Surface & Coatings Technology 228 (2013) 27–33

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A Pd-free activation method for electroless nickel deposition on copper Dong Tian a, De Y. Li a, Fang F. Wang b, Ning Xiao a, Rui Q. Liu a, Ning Li a,⁎, Qing Li c, Wei Gao c, Gang Wu c a b c

School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States

a r t i c l e

i n f o

Article history: Received 4 June 2012 Accepted in revised form 30 March 2013 Available online 6 April 2013 Keywords: Pd-free activation Immersion nickel layers Copper Thiourea Electroless nickel deposition

a b s t r a c t In this work, a Pd-free activation method for electroless nickel deposition on copper via an immersion nickel technique was developed. In the very solution we studied, high concentration of thiourea resulted in a negative shift of the steady potential of copper, making it possible to realize immersion nickel. The obtained immersion nickel layers were characterized by scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray photoelectron spectroscopy, demonstrating a co-deposition of sulfur in the nickel layer. Importantly, the post-treatment in 1.0 M NaH2PO2 + 1.0 M NaOH solution was able to eliminate the adsorbed thiourea and stimulate the catalytic activity of the immersion nickel layer for electroless nickel deposition. A combination of open circuit potential measurements and morphology studies indicated that an incubation step was required during the electroless nickel deposition on the immersion nickel layers after post-treatment. Although the catalytic activity of this Ni-activation method was slightly lower as compared to the conventional Pd-activation, both obtained electroless Ni–P layers exhibited similar morphology, chemical composition, corrosion resistance, and adhesion strength. Thus, this work demonstrated that the newly developed Ni-activation method was cost-effective and could be a promising replacement to expensive Pd-activation method currently used in printed circuit board industries. © 2013 Published by Elsevier B.V.

1. Introduction The ever-increasing requirements for higher interconnection densities on printed circuit board (PCB) promote the development of copper circuit in PCB manufacturing [1,2]. In order to improve the corrosion resistance, the copper circuits need to be deposited with protective coatings [3,4]. Due to the excellent corrosion resistance [5–7], electroless nickel layers have been extensively employed to protect the copper circuits [3,4]. Electroless nickel deposition with hypophosphite as the reducing agent is an autocatalytic process. In this process, nickel ions can be spontaneously reduced by hypophosphite on a catalytic substrate [8,9]. However, copper is inert for the hypophosphite oxidation reaction [10–12] and electroless nickel deposition cannot take place on copper surface automatically. Therefore, activation for copper surface is required to generate the active sites for hypophosphite oxidation. Currently, precious metal Pd is extensively used as an activator to initiate the electroless deposition on inactive substrates such as copper [13–17]. Typically, immersing copper into a Pd2+-containing solution for about 1–2 min to obtain Pd active sites is the standard activation method. Unfortunately, high price of Pd strictly limits its application in PCB industries [18,19]. Therefore, the development of highly efficient and cost-effective activation methods is desperately needed. Because nickel is inherently catalytic active for hypophosphite oxidation ⁎ Corresponding author. Tel.: +86 451 86413721; fax: +86 451 86418270. E-mail address: [email protected] (N. Li). 0257-8972/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.surfcoat.2013.03.048

[9–12], the preparation of nickel layer on inert substrate attracts much attention to replace the Pd-activation method. As a result, several Ni-activation methods for specified materials have been developed. Si-based substrates are able to be active by a replacement deposition of nickel in the presence of OH − [20,21]. ABS plastic can be activated by depositing Ni(0) nanoparticles [22,23]. Usually, the reported methods for these specified materials require complicated procedures that always take 5–30 min. And a relatively long incubation step (ca. 2 min) is needed before the obtained nickel layers can exhibit sufficient catalytic activity [20–23]. However, the simple and efficient Ni-activation method for copper substrate has not been developed. The unique electrochemical properties of copper in the presence of thiourea may provide a viable approach for Ni-activation [24–26]. Thiourea can inhibit copper corrosion at low concentration, while high concentration of thiourea can result in a negative shift of the steady potential of copper [26–31]. Due to the negative shift of copper potential, replacement deposition such as immersion tin on Cu was reported [32–34]. In doing so, the spontaneous replacement deposition of nickel onto copper may be realized, which can potentially be an effective Ni-activation method for subsequent electroless nickel deposition. Although the influences of thiourea on copper potential [26–31] as well as on electroless nickel deposition [35–37] have been investigated before, there is little research focusing on immersion nickel on copper in thiourea-containing solution and its catalytic activity for electroless nickel deposition.

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In this work, immersion nickel on copper was studied in acidic solution containing a high concentration of thiourea, and an effective activation method for electroless nickel deposition was developed. The thermodynamic plausibility of the replacement deposition of nickel on copper was studied by open circuit potential (OCP) measurements. The obtained immersion nickel layers were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and X-ray photoelectron spectroscopy (XPS). The effect of post-treatment in 1.0 M NaH2PO2 + 1.0 M NaOH solution on the catalytic activity of the immersion nickel layer was investigated. The possible process of electroless nickel deposition on the immersion nickel layer was discussed. More importantly, the newly developed Ni-activation method was systematically compared to the conventional Pd-activation method in terms of the properties of resulting electroless nickel layers, including surface appearance, phosphorus content, corrosion resistance, and adhesion strength. Fig. 1. A schematic of the shear strength test setting.

2. Experimental 2.1. Immersion nickel Copper sheets (99.9% Cu with a thickness of 0.05 cm) were used as the substrates for immersion nickel experiments. Prior to the immersion plating, the copper sheets were degreased in acetone at room temperature for 2 min and then etched in 5 wt.% H2SO4 solution for 2 min to remove the surface oxides. After rinsing with deionized water, the pretreated copper sheets were put into the immersion nickel bath comprising of NiSO4·6H2O (40 g/L), (NH2)2CS (170 g/L), and H3BO3 (30 g/L). The pH value was controlled at 1.0 and the temperature was controlled at 60 °C. For a comparison, the commercial Pd-activation bath was also employed to activate the copper sheets.

The shear strength testing method was used to study the adhesion strength between the Ni–P layers and the copper substrates [38,39]. As illustrated in Fig. 1, the copper sheet coated with Ni–P layers on both sides (area = 0.785 cm 2, thickness = 0.05 cm) was bonded to two rigid cylinders using high strength adhesive. A load parallel to the sample was continually imposed on the cylinder at a fixed rate (0.05 cm/min) using the Instron-4467 stretcher. Until adhesion failure happened, the instant maximum load (Fmax) was recorded. The adhesion strength τ was determined by the equation: τ = Fmax/0.785 cm2. According to the principle of statistics, eight identical measurements were carried out and then the maximum and minimum values were removed. The rest of the calculated values were averaged to obtain the adhesion strength.

2.2. Electroless nickel deposition Prior to the electroless nickel deposition, the activated copper sheets were dipped into a solution (60 °C) containing 1.0 M NaH2PO2 and 1.0 M NaOH for 5 s, and then rinsed by deionized water. The subsequent electroless nickel deposition was performed by immediately transferring the activated copper sheets into the electroless nickel bath at 85 °C. The electroless nickel bath (pH = 5.2) was comprised of NiSO4·6H2O (20 g/L), NaH2PO2·H2O (25 g/L), C3H6O3 (lactic acid, 20 ml/L), C3H6O2 (propionic acid, 5 ml/L), and KIO3 (2 mg/L).

3. Results and discussion 3.1. OCP measurements during immersion nickel process The OCPs of copper electrode in different acidic solutions (pH = 1.0, 60 °C) are shown in Fig. 2. Without thiourea and Ni2+, copper potential is relatively positive (ca. −0.43 V vs SMSE) as shown in Fig. 2(a). The addition of Ni2+ into this solution system does not change the OCP of copper (Fig. 2(b)). According to the thermodynamic principles, the spontaneous deposition of nickel cannot proceed because copper is

2.3. Electrochemical measurements

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2.4. Physical characterization The morphology of deposited nickel layers as a function of the plating time during the plating processes was studied with SEM (HITACHI S-4800). The EDX system mounted on the SEM was employed to determine the elemental compositions of the immersion nickel layers and the electroless nickel layers. The immersion nickel layers were also analyzed by XPS (K-Alpha, VG). The binding energies were calibrated using contamination carbon (binding energy = 284.6 eV) as a reference. A Shirley background was subtracted when fitting the XPS peaks.

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All electrochemical measurements were performed on a PARSTAT 2273 (AMETEK) electrochemical workstation in a three-electrode cell. Depending on the specific electrochemical measurement, copper electrode (0.785 cm2), activated copper electrode (0.785 cm2) or copper electrode coated with electroless nickel layer (0.785 cm2) was used as the working electrode. The saturated mercurous sulfate electrode (SMSE) and saturated calomel electrode (SCE) were used as the reference electrode in OCP and corrosion resistance measurements, respectively.

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Time, s Fig. 2. OCPs of copper electrode in different acidic solutions (pH = 1.0, 60 °C). (a) 30 g/L H3BO3; (b) 30 g/L H3BO3 + 40 g/L NiSO4·6H2O; (c) 30 g/L H3BO3 + 170 g/L thiourea; and (d) 30 g/L H3BO3 + 170 g/L thiourea + 40 g/L NiSO4·6H2O.

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more electrochemically stable than nickel. However, as exhibited in Fig. 2(c), in an acidic solution containing a high concentration of thiourea, the OCP immediately shifts to the negative direction until a steady state is reached. When Ni2+ is added (Fig. 2(d)), the replacement reaction of Ni2+ by Cu takes place because the potential is negative enough for nickel deposition. It is observed that the OCP increases rapidly during the first 60 s and then gradually keeps stable. This measured OCP profile indicates that the immersion nickel process includes two different steps [33]. At the first step, the replacement reaction proceeds quickly, leading to a rapid deposition of nickel on copper surface. After 60 s, the copper surface may be fully covered by the nickel layer, resulting in a stable OCP. 3.2. Physical characterization of the immersion nickel layer The morphology of the immersion nickel layer as a function of the immersion time was studied with SEM as shown in Fig. 3. In good

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agreement with the OCP measurements, the immersion process can be divided into two steps. In the first 60 s, the significant changes in morphology are observed. Before deposition, the surface of copper substrate is relatively smooth and uniform as shown in Fig. 3(a). After 2 s, some irregular dots appear on copper surface. From 5 s to 60 s, the dots become bigger and bigger (Fig. 3(c)) and gradually coalesce into clusters (Fig. 3(d)–(g)). However, further increasing the immersion time results in no significant change in morphology (Fig. 3(h)–(i)). The EDX spectra of area A and B (Fig. 3(j) and (k)) clearly indicate that both nickel and sulfur present on the entire surface in the form of uniformly distributed clusters. The immersion nickel layer was further analyzed using XPS. The XPS spectra of the activated copper sheet before and after Ar+ sputtering are shown in Fig. 4. Before sputtering (Fig. 4(a)), the main peak at 855.7 eV in the zoom-in of survey XPS spectrum is corresponding to Ni 2+ species [40,41]. This is mainly due to that the active nickel sites on the surface

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are easily oxidized to form oxides in the ambient air. After sputtering, the metallic nickel with a peak at 852.4 eV becomes dominant (Fig. 4(b)) [40–43], indicating the deposition of metallic nickel on the copper surface during the immersion process. It should be noted that the peaks corresponding to sulfur are observed before and after sputtering [44,45], and nitrogen peaks can only be detected before sputtering [46]. Thus, the immersion nickel process leads to a simultaneous co-deposition of nickel and sulfur. The nitrogen can be assigned to the adsorbed species on the surface. Based on these analyses, the immersion nickel process on copper is schematically represented in Fig. 5, including the adsorption of thiourea on copper surface, the corrosion and dissolution of copper, and the co-deposition of nickel and sulfur.

3.3. Catalytic activity of the immersion nickel layers The OCPs of the activated copper electrodes in the electroless nickel bath are shown in Fig. 6. In principle, when the OCPs reach the typical potential of the Ni–P layer/electroless nickel bath system (ca. −1.05 V vs SMSE), the immersion nickel layers can be active for electroless nickel deposition [17,47]. If the post-treatment in 1.0 M NaH2PO2 + 1.0 M NaOH solution is omitted, the OCPs of the activated copper electrodes will stabilize at ca. −0.95 V (vs SMSE) and then slowly shift to the positive direction as shown in Fig. 6(a). No matter how long the Ni-activation step takes, the activated copper electrodes cannot reach the typical potential of the Ni–P layer/electroless nickel bath system [47]. These results imply that electroless nickel deposition cannot occur and the copper electrodes coated with immersion nickel layers are still inert for electroless nickel deposition. However, after the post-treatment in 1.0 M NaH2PO2 + 1.0 M NaOH solution for 5 s at 60 °C, the required electrochemical conditions for electroless nickel deposition can be achieved. As shown in Fig. 6(b), the OCPs will shift to the negative direction and reach the typical potential (ca. −1.05 V vs SMSE) [47]. Thus, the catalytic activity of the immersion nickel layer can be stimulated by the post-treatment in the alkaline solution. In order to study the roles of the post-treatment in activating the immersion nickel layers, XPS was employed to analyze the immersion nickel layer before and after the post-treatment (Fig. 7). Before the post-treatment, two peaks are required to obtain a satisfactory fit for the N 1s spectrum (Fig. 7(a)) [46,48]. According to literature, the peak at 399.5 eV is assignable to the nitrogen atoms in the thiourea molecule (NH2-C), and the other peak at 397.5 eV can be assigned to the nitrogen atoms in the thiourea molecule bonded with the metal substrate (M-NH2-C) [48]. The shift of −2.0 eV in the binding energy is likely due to electron transfers from the metal substrate to the nitrogen atoms, suggesting a strong chemical adsorption of thiourea on the immersion nickel layer. As thiourea is the stabilizer or inhibitor for electroless nickel deposition [35–37], the chemical adsorption of thiourea

on the immersion nickel layer will suppress its catalytic activity for electroless nickel deposition. However, as shown in Fig. 7(b), after the post-treatment in alkaline solution, the peaks corresponding to the nitrogen atoms of thiourea molecule disappear, and only one peak at 400.1 eV can be observed. It is indicated that the post-treatment in 1.0 M NaH2PO2 + 1.0 M NaOH solution can eliminate the adsorbed thiourea. Therefore, the catalytic activity of the immersion nickel layer for electroless nickel deposition can be stimulated after the post-treatment. Based on the changes of OCPs as shown in Fig. 6(b), there are obviously two steps during the electroless nickel deposition on the immersion nickel layers after post-treatment, including an incubation step and an electroless nickel deposition step. In the incubation step, the OCPs shift towards the negative direction until the typical potential of the Ni–P layer/electroless nickel bath system is established. During the electroless nickel deposition step, the OCPs are stable. It is found that the duration of the incubation step strongly depends on the activation time. Usually, a longer activation time corresponds to a prolonged incubation step. In other words, the catalytic activity

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D. Tian et al. / Surface & Coatings Technology 228 (2013) 27–33

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of the immersion nickel layer for electroless nickel deposition will be decreased with the increasing activation time and ultimately becomes insignificant after 5 min. According to our previous research [49], the extension of activation time leads to an increase of sulfur content in the immersion nickel layers, thereby resulting in passivation of the immersion nickel layers. As a result, the catalytic activity is decreased with the activation time.

3.4. Electroless nickel deposition on activated copper In order to study the autocatalytic process of electroless nickel deposition on the immersion nickel layer after post-treatment, SEM images with different deposition times were studied (Fig. 8). As shown in Fig. 8(a), irregular nickel clusters present on the activated copper sheet prior to electroless nickel deposition. No further change

Fig. 8. SEM images of the activated (20 s) copper as a function of the electroless nickel deposition time. (a) 0 s, (b) 30 s, (c) 1 min, (d) 5 min, (e) 10 min and (f) 30 min.

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3.5. Comparison between Ni- and Pd-activation methods

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is observed even after 30 s (Fig. 8(b)). This suggests that the autocatalytic process of electroless nickel deposition on the activated copper sheet requires an incubation step. During this step, obvious electroless nickel grains cannot be observed and only irregular nickel clusters deposited during the activation process appear on the surface. The OCP measurements indicate that the duration of the incubation step is around 50 s under the studied experimental conditions. In Fig. 8(c), fine and uniform Ni–P grains are formed after 1.0 min. The observed morphology changes illustrate how electroless nickel deposition gradually occurs on the activated Cu sheet. After that, the Ni–P grains grow with the increasing deposition time as evident in Fig. 8(d)–(f). Ultimately, a typical electroless nickel layer is obtained on the inert copper sheet due to the effective Ni-activation method.

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In order to show the practicability of the newly developed Ni-activation method for PCB industries, we systematically compared our recipe with the conventional Pd-activation method in terms of their catalytic activity, and resulting properties of electroless nickel layers including surface appearance, chemical composition, corrosion resistance, and adhesion strength. At first, the OCPs of both activated copper electrodes in electroless nickel bath were recorded to compare their catalytic activity. As shown in Fig. 9, the incubation step is about 25 s for the best performing Ni-activation, while the Pd-activation requires a transition period of 7 s. Thus, the OCP measurements imply that the catalytic activity of the Ni-activation is slightly lower than that of the Pd-activation, likely due to the difference in intrinsic activity between Ni and Pd. However, relative to the reported Ni-activation methods for other substrates (ca. 2 min for the transition period) [20–23], the catalytic activity of this Ni-activation is better. The morphology and chemical composition of the obtained electroless nickel layers from both Ni-activation and Pd-activation are compared in Fig. 10. As shown in Fig. 10(a) and (b), the surface appearance and uniformity of the electroless nickel layer obtained from Ni-activation are comparable to those from Pd-activation. Although thiourea was found to have a significant influence on the surface appearance of the electroless nickel layer [35–37], the developed post-treatment for the Ni-activation can completely eliminate the adsorbed thiourea. So, the presence of thiourea in the Ni-activation method does not affect the morphology of the electroless nickel layers at all. In addition, the chemical compositions of the Ni–P layers derived from Ni- and Pd-activations are compared in Fig. 10(c) and (d), respectively. The results show that the phosphorus contents in both Ni–P layers are almost identical (9.63 wt.% and 9.18 wt.%). Corrosion resistance and adhesion strength of the Ni–P layers obtained from Ni- and Pd-activation were compared. Tafel plots

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D. Tian et al. / Surface & Coatings Technology 228 (2013) 27–33

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Potential, V vs SCE Fig. 11. Tafel plots (5 mV/s) of the Ni–P layers in 3.5% NaCl solution.

recorded in 3.5% NaCl solution were used to determine the corrosion resistance of these Ni–P layers. As shown in Fig. 11, both Ni–P layers exhibit similar corrosion behavior, attested by the comparable icorr. In addition, the average adhesion strength (τ) is calculated to be 25.2 MPa for Ni-activation and 23.9 MPa for Pd-activation. Thus, compared to Pd-activation, Ni-activation can yield Ni–P layers with matched electrochemical and mechanic properties, holding a great promise to be implemented in PCB manufacturing. 4. Conclusion A Ni-activation method for electroless nickel deposition on inert copper substrates was developed through negatively shifting copper potential in the presence of a high concentration of thiourea in acidic solution. This method was proved effective to deposit an immersion nickel layer onto copper substrate as an activator, which consisted of nickel and sulfur. The chemical adsorption of thiourea on the immersion nickel layer could suppress the catalytic activity for subsequent electroless nickel deposition. However, an effective post-treatment in 1.0 M NaH2PO2 + 1.0 M NaOH solution was developed to eliminate the adsorbed of thiourea and stimulate the catalytic activity. Both OCP measurements and SEM images indicated that an incubation step was required during the catalyzing process for electroless nickel deposition. Importantly, compared to the conventional Pd-activation method, the Ni-activation was able to obtain comparable electroless Ni–P layers, exhibiting similar morphology, chemical composition, corrosion resistance, and adhesion strength. Thus, the cost-effective Ni-activation method is promising to be applied in PCB manufacturing to replace the current Pd-activation strategy. Acknowledgments The financial support by the Highnic Group in this work is gratefully acknowledged.

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