A low silver consumption composite electroplating metallization method for TOPCon solar cells

Abstract

This invention discloses a low-silver-consumption composite electroplating method for metallizing TOPCon solar cells, comprising the following steps: providing a silicon wafer with a TOPCon structure, the silicon wafer being based on a semiconductor substrate; forming a silver paste layer on the surfaces of a second passivation layer B and a first passivation layer of the semiconductor substrate by screen printing, and after sintering, using the silver paste layer as a seed layer for subsequent electroplating; selectively electroplating copper using the silver paste layer as the seed layer, depositing a copper layer on the surface of the silver paste layer; and electroplating a tin layer on the surface of the copper layer. This invention forms a narrow-linewidth, low-height silver paste seed layer through high-mesh screen printing, and then achieves grid line thickening through copper plating, reducing silver paste consumption by 40-60%; the electroplated copper layer has a higher conductivity than the silver paste, reducing series resistance and increasing the fill factor; the narrow width of the silver paste layer results in a smaller corrosion area on the passivation layer, thus improving the open-circuit voltage.

Description

Technical Field

[0001] This invention relates to the field of solar cell technology, and specifically to a method for metallizing TOPCon solar cells using low-silver-loss composite electroplating. Background Technology

[0002] With the rapid development of the global photovoltaic industry, crystalline silicon solar cell technology is undergoing a comprehensive transformation from PERC technology to Topcon technology. Topcon technology, with its tunneling oxide passivated contact structure, exhibits significant advantages in carrier collection efficiency, open-circuit voltage, and overall conversion efficiency. Its mass-produced conversion efficiency has already exceeded 26%, and it is expected to capture up to 60% of the crystalline silicon photovoltaic market share within the next decade. However, the metallization process has become a key bottleneck restricting the cost competitiveness and further development of Topcon technology.

[0003] Currently, metallization in Topcon solar cells commonly employs screen-printed silver paste technology. While this technology is relatively mature and has low equipment costs, it suffers from several inherent drawbacks. Firstly, silver, the second most expensive material in photovoltaic cells, has seen its price rise from 3.87 yuan / gram in 2017 to 20.62 yuan / gram in January 2026, an increase of 388%. Silver paste accounts for 40-50% of the cell manufacturing cost and 20-30% of the total cost, becoming a major obstacle to cost reduction. Secondly, the global silver supply-demand imbalance is increasingly prominent. In 2025, supply is estimated at approximately 32,472 tons, while demand is projected at 35,499 tons, resulting in a shortfall of approximately 3,027 tons. Furthermore, silver mining production is experiencing sluggish growth, and the resource security period has been shortened to 24-25 years, posing a severe challenge to supply chain stability. Additionally, silver paste, due to the presence of glass and organic components, has a bulk resistivity of 10⁻⁶. -6 -10 -4 Ω·m, contact resistance with silicon is 0.3-5Ω·cm 2 Its performance is far inferior to that of pure silver, and its electrical conductivity is significantly limited.

[0004] To address the cost pressures of silver paste technology, the industry has explored various alternatives. Some technologies employ laser-grooved silicon nitride followed by direct deposition of a seed layer on the silicon substrate. However, the laser can cause irreparable damage to the alumina passivation layer and the silicon substrate, leading to open-circuit voltage drops and EL darkening. Another technology uses PVD sputtering to prepare a nickel seed layer, but this method is expensive, complex, and susceptible to the "line-of-sight effect," making it difficult to achieve uniform and continuous coverage in the complex textured surface and deep, narrow trenches of Topcon cells. The density of the deposited film is also insufficient. These technological solutions, either due to performance degradation or excessive cost, have failed to effectively solve the core problem of metallization in Topcon cells.

[0005] Therefore, there is an urgent need to develop a low-cost metallization technology that can significantly reduce the amount of silver paste used, maintain good conductivity and reliability, and avoid damage to the battery passivation layer and substrate. Summary of the Invention

[0006] In view of the problems existing in the background technology, the present invention provides a metallization method for TOPCon solar cells with low silver consumption composite electroplating, so as to reduce the amount of silver paste consumed, reduce manufacturing costs, and at the same time ensure good electrical performance and reliability.

[0007] This invention is implemented through the following technical solutions: This invention discloses a method for metallizing TOPCon solar cells using low-silver-loss composite electroplating, comprising the following steps: S1. A silicon wafer having a TOPCon structure is provided, the silicon wafer being based on a semiconductor substrate, a second conductivity type semiconductor region being disposed on one side of the semiconductor substrate, a second passivation layer A being disposed on the side of the second conductivity type semiconductor region away from the semiconductor substrate, a second passivation layer B being disposed on the side of the second passivation layer A away from the second conductivity type semiconductor region, a tunneling layer being disposed on the side of the semiconductor substrate away from the second conductivity type semiconductor region, a first conductivity type semiconductor region being disposed on the side of the tunneling layer away from the ultrathin amorphous silicon layer, and a first passivation layer being disposed on the side of the first conductivity type semiconductor region away from the tunneling layer; S2. A silver paste layer is formed on the surfaces of the second passivation layer B and the first passivation layer by screen printing. After sintering, the silver paste layer serves as a seed layer for subsequent electroplating. S3. Using the silver paste layer as a seed layer, selectively electroplate copper to deposit a copper layer on the surface of the silver paste layer; S4. Electroplating a tin layer onto the surface of the copper layer.

[0008] Furthermore, in step S1, the second passivation layer A is an aluminum oxide layer; The second passivation layer B and the first passivation layer are single-layer films selected from silicon nitride film, hydrogen-containing silicon nitride film, silicon oxide film, silicon oxynitride film, magnesium fluoride MgF2, zinc sulfide ZnS, titanium dioxide TiO2 and cerium oxide CeO2, or multilayer film structures composed of at least two layers of the above materials. The first type of conductivity semiconductor region is doped with a Group 5 element, including at least one of phosphorus, arsenic, antimony or bismuth; The semiconductor region of the second conductivity type is doped with a third group element, including at least one of boron, aluminum, gallium or indium; The tunneling layer is any one of oxide, nitride, semiconductor or conductive polymer.

[0009] Furthermore, in step S2, the screen printing uses a high mesh count screen, with a mesh count of 600-700, a film thickness of 5-10μm, and a screen line width of 5-12μm.

[0010] Furthermore, in step S2, the screen printing uses a steel plate screen with a wire diameter of 5-10 μm and a thickness of 15-25 μm.

[0011] Furthermore, in step S2, the linewidth of the silver paste layer formed on the front side of the silicon wafer is 15-20 μm, and the height is 1-3 μm; the linewidth of the silver paste layer formed on the back side of the silicon wafer is 20-30 μm, and the height is 1-2 μm; both sides have a lower linewidth and height than conventional printed lines to obtain lower silver paste consumption; at the same time, the line type obtained by steel plate printing is better.

[0012] Further, in step S3, the electroplating solution for electroplating copper contains 150-300 g / L copper sulfate pentahydrate, 10-50 g / L sulfuric acid and additives, the electroplating current is 100-500 mA, and the electroplating time is 180-360 s; based on the principle of minimum specific surface area and minimum surface energy, the electroplating limit is a semi-circle (height-to-width ratio of 1:2).

[0013] Furthermore, in step S3, the electroplating of copper uses a phosphorus copper anode or an insoluble anode, and copper phosphide can promote the oxidation of cuprous ions into copper ions.

[0014] Furthermore, in step S4, the electroplating solution for tin plating is either a sulfuric acid system or a methanesulfonic acid system. The sulfuric acid electroplating solution contains 20-40 g / L stannous sulfate solution, 80-150 mL / L sulfuric acid, and additives. The methanesulfonic acid system comprises 80-180 g / L of methanesulfonic acid, 80-160 g / L of stannous methanesulfonate, 5-30 g / L of bismuth methanesulfonate, additives, and precipitants.

[0015] Further, the additive comprises: a softener of 20-50 mL / L, at least one of an aromatic sulfonate or polyether derivative, used to refine grains, reduce internal stress, and inhibit tin whiskers; a brightener of 1-10 mL / L, at least one of a heterocyclic compound or aldehyde derivative, used to level the plating layer and improve surface brightness and density; and a stabilizer of 0.5-5 g / L, at least one of phenolsulfonic acid or ascorbic acid, used to inhibit Sn. 2+ Oxidized to Sn 4+ Stabilizes the plating solution; wetting agent 0.1-1.0g / L, a nonionic surfactant, used to reduce surface tension and eliminate pinholes and pitting; The precipitant is at least one of the following: a mixture of 0.05-0.5 g / L of an acrylic acid derivative and 0.01-0.2 g / L of sodium dodecyl sulfate as an accelerator; 0.03-0.3 g / L of anisaldehyde dimethyl acetal; or 1-10 ml / L of HN100 as a settling agent.

[0016] Divalent tin has a standard electrode potential of E° = -0.136V, is a strong reducing agent, and preferentially reacts with oxygen (4Sn). 2+ +O2+4H + →4Sn 4+ +2H2O), protecting the copper grid lines from oxidation; the methanesulfonic acid system includes acid concentrate (adjusting pH value), tin concentrate (providing tin source), bismuth concentrate (lowering the melting point of tin to facilitate soldering and prevent tin contamination), additives (improving cathode polarization, making the deposition process of tin ions more controllable, thereby obtaining a finely crystalline plating layer and avoiding defects such as roughness, looseness or dullness) and precipitant (in the long-term use of electroplating solution, divalent tin will be oxidized to tetravalent tin, tetravalent tin is easily hydrolyzed to generate insoluble metastannic acid colloid. These suspended colloidal particles will cause a series of quality problems such as pinholes, roughness, and yellowing of the plating layer. The precipitant, through its special components, can quickly capture and neutralize these negatively charged colloidal particles and suspended impurities).

[0017] Furthermore, the electroplating processes in steps S3 and S4 employ photo-induced electroplating.

[0018] The beneficial effects of this invention are: 1. This invention utilizes high-mesh screen printing or steel plate screen printing to form a low-height, narrow-linewidth silver paste layer as a seed layer, significantly reducing silver paste usage by 40-60%, thereby substantially lowering manufacturing costs and reducing reliance on silver resources. Electroplated copper is used to form the main conductive layer. Copper's conductivity is close to that of silver, and the electroplated copper layer has a dense crystalline structure with a volume resistivity far lower than that of silver paste, effectively reducing the series resistance of the gate lines. A tin layer is electroplated on the copper layer surface as a protective layer. Tin preferentially reacts with oxygen to prevent copper oxidation and also has good solderability, ensuring the welding quality and long-term reliability of the component packaging.

[0019] 2. This invention uses silver paste to directly print onto the surface of a silicon nitride layer. During sintering, the glass powder in the silver paste penetrates the silicon nitride layer and forms contact with the silicon substrate, eliminating the need for laser grooving and avoiding damage to the passivation layer and silicon substrate caused by laser, thus preventing a drop in open-circuit voltage. The screen printing process is compatible with existing production line equipment, requiring only the addition of electroplating equipment, resulting in a significantly lower equipment investment cost than PVD sputtering equipment. The electroplating process uses photo-induced electroplating to provide additional driving force, resulting in better uniformity and density of the coating. This invention significantly reduces the manufacturing cost of TOPCon solar cells while maintaining or even improving electrical performance, and has promising prospects for industrial application. Detailed Implementation

[0020] The technical solution of the present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0021] Example 1: This example provides a method for metallizing TOPCon solar cells using low-silver-loss composite electroplating, including the following steps: S1. An N-type silicon wafer with a TOPCon structure is provided: an N-type silicon wafer with dimensions of 210×210mm and a silicon substrate thickness of 130μm is selected. The front side of the N-type silicon wafer has a second conductivity type semiconductor region (boron diffusion layer), a second passivation layer A (alumina layer), and a second passivation layer B (silicon nitride antireflection layer) in sequence, and the back side has a tunneling layer (tunneling oxide layer), a first conductivity type semiconductor region (polysilicon layer), and a first passivation layer (silicon nitride layer) in sequence. The thickness of the silicon nitride layer on both the front and back sides is 70nm.

[0022] S2. Screen Printing to Form a Seed Layer of Silver Paste: High-mesh PI screens are used for screen printing. The front screen has a mesh count of 650, a film thickness of 8μm, and a screen linewidth of 10μm, resulting in a silver paste layer with a linewidth of 18μm and a height of 2μm. The back screen has a mesh count of 650, a film thickness of 8μm, and a screen linewidth of 12μm, resulting in a silver paste layer with a linewidth of 25μm and a height of 1.5μm. After printing, a sintering process is performed to allow the glass powder in the silver paste to penetrate the silicon nitride layer and form an ohmic contact with the silicon substrate. This silver paste layer serves as a seed layer for subsequent electroplating.

[0023] S3. Selective Copper Electroplating: Selective copper electroplating is performed using the silver paste layer as a seed layer. The electroplating solution contains 200 g / L copper sulfate pentahydrate, 30 g / L sulfuric acid, and additives. Copper ions are provided by a phosphorus copper anode. The electroplating current is 300 mA, and the electroplating time is 270 s. Photo-induced electroplating is used in the electroplating process to overcome insufficient driving force caused by insufficient potential difference. Based on the principle of minimum specific surface area and minimum surface energy, the electroplated copper layer has a semi-circular cross-section (height-to-width ratio of approximately 1:2), forming a dense crystalline copper layer.

[0024] S4. Tin Plating Protective Layer: A tin layer is electroplated onto the surface of the copper layer. The plating solution contains 120 g / L methanesulfonic acid, 120 g / L stannous methanesulfonate, 15 g / L bismuth methanesulfonate, additives, and a precipitant. The additives include a brightener, a dispersant, and a stabilizer. The brightener is an aldehyde compound, the dispersant is a nonionic surfactant, and the stabilizer is a phenol sulfonic acid derivative. The precipitant is an acrylic acid derivative (used to capture and neutralize the stannic acid colloidal particles generated by the hydrolysis of tetravalent tin). The electroplating process also employs photo-induced electroplating.

[0025] Comparative Example 1: This comparative example uses a conventional screen printing silver paste process. The battery size is 210mm × 210mm, and includes the following steps: S1. Provides an N-type silicon wafer with a TOPCon structure, the silicon substrate having a thickness of 130 μm. The front side of the silicon wafer has a second conductivity type semiconductor region (boron diffusion layer), a second passivation layer A (alumina layer), and a second passivation layer B (silicon nitride antireflection layer) in sequence, and the back side has a tunneling layer (tunneling oxide layer), a first conductivity type semiconductor region (polysilicon layer), and a first passivation layer (silicon nitride layer) in sequence. The thickness of the first / second passivation layer on both the front and back sides is 70 nm.

[0026] S2. Screen printing is performed using conventional screen printing plates. The front screen has a mesh count of 600 and a film thickness of 15μm, resulting in a silver paste layer with a linewidth of 30μm and a height of 12μm. The back screen has a mesh count of 600 and a film thickness of 20μm, resulting in a silver paste layer with a linewidth of 40μm and a height of 10μm. Sintering is then performed after printing.

[0027] This comparative example does not perform copper and tin electroplating steps, and directly uses silver paste grid lines as metallization electrodes.

[0028] The TOPCon cells prepared in Example 1 and Comparative Example 1 were tested using an IV tester (test parameters included: photoelectric conversion efficiency Eta, open circuit voltage Uoc, short circuit current Isc, current density Jsc, fill factor FF, series resistance Rs, parallel resistance Rsh, and leakage current IRev2 under 12V reverse voltage). Each group was tested twice, and the results of the two tests are shown in Table 1.

[0029] Table 1 Performance test results of Example 1 and Comparative Example 1 Comparative Example 1-1 26.04 0.7290 18.254 41.395 86.28 0.66 855 0.0868 Comparative Examples 1-2 26.07 0.7297 18.250 41.388 86.33 0.64 839 0.0781 Example 1-1 26.07 0.7295 18.224 41.228 86.47 0.61 192 0.1034 Examples 1-2 26.11 0.7301 18.218 41.203 86.56 0.61 200 0.1217 Difference 1 0.03 0.0005 -0.030 -0.167 0.20 -0.04 -664 0.0166 Difference 2 0.04 0.0004 -0.032 -0.185 0.23 -0.03 -639 0.0436 The test results show that: 1. Improved conversion efficiency (Eta): The conversion efficiency of Example 1 is 0.03-0.04% higher than that of Comparative Example 1, indicating that the composite electroplating process of the present invention can effectively improve the photoelectric conversion performance of the battery.

[0030] 2. Slightly increased open-circuit voltage (Uoc): The open-circuit voltage of Example 1 is approximately 0.0004-0.0005V higher than that of Comparative Example 1. This is because the present invention uses a narrow linewidth silver paste layer, which reduces the corrosion area of ​​the passivation region by the silver paste, thus better maintaining the passivation effect and thereby increasing the open-circuit voltage.

[0031] 3. Significantly improved fill factor (FF): The fill factor of Example 1 was 0.20-0.23% higher than that of Comparative Example 1. This is because the copper layer formed by electroplating has a dense crystalline copper structure, which has a much higher conductivity than silver paste and a significantly lower bulk resistivity; at the same time, the copper layer and the silver paste seed layer form a dense surface contact, which greatly reduces the contact resistivity, thereby improving the fill factor.

[0032] 4. Reduced series resistance (Rs): The series resistance of Example 1 was reduced by 0.03-0.04 mΩ compared with Comparative Example 1, further verifying that the electroplated copper layer has excellent conductivity.

[0033] 5. Short-circuit current (Isc) is slightly reduced: The short-circuit current in Example 1 is reduced by approximately 0.030-0.032A compared to Comparative Example 1, corresponding to a reduction in short-circuit current density of approximately 0.167-0.185 mA / cm². 2 This is due to limitations in current silver paste printing technology. Although the width of the silver paste seed layer is narrower than that of a conventional silver grid, the total grid line width after the copper plating layer is slightly larger than that of a conventional silver grid, resulting in a slight increase in the light-blocking area. This problem can be improved with further optimization of the printing process.

[0034] This invention utilizes high-mesh screens or steel screens to form a low-height, narrow-linewidth silver paste seed layer, reducing silver paste consumption by 40-60% compared to conventional processes, thus significantly lowering manufacturing costs. The low-silver-consumption composite electroplating metallization method provided by this invention, while drastically reducing silver paste consumption, can maintain or even improve the battery's conversion efficiency, open-circuit voltage, and fill factor, demonstrating significant technical advantages and economic benefits.

[0035] Example 2: The difference between this example and Example 1 is as follows: In step S2, a steel plate screen is used for screen printing. The front steel plate has a line diameter of 8μm and a thickness of 20μm, forming a silver paste layer with a line width of 15μm and a height of 1μm after printing. The back steel plate has a line diameter of 10μm and a thickness of 20μm, forming a silver paste layer with a line width of 20μm and a height of 1μm after printing.

[0036] In step S3, the electroplating solution contains 150 g / L copper sulfate pentahydrate, 10 g / L sulfuric acid, and additives. The electroplating current is 100 mA and the electroplating time is 360 s.

[0037] The remaining steps are the same as in Example 1.

[0038] Example 3: The difference between this example and Example 1 is as follows: In step S2, the front screen has a mesh count of 700, a film thickness of 5μm, and a screen line width of 5μm, resulting in a silver paste layer with a line width of 15μm and a height of 1μm after printing; the back screen has a mesh count of 700, a film thickness of 5μm, and a screen line width of 10μm, resulting in a silver paste layer with a line width of 20μm and a height of 1μm after printing.

[0039] In step S3, the electroplating solution contains 300 g / L copper sulfate pentahydrate, 50 g / L sulfuric acid, and additives. An insoluble anode is used, the electroplating current is 500 mA, and the electroplating time is 180 s.

[0040] The remaining steps are the same as in Example 1.

[0041] Example 4: The difference between this example and Example 1 is as follows: In step S2, the front screen has a mesh count of 600, a film thickness of 10μm, and a screen line width of 12μm, resulting in a silver paste layer with a line width of 20μm and a height of 3μm after printing; the back screen has a mesh count of 600, a film thickness of 12μm, and a screen line width of 15μm, resulting in a silver paste layer with a line width of 30μm and a height of 2μm after printing.

[0042] The remaining steps are the same as in Example 1.

[0043] Comparative Example 2: The difference between this comparative example and Example 1 is that: In step S2, a grooved area is formed on the silicon nitride layer using a laser grooving process to expose the silicon substrate, and then silver paste is printed in the grooved area to form a seed layer.

[0044] The remaining steps are the same as in Example 1.

[0045] The prepared batteries were subjected to performance tests, and the results are shown in Table 1.

[0046] Comparative Example 3: The difference between this comparative example and Example 1 is that: In steps S3 and S4, photo-induced electroplating is not used in the electroplating process.

[0047] The remaining steps are the same as in Example 1.

[0048] The performance of the TOPCon batteries prepared in Examples 2-4 and Comparative Examples 2-3 was tested, and the test results are shown in Table 2.

[0049] Table 2 Performance test results of Examples 2-4 and Comparative Examples 2-3 Example 2 26.29 0.7335 18.328 41.562 86.23 0.64 956 0.0643 Example 3 26.24 0.7326 18.319 41.542 86.21 0.66 1516 0.0813 Example 4 26.13 0.7306 18.280 41.453 86.28 0.65 1568 0.0751 Comparative Example 2 25.55 0.7165 18.245 41.374 86.18 0.69 956 0.0846 Comparative Example 3 26.09 0.7305 18.230 41.340 86.38 0.60 1566 0.0643 As shown in Table 2, the solar cells prepared in Examples 2-4 of this invention significantly outperform Comparative Examples 2-3 in key performance indicators such as photoelectric conversion efficiency, fill factor, open-circuit voltage, and short-circuit current. Comparative Example 2 uses laser grooving instead of screen printing, which achieves finer grid lines, but the thermal damage generated during laser processing creates defects on the silicon substrate surface, increasing carrier recombination, resulting in a photoelectric conversion efficiency of 25.55%. Comparative Example 3 uses the same screen printing process as Example 1, but does not employ photo-induced electroplating. During electroplating, the copper layer exhibits random crystal orientation, numerous grain boundary defects, and poor deposition, leading to increased copper layer resistivity and a photoelectric conversion efficiency of 26.09%.

[0050] Finally, it should be noted that the above embodiments are merely illustrative of several implementations of the present invention and are not intended to limit the scope of the invention. For those skilled in the art, any modifications, equivalent substitutions, or improvements made without departing from the concept of the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for metallizing TOPCon solar cells using low-silver-loss composite electroplating, characterized in that, Includes the following steps: S1. A silicon wafer having a TOPCon structure is provided, the silicon wafer being based on a semiconductor substrate, a second conductivity type semiconductor region being disposed on one side of the semiconductor substrate, a second passivation layer A being disposed on the side of the second conductivity type semiconductor region away from the semiconductor substrate, a second passivation layer B being disposed on the side of the second passivation layer A away from the second conductivity type semiconductor region, a tunneling layer being disposed on the side of the semiconductor substrate away from the second conductivity type semiconductor region, a first conductivity type semiconductor region being disposed on the side of the tunneling layer away from the ultrathin amorphous silicon layer, and a first passivation layer being disposed on the side of the first conductivity type semiconductor region away from the tunneling layer; S2. A silver paste layer is formed on the surfaces of the second passivation layer B and the first passivation layer by screen printing. After sintering, the silver paste layer serves as a seed layer for subsequent electroplating. S3. Using the silver paste layer as a seed layer, selectively electroplate copper to deposit a copper layer on the surface of the silver paste layer; S4. Electroplating a tin layer onto the surface of the copper layer.

2. The method according to claim 1, characterized in that, In step S1, the second passivation layer A is an aluminum oxide layer; The second passivation layer B and the first passivation layer are single-layer films selected from silicon nitride film, hydrogen-containing silicon nitride film, silicon oxide film, silicon oxynitride film, magnesium fluoride MgF2, zinc sulfide ZnS, titanium dioxide TiO2 and cerium oxide CeO2, or multilayer film structures composed of at least two layers of the above materials. The first type of conductivity semiconductor region is doped with a Group 5 element, including at least one of phosphorus, arsenic, antimony or bismuth; The semiconductor region of the second conductivity type is doped with a third group element, including at least one of boron, aluminum, gallium or indium; The tunneling layer is any one of oxide, nitride, semiconductor or conductive polymer.

3. The method according to claim 1, characterized in that, In step S2, the screen printing uses a high mesh count screen, with a mesh count of 600-700, a film thickness of 5-10μm, and a screen line width of 5-12μm.

4. The method according to claim 1, characterized in that, In step S2, the screen printing uses a steel plate screen with a wire diameter of 5-10 μm and a thickness of 15-25 μm.

5. The method according to claim 1, characterized in that, In step S2, the linewidth of the silver paste layer formed on the front side of the silicon wafer is 15-20 μm and the height is 1-3 μm; the linewidth of the silver paste layer formed on the back side of the silicon wafer is 20-30 μm and the height is 1-2 μm.

6. The method according to claim 1, characterized in that, In step S3, the electroplating solution for electroplating copper contains 150-300 g / L copper sulfate pentahydrate, 10-50 g / L sulfuric acid, and additives. The electroplating current is 100-500 mA, and the electroplating time is 180-360 s.

7. The method according to claim 1, characterized in that, In step S3, the electroplated copper uses a phosphorus copper anode or an insoluble anode.

8. The method according to claim 1, characterized in that, In step S4, the electroplating solution for tin plating is either a sulfuric acid system or a methanesulfonic acid system. The sulfuric acid electroplating solution contains 20-40 g / L stannous sulfate solution, 80-150 mL / L sulfuric acid, and additives. The methanesulfonic acid system comprises 80-180 g / L methanesulfonic acid, 80-160 g / L stannous methanesulfonate, 5-30 g / L bismuth methanesulfonate, additives, and precipitants.

9. The method according to claim 8, characterized in that, The additives include: a softener of 20-50 mL / L, which is at least one of an aromatic sulfonate or a polyether derivative; a brightener of 1-10 mL / L, which is at least one of a heterocyclic compound or an aldehyde derivative; a stabilizer of 0.5-5 g / L, which is at least one of phenolic sulfonic acid or ascorbic acid; and a wetting agent of 0.1-1.0 g / L, which is a nonionic surfactant. The precipitant is at least one of the following: a mixture of 0.05-0.5 g / L of an acrylic acid derivative and 0.01-0.2 g / L of sodium dodecyl sulfate as an accelerator; 0.03-0.3 g / L of anisaldehyde dimethyl acetal; or 1-10 ml / L of HN100 as a settling agent.

10. The method according to claim 1, characterized in that, The electroplating processes in steps S3 and S4 employ photo-induced electroplating.