Copper electroplating solution suitable for filling copper in through hole of printed circuit board and electroplating method

Abstract

The invention discloses a copper electroplating solution suitable for filling copper in a through hole of a printed circuit board and an electroplating method, and belongs to the technical field of electroplating. The invention discloses a copper electroplating solution suitable for filling copper into a through hole of a printed circuit board. The copper electroplating solution is prepared from copper sulfate pentahydrate, sulfuric acid, halide ions and a combined additive, through the synergistic effect of the alkaline blue 1 and the 5 '-adenine nucleotide disodium salt in the composite leveling agent, the through hole copper plating filling capacity can be achieved, the uniformity of a copper interconnection plating layer is effectively improved, and the interconnection structure quality is improved. The method comprises the following steps: firstly, pretreating the printed circuit board with the through hole, immersing the pretreated printed circuit board with the through hole into a copper electroplating solution suitable for copper filling of the through hole of the printed circuit board, and electroplating the through hole of the printed circuit board with the through hole for a period of time by adopting a one-way pulse electroplating process, the electroplated printed circuit board is obtained; the composite leveling agent is matched with a pulse process, so that the uniformity of a plating layer is relatively good, the thickness of surface copper in the through hole is low, and the TP value is remarkably improved.

Description

Technical Field

[0001] The invention belongs to the technical field of electroplating, and in particular relates to a copper electroplating solution and an electroplating method suitable for filling copper in through holes of a printed circuit board. Background Art

[0002] In the technological wave of the 21st century, aerospace, communications, industrial manufacturing, energy and other fields have put forward more stringent requirements for the miniaturization, functionalization and high integration of electronic products. As the heart of electronic products, printed circuit boards (PCBs) carry the interconnection layout of electronic components. In the manufacturing process of PCBs, electronic electroplating technology is currently the most mature and reliable micro-nanoscale electronic component interconnection and forming process, among which acidic copper sulfate aqueous solution is widely used as a commercial electroplating solution system due to its maturity. However, in the PCB electroplating process, the scale effect caused by the complex pore structure causes kinetic limitations on the diffusion and mass transfer of local copper ions under the action of the electric field. With the extension of electroplating time, the intensification of concentration polarization effect leads to a lower copper deposition rate in the pore than outside the pore. At the same time, the tip effect of the pore edge structure will induce electrochemical polarization changes, both of which will directly affect the density and uniformity of the coating. These problems have a serious negative impact on the quality of PCB copper interconnection. Therefore, regulating the electroplated copper growth process and optimizing the coating quality have become the key to ensuring the practical performance of PCBs. Upgrading the electroplating process, such as adding functional plating solution additives, switching the electroplating mode and adjusting the physical conditions of the electroplating, are all feasible technical approaches.

[0003] In the existing PCB copper electroplating process, a variety of organic compounds are usually selected as accelerators, inhibitors and levelers, respectively, and the three are used in a compounding manner to trigger synergistic and antagonistic effects, ensuring that a difference in plating speed is generated at the hole positions of different geometric structures, and finally achieving the copper deposition target from the inside out or from the bottom up. Brightening promoters, also known as accelerators, usually refer to organic molecules containing sulfonate groups and thiol groups. These molecules are small in size and their function is to accelerate the formation of the copper layer. On the other hand, high molecular weight oxygen-containing compounds such as polyethylene glycol and polypropylene glycol, which are used as grain refiners, play a role in slowing down the crystallization rate of copper during the electroplating process. In addition, leveling agents, also known as leveling agents or second-class inhibitors, are mainly composed of nitrogen-containing organic substances with larger molecular weights. These substances tend to preferentially adsorb on the protruding parts and current-intensive areas of the cathode due to their positive charge characteristics during the electroplating process, thereby inhibiting the deposition of copper at the hole mouth, thereby achieving the purpose of leveling. Based on the functional effect produced by the compounding of the three additives, the plating speed can be accurately controlled to make copper grow from the inside out in the micropores, and finally meet the practical needs of copper interconnects. The levelers used in the current mature production processes are generally single components. However, due to the substitution adsorption mechanism between inhibitors, accelerators and levelers, the function of a single leveler cannot be maximized, so the ability to regulate the growth behavior of high aspect ratio through-hole copper plating is very limited. At the same time, the cathode polarization caused by the DC plating mode in the traditional manufacturing process causes excessive copper ion mass transfer rate differences at different positions of the hole, which also has a negative impact on the function of the additive, so that the quality of the coating prepared under high current conditions cannot meet practical needs. In summary, the development of electroplating leveler compound components that can achieve high-quality copper interconnection and the reasonable optimization of electroplating process conditions are not only of great significance to the development of semiconductor manufacturing technology, but also show great application potential and market prospects in the future electronics manufacturing industry. Summary of the invention

[0004] In order to solve the above-mentioned problems, the present invention provides a copper electroplating solution and an electroplating method suitable for filling copper in through holes of a printed circuit board.

[0005] A copper electroplating solution suitable for copper filling of through holes in printed circuit boards, prepared from copper sulfate pentahydrate, sulfuric acid, halogen ions and combined additives;

[0006] The combined additives include a composite leveler, an accelerator and an inhibitor;

[0007] The copper electroplating solution suitable for copper filling of through-holes in printed circuit boards has a concentration of copper sulfate pentahydrate of 75 g / L, a concentration of sulfuric acid of 240 g / L, a concentration of halogen ions of 60 mg / L, a concentration of a composite leveler of 30 mg / L to 300 mg / L, a concentration of an accelerator of 2 mg / L to 3 mg / L, and a concentration of an inhibitor of 100 mg / L to 750 mg / L.

[0008] The method of electroplating using a copper electroplating solution suitable for copper filling of through holes of a printed circuit board is specifically completed in the following steps:

[0009] 1. Pre-processing of printed circuit boards with through holes:

[0010] First, immerse the printed circuit board containing through holes in a degreasing liquid at a temperature of 70°C to 90°C for 5min to 10min, take it out and wash it with water at room temperature for 1min to 3min, then immerse it in acetone for 3min to 8min, take it out and wash it with water at room temperature for 1min to 3min, then immerse it in sulfuric acid with a mass fraction of 10% for 5min to 8min, take it out and wash it with water at room temperature for 1min to 3min, immerse it in water for 1min to 3min, then take it out and wash it with water at room temperature for 1min to 3min, to obtain a printed circuit board containing through holes after pretreatment;

[0011] 2. Immersing the pre-treated printed circuit board with through holes into a copper electroplating solution suitable for copper filling of through holes of the printed circuit board, electroplating the through holes of the printed circuit board with through holes for a period of time using a unidirectional pulse electroplating process to obtain a printed circuit board after electroplating;

[0012] The cathode current of the unidirectional pulse electroplating process described in step 2 is set to 1.5ADS to 4.5ADS, and the pulse peak current density is 2.5A / dm 2 ~4.5A / dm 2 , pulse time 15ms~30ms, duty cycle 40%~60%, electroplating time 45min~80min.

[0013] Principle of the present invention:

[0014] The present invention utilizes the synergistic effect of basic blue 1 and 5'-adenine nucleotide disodium salt in the composite leveler to achieve through-hole copper plating filling capability, effectively improve the uniformity of the copper interconnection plating layer, and enhance the quality of the interconnection structure.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] 1. The present invention is suitable for high current working conditions, effectively shortens the actual production time of PCB metal interconnection, and reduces manufacturing costs;

[0017] Second, the present invention is applicable to the electroplating of through holes of printed circuit boards;

[0018] 3. The composite leveler in the present invention is combined with the pulse process to achieve better uniformity of the coating, lower copper thickness in the through hole, and significantly improved TP value. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The molecular structures of Basic Blue 1 and 5'-adenine nucleotide disodium salt used in the present invention;

[0020] Figure 2 The three-dimensional microscope image of the copper plating layer after through-hole electroplating of the printed circuit board, in which a is the control example 1, b is the control example 2, and c is the embodiment 1;

[0021] Figure 3 The metallographic slice diagram of the cross section of the copper plating layer after through-hole electroplating of the printed circuit board, (a) is the comparative example 3, (b) is the embodiment 1, and (b) is the comparative example 2;

[0022] Figure 4 The metallographic slices are of the cross-section of the copper plating layer after through-hole electroplating of the printed circuit board, wherein (a) is Example 2 and (b) is Example 3. DETAILED DESCRIPTION

[0023] Specific implementation method 1: This implementation method is a copper electroplating solution suitable for copper filling of through holes in printed circuit boards, which is prepared from copper sulfate pentahydrate, sulfuric acid, halogen ions and combined additives;

[0024] The combined additives include a composite leveler, an accelerator and an inhibitor;

[0025] The copper electroplating solution suitable for copper filling of through-holes in printed circuit boards has a concentration of copper sulfate pentahydrate of 75 g / L, a concentration of sulfuric acid of 240 g / L, a concentration of halogen ions of 60 mg / L, a concentration of a composite leveler of 30 mg / L to 300 mg / L, a concentration of an accelerator of 2 mg / L to 3 mg / L, and a concentration of an inhibitor of 100 mg / L to 750 mg / L.

[0026] Specific embodiment 2: This embodiment differs from specific embodiment 1 in that the accelerator is sodium dodecyl sulfonate, sodium 3-mercapto-1-propane sulfonate, sodium 2,3-dimercaptopropane sulfonate, 1,3-propanedithiol, sodium thiazoline dithiopropane sulfonate or sodium N,N-dimethyldithiocarboxamide propane sulfonate. The other steps are the same as those of specific embodiment 1.

[0027] Specific implementation method 3: This implementation method is different from specific implementation method 1 or 2 in that the inhibitor is polyethylene glycol. The other steps are the same as those of specific implementation method 1 or 2.

[0028] Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that the molecular weight of the polyethylene glycol is 8000 to 20000. The other steps are the same as those of specific embodiments 1 to 3.

[0029] Specific embodiment 5: This embodiment differs from specific embodiments 1 to 4 in that the composite leveler is a mixture of basic blue 1 and 5'-adenine nucleotide disodium salt, wherein the mass ratio of basic blue 1 to 5'-adenine nucleotide disodium salt is 120:30. The other steps are the same as those of specific embodiments 1 to 4.

[0030] Specific implementation method 6: This implementation method is different from specific implementation methods 1 to 5 in that the mass fraction of the sulfuric acid is 98% and the halogen ions are chloride ions. The other steps are the same as those of specific implementation methods 1 to 5.

[0031] Specific embodiment 7: This embodiment differs from specific embodiments 1 to 6 in that the concentration of copper sulfate pentahydrate in the electroplating copper solution suitable for copper filling of through holes of printed circuit boards is 75 g / L, the concentration of sulfuric acid is 240 g / L, the concentration of halogen ions is 60 mg / L, the concentration of the composite leveler is 50 mg / L to 150 mg / L, the concentration of the accelerator is 2 mg / L to 3 mg / L, and the concentration of the inhibitor is 200 mg / L to 500 mg / L. The other steps are the same as those of specific embodiments 1 to 6.

[0032] Specific embodiment eight: This embodiment is a method for electroplating using a copper electroplating solution suitable for filling copper through holes in printed circuit boards, which is specifically completed in the following steps:

[0033] 1. Pre-processing of printed circuit boards with through holes:

[0034] First, immerse the printed circuit board containing through holes in a degreasing liquid at a temperature of 70°C to 90°C for 5min to 10min, take it out and wash it with water at room temperature for 1min to 3min, then immerse it in acetone for 3min to 8min, take it out and wash it with water at room temperature for 1min to 3min, then immerse it in sulfuric acid with a mass fraction of 10% for 5min to 8min, take it out and wash it with water at room temperature for 1min to 3min, immerse it in water for 1min to 3min, then take it out and wash it with water at room temperature for 1min to 3min, to obtain a printed circuit board containing through holes after pretreatment;

[0035] 2. Immersing the pre-treated printed circuit board with through holes into a copper electroplating solution suitable for copper filling of through holes of the printed circuit board, electroplating the through holes of the printed circuit board with through holes for a period of time using a unidirectional pulse electroplating process to obtain a printed circuit board after electroplating;

[0036] The cathode current of the unidirectional pulse electroplating process described in step 2 is set to 1.5ADS to 4.5ADS, and the pulse peak current density is 2.5A / dm 2 ~4.5A / dm 2 , pulse time 15ms~30ms, duty cycle 40%~60%, electroplating time 45min~80min.

[0037] Specific embodiment 9: The difference between this embodiment and specific embodiments 1 to 8 is that the degreasing liquid described in step 1 is a mixture of NaOH, Na2SiO3·9H2O, Na3PO4·12H2O, Na2CO3 and water, wherein the concentration of NaOH is 0.2 mol·L -1 ~0.3mol·L -1 , the concentration of Na2SiO3·9H2O is 0.03 mol·L -1 ~0.05mol·L -1 , the concentration of Na3PO4·12H2O is 0.1 mol·L -1 ~0.2mol·L -1 , the concentration of Na2CO3 is 0.40 mol·L -1 ~0.60mol·L -1 The other steps are the same as those in Specific Embodiments 1 to 8.

[0038] Specific embodiment 10: This embodiment differs from specific embodiments 1 to 9 in that the size of the printed circuit board with through holes described in step 1 is 10 cm×5 cm, the diameter of the through holes is 300 μm, and the aspect ratio is 10:1; the capacity of the Haring cell used for electroplating in step 2 is 1.5 liters. The other steps are the same as those of specific embodiments 1 to 9.

[0039] The following examples are used to verify the beneficial effects of the present invention:

[0040] Embodiment 1: A copper electroplating solution suitable for copper filling of through holes in printed circuit boards, prepared from copper sulfate pentahydrate, sulfuric acid, halogen ions and a combination of additives;

[0041] The combined additives include a composite leveler, an accelerator and an inhibitor;

[0042] The accelerator is sodium dodecyl sulfate;

[0043] The inhibitor is polyethylene glycol with a molecular weight of 10,000;

[0044] The composite leveler is a mixture of basic blue 1 and 5'-adenine nucleotide disodium salt, wherein the mass ratio of basic blue 1 to 5'-adenine nucleotide disodium salt is 120:30;

[0045] The mass fraction of the sulfuric acid is 98%; the halogen ions are chloride ions;

[0046] The copper electroplating solution suitable for copper filling of through-holes in printed circuit boards has a concentration of 75 g / L of copper sulfate pentahydrate, 240 g / L of sulfuric acid, 60 mg / L of halogen ions, 150 mg / L of composite leveling agent, 2 mg / L of accelerator, and 500 mg / L of inhibitor.

[0047] The method of electroplating using a copper electroplating solution suitable for copper filling of through holes of a printed circuit board is specifically completed in the following steps:

[0048] 1. Pre-processing of printed circuit boards with through holes:

[0049] First, immerse the printed circuit board with through holes in a degreasing liquid at a temperature of 80°C for 5 minutes, take it out and wash it with water at room temperature for 1 minute, immerse it in acetone for 5 minutes, take it out and wash it with water at room temperature for 1 minute, immerse it in sulfuric acid with a mass fraction of 10% for 5 minutes, take it out and wash it with water at room temperature for 1 minute, immerse it in water for 1 minute, take it out and wash it with water at room temperature for 1 minute, and obtain a printed circuit board with through holes after pretreatment;

[0050] The size of the printed circuit board with through holes described in step 1 is 10 cm×5 cm, the diameter of the through holes is 300 μm, and the aspect ratio is 10:1;

[0051] The degreasing solution described in step 1 is a mixture of NaOH, Na2SiO3·9H2O, Na3PO4·12H2O, Na2CO3 and water, wherein the concentration of NaOH is 0.25 mol·L -1 , the concentration of Na2SiO3·9H2O is 0.04 mol·L -1 , the concentration of Na3PO4·12H2O is 0.15 mol·L -1 , the concentration of Na2CO3 is 0.50 mol·L -1 ;

[0052] 2. Immerse the pretreated printed circuit board with through holes in a copper electroplating solution at a temperature of 25° C. suitable for copper filling of through holes of the printed circuit board, and electroplate the through holes of the printed circuit board with through holes for a period of time using a unidirectional pulse electroplating process to obtain a printed circuit board after electroplating;

[0053] The cathode current of the unidirectional pulse plating process described in step 2 is set to 3ADS, and the pulse peak current density is 3A / dm 2 , pulse time 30ms, duty cycle 40%, electroplating time 80min;

[0054] The capacity of the Haring cell used for electroplating in step 2 is 1.5 liters.

[0055] Example 2: This example is different from Example 1 in that the composite leveler is a mixture of basic blue 1 and 5'-adenine nucleotide disodium salt, wherein the mass ratio of basic blue 1 to 5'-adenine nucleotide disodium salt is 100:50. Other steps and parameters are the same as those in Example 1.

[0056] Example 3: This example is different from Example 1 in that the composite leveler is a mixture of basic blue 1 and 5'-adenine nucleotide disodium salt, wherein the mass ratio of basic blue 1 to 5'-adenine nucleotide disodium salt is 140:10. Other steps and parameters are the same as those in Example 1.

[0057] Comparative Example 1: This example is different from Example 1 in that no leveling agent is used, that is, the combined additive includes an accelerator and an inhibitor. Other steps and parameters are the same as those in Example 1.

[0058] Comparative Example 2: The difference between this example and Example 1 is that the combined additive includes a leveler, an accelerator and an inhibitor; the leveler is basic blue 1, and the concentration of the leveler is 150 mg / L. The other steps and parameters are the same as those in Example 1.

[0059] Comparative Example 3: The difference between this embodiment and embodiment 1 is that in step 2, the printed circuit board containing through holes after pretreatment is immersed in a copper electroplating solution suitable for copper filling of through holes of the printed circuit board, and the through holes of the printed circuit board containing through holes are electroplated for a period of time using a direct current electroplating process to obtain a printed circuit board after electroplating; the cathode current of the direct current electroplating process described in step 2 is set to 3ADS, and the electroplating time is 80min. The other steps and parameters are the same as those in embodiment 1.

[0060] Figure 2 The three-dimensional microscope image of the copper plating layer after through-hole electroplating of the printed circuit board, in which a is the control example 1, b is the control example 2, and c is the embodiment 1;

[0061] from Figure 2 It can be seen that the roughness of a and b is higher than that of c, indicating that compared with no leveling agent or using only basic blue 1 as a leveling agent, the copper plating layer using basic blue 1 and 5'-adenine nucleotide disodium salt as a leveling agent in Example 1 of the present invention has fine and smooth crystals and has excellent bright leveling ability.

[0062] Figure 3 The metallographic slice diagram of the cross section of the copper plating layer after through-hole electroplating of the printed circuit board, (a) is the comparative example 3, (b) is the embodiment 1, and (b) is the comparative example 2;

[0063] from Figure 3 It can be seen that: in the 300-micron through-hole, Example 1 achieved high-quality interconnection of the compound leveler under pulse current conditions, with a TP value of 95.8% and good plating uniformity. In contrast, the copper filling quality of the sample under the compound leveler + DC plating mode (Comparative Example 3) was poor, with an average surface copper thickness of 22.63 microns and a TP value of only 55.8%. The single leveler (Comparative Example 2) also did not work well under pulse plating conditions, with an average surface copper thickness of 15.28 microns and a TP value of only 66.8%. This shows that both the compound leveler and the pulse plating mode are necessary conditions for high-quality through-hole electroplating copper filling.

[0064] Figure 4 The metallographic slice diagram of the cross section of the copper plating layer after through-hole electroplating of the printed circuit board, (a) is Example 2, (b) is Example 3;

[0065] from Figure 4 It can be seen that after adjusting the concentration of the composite leveling agent, it can be seen that in the 300-micron through-hole, the copper filling quality of the sample in Example 2 is poor, the average surface copper thickness is 15.022 microns, and the TP value is only 57.7%; the copper filling quality of the sample in Example 3 is improved compared with that in Example 2, but the copper plating effect is still not good, the average surface copper thickness is 15.204 microns, and the TP value is 75.8%.

Claims

1. A copper electroplating solution suitable for copper filling of through holes in printed circuit boards, characterized in that The electroplating copper solution suitable for copper filling of through-holes in printed circuit boards is prepared from copper sulfate pentahydrate, sulfuric acid, halogen ions and a combination of additives; The combined additives include a composite leveler, an accelerator and an inhibitor; The copper electroplating solution suitable for copper filling of through-holes in printed circuit boards has a concentration of copper sulfate pentahydrate of 75 g / L, a concentration of sulfuric acid of 240 g / L, a concentration of halogen ions of 60 mg / L, a concentration of a composite leveler of 30 mg / L to 300 mg / L, a concentration of an accelerator of 2 mg / L to 3 mg / L, and a concentration of an inhibitor of 100 mg / L to 750 mg / L.

2. The copper electroplating solution suitable for copper filling of through holes in printed circuit boards according to claim 1, characterized in that The accelerator is sodium dodecyl sulfonate, sodium 3-mercapto-1-propane sulfonate, sodium 2,3-dimercaptopropane sulfonate, 1,3-propanedithiol, sodium thiazoline dithiopropane sulfonate or sodium N,N-dimethyldithiocarboxamide propane sulfonate.

3. The copper electroplating solution suitable for copper filling of through holes in printed circuit boards according to claim 1, characterized in that The inhibitor is polyethylene glycol.

4. The copper electroplating solution suitable for copper filling of through holes in printed circuit boards according to claim 3, characterized in that The molecular weight of the polyethylene glycol is 8000-20000.

5. The copper electroplating solution suitable for copper filling of through holes in printed circuit boards according to claim 1, characterized in that The composite leveler is a mixture of basic blue 1 and 5'-adenine nucleotide disodium salt, wherein the mass ratio of basic blue 1 to 5'-adenine nucleotide disodium salt is 120:

30.

6. The copper electroplating solution suitable for copper filling of through holes in printed circuit boards according to claim 1, characterized in that The mass fraction of the sulfuric acid is 98%; the halogen ions are chloride ions.

7. The copper electroplating solution suitable for copper filling of through holes in printed circuit boards according to claim 1, characterized in that The copper electroplating solution suitable for copper filling of through-holes in printed circuit boards has a concentration of copper sulfate pentahydrate of 75 g / L, a concentration of sulfuric acid of 240 g / L, a concentration of halogen ions of 60 mg / L, a concentration of a composite leveler of 50 mg / L to 150 mg / L, a concentration of an accelerator of 2 mg / L to 3 mg / L, and a concentration of an inhibitor of 200 mg / L to 500 mg / L.

8. A method for electroplating using the copper electroplating solution suitable for filling copper in through holes of printed circuit boards as claimed in any one of claims 1 to 7, characterized in that The method is specifically completed according to the following steps:

1. Pre-processing of printed circuit boards with through holes: First, immerse the printed circuit board containing through holes in a degreasing liquid at a temperature of 70°C to 90°C for 5min to 10min, take it out and wash it with water at room temperature for 1min to 3min, then immerse it in acetone for 3min to 8min, take it out and wash it with water at room temperature for 1min to 3min, then immerse it in sulfuric acid with a mass fraction of 10% for 5min to 8min, take it out and wash it with water at room temperature for 1min to 3min, immerse it in water for 1min to 3min, then take it out and wash it with water at room temperature for 1min to 3min, to obtain a printed circuit board containing through holes after pretreatment; 2. Immersing the pre-treated printed circuit board with through holes into a copper electroplating solution suitable for copper filling of through holes of the printed circuit board, electroplating the through holes of the printed circuit board with through holes for a period of time using a unidirectional pulse electroplating process to obtain a printed circuit board after electroplating; The cathode current of the unidirectional pulse electroplating process described in step 2 is set to 1.5ADS to 4.5ADS, and the pulse peak current density is 2.5A / dm 2 ~4.5A / dm 2 , pulse time 15ms~30ms, duty cycle 40%~60%, electroplating time 45min~80min.

9. The method for electroplating using a copper electroplating solution suitable for copper filling of through holes in printed circuit boards according to claim 8, characterized in that The degreasing liquid described in step 1 is a mixture of NaOH, Na2SiO3·9H2O, Na3PO4·12H2O, Na2CO3 and water, wherein the concentration of NaOH is 0.2 mol·L -1 ~0.3mol·L -1 , the concentration of Na2SiO3·9H2O is 0.03 mol·L -1 ~0.05mol·L -1 , the concentration of Na3PO4·12H2O is 0.1 mol·L -1 ~0.2mol·L -1 , the concentration of Na2CO3 is 0.40 mol·L -1 ~0.60mol·L -1 .

10. The method for electroplating using a copper electroplating solution suitable for filling copper in through holes of a printed circuit board according to claim 8, characterized in that The size of the printed circuit board with through holes described in step 1 is 10 cm×5 cm, the diameter of the through holes is 300 μm, and the aspect ratio is 10:1; the capacity of the Haring cell used for electroplating in step 2 is 1.5 liters.

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