Chemical tin solution for flexible printed circuit board

Abstract

The invention discloses a chemical tin solution for a flexible printed circuit board, and belongs to the technical field of circuit boards. The chemical tin solution comprises stannous mono-sulphate, methanesulfonic acid, a reducing agent, thiourea, a complexing agent, phytic acid, disodium ethylene diamine tetraacetate, an active agent, lanthanum chloride and a fluorine-substituted benzimidazole protective agent. Compared with the prior art, a hydrophobic network constructed by the fluorine-substituted benzimidazole protective agent effectively blocks environmental moisture permeation, the salt mist resistance and the tin immersion thickness of a chemical tin solution are improved, the grain boundary structure is synergistically optimized through the rare earth metal additive and the composite complexing agent, the compactness and the uniformity of a plating layer are remarkably improved, and the corrosion resistance of the plating layer is improved. And the fine-pitch wiring requirement is met. In addition, uniform nucleation of tin ions can be promoted, growth of tin whiskers is inhibited, a high-flatness coating is achieved, local micropores and cracks are reduced, and the method has high reliability and good application prospects.

Description

Technical Field

[0001] The present invention relates to the technical field of circuit boards, in particular to a chemical tin solution for flexible printed circuit boards. Background Art

[0002] In modern electronics manufacturing, flexible printed circuits (FPCs) are widely used in consumer electronics, automotive electronics, medical devices, and other fields due to their lightweight, flexible nature. Chemical tin plating, a key surface treatment process, provides FPCs with excellent conductivity, oxidation resistance, and solderability, thereby meeting the high-performance and high-reliability requirements of electronic devices.

[0003] Traditional chemical tin plating solution is mainly composed of stannous ion, reducing agent, chelating agent, etc. For example, Chinese patent application CN106939417A discloses a chemical tin plating solution for printed circuit boards, whose formula includes stannous ion, thiourea, methylsulfonic acid, citric acid, tetrasodium iminodisuccinate, carbohydrazide, surfactant, bismuth acetate and polyaminopolyether methylene phosphonic acid. This formula stabilizes stannous ion by multiple chelating agents, utilizes reducing agent to remove oxygen dissolved in the plating solution, prevents stannous ion oxidation, thereby achieving stable tin deposition rate and good solution stability. However, the addition of bismuth can only partially alleviate whisker growth, and in high humidity environments, it is easy to cause the coating to delaminate and fall off due to interfacial stress concentration.

[0004] Another Chinese patent application, CN117888094A, discloses a chemical tin precipitation composite solution comprising a tin base agent, a tin correction agent, a SnTech tin solution, a special acid SF, and a tin additive. The chemical tin precipitation process of this invention has excellent tinning performance and tin whisker suppression capabilities, and the maximum tin whisker length and number of the obtained tin precipitation layer during storage can meet the strict IPC-4554 standard. However, the process of this invention is relatively complex, involving multiple steps and a variety of specialized chemicals, which increases production costs and the difficulty of process control.

[0005] While existing technologies have made some progress in the field of electroless tin plating, several pressing challenges remain. For example, conventional electroless tin plating solutions are prone to producing tin whiskers in high-temperature and high-humidity environments, leading to coating failure and impacting the reliability and service life of electronic devices. Furthermore, some existing electroless tin plating solutions still need to be improved in terms of coating uniformity, density, and corrosion resistance, making it difficult to meet the increasingly stringent performance requirements, particularly in the manufacture of fine-pitch wiring and high-precision electronic devices.

[0006] Therefore, it is very important to develop a new type of chemical tin solution for flexible printed circuit boards and solve the problems existing in the existing technology by optimizing the formula and process. Summary of the Invention

[0007] In order to solve the deficiencies in the prior art, the present invention aims to provide a chemical tin solution for flexible printed circuit boards.

[0008] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions: A chemical tin solution for flexible printed circuit boards, comprising the following components: Stannous sulfate 10-30g / L, methanesulfonic acid 20-60g / L, reducing agent 10-30g / L, thiourea 80-120g / L, complexing agent 20-60g / L, phytic acid 20-40g / L, disodium edetate 20-40g / L, active agent 20-60mg / L, lanthanum chloride 0.05-0.15g / L, fluorinated benzimidazole protective agent 1-3g / L; The above-mentioned component solutions are mixed to form a chemical tin plating solution.

[0009] The preparation method of the fluorine-substituted benzimidazole protective agent is as follows: An o-phenylenediamine compound, a phenylacetic acid compound, and boric acid are mixed, heated, and stirred for reaction; cooled to room temperature, methanol is added to the reactant, stirred and dissolved, the solution is added to a triethylamine aqueous solution, and filtered to obtain a crude product; the crude product is dissolved in methanol, decolorized, filtered, and the filtrate is collected and cooled to room temperature. The filtrate is then added to water, filtered, and dried to obtain a fluorinated substituted benzimidazole protective agent.

[0010] The mixing temperature is 60-80° C. and the mixing time is 10-30 minutes.

[0011] The reducing agent is at least one of carbohydrazide, sodium hypophosphite, ascorbic acid, and hydroxylamine hydrochloride.

[0012] The complexing agent is at least one of citric acid and oxalic acid.

[0013] The active agent is fatty alcohol polyoxyethylene ether.

[0014] The preparation method of the fluorine-substituted benzimidazole protective agent is as follows, in parts by weight: 10-20 parts of an o-phenylenediamine compound, 15-25 parts of a phenylacetic acid compound, and 0.05-0.2 parts of boric acid are mixed, heated to 140-150° C., stirred and reacted for 4-8 hours, cooled to room temperature, 4000-6000 parts of methanol are added to the reactants, stirred and dissolved, the solution is added dropwise to 15000-25000 parts of a triethylamine aqueous solution with a pH of 8-9, and filtered to obtain a crude product; the crude product is dissolved in 4000-6000 parts of methanol at 40-60° C., decolorized with activated carbon at 40-60° C. for 1-3 hours, filtered, and the filtrate is collected and cooled to room temperature. The filtrate is then added dropwise to 8000-12000 parts of water, filtered, and dried in vacuo at 40-60° C. to obtain a fluoro-substituted benzimidazole protective agent.

[0015] The o-phenylenediamine compound is at least one of 4-bromo-o-phenylenediamine, 4-nitro-o-phenylenediamine, and 4-chloro-5-fluoro-o-phenylenediamine.

[0016] The phenylacetic acid compound is at least one of 2,4-difluorophenylacetic acid, 4-(trifluoromethyl)phenylacetic acid, and 5-fluorophenylacetic acid.

[0017] The functions of the components in the chemical tin solution of the present invention are as follows: Stannous sulfate is used as a source of tin ions (Sn²⁺) and is deposited under the action of a reducing agent to form a metallic tin coating.

[0018] Methanesulfonic acid provides an acidic environment to maintain solution stability and tin ion solubility.

[0019] Reducing agents (carbohydrazide / hydroxylamine hydrochloride, etc.) reduce Sn²⁺ to metallic tin (Sn 0 ), driving the chemical deposition reaction.

[0020] Thiourea complexes tin ions, controls the deposition rate and inhibits tin whisker growth.

[0021] Complexing agents (oxalic acid / citric acid) stabilize stannous ions by chelating to prevent oxidation and hydrolytic precipitation.

[0022] Phytic acid enhances the dispersibility of the plating solution, helps form a dense tin layer and improves bonding strength.

[0023] Disodium EDTA strengthens the chelating ability of heavy metal ions and reduces interference from impurities.

[0024] Fatty alcohol polyoxyethylene ether (surfactant) reduces the surface tension of the solution, improves substrate wettability and coating uniformity.

[0025] Lanthanum ions of lanthanum chloride co-precipitate to form tin-lanthanum alloy, which suppresses tin whiskers and improves the hardness of the coating.

[0026] Fluoro-substituted benzimidazole protective agents construct a hydrophobic network through the condensation product of o-phenylenediamine compounds and phenylacetic acid compounds, forming coordination bonds and hydrophobic barriers on the copper surface, blocking salt spray penetration and enhancing corrosion resistance.

[0027] The fluorine-substituted benzimidazole protective agent of the present invention exhibits excellent chemical stability, process compatibility and functional synergistic enhancement characteristics in chemical tin solutions. In an acidic plating solution environment, its benzimidazole ring structure is stable, does not react with other components in the plating solution, and keeps the solution uniform and transparent. Its high-temperature resistance allows it to maintain molecular integrity in the tin plating process and significantly improves the thickness of the tin plating. The formed double-fluorine hydrophobic network effectively blocks environmental moisture, reduces interfacial tension, improves the uniformity and corrosion resistance of the coating, and synergistically inhibits tin whisker growth with rare earth additives without triggering component competition, and has no negative effect on the performance of the chemical tin solution.

[0028] Compared with the existing technology, it has the following beneficial effects: 1) The present invention optimizes the grain boundary structure by using rare earth metal additives (such as lanthanum ions), significantly improving the density and uniformity of the coating and meeting the wiring requirements of fine pitch.

[0029] 2) The hydrophobic network constructed by the fluorine-substituted benzimidazole protective agent in the present invention blocks the penetration of environmental moisture, so that the coating still does not produce whiskers after aging, breaking through the failure bottleneck of traditional pure tin coatings in high temperature and high humidity environments.

[0030] 3) The present invention can promote uniform nucleation of tin ions, reduce grain size, and significantly reduce surface undulations; at the same time, it can fill the voids in the tin lattice, inhibit stress concentration caused by volume expansion, block tin whisker growth at the source, and achieve a highly flat coating with reduced local micropores and cracks. DETAILED DESCRIPTION

[0031] Main sources of substances: Fatty alcohol polyoxyethylene ether, product number: AEO-3, brand: BASF, Germany.

[0032] Activated carbon, product number: 6741, particle size: 200 mesh, function: decolorization, Hubei Tongke Activated Carbon Co., Ltd.

[0033] The remaining raw materials in the examples and comparative examples of the present invention are all commercially available products.

[0034] Example 1 A chemical tin solution for flexible printed circuit boards, comprising the following components: Stannous sulfate 20g / L, methanesulfonic acid 40g / L, carbohydrazide 20g / L, thiourea 100g / L, oxalic acid 40g / L, phytic acid 30g / L, disodium ethylenediaminetetraacetic acid 30g / L, fatty alcohol polyoxyethylene ether 40mg / L, lanthanum chloride 0.1g / L, fluorinated benzimidazole protective agent 2g / L; The above-mentioned component solutions were mixed at 70° C. for 20 minutes to prepare the chemical tin solution for the flexible printed circuit board.

[0035] The preparation method of the fluorine-substituted benzimidazole protective agent is as follows, in parts by weight: 15 parts of 4-chloro-5-fluoro-o-phenylenediamine, 19 parts of 5-fluorophenylacetic acid, and 0.12 parts of boric acid were mixed, heated to 145° C., stirred and reacted for 6 hours, cooled to room temperature, 5000 parts of methanol were added to the reactant, stirred and dissolved, and the solution was added dropwise to 20,000 parts of a triethylamine aqueous solution with a pH of 8.5, and filtered to obtain a crude product; the crude product was dissolved in 5000 parts of methanol at 50° C. and decolorized with activated carbon at 50° C. for 2 hours. After filtering, the filtrate was collected and cooled to room temperature. The filtrate was then added dropwise to 10,000 parts of water, filtered, and dried in vacuo at 50° C. to obtain a fluorinated substituted benzimidazole protective agent.

[0036] Example 2 A chemical tin solution for flexible printed circuit boards is substantially the same as that in Example 1, the only difference being the preparation method of the fluorine-substituted benzimidazole protective agent.

[0037] The preparation method of the fluorine-substituted benzimidazole protective agent is as follows, in parts by weight: 15 parts of 4-nitro-o-phenylenediamine, 19 parts of 5-fluorophenylacetic acid, and 0.12 parts of boric acid were mixed, heated to 145° C., stirred and reacted for 6 hours, cooled to room temperature, 5000 parts of methanol were added to the reactant, stirred and dissolved, and the solution was added dropwise to 20,000 parts of a triethylamine aqueous solution with a pH of 8.5, and filtered to obtain a crude product; the crude product was dissolved in 5000 parts of 50° C. methanol and decolorized with activated carbon at 50° C. for 2 hours, filtered, and the filtrate was collected and cooled to room temperature. The filtrate was then added dropwise to 10,000 parts of water, filtered, and dried in vacuo at 50° C. to obtain a fluorinated benzimidazole protective agent.

[0038] Example 3 A chemical tin solution for flexible printed circuit boards is substantially the same as that in Example 1, the only difference being the preparation method of the fluorine-substituted benzimidazole protective agent.

[0039] The preparation method of the fluorine-substituted benzimidazole protective agent is as follows, in parts by weight: A chemical tin solution for a flexible printed circuit board is substantially the same as that of Example 1, except that the preparation method of the fluorine-substituted benzimidazole protectant is different.

[0040] Example 4 A chemical tin solution for a flexible printed circuit board is substantially the same as that of Example 1, except that the preparation method of the fluorine-substituted benzimidazole protectant is different.

[0041] The preparation method of the fluorine-substituted benzimidazole protectant is as follows, in terms of weight parts: A chemical tin solution for a flexible printed circuit board is substantially the same as that of Example 1, except that the preparation method of the fluorine-substituted benzimidazole protectant is different.

[0042] Example 5 A chemical tin solution for a flexible printed circuit board is substantially the same as that of Example 1, except that the preparation method of the fluorine-substituted benzimidazole protectant is different.

[0043] The preparation method of the fluorine-substituted benzimidazole protectant is as follows, in terms of weight parts: A chemical tin solution for a flexible printed circuit board is substantially the same as that of Example 1, except that the preparation method of the fluorine-substituted benzimidazole protectant is different.

[0044] Example 6 A chemical tin solution for flexible printed circuit boards is substantially the same as that in Example 1, except that the carbohydrazide is replaced by an equal amount of hydroxylamine hydrochloride.

[0045] Comparative Example 1 A chemical tin solution for flexible printed circuit boards is substantially the same as that in Example 1, the only difference being the preparation method of the fluorine-substituted benzimidazole protective agent.

[0046] The preparation method of the fluorine-substituted benzimidazole protective agent is as follows, in parts by weight: 15 parts of 4-chloro-o-phenylenediamine, 19 parts of 5-fluorophenylacetic acid, and 0.12 parts of boric acid were mixed, heated to 145° C., stirred and reacted for 6 hours, cooled to room temperature, 5000 parts of methanol were added to the reactant, stirred and dissolved, and the solution was added dropwise to 20,000 parts of a triethylamine aqueous solution with a pH of 8.5, and filtered to obtain a crude product; the crude product was dissolved in 5000 parts of methanol at 50° C. and decolorized with activated carbon at 50° C. for 2 hours, filtered, and the filtrate was collected and cooled to room temperature. The filtrate was then added dropwise to 10,000 parts of water, filtered, and dried in vacuo at 50° C. to obtain a fluorinated substituted benzimidazole protective agent.

[0047] Comparative Example 2 A chemical tin solution for flexible printed circuit boards is substantially the same as that in Example 1, the only difference being the preparation method of the fluorine-substituted benzimidazole protective agent.

[0048] The preparation method of the fluorine-substituted benzimidazole protective agent is as follows, in parts by weight: 15 parts of 4-chloro-5-fluoro-o-phenylenediamine, 19 parts of 2-chloro-4-fluorophenylacetic acid, and 0.12 parts of boric acid were mixed, heated to 145° C., stirred and reacted for 6 hours, cooled to room temperature, 5000 parts of methanol were added to the reactant, stirred and dissolved, and the solution was added dropwise to 20,000 parts of a triethylamine aqueous solution with a pH of 8.5, and filtered to obtain a crude product; the crude product was dissolved in 5000 parts of methanol at 50° C. and decolorized with activated carbon at 50° C. for 2 hours, filtered, and the filtrate was collected and cooled to room temperature. The filtrate was then added dropwise to 10,000 parts of water, filtered, and dried in vacuo at 50° C. to obtain a fluorinated substituted benzimidazole protective agent.

[0049] Comparative Example 3 A chemical tin solution for flexible printed circuit boards is substantially the same as that in Example 1, the only difference being the preparation method of the fluorine-substituted benzimidazole protective agent.

[0050] The preparation method of the fluorine-substituted benzimidazole protective agent is as follows, in parts by weight: 15 parts of 4-chloro-o-phenylenediamine, 19 parts of 2-chloro-4-fluorophenylacetic acid, and 0.12 parts of boric acid were mixed, heated to 145° C., stirred and reacted for 6 hours, cooled to room temperature, 5000 parts of methanol were added to the reactant, stirred and dissolved, and the solution was added dropwise to 20,000 parts of a triethylamine aqueous solution with a pH of 8.5, and filtered to obtain a crude product; the crude product was dissolved in 5000 parts of methanol at 50° C. and decolorized with activated carbon at 50° C. for 2 hours, filtered, and the filtrate was collected and cooled to room temperature. The filtrate was then added dropwise to 10,000 parts of water, filtered, and dried in vacuo at 50° C. to obtain a fluorine-substituted benzimidazole protective agent.

[0051] Comparative Example 4 A chemical tin solution for flexible printed circuit boards is substantially the same as that in Example 1, except that the fluorine-substituted benzimidazole protective agent is not added.

[0052] Test Example 1 Flexible printed circuit boards (FPCs) were treated with the chemical tin solutions prepared in the examples of the present invention and the comparative example, respectively. The pre-dip plating time was 1 minute at a temperature of 30° C., and the immersion tin plating time was 10 minutes at a temperature of 65° C.

[0053] Salt spray resistance test: refer to GB / T 2423.17-2024 "Environmental testing Part 2: Test method Test Ka: Salt spray" for testing.

[0054] The test results are shown in Table 1.

[0055] Table 1

[0056] Test Example 2 The thickness of the tin layer after treatment with the chemical tin solution prepared in the embodiment of the present invention and the comparative example in Test Example 1 was tested. The tin thickness was measured at 10 points on the front side of the tin-coated flexible printed circuit board (FPC) using X-ray.

[0057] The test results are shown in Table 2.

[0058] Table 2

[0059] The advantage of 4-chloro-5-fluoro-o-phenylenediamine in the 5-fluorophenylacetic acid system stems from the synergistic effect of its molecular structure. The ortho-chlorine and fluorine form a highly electronegative region, significantly enhancing coordination with the copper substrate. Through chelation, a stable six-membered ring structure is constructed, increasing bond energy. Simultaneously, the 5-fluoro-substituted benzene ring, due to the small van der Waals radius of the fluorine atom, significantly reduces steric hindrance, allowing the fluorine-substituted benzimidazole protective agent molecules to adsorb in a highly ordered and flat manner on the copper surface, optimizing crystallization uniformity. Furthermore, this combination forms a difluorohydrophobic network with 5-fluorophenylacetic acid, effectively blocking salt spray penetration. However, the monochloro-substituted agent, due to the lack of a fluorine atom, results in weak interfacial bonding, causing delamination and blistering. This synergistic effect of the molecular structure allows the protective agent to form a uniform and dense protective layer on the copper substrate, providing an excellent foundation for the uniform deposition of tin ions. This helps improve the uniformity and stability of the tin deposition thickness, reduces uneven tin deposition caused by local defects, and ultimately increases the overall tin deposition thickness.

[0060] The para-fluorine substitution of 5-fluorophenylacetic acid forms a complementary electron cloud distribution with the ortho-difluorine of the amine component, achieving strong coordination anchoring. The small size of the fluorine atom simultaneously maintains molecular planarity, reduces steric hindrance, and promotes the crystallization of a dense tin layer. Furthermore, the two synergistically construct a continuous hydrophobic network, effectively blocking salt spray penetration. This combination simultaneously improves tin deposition efficiency and corrosion resistance through dual electronic and spatial optimization, resulting in high reliability. This structural characteristic of 5-fluorophenylacetic acid facilitates the uniform adsorption and deposition of tin ions on its surface, reducing the aggregation and uneven distribution of tin ions during the deposition process, thereby improving the density and uniformity of the tin deposition layer and further increasing the thickness of the tin deposition layer.

[0061] The possible reason for the improved salt spray resistance after using hydroxylamine hydrochloride instead of carbohydrazide in Example 6 is that the reduction potential of hydroxylamine hydrochloride is lower than that of carbohydrazide, which can more thoroughly reduce Sn²⁺ to Sn 0 , reducing residual stannous ions; the latter are easily oxidized to SnO in a salt spray environment, causing volume expansion and delamination risks. Hydroxylamine hydrochloride has a stronger reducing ability and can more effectively reduce Sn²⁺ to Sn 0 , thereby improving the deposition efficiency of tin ions, allowing more tin ions to be deposited on the substrate surface, thereby increasing the thickness of the tin layer. At the same time, reducing residual stannous ions can avoid uneven deposition and defects that may be caused by them during the deposition process, further improving the quality and thickness uniformity of the tin layer.

Claims

1. A chemical tin solution for flexible printed circuit boards, characterized in that: Includes the following components: Stannous sulfate 10-30g / L, methanesulfonic acid 20-60g / L, reducing agent 10-30g / L, thiourea 80-120g / L, complexing agent 20-60g / L, phytic acid 20-40g / L, disodium edetate 20-40g / L, active agent 20-60mg / L, lanthanum chloride 0.05-0.15g / L, fluorinated benzimidazole protective agent 1-3g / L; Mixing the above-mentioned component solutions to form a chemical tin plating solution; The preparation method of the fluorine-substituted benzimidazole protective agent is as follows: An o-phenylenediamine compound, a phenylacetic acid compound, and boric acid are mixed, heated, and stirred for reaction; cooled to room temperature, methanol is added to the reactant, stirred and dissolved, the solution is added to a triethylamine aqueous solution, and filtered to obtain a crude product; the crude product is dissolved in methanol, decolorized, filtered, and the filtrate is collected and cooled to room temperature. The filtrate is then added to water, filtered, and dried to obtain a fluorinated substituted benzimidazole protective agent.

2. The chemical tin solution for flexible printed circuit boards according to claim 1, wherein: The mixing temperature is 60-80° C. and the mixing time is 10-30 minutes.

3. The chemical tin solution for flexible printed circuit board according to claim 1, wherein: The reducing agent is at least one of carbohydrazide, sodium hypophosphite, ascorbic acid, and hydroxylamine hydrochloride.

4. The chemical tin solution for flexible printed circuit boards according to claim 1, wherein: The complexing agent is at least one of citric acid and oxalic acid.

5. The chemical tin solution for flexible printed circuit board according to claim 1, wherein: The active agent is fatty alcohol polyoxyethylene ether.

6. The chemical tin solution for flexible printed circuit boards according to claim 1, wherein: The preparation method of the fluorine-substituted benzimidazole protective agent is as follows, in parts by weight: 10-20 parts of an o-phenylenediamine compound, 15-25 parts of a phenylacetic acid compound, and 0.05-0.2 parts of boric acid are mixed, heated to 140-150° C., stirred and reacted for 4-8 hours, cooled to room temperature, 4000-6000 parts of methanol are added to the reactants, stirred and dissolved, the solution is added dropwise to 15000-25000 parts of a triethylamine aqueous solution with a pH of 8-9, and filtered to obtain a crude product; the crude product is dissolved in 4000-6000 parts of methanol at 40-60° C., decolorized with activated carbon at 40-60° C. for 1-3 hours, filtered, and the filtrate is collected and cooled to room temperature. The filtrate is then added dropwise to 8000-12000 parts of water, filtered, and dried in vacuo at 40-60° C. to obtain a fluoro-substituted benzimidazole protective agent.

7. The chemical tin solution for flexible printed circuit boards according to claim 6, wherein: The o-phenylenediamine compound is at least one of 4-bromo-o-phenylenediamine, 4-nitro-o-phenylenediamine, and 4-chloro-5-fluoro-o-phenylenediamine.

8. The chemical tin solution for flexible printed circuit boards according to claim 6, wherein: The phenylacetic acid compound is at least one of 2,4-difluorophenylacetic acid, 4-(trifluoromethyl)phenylacetic acid, and 5-fluorophenylacetic acid.

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