1 Introduction
Electrochemical plating is a widely utilized method for film deposition in various industries. Presently, electrochemical plating technology can be categorized into two main types: electroplating deposition technology, which involves applying current, and electroless deposition technology, which does not require applied current. Electroplating is a more intricate process compared to electroless plating due to its need for a clean environment and the use of potentially hazardous equipment [1].
In the electroless plating process, components are immersed in an aqueous solution containing metal ions and anti-oxidation elements, such as additives. These additives prevent the metal ions from oxidizing and forming an oxide layer on the substrate’s surface. Consequently, the metal ions can be reduced to their elemental form and
deposited onto the substrate. Electroless plating is commonly performed on metals like Cu, Ni, Au, Pd, Co, Cr, Ag, and Pt [2]. The advantage of electroless plating is that it can be conducted at room temperature, mitigating concerns about thermal shock in subsequent processes. Within the realm of electroless plating, there are subcategories such as general electroless plating with reducing agents, contact electroless plating, and galvanic displacement electroless plating [3]. Galvanic displacement electroless plating is a spontaneous reduction process, governed by the redox potentials of the metal/metal ion in the electrolyte, which determine the thermodynamic feasibility of the process [4, 5]. The reaction follows the redox potentials in the standard electrochemical series and depends
on factors like metal ion concentration, pH, and temperature. However, galvanic displacement electroless deposition is generally limited to only a few monolayers, resulting in a very thin maximum thickness and weaker bonding force compared to general electroless plating. As a result, the application of galvanic displacement electroless plating is not universally suitable.
To address the limitations of existing galvanic displacement electroless plating methods, this work proposes an innovative electroless plating technique that combines a thick-film Al conductor using screen printing with a modified galvanic displacement reaction. Thick-film aluminum electrodes are printed on various non-conductive substrates [6], showcasing three distinct characteristics for the sequential implementation of innovative galvanic displacement electroless plating deposition: Aluminum metal, with a high oxidation potential (-1.66V), readily undergoes oxidation, facilitating the deposition of various metals. The porous structure of the thick-film aluminum electrode allows for the replacement of reduced metals throughout the entire aluminum electrode, not just on its surface.
Utilizing metal aluminum powder, which is abundantly available on Earth, makes aluminum an affordable choice for use as a replacement sacrificial layer, without increasing manufacturing costs [7].
In summary, this innovative electroless plating technique offers a promising approach to overcome the limitations of traditional galvanic displacement electroless plating, combining the benefits of thick-film Al conductors and modified galvanic displacement reactions for advanced metal deposition on non-conductive substrates.
2 Experiments
The study is divided into three parts including preparation of the thick film Al conductor on an Al2O3 substrate, the galvanic reduction-oxidation displacement reaction, and analysis and measurements.
2.1 Preparation of the thick film Al conductor on the Al2O3 substrate
Aluminum paste was prepared based on 75 wt% solid content (70 wt% Al powder 3-6 μm obtained from Kanto Chemical, 5 wt% bismuth-borosilicate-based glass obtained from Ashahi Glass), and a 25wt% organic formulation (15 wt% terpineol solvent, 4 wt% thixotropic agent 6 wt% ethyl cellulose binder obtained from Thixatrol Max.). The Al paste was screen printed onto the Al2O3 substrate (99.6%) (Leatec fine ceramic 3×3 cm2)
using a 325-steel mesh with a thickness of 30μm and an emulsion film with thickness of 15μm. The screen-printed Al conductors were dried at 120 ℃ for 30 min and then sintered at 600 ℃ for 10 min.
2.2 Galvanic reduction-oxidation displacement reaction in the NiSO4 and CuSO4 solutions
The Al conductor specimens were put into aqueous NiSO4 and CuSO4 solutions to carry out the galvanic displacement reaction. All the aqueous solutions were put in a water bath to control the reaction temperature. The nickel sulphate solutions (NiSO4(aq)) were prepared based on NiSO4‧6H2O(s)(BASF)(20g/L) and HCL (BASF) (3g/L); the copper sulphate solution (CuSO4(aq)) is prepared based on CuSO4∙5H2O(s) (BASF)
(98g/L) and H2SO4 (aq) (BASF)(37g/L).
2.3 Analysis and measurements
The specimens were analyzed using several methods, including electrical measurement with a modified four-point probe, microstructure observation with a scanning electron microscope, and element analyses with an energy-dispersive spectrometer, X-ray diffractometer, and an inductively coupled plasma-mass spectrometer (ICPMS).
3 Results and Discussion
In our studies, innovative galvanic reduction-oxidation displacement reaction is adopted to deposit electroless plating Ni film and Cu films.
3.1 Innovative galvanic displacement electroless-plating for Ni
The innovative galvanic reduction-oxidation displacement reaction includes oxidation reaction of Al and reduction reaction of Ni+2 ion.
3.1.1 Oxidation reaction of Al
When the thick film Al electrode was immersed in the aqueous NiSO4 solution, the thick film Al electrode oxidation reaction occurred.
Since aluminum has a very high oxidation potential, which was described before, aluminum is an easily released electron that produces ions in an acidic NiSO4 solution. Aluminum metal oxidizes to produce Al3+ ions and electrons in an acidic NiSO4 solution. The concentration of Al3+ ions is inversely correlated with the solution’s electron concentration. The oxidation reaction is significantly impacted by the temperature and duration of immersion in the NiSO4 solution. An increased concentration of Al+3 ions in the NiSO4 (aq) solution is the result of a more aggressive oxidation reaction, which is triggered by higher solution temperatures and longer immersion durations. As shown in Figure 1, different solution temperatures and immersion times were used to determine the Al+3 ion concentration using an inductively coupled plasma-mass spectrometer (ICP-MS). The concentration of Al+3 ions increase rapidly within the first 5 minutes at an initial NiSO4(aq) solution temperature (Tex) of 80 °C, but the rate of increase slows down beyond that time. Additionally, the concentration of the Al+3 ions is 292.80 ppm at Tex=80 ℃, which is nearly as high as that at Tex=90 ℃ (300 ppm), but it is only 236 ppm at Tex=60 ℃ when the soak time is set at 15 minutes. Based on the ICP-MS analysis of the Al+3 in the NiSO4 solution, the galvanic displacement reaction temperature was carried out at 80 ℃ for 15-30 minutes.
3.1.2 Reduction reaction of Ni+2 ion
A nickel ion A nickel ion accepts a free electron to reduce to a metal Ni film accepts a free electrode to reduce a metal Ni film; therefore, observation of the conversion of the Al electrode into Ni electrode is an indicator of the reduction reaction.
Figure 2 shows the appearance and SEM images of BSE and SE for Al specimens with different soak times at Tex=80 ℃ in the NiSO4(aq)solution. The Ni is brown, but it appears to be a little dark when the immersion time is short, and became whiter as the soaking time is increased, indicating that more of the Al electrode is replaced with Ni. The BSE (backscattered electron) image in this study is used to observe the distribution of Ni substituted for thick film Al electrode, where the light part is related to Ni, and the dark part is related to Al owing to the difference in their atomic numbers. The SE image can be used to observe the microstructure of the Ni and Al on the specimen.

Fig. 2: Microstructure analysis using (a) photographs, (b) BSE images, and (c) SE images of thick film Ni substituted for Al at 80℃ for different soaking times (A) 5 min, (B) 15 min, (C) and 30 min
According to the BSE images, it is found that the surface coverage has increased somewhat with increased soaking time. This result is consistent with the concentration of Al+3 in NiSO4(aq) at 80 ℃ from 5 minutes to 30 minutes.
Figure 3-a shows a diagram of the thickness of Ni with soak times of 5, 15, and 30 minutes at Tex=80 ℃. It can be observed that the thickness of Ni increased slowly from 5.6, 5.8, and 6.0 μm, which is reflected in the resistance of Ni at different soaking times, as shown in Figure 3-b. This can be explained by the fact that the displacement reaction rate at 80 ℃ was very fast, so the displacement reaction is nearly completed within 5 minutes, and the surface is almost covered by Ni. Thus, the growth rate of the thickness of Ni become slower after 5 minutes at Tex=80 ℃.

Fig. 3.-a: Cross-section the microstructure of thick film Ni substituted for Al at 80℃ for different soaking times (a) 5 min, (b) 15 min, and (c) 30 min. 3-b Thickness and resistance of the Ni layer after the galvanic displacement reaction at 80°C as function of soaking time
The resistance of Al specimen is approximately 140.8 mΩ. Figure 3-b shows that there is a dramatic decrease in the resistance from 140.8 mΩ to 39.8 mΩ when the specimen was immersed in NiSO4(aq) at 80 ℃ for 5 minutes, and it continues to decrease slightly as the soaking time is increased [8].
3.2 Innovative displacement electrolessplating for Cu
3.2.1 Oxidation reaction of Al
When the thick film aluminum electrode was immersed in the copper sulfate solution, the following oxidation reaction occurred:
The concentration of reactant, the aluminum (Al) electrode, that dissolved in the reaction solution of CuSO4(aq) were analyzed using an ICP-MS. Figure 4 shows a diagram of the concentration of Al+3 ion in CuSO4(aq) at 40, 60, and 80 °C solution temperatures for different soaking times. At a 40 ℃ solution temperature, the Al+3 ion concentration is 85 ppm after a soaking time of 15 minutes When the temperature of the solution rose to 60 ℃ and 80 ℃ and immersion time was for 15 minutes, Al+3 ion concentration rose to approximately 300 ppm. It is obvious that solution temperature and immersion time strongly affected oxidation reaction [8]. When the solution temperature reaches 80 ℃, even if the specimen is only immersed for 5 minutes, Al+3 ion concentration is as high as 250 ppm, which is much higher than that at 40 ℃.
After immersing for 15 minutes at 80 ℃, Al+3 ion concentration increased slowly with the immersion time. When the immersion time reached 60 minutes, Al+3 ion concentration reached approximately 400 ppm.
3.2.2 Reduction reaction of Cu+2
3.2.2.1 Effects of solution temperature and immersion time
The thick film aluminum electrode was immersed in a copper sulfate solution at 40, 60, and 80 °C from 10 to 120 minutes.
It is obvious that the following reduction reaction occurred:

Fig. 5: BSE images of thick film Cu substituted for Al at different temperatures and soaking time, Tex=40℃, (a) 5 min, (b) 30 min, (c) 60 min, (d) 120 min, Tex=60℃, (e) 5 min, (f) 30 min, (g) 60 min, (h) 120 min, Tex=80℃, (i) 5 min, (j) 30 min, (k) 60 min, (l) 120 min
In this study, we want to investigate the impact of CuSO4 displacement solution at different temperatures and immersion durations. To further investigate the microstructure of the thick film aluminum electrode that was changed into a copper electrode at various solution temperatures and immersion times, an electron microscope was used, as shown in Figure 5. At 40 °C, it is evident that the copper grains have insufficient copper deposition and have a porous structure due to slow reaction rate and insufficient immersion time. In contrast to a short immersion duration, the density of the copper grains tended to grow when the immersion time was extended above 30 minutes for 60 and 120 minutes. However, copper deposition increased, but the porousness decreased. At higher temperatures and shorter immersion times, copper electrode surface densities are comparable to those obtained at 40°C of prolonged immersion. At solution temperatures of 60 °C and 80 °C, the microstructures of the copper electrodes are almost identical. Conversely, the surface features a denser copper grain surface with a better porosity structure at high temperatures and prolonged immersion times. This result is consistent with the concentration of Al+3 ions analyzed by using ICP-MS in oxidation reaction, which indicates that the reduction-oxidation displacement reaction in the solution were undergoing at the same time.

Fig. 6: Cross-section images of thick film Cu substituted for Al at different temperature and soaking time, Tex=40℃, (a) 5 min, (b) 30 min, (c) 60 min,(d) 120 min, Tex=60℃, (e) 5 min, (f) 30 min, (g) 60 min,(h) 120 min, Tex=80℃, (i) 5 min, (j) 30 min, (k) 60 min,(l) 120 min

Fig. 7: Thickness of thick film Cu substituted for Al at different solution temperature as function of soaking time
Figure 6 shows the cross-sectional microstructure of the Al electrode was altered by replacing it with Cu through a galvanic displacement reaction at varying solution temperatures and immersion times. In comparison with Figure 5 and Figure 6, the result of cross section also proves at lower temperatures with short immersion times, but the Al electrode itself had no impact by copper. However, at higher temperatures compared to lower temperature, the density of the Cu-replaced Al electrode in the upper layer increased significantly. Using a lowtemperature solution (40 °C) and maintaining a prolonged immersion duration (120 minutes), a thick Cu electrode in the Al film is generated. On the other hand, copper deposition was feasible with shorter immersion times and higher solution temperatures (60°C and 80°C), although this method led to grain agglomeration. The bonding between the Al film and the copper electrode may be impacted by the rapid reduction rate of copper ions under high temperatures, which leads to the quick deposition of partial clusters of copper grains.

Fig. 8: Microstructure of thick film Cu substituted for Al at different solution temperatures for 30 min soaking time (a) Tex=40℃, (b) Tex=60℃, and (c) Tex=80℃

Fig. 9: Microstructure of thick film Cu substituted for Al at a Tex=80℃ solution temperature for different soaking times (a) 5 min, (b) 30 min, and (c) 60 min
Figure 7 illustrates that the thickness of the copper electrode exhibits rapid growth with a very short immersion time when the temperature is high (80 °C). Conversely, achieving a thicker copper electrode requires a longer immersion time of 60 minutes at a lower temperature of 40 °C. Evidently, the thickness of the copper electrode obtained at a higher solution temperature for a short time is comparable to that achieved at a lower solution temperature for a longer time. However, it’s important to note that their microstructure differs significantly. The microstructure of the copper electrode through the galvanic displacement reaction at 80 °C for 5 min is loose and agglomerated. In contrast, the microstructure of the copper electrode through the galvanic displacement reaction at 40 °C for 60 min is very dense and highly crystalline. The impact of the solution temperature on the microstructure after 30 minutes is depicted in Figure 8. The aggregation of the small crystal grains becomes worse as the temperature of the solution increases. When
the Al electrode is replaced with Cu through the galvanic displacement reaction in a high temperature CuSO4 solution, the displacement reaction rate is very quick. Thus, the soaking time is too short to align these crystalline grains and form an agglomerated microstructure [9]. Figure 9 shows the effect of immersion time at 80 °C on microstructure of Cu. With increasing immersion time for the Al electrode to be replaced with Cu through the galvanic displacement reaction, the grain size of Cu becomes larger [10]. Therefore, in the galvanic redox displacement reaction of the Al electrode replaced with Cu, the temperature of solution determined the degree of copper nucleation, and the immersion time determined the growth rate of the copper grains [11, 12]. As a result, the more effective approach is to employ a low-temperature solution with a longer immersion duration to improve the adhesion between the Al layer and copper deposition because it
ensures a constant and controlled pace of galvanic displacement reaction.
3.2.2.2 Effects of PEG additive in CuSO4 solution
According to the results previously described, just a little amount of the Al electrode is converted to a Cu electrode during the galvanic displacement reaction. The Cu electrode is meant to completely replace the Al electrode. According to previous studies [13], polyethylene glycols PEG can be used as a suppressor to significantly inhibit Cu+2 ion reduction in electrochemical copper plating. According to the impact on the kinetics of copper electrodeposition, we will discuss the PEG suppressor with various molecular weights [14] [15] and the combination of other organic additives as an accelerator and leveler. In this experiment, several molecular sizes were utilized and the most effective molecular weight of PEG 300O results were shown. The impact of extra molecular size results and performance, as well as the change in surface properties such roughness,
glossiness, preferred plane, and grain size, will also be comprehensively discussed in future works. Polyethylene glycol PEG was thus added to the copper sulfate solution to delay the reduction reaction of Cu+2 ions into Cu film [16], so that the Cu+2 ions in the solution could diffuse into the interior of the porous aluminum electrode film. When the Cu+2 ions are fully distributed in the porous Al electrode, the Al electrode could completely convert into a Cu electrode through the galvanic reduction-oxidation displacement reaction. Figure 10 shows the microstructure and the resistance of the Al electrode replaced with Cu through the galvanic displacement reaction in CuSO4(aq) with 1 wt% PEG for soaking time from 5 to 60 min. It can be clearly observed that as the immersion time is increased, the thick film Al electrode is gradually converted into a thick film Cu electrode. All the thick film Al electrodes are completely converted into thick film Cu electrodes after immersing for 60 minutes. Compared with the original thick-film aluminum electrode, the resistance of the thickfilm electrode was remarkably reduced due to the top surface of the Al electrode being replaced with Cu after immersing for 10 min. Then, the resistance decreased steadily when the immersion time is increased. The resistance of the thickfilm Al that is completely replaced with Cu after immersing for 60 min. reached a single-digit ohm, which is equivalent to that of a thick-film copper electrode sintered at high-temperature in nitrogen atmosphere [15][16].

Fig. 10: Microstructure and resistance for the galvanic displacement reaction of thick film Al electrode in CuSO4(aq) solution at 60°C in term of soaking times
The effects of the PEG additive on the galvanic reduction-oxidation displacement reaction on the Al electrode replaced with Cu in a CuSO4 solution is addressed based on the following two conditions:
(a) Pure copper sulfate solution:
Figure 11-a depicts the mechanism of the galvanic reduction-oxidation displacement reaction of the Al electrode replaced with Cu in a pure copper sulfate solution without adding PEG. The entire redox reaction is divided into four stages.
In the first stage, the thick film printed aluminum electrode is immersed in a solution of copper sulfate containing copper ions (Cu2+) and sulfate ions (SO42-). In the second stage, the thick film printed Al electrode begins to undergo an oxidation reaction to generate Al+3 ions and free electrons. In the third stage, the Cu+2 ions in the solution obtain the free electrons to reduce into metallic copper [17]. In the fourth stage, the upper layer of the thick film Al electrode is fully covered with the Cu electrode through the reduction reaction. The Cu+2 ion in the solution is isolated in the interior of the Al electrode, after which the galvanic reduction-oxidation displacement reaction is terminated.

Fig. 11.-a: Mechanism of the galvanic displacement reaction of the Al electrode with the Cu+2 ion in the CuSO4 solution without a suppressor 11-b Mechanism of the galvanic displacement reaction of the Al electrode with the Cu+2 ion in the CuSO4 solution with the PEG suppressor
(b) Pure copper sulfate solution with organic additives
Figure 11-b depicts the mechanism of galvanic reduction-oxidation displacement reaction of Al electrode replaced with Cu in a pure copper sulfate solution with PEG. The entire redox reaction is divided into five stages. In the first stage, the thick film printed Al electrode is immersed in a copper sulfate solution filled with copper ions, inhibitors, and sulfate radicals. In the second stage, the thick film printed with Al electrode begins to undergo oxidation reaction to generate Al+3 ions and free the electrons.
In the third stage, the reduction reaction of Cu+2 in the solution is inhibited due to the presence of the inhibitor (PEG) [18][19]. Therefore, part of the copper ions and free electrons diffuse and immerse into the interior of the porous, thick film Al electrode. In the fourth stage, Cu+2 ions in the solution or in the interior of the porous thick-film Al electrode simultaneously obtain free electrons and undergo a reduction reaction to form metallic copper. In the fifth step, the thick-film Al electrode is completely replaced with Cu electrode when the galvanic reduction-oxidation displacement reaction is terminated.
4 Conclusions
- A novel environmental electroless-plating technique that combines a thick film Al electrode by using screen printing and a galvanic displacement reaction has been successfully demonstrated to generate the electroless plating deposition of Cu and Ni films.
- In the galvanic displacement reaction, the solution temperature, soaking time and additive are found to play important roles in determining the quality of the electroless plating film. Solution temperature is closely related to degree of nucleation of Cu plating; the immersion time is closely related to the grain growth of Cu film and the suppressor of additive is closely related the conversion ratio of the Al electrode replaced with Cu+2. The performance of the thick film Cu electrode fabricated using the novel electroless plating method based on a CuSO4 solution with the addition of 1wt% PEG at 70 ℃ for 60 min is comparable to that of the conventional thick film Cu electrode fired in a reducing atmosphere at 800-900℃.
- A low-cost thick film Al electrode with high oxidation potential and a porous structure is suitable for using as a sacrificial layer to carry out the galvanic displacement reaction with other metal ions with high reduction potentials. That means that not only Cu and Ni films but also other metal films that cannot be made using the existing electroless plating technique can be fabricated using the novel environmental electroless-plating technique proposed in this work.
Author Contributions
All listed authors have made a substantial, direct and intellectual contribution to the work and approved it for publication.
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