Cu, and its size was few micrometers. If the Cu electrically connected to Au was severely over-etched by galvanic corrosion in pretreatment solutions, it leads to degradation of signal integrity and subsequently to the failure of electronic device. Figure 2. show patterns of Cu coupled to Au on PCB after OSP pretreatment process. For narrow Cu patterns coupled to Au, electrical short phenomenon caused by galvanic corrosion was clearly observed (Fig. 2(c)). Accordingly, to

2 782 SeKwon Oh et al. Fig. 1. Process of organic solderability preservatives (OSP). Fig. 2. Pattern of PCB substrate and optical microscope image of (a) PCB board, (b) normal pattern and (c) defective pattern caused by galvanic corrosion after the OSP pretreatment process. overcome this problem, it is very important to understand the galvanic corrosion behavior between Au and Cu on PCBs in given OSP process conditions. Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when dissimilar metals with different corrosion potentials (E corr ) are in electrical contact and immersed in an electrolyte [6]. The potential difference between the dissimilar metals is the driving force for the accelerated corrosion on the anode side of the galvanic couple. Additionally, other factors in galvanic corrosion include the area ratio of the cathode/anode [7,8], solution composition [9], solution temperature [10], ph, etc. Many researchers have focused on the galvanic corrosion of Cu coupled to various metal systems [11-19]. However, the galvanic corrosion behaviors between Cu and Au on the PCB have rarely been investigated. Especially, galvanic corrosion behavior of Cu electrically coupled to Au on PCB in the OSP pickling and soft etching solution were not reported yet. In this work, we researched the galvanic corrosion of Cu electrically coupled to Au on PCB in pretreatment (pickling and soft etching) solutions of OSP process and the effects of etchant concentration (sulfuric acid, oxone) of an OSP soft etching solution on the galvanic corrosion behaviors of Cu coupled to Au. 2. EXPERIMENTAL PROCEDURES To replicate real PCB specimen, pure copper foil on CCL (copper clad laminate) was used as a Cu specimen and then patterned for electrodepositon of Au. For electrodeposition of Au, Ni was preferentially electroplated on the Cu substrate for buffer layer to inhibit intermetallic compound Au and Cu. Electrodeposition was conducted with cathodic current density of 100 ma/cm 2 for 15 min at 40 C from solution containing 0.45 M NiSO 4 6H 2 O, M NiCl 2 6H 2 O and 0.8 M H 3 BO 3. Then, Au was electroplated on the Ni substrate at a cathodic current density of 1.5 ma/cm 2 for 10 min 30 s at 50 C from solution containing M KAu(CN) 2. Figure 3 shows the specimen of Cu and Au for potentiodynamic polarization tests. Circular shape of Cu and Au were used as electrode and their diameter were 1 cm. Each electrode was electrically connected to rectangular shape of Cu with 2.9 cm of distance, and distance of circular shape electrode between Au and Cu was 1 cm. To analyze the corrosion phenomenon of Cu coupled to Au, two types of test solution (pickling solution and soft etching solution) were prepared. The composition of the pickling solution was 0.35 M H 2 SO 4 aqueous solution, and soft etching solution was 0.25 M sulfuric acid (H 2 SO 4 ) and 0.65 M oxone (KHSO 5 ) aqueous mixture solution. In addition, to examine the effects of etchants concentration on the corrosion behavior of Cu coupled to Au in a soft etching solution, different concentration of sulfuric acid ( M) and oxone ( M) were prepared, respectively. To examine the corrosion behavior of Cu and Au in pickling solution and soft etching solution, potentiodynamic polarization tests of Cu and Au were conducted by a potentiostat (Gamry) in pickling and a soft etching solution. The effect of etchants concentration on the corrosion behavior of single Cu and Au was examined. Potentiodynamic polarization tests of Cu and Au were conducted with various sulfuric acid ( M) and oxone concentrations ( M). The electrochemical cell was equipped with a Pt counter electrode, a saturated calomel reference electrode (SCE), and Fig. 3. Specimen arrangement of Cu and Au electrode on PCB (a), Schematic diagram of ZRA test (b) and photograph of real test (c).

3 a test specimen (Cu, Au) as a working electrode. The exposed area of the working electrode was cm 2. Polarization curves were obtained in the noble direction from -300 mv SCE to about 400 mv SCE based on the open circuit potential with a scan rate of 1 mv/s. Galvanic coupled current was measured by the zero resistance ammeter (ZRA) mode of a potentiostat to investigate the effects of the electrolyte (pickling and soft etching solution), and the different composition of sulfuric acid (0.1 M ~ 0.56 M) and oxone (0.39 M ~ 0.79 M) in soft etching solution. The electrochemical system consisted of Au as counter electrode, saturated calomel reference electrodes and Cu as working electrode as shown in Fig. 3(b-c). Each electrode was electrically connected to potentiostat and the exposed area of each electrode was cm 2 (area ratio of Cu:Au = 1:1). The surface morphology of Cu was observed by scanning electron microscopy (SEM). 3. RESULTS AND DISCUSSION Galvanic Corrosion of Cu Coupled to Au on a Print Circuit Board Galvanic corrosion behavior of Cu coupled to Au in the pickling and soft etching solutions of the OSP process Figure 4(a) shows the polarization response of Cu and Au in pickling solution (0.35 M H 2 SO 4 ) at 25 C. The Cu showed active corrosion behavior in pickling solution and continuous increase of anodic current density was observed as the applied potential was increased. From the results, the measured corrosion potential (E corr ) of Cu was V SCE and corrosion current density (i corr ) was ma/cm 2, which is the target etching rate of Cu in the pickling solution. In case of Au in pickling solution, the measured E corr and i corr were V SCE and 2.71?10-2 ma/cm 2, which were caused by the oxidation and reduction reaction of hydrogen on the surface of Au. To investigate the galvanic corrosion rate of Cu coupled to Au (area ratio of 1:1) in pickling solution, a ZRA test was conducted. Figure 4(b) shows the results of the ZRA test of Cu coupled to Au in the pickling solution at 25 C. The measured galvanic coupled current, i couple, (Cu-Au), was ma/ cm 2, which was slightly higher than that of corrosion rate of single Cu (i corr, Cu : ma/cm 2 ). The corrosion rate difference between the i couple, (Cu-Au) and i corr, Cu in picking solution is expressed as i pickling = i couple, (Cu-Au) i corr, Cu. The calculated i pickling was ma/cm 2, which indicates the quantity of over-etching of Cu coupled to Au as a result of galvanic corrosion in pickling solution. Figure 5(a) shows the polarization responses of Cu and Au in soft etching solution (0.25 M H 2 SO 4 and 0.65 M KHSO 5 ) at 32 C. As a result, Cu exhibited active corrosion behavior with a continuous increase of anodic current density as the applied potential was increased. The measured E corr of Cu was V SCE which was slightly higher value than that of Cu in pickling solution. On the other hand, the i corr of Cu was 25.4 ma/cm 2, which was about 200 times higher than that of Fig. 4. Polarization behavior of Cu and Au (a), and galvanic current density of Cu coupled to Au (b) in the pickling solution at 25 C. Cu in pickling solution ( ma/cm 2 ). In the case of polarization responses of Au in soft etching solution, the measured E corr and i corr of Au were V SCE and 1.02 ma/cm 2, respectively. Unlike pickling solution, anodic polarization reaction of Au was observed and it is thought be caused by highly corrosive environment due to oxone (KHSO 5 ). The standard electrode potential of KHSO 5 is 1.85 V SHE, corresponding to following reaction (Eq. (1) and (2)), which is higher than that of Au (1.5 V SHE ) [20]. Accordingly, anodic reaction of Au was active corrosion and cathodic reaction is decomposition of HSO 5 - to HSO 4 - in soft etching solution. Consequently, it was clearly demonstrated that the soft etching solution is more corrosive condition than that of the pickling solution, and the more severe corrosion behavior of Cu in soft etching solution seems to be caused by high oxidation environments due to oxone (KHSO 5 ). KHSO 5 K HSO 5 (1) - HSO 5 + 2H + + 2e - - HSO 4 + H 2 O (2) To examine the galvanic corrosion of Cu coupled to Au

4 784 SeKwon Oh et al. Fig. 6. Surface morphologies of Cu coupled to Au (a) before etching, (b) after etching in the pickling solution for 10 min at 25 C, (c) after etching in soft etching solution for 10 min at 32 C. pretreatment. On the other hand, relatively smooth surface of Cu was observed in case of Cu coupled to Au in soft etching solution, resulting in a severe etching of Cu surface. These results agree well with the results of the polarization test with higher corrosion rate of Cu in soft etching solution. Therefore, it was clearly confirmed that pretreatment process in soft etching solution is more critical step for galvanic corrosion occurrence between Cu and Au on PCB substrate than pickling solution. Fig. 5. Polarization behavior of Cu and Au (a), and galvanic current density of Cu coupled to Au (b) in soft etching solution at 25 C. (area ration of 1:1) in soft etching solution (0.25 M H 2 SO 4, 0.65 M KHSO 5 ), ZRA test was conducted. Figure 5(b) shows the results of the ZRA test in the soft etching solution at 32 C. As a result, the measured i couple, (Cu-Au) was ma/ cm 2, which is higher than that of i corr, Cu (25.4 ma/cm 2 ). The corrosion rate difference in soft etching solution between a single Cu and a Cu coupled to Au, i soft etching, was 6.68 ma/cm 2, which was approximately 3000 times higher than that of the corrosion rate difference in pickling solution ( i pickling = ma/cm 2 ). Figure 6 shows the effects of pretreatment (picking and soft etching solution) on the surface morphology of Cu. Figure 6(a) shows surface morphology of Cu without pretreatment, which relatively higher roughness than other pretreatment cases. Figure 6(b) and (c) shows the surface of Cu coupled to Au after immersion in pickling and soft etching solution for 10 min, respectively. For immersion of Cu coupled to Au in pickling solution, evidence of corrosion was not observed on the surfaces of Cu, which was very similar with surface of Cu without 3.2. Influence of oxone concentration in soft etching solution on galvanic corrosion of Cu coupled to Au The soft etching solution consists of sulfuric acid (0.25 M H 2 SO 4 ) and oxone (0. 65 M KHSO 5 ). Among these, oxone is known as a strong oxidant affecting the corrosion rate of metal. In OSP process, the Cu coupled to Au on the PCB is immersed in soft etching solution and Au acts as a cathode and Cu acts as an anode due to the difference of the electrochemical potential. Anodic reaction of current galvanic cell is dissolution of Cu, and cathodic reaction is decomposition of HSO 5 -. With an increase of oxone concentration in soft etching solution, the galvanic corrosion rate of Cu coupled to Au increased resulting from the increase of HSO 5 - concentration (Eq. 2). The corrosion rate determining step of the overall galvanic corrosion reaction of Cu coupled to Au is cathodic reaction, therefore, the galvanic corrosion rate of Cu coupled to Au increased with an increase of concentration of the reactant (HSO 5 - ) in the cathodic reaction. In order to evaluate the effects of the oxone concentration in the soft etching solution on the galvanic corrosion rate of Cu coupled to Au (area ratio is 1:1), a ZRA test was conducted to the change of the oxone concentration in a range from 0.39 M to 0.79 M at 32 C with fixed concentration of sulfuric acid (0.25 M), as listed in Table 1. As a result, i couple, (Cu-Au)

5 Galvanic Corrosion of Cu Coupled to Au on a Print Circuit Board 785 Table 1. Galvanic current density of Cu coupled to Au with the change of oxone concentration in soft etching solution Concentration of Oxone (M) i coupleof Cu coupled to Au (ma/cm 2 ) Fig. 8. (a) Effect of oxone concentration on the corrosion behavior of single Cu, (a) the difference of the corrosion rate between Cu coupled to Au and single Cu according to the concentration of oxone in OSP soft etching solution at 32 C (b). Fig. 7. Effects of oxone concentration on the galvanic current density of Cu coupled to Au in soft etching solution at 32 C (a) and the change of galvanic current density in terms of oxone concentration (b). increased from 21.2 ma/cm 2 to 34.7 ma/cm 2 with an increase of the oxone concentration from 0.39 M to 0.79 M, as shown in Fig. 7(a) and Table 1. Based on the results of Table 1, we experimentally confirm that the galvanic corrosion rate of Cu coupled to Au increased with an increase of the oxone concentration in soft etching solution in the form of an exponential function, as shown in Fig. 7(b), and the results are formulated as Eq.(3): Y = exp X where, X is concentration of oxone and Y is i couple, (Cu-Au) in (3) Table 2. Corrosion current density of single Cu with the change of oxone Concentration in soft etching solution Concentration of Oxone (M) i corrof Cu (ma/cm 2 ) E corr (mv) soft etching solution. As mentioned above, in order to suppress the galvanic corrosion rate of Cu coupled to Au and to maintain the standard etching rate, the concentration of oxone should be decreased below 0.66 M. Figure 8(a) shows the results of polarization test of single Cu in soft etching solution, according to the change of the oxone concentration from 0.39 M to 0.66 M, which were listed in Table 2. The measured i corr of Cu was decreased from 25.4 ma/cm 2 to ma/cm 2 with a decrease of the oxone concentration, as listed in Table 2. According to previous results, galvanic corrosion rate of Cu coupled to Au is increased expo-

6 786 SeKwon Oh et al. nentially with the increase of oxone concentration. However, the corrosion rate of single Cu is increased linearly with the increase of oxone concentration as shown in Fig. 8(b). The corrosion rate difference between Cu coupled to Au and single Cu ( i soft etching = i couple, (Cu-Au) i corr, Cu ) at the same oxone concentration decreased from 6.64 ma/cm 2 to 1.48 ma/ cm 2 with the decrease of the oxone concentration from 0.65 M to 0.39 M in OSP soft etching solution. These results clearly indicate that the degree of galvanic corrosion rate decreased with a decrease of the oxone concentrations. Accordingly, the optimum oxone concentration in present study was 0.39 M to minimize the difference of the corrosion rate between Cu coupled to Au and single Cu in soft etching solution Influence of concentration of sulfuric acid in the soft etching solution on galvanic corrosion of Cu coupled to Au Sulfuric acid (H 2 SO 4 ) is one of the main components of the soft etching solution with oxone. With an increase of the sulfuric acid concentration in soft etching solution, by the same explanation as provided for oxone in the previous chapter, the increase of galvanic corrosion rate of Cu coupled to Au was expected due to the increase of hydrogen ion concentration, which is a reactant in the cathodic reaction, as delineated in Eq. 2. In order to evaluate the effects of the sulfuric acid concentration in soft etching solution on the corrosion rate of single Cu and Cu coupled to Au (area ration is 1:1), a ZRA test was conducted according to the sulfuric acid concentration in a range from 0.1 M to 0.56 M at 32 C with fixed content of oxone (0.66 M), as listed in Table 3. The ph value of soft etching solution was decreased from 1.32 to 0.87 with an increase of sulfuric acid concentration from 0.1 M to 0.56 M. As a result of ZRA test, i couple, (Cu-Au) increased from ma/cm 2 to 50.3 ma/cm 2 with an increase of the sulfuric acid concentration from 0.1 M to 0.56 M, as shown in Fig. 9(a) and Table 3. Based on the results, we experimentally confirm that the galvanic corrosion rate of Cu coupled to Au increased with an increase of the sulfuric acid concentration in soft etching solution with linear function, as shown in Fig. 9(b), and the results are formulated as Eq. (4). The increasing rate of i couple, (Cu-Au) with an increase of sulfuric acid concentration was higher than that of i couple, (Cu-Au) with increasing oxone concentration. Therefore, the control of sulfuric acid concentration is considered much important factor to suppress galvanic corrosion effect on Cu surface. Table 3. Galvanic current density of Cu coupled to Au with the change of sulfuric acid concentration in soft etching solution Concentration of Sulfuric i coupleof Cu coupled to Au acid (M) (ma/cm 2 ) ph Fig. 9. Effects of sulfuric acid concentration on the galvanic current density of Cu coupled to Au in OSP soft etching solution at 32 C (a) and the change of galvanic current density in terms of sulfuric acid concentration (b). Y = 0.63 X (4) where, X is concentration of sulfuric acid and Y is i couple, (Cu-Au) in soft etching solution. In present study, the concentration of sulfuric acid should be decreased below 0.25 M to suppress the galvanic corrosion of Cu coupled to Au. Figure 10(a) shows the results of polarization test of single Cu in soft etching solution, according to the change of the sulfuric acid concentration from 0.1 M to 0.25 M, which were listed in Table 4. The i corr of Cu was decreased from 25.4 ma/cm 2 to 18.5 ma/cm 2 with a decrease of the sulfuric acid concentration from 0.25 M to 0.1 M, as described in Table 4. Figure 10(b) shows the difference in the corrosion rate ( i soft etching = i couple, (Cu-Au) i corr, Cu ) between Cu coupled to Au and single Cu with the change of sulfuric acid concentration in OSP soft etching solution. According to previous results, galvanic corrosion rate of Cu coupled to Au is increased exponentially with the increase

7 Galvanic Corrosion of Cu Coupled to Au on a Print Circuit Board 787 conditions was 0.1 M in soft etching solution to minimize the difference of the corrosion rate between Cu coupled to Au and single Cu. 4. CONCLUSIONS Fig. 10. Effect of sulfuric acid concentration on the corrosion behavior of single Cu, (a) the difference of the corrosion rate between Cu coupled to Au and single Cu according to the concentration of sulfuric acid in OSP soft etching solution at 32 C (b). Table 4. Corrosion current density of single Cu with the change of sulfuric acid Concentration in soft etching solution Concentration of sulfuric acid (M) i corrof Cu (ma/cm 2 ) E corr (mv) of oxone concentration. However, the corrosion rate of single Cu is increased linearly with the increase of oxone concentration as shown in Fig. 10(b). The i soft etching at the same sulfuric acid concentration decreased from 6.64 ma/cm 2 to 2.85 ma/cm 2 with the decrease of the sulfuric acid concentration from 0.25 M to 0.1 M, indicating that the degree of galvanic corrosion rate decreased with a decrease of the sulfuric acid concentration. Accordingly, the optimum sulfuric acid concentration in given experimental We examined the galvanic corrosion of Cu coupled to Au in pickling and soft etching solution and evaluated the extent of galvanic corrosion (over-etching) in OSP process. Additionally, the etchant concentration such as oxone and sulfuric acid were investigated to evaluate the galvanic corrosion of Cu coupled to Au in soft etching solutions, and the conclusions drawn from this work are as follows: (1) The difference in the corrosion rate between single Cu and Cu coupled to Au in soft etching solution was 6.68 ma/ cm 2 ( i soft etching = i couple, (Cu-Au) Ai corr, Cu ), which was approximately 3,000 times higher than that of the pickling solution ( i pickling = ma/cm 2 ). Therefore, the galvanic corrosion of Cu coupled to Au was more appreciable when it was exposed to the soft etching solution rather than to the pickling solution. The main cause of this phenomenon was due to the existence of oxone (KHSO 5 ) in soft etching solution. (2) The effects of oxone concentration on the corrosion rate of single Cu and Cu coupled to Au in soft etching solution were investigated. The galvanic corrosion rate of Cu coupled to Au increased in the form of an exponential function with an increase of the oxone concentration resulting from an increase of the HSO 5 - concentration. The corrosion rate of single Cu was also increased with an increase of the oxone concentration. (3) The effects of sulfuric acid concentration on the corrosion rate of single Cu and Cu coupled to Au in soft etching solution were investigated. The corrosion rate of Cu coupled to Au and single Cu increased as a linear function with an increase of the sulfuric acid concentration resulting from an increase of the hydrogen ion concentration. The increasing rate of corrosion with increasing sulfuric acid concentration was higher than that of corrosion rate with increasing oxone concentration. Accordingly, sulfuric acid concentration is considered much important factor to suppress galvanic corrosion effect on Cu surface. ACKNOWLEDGEMENTS This work was supported by Samsung Electro-Mechanics and the National Research Foundation of Korea (NRF), REFERENCE 1. K. C. Yung, H. Liem, H. S. Choy, and W. K. Lun, Int. J. Heat Mass Transfer 37, 1266 (2010). 2. R. Ambat and P. Moller, Corros. Sci. 49, 2866 (2007).

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