Nickel plating film, electroless nickel plating solution, and electroless plating method
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2026-02-03
- Publication Date
- 2026-08-13
Smart Images
Figure JP2026003713_13082026_PF_FP_ABST
Abstract
Description
Nickel plating film, electroless nickel plating solution, and electroless plating method
[0001] The present invention relates to a nickel plating film formed on a metal substrate by electroless plating and an electroless nickel plating solution for forming the nickel plating film. More specifically, the present invention relates to an electroless nickel plating solution that can directly form a nickel plating film with good smoothness regardless of the constituent metal of the substrate or whether or not a catalyst is applied to the substrate, and that also exhibits excellent bath stability.
[0002] In printed circuit boards, metal wiring and metal electrodes are typically surface-treated at the joints for wire bonding and soldering. This surface treatment involves depositing a nickel plating film on the wiring and electrodes, followed by a gold plating film. Alternatively, silver plating or palladium plating may be used instead of gold plating, and multilayer films such as palladium plating and gold plating may also be applied. These surface treatments ensure the strength of the joints and solderability required for wire bonding and soldering.
[0003] The nickel plating film deposited on the wiring using the surface treatment described above can sometimes be formed by electrolytic plating, but electroless plating is considered a more promising method. The miniaturization and high integration of wiring in circuit boards and semiconductor devices continues unabated, and electroless plating is useful in meeting these trends.
[0004] Electroless plating solutions for forming nickel plating films typically contain nickel salts as the metal source, along with reducing agents and complexing agents as essential components, and usually include additives such as pH buffers as appropriate. There is a large amount of prior art relating to electroless nickel plating solutions with such a configuration. However, the electroless nickel plating solution that is currently widely used industrially is the hypophosphorous acid bath (hypophosphorous acid bath) (for example, Patent Document 1, Non-Patent Document 1). In addition, although less industrially used than the hypophosphorous acid bath, a well-known electroless nickel plating solution is the boron compound bath (boron compound bath) that uses boron-based compounds such as dimethylamine borane (DMAB) as the reducing agent (for example, Patent Document 1, Non-Patent Document 2).
[0005] Japanese Patent No. 6474860, Kenshi Yamagishi, Naoki Okamoto, Hiroyuki Uchikawa, Naoki Fukumuro, Shinji Yaeba, Hitoshi Matsuda, Surface Technology, Vol. 55, No. 6, 2004, pp. 417-422, Hideyoshi Nawa, Yasuto Kobashi, Shozo Mizumoto, Surface Technology, Vol. 46, No. 10, 1995, pp. 951-955
[0006] The above two electroless nickel plating solutions have well-known advantages and disadvantages. That is, in the case of an electroless nickel plating solution using hypophosphite as a reducing agent, when the substrate is copper, a nickel plating film cannot be directly deposited. Therefore, when applying a hypophosphite bath to a copper substrate, a catalyst such as palladium is applied to the surface of the copper substrate before nickel plating. However, the addition of the palladium catalyst application step will affect the manufacturing efficiency of printed circuit boards and the like. In addition, by applying a palladium catalyst, a potential difference between palladium and copper may occur, and there is a risk of forming corrosion holes in the copper wiring. Furthermore, although it is necessary to apply it only to the wiring and electrodes for forming the nickel plating film, there is a risk that the catalyst may adhere to unnecessary insulation sections between the wirings. Copper is a central metal material for forming wiring and electrode patterns of various circuit boards. Considering that the trend of their precision and high definition continues even now, an electroless nickel plating solution capable of plating copper regardless of the presence or absence of palladium catalyst application is desired.
[0007] On the other hand, according to an electroless nickel plating solution using a boron compound as a reducing agent, a nickel plating film can be formed on a copper substrate without applying a catalyst such as palladium. According to the boron compound bath, efficient surface treatment of copper wiring and electrodes becomes possible. However, the boron compound bath has a problem of poor bath stability. The bath stability is affected by the activity of the reducing agent. Since the boron compound has high activity, it is not easy to control the liquid temperature adjustment and stirring conditions during the plating operation.
[0008] Furthermore, from the standpoint of bath stability, hypophosphorous acid baths offer good stability and are less expensive than boron compound baths. For this reason, hypophosphorous acid baths are more commonly used in industrial applications. However, considering the disadvantage of hypophosphorous acid baths, namely their direct deposition properties on copper substrates, there is a need for an electroless nickel plating solution that can form nickel plating films regardless of the substrate metal type and also exhibits excellent bath stability.
[0009] Furthermore, in order to support high-frequency bands (5G, 6G) and increase transmission speed for circuit boards and semiconductor devices, the requirements for nickel plating films on wiring and other components of circuit boards are expected to become more diverse in the future. These requirements for nickel plating films include higher purity through increased nickel content, smoother surfaces, and thinner film thicknesses.
[0010] Increasing the purity of nickel plating films contributes to lowering the resistivity of wiring and other components. Conventional electroless nickel plating solutions, such as hypophosphorous acid baths and boron compound baths, often refer to nickel plating films as Ni-P films and Ni-B films, respectively, and both contain approximately a few mass percent of phosphorus (P) or boron (B) derived from the reducing agent. These nickel plating films are formed when phosphorus or boron co-deposited with nickel penetrates between the nickel lattice and forms an alloy (interstitial solid solution). From the viewpoint of lowering resistivity, it is preferable that the nickel plating film be composed of nickel of higher purity than that which is alloyed with P or B.
[0011] Furthermore, the smoothness of the nickel plating film is related to the transmission loss of wiring and other components. The smoothness of the underlying nickel plating film affects the surface roughness of the gold plating film, silver plating film, palladium film, etc., that are deposited on top of it. As the surface roughness of wiring and other components increases, the transmission loss (scattering loss) increases, making it difficult to apply them to high-frequency applications.
[0012] Furthermore, research is underway to reduce the thickness of the underlying nickel plating film. While the thickness of the underlying nickel plating film has traditionally been considered to be 10 μm or less, further reductions to 2-3 μm or less are being considered, and even to the point of considering extremely thin films on the submicron order of 1 μm or less. This reduction in the thickness of the underlying plating is driven by considerations for higher transmission speeds and higher density in wiring and other applications. Even when considering this reduction in nickel plating film thickness, the conventional alloying of nickel plating films using hypophosphorous acid baths or boron compound baths is not desirable. This is because residual stress caused by lattice distortion due to the penetration of phosphorus and boron into the nickel lattice can lead to cracking and delamination in the thinned plating film.
[0013] As explained above, electroless nickel plating solutions used as undercoat plating films for wiring and the like have the advantage of direct deposition regardless of the type of metal of the substrate or whether or not a catalyst is applied to the substrate, as well as good bath stability. In addition, there is room for improvement in many aspects, such as increasing the purity of the formed nickel plating film, improving the smoothness, and reducing the film thickness.
[0014] Given the existence of these various improvements, the application of undercoat plating films made of metals other than nickel could be considered, but this is not practical. Nickel plating films have a proven track record of being used for several decades in terms of ensuring solder joint strength through undercoat plating films, and have high reliability. Applying metals other than nickel would require evaluation of the physical properties of the new intermetallic compounds formed at the joint, which would take at least several years. Therefore, simply replacing the undercoat plating film from nickel plating films to other metals would be a more difficult undertaking.
[0015] The present invention was made against the background described above, and aims to provide an electroless nickel plating solution that can form an electroless nickel plating film that can address the various improvements mentioned above regarding nickel plating films that serve as undercoat plating films for metal wiring and metal electrodes. Furthermore, the present invention clarifies the structure of a nickel plating film formed by the electroless nickel plating solution, which is of high purity and has improved smoothness.
[0016] To solve the above problems, the inventors decided to investigate the application of an electroless nickel plating solution (hydrazine bath) using hydrazine or a hydrazine compound as a reducing agent. Hydrazine and its compounds are known to be compounds with reducing properties, but their constituent elements are nitrogen (N) and hydrogen (H), and they do not contain elements such as phosphorus or boron, which can affect the composition of the nickel plating film through co-deposition. Therefore, a nickel plating film made of high-purity nickel can be formed.
[0017] As mentioned in Patent Document 1 above, the use of hydrazine as a reducing agent in electroless nickel plating solutions is well known. It is also known that a high-purity nickel plating film can be formed by using hydrazine as a reducing agent. However, according to the inventors' research, the hydrazine baths with configurations derived from the above-mentioned patent documents are as unstable as boron compound baths, and the smoothness of the resulting nickel plating film is undesirable. Furthermore, unlike hypophosphorous acid baths and boron compound baths, there are few industrial applications of hydrazine baths as electroless nickel plating solutions. Therefore, the inventors decided to conduct further research on electroless nickel plating solutions using hydrazine as a reducing agent. As will be described in detail later, in this research, the inventors aimed to optimize the hydrazine bath for the formation of a low-thickness nickel plating film.
[0018] Furthermore, the inventors investigated the structure of the nickel plating film formed by the electroless nickel plating solution optimized above, and found that it has a distinctive structure not only in terms of nickel content but also in terms of color, which led to the present invention.
[0019] In other words, the present invention relates to a nickel plating film formed on a metal substrate by electroless plating, characterized in that the nickel content is 98% by mass or more, and the spectral reflectance on the surface of the plating film for light with a wavelength of 650 nm is 20% or more and 80% or less.
[0020] In the nickel plating film of the present invention described above, it is preferable that the hydrogen storage capacity, indicated by the ratio of the number of hydrogen atoms to the number of nickel atoms (H / Ni) in the nickel plating film, is 1% or less.
[0021] Furthermore, the electroless nickel plating solution according to the present invention is an electroless nickel plating solution for forming a nickel plating film on a metal substrate by an electroless plating method, and is characterized in that it contains a nickel salt, a reducing agent, and a complexing agent as essential components, and the reducing agent contains 0.001 mol / L or more and 0.1 mol / L or less of hydrazine or a hydrazine compound.
[0022] In the present invention, suitable complexing agents include glycine, citric acid, tartaric acid, malic acid, succinic acid, malonic acid, gluconic acid, nitrilotriacetic acid, ethylenediaminetetraacetic acid, and at least one of these compounds.
[0023] Furthermore, the electroless nickel plating solution according to the present invention may contain any additives. These additives may include at least one of the following as deposition accelerators: bismuth compounds, lead compounds, thallium compounds, tellurium compounds, antimony compounds, tin compounds, germanium compounds, selenium compounds, ethylenediamine and its compounds, diethylenetriamine and its compounds, triethylenetetramine and its compounds, tetraethylenepentamine and its compounds, bipyridine and its compounds.
[0024] Other additives may include, as pH buffering agents, boric acid, carbonic acid, phosphoric acid, and at least one of these compounds.
[0025] Furthermore, the present invention provides an electroless nickel plating method. This electroless nickel plating method is characterized by using the electroless nickel plating solution described above as the electroless nickel plating solution. In this case, it is preferable to form the nickel plating film with the pH of the electroless nickel plating solution set to 10 or higher.
[0026] As described above, the nickel plating film according to the present invention is made of high-purity nickel and has good smoothness. Furthermore, the electroless nickel plating solution according to the present invention can directly deposit a nickel plating film on a copper substrate and has good bath stability. In addition, it can deposit the aforementioned suitable nickel plating film and can accommodate the reduction of nickel plating film thickness.
[0027] SEM images of the nickel plating film surface formed with the electroless nickel plating films of Example 1 and Comparative Example 1 of the second embodiment. Photographs of the appearance of nickel plating films formed with the electroless nickel plating solutions of Examples 1 to 5 and Comparative Examples 1 and 2 of the second embodiment. SEM image of the nickel plating film surface formed with Example 1 (without deposition accelerator) of the third embodiment. SEM image of the nickel plating film surface formed with Example 6 (deposition accelerator: bismuth compound) of the third embodiment. SEM image of the nickel plating film surface formed with Reference Example 1 (with potassium thiocyanate added) of the third embodiment.
[0028] The following describes specific embodiments of the electroless nickel plating film and the reducing electroless nickel plating solution according to the present invention.
[0029] (A) Electroless nickel plating film according to the present invention As described above, the electroless nickel plating film according to the present invention is made of high-purity nickel, specifically, the nickel content is 98% by mass or more. This is to eliminate interstitial elements such as phosphorus and boron that cause internal stress, and also to improve electrical properties such as resistivity. The nickel purity of the electroless nickel plating film is preferably 99% by mass or more. The electroless nickel plating film according to the present invention may contain unavoidable impurities. Examples of these unavoidable impurities include hydrogen, carbon, nitrogen, oxygen, and chlorine. These unavoidable impurities are due to components such as nickel salts, reducing agents, and complexing agents that constitute the electroless nickel plating solution. The total content of unavoidable impurities is preferably 10,000 ppm or less.
[0030] The nickel content and impurity content of a nickel plating film can be measured by performing X-ray fluorescence analysis (XRF), electron probe microprobe analysis (EPMA), energy dispersive X-ray spectroscopy (EDX), etc., on the surface or cross-section of the film. Furthermore, if the nickel plating film can be peeled from the substrate, the film can be dissolved and analyzed by inductively coupled plasma atomic emission spectroscopy (ICP).
[0031] Furthermore, the electroless nickel plating film of the present invention has a distinctive appearance, exhibiting a gray color. This appearance is due to the application of the electroless nickel plating solution using a predetermined concentration of hydrazine or a hydrazine compound as a reducing agent. Even with plating using hydrazine or the like as a reducing agent, if a concentration outside the predetermined range is applied, the nickel plating film becomes black, resulting in an appearance clearly different from that of the nickel plating film of the present invention. The appearance and color of such a nickel plating film are mainly determined by surface smoothness and other factors resulting from the shape of the nickel crystals constituting the nickel plating film. In the present invention, the distinctive appearance of the nickel plating film can be defined by the spectral reflectance on the surface of the plating film. The nickel plating film of the present invention is required to have a spectral reflectance of 20% or more and 80% or less for light with a wavelength of 650 nm. The lower the spectral reflectance (the closer it is to 0%), the blacker the color of the nickel plating film becomes, and a nickel plating film with a spectral reflectance of less than 10% is clearly recognizable as black to the naked eye. A spectrophotometer can be used to measure the spectral reflectance of the nickel plating film.
[0032] Furthermore, the nickel plating film according to the present invention is preferably one with a low hydrogen storage capacity. Hydrogen absorbed by the nickel plating film can lead to an increase in internal stress, which can cause cracks and delamination. The nickel plating film according to the present invention is composed of high-purity nickel that does not contain interstitial elements, thereby reducing the hydrogen storage capacity. The hydrogen storage capacity in the present invention can be defined by the ratio (H / Ni) of the number of hydrogen atoms to the number of nickel atoms in the nickel plating film. The nickel plating film according to the present invention is preferably one in which the hydrogen storage capacity based on the above ratio (H / Ni) is 1% or less. The hydrogen storage capacity of the plating film can be measured by temperature-controlled desorption gas analysis (TDS).
[0033] There are no particular limitations on the thickness of the nickel plating film of the present invention. However, considering that the present invention is intended to be used as an undercoat plating film for surface treatment of metal wiring and metal electrodes, and considering the demand for thinner undercoat plating films, a film thickness of 3 μm or less is preferred, and 2 μm or less is more preferred. Furthermore, the nickel plating film of the invention can have the desired function even when it is extremely thin, at a thickness of 1 μm or less. The lower limit of the film thickness is 0.01 μm or more.
[0034] There are no particular restrictions on the metal of the substrate on which the nickel plating film according to the present invention is formed. The electroless nickel plating solution according to the present invention, described later, can directly form a nickel plating film on copper as well. When copper is used as the substrate, pure copper or copper alloys containing 60% by mass or more of copper (copper-beryllium alloy, copper-nickel alloy, copper-zinc alloy, copper-tin) can be used. Furthermore, the present invention is also applicable to metal substrates other than copper and copper alloys, and is effective on substrates made of nickel or nickel alloys, iron or iron alloys, etc. There are no restrictions on the dimensions or shape of the substrate; it may be a thin film such as wiring or electrodes provided on a circuit board or semiconductor element, or it may be a bulk material such as a plate.
[0035] (B) Electroless nickel plating solution according to the present invention The electroless nickel plating solution according to the present invention contains a nickel salt, a reducing agent, and a complexing agent as essential components. It may also contain optional additives such as a deposition accelerator to suppress non-deposition and a pH buffer to optimize the pH of the plating bath.
[0036] (B-1) Nickel salts Nickel salts are essential as a metal source for depositing nickel that constitutes the nickel plating film. Preferred nickel salts include nickel chloride, nickel sulfate, nickel carbonate, nickel acetate, nickel sulfamate, and nickel citrate. The nickel salt content in the plating solution is preferably in the range of 0.001 mol / L to 0.2 mol / L in terms of metallic nickel. If it is less than 0.001 mol / L, the amount of nickel deposited will be insufficient, making it difficult to obtain the required deposition rate. On the other hand, a plating solution containing an excess amount of nickel salt exceeding 0.2 mol / L may have reduced stability. It is more preferable that the nickel salt content be 0.01 mol / L to 0.1 mol / L in terms of metallic nickel.
[0037] (B-2) Reducing agent (hydrazine or hydrazine compound) The electroless nickel plating solution according to the present invention uses hydrazine or a hydrazine compound at a concentration of 0.001 mol / L or more and 0.1 mol / L or less as a reducing agent. The nickel plating solution according to the present invention is characterized in that it specifies the type of reducing agent and the range of low content. The reason for using hydrazine or a hydrazine compound as a reducing agent is to form a plating film made of high-purity nickel, as described above. Furthermore, with hydrazine or a hydrazine compound, it is possible to form a nickel plating film directly on a copper substrate without the application of a catalyst.
[0038] Furthermore, in the electroless nickel plating solution according to the present invention, the content of hydrazine or a hydrazine compound, which is a reducing agent, is set to 0.001 mol / L or more and 0.1 mol / L or less. The reasons for setting such a low concentration of reducing agent are, firstly, to ensure suitable surface smoothness of the nickel plating film, and secondly, to ensure the bath stability of the electroless nickel plating solution.
[0039] To explain the reasons for these issues, first, regarding the smoothness of the nickel plating film, the inventors have confirmed through their research that the content of reducing agents such as hydrazine affects the crystallinity and crystal shape of the deposited and growing nickel plating film. In electroless nickel plating solutions using hydrazine or hydrazine compounds as a reducing agent, deposition of the nickel plating film will occur even if the reducing agent content exceeds the above range. However, the nickel deposited at this time will grow in the preferred direction, resulting in needle-shaped crystals that protrude perpendicularly to the substrate. These needle-shaped crystals create irregularities on the surface of the nickel plating film, thus impairing its smoothness. In particular, when reducing the thickness of the nickel plating film, this crystal shape significantly reduces smoothness. This tendency can be suppressed by reducing the concentration of hydrazine in the electroless nickel plating solution, and below a certain concentration, crystal growth proceeds uniformly, and growth in the planar direction also progresses, thus improving smoothness. In this invention, which emphasizes the smoothness of the nickel plating film, the concentration of hydrazine is set low from this viewpoint.
[0040] Furthermore, the concentration of hydrazine or hydrazine compounds correlates with the bath stability of the plating solution. When the concentration of hydrazine or similar compounds exceeds a certain level, the bath stability decreases, making decomposition (nickel deposition) more likely. Bath stability is also a major issue in this invention.
[0041] As described above, the inventors considered that the concentration of hydrazine or a hydrazine compound necessary to ensure the smoothness of the nickel plating film and the bath stability of the electroless nickel plating solution is 0.1 mol / L or less, and therefore set the above range. Furthermore, since the reducing agent effect is almost nonexistent at concentrations below 0.001 mol / L, a lower limit was set. A more preferable range for the concentration of hydrazine or a hydrazine compound is 0.01 mol / L or more and 0.1 mol / L or less.
[0042] In the electroless nickel plating solution of the present invention using hydrazine or a hydrazine compound as a reducing agent, no reducing agent other than hydrazine or a hydrazine compound is contained. This is because even if other reducing agents are contained, there is no effect of improving the smoothness of the plating film or the bath stability, and rather, a decrease in bath stability is a concern. However, if it is hydrazine or a hydrazine compound, a plurality of types may be contained. As the hydrazine compound, hydrazine hydrochloride, hydrazine sulfate, hydrazine carbonate, etc. can be applied.
[0043] (B-3) Complexing agent The complexing agent is a component for maintaining the bath stability by forming a complex with nickel ions in the plating solution to form a nickel complex. In the electroless nickel plating solution according to the present invention, as the complexing agent, glycine, citric acid, tartaric acid, malic acid, succinic acid, malonic acid, gluconic acid, nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), and at least one or more of these compounds can be contained. In order to enable the above-described effects, the content of the complexing agent is preferably in the range of 1.2 times mol or more and 16 times mol or less, more preferably 2 times mol or more and 12 times mol or less, relative to the nickel salt concentration (mol / L).
[0044] (B-4) Additive (optional component) The reducing type electroless nickel plating solution according to the present invention essentially comprises the above-described nickel salt, reducing agent (hydrazine or a hydrazine compound), and complexing agent, but may contain other components as necessary.
[0045] A particularly useful additive in this invention is a deposition accelerator. By adding a deposition accelerator, the non-deposition of nickel can be suppressed, improving coverage (adhesion) and other properties. This invention takes into consideration the reduction of nickel plating film thickness, and even slight non-deposition can cause uneven plating. By adding a deposition accelerator, adhesion can be ensured, and a uniform nickel plating film can be formed. Examples of compounds that serve as deposition accelerators include inorganic or organic compounds of metals such as bismuth (Bi) compounds, lead (Pb) compounds, thallium (Tl) compounds, tellurium (Te) compounds, antimony (Sb) compounds, tin (Sn) compounds, germanium (Ge) compounds, and selenium (Se) compounds. In this invention, tellurium and germanium, which are called metalloids, are also treated as metals. As for these metal compounds, chlorides, oxides, suboxides, hydroxides, sulfates, nitrates, carbonates, formates, tartrates, and phosphates are preferred. The amount of these metal compounds added is preferably 0.001 ppm to 100 ppm based on the above-mentioned metal element, and more preferably 0.01 ppm to 10 ppm.
[0046] Furthermore, organic compounds such as ethylenediamine and its compounds, diethylenetriamine and its compounds, triethylenetetramine and its compounds, tetraethylenepentamine and its compounds, bipyridine and its compounds can also be used as precipitation accelerators. Compounds such as ethylenediamine include sulfates, etc. When organic compounds are used as additives, their concentration is preferably 0.0001 g / L or more and 10 g / L or less, more preferably 0.001 g / L or more and 1 g / L or less, and particularly preferably 0.01 g / L or more and 1 g / L or less.
[0047] Furthermore, the electroless nickel plating solution according to the present invention may contain an additive included in conventional electroless nickel plating. Examples of such an additive include a pH buffer. The pH buffer is added to stabilize the pH of the plating solution. As will be described later, in the plating process using the electroless nickel plating solution of the present invention, there is a suitable range for the pH of the plating bath. The pH of the plating bath may be adjusted during the plating process, or a pH buffer may be added to the electroless nickel plating solution in advance. Examples of the pH buffer include boric acid or a boric acid compound, carbonic acid or a carbonic acid compound, and phosphoric acid or a phosphoric acid compound. The pH additive can be added within a range where the pH of the electroless nickel plating solution becomes 10 or higher.
[0048] In addition, the electroless nickel plating solution according to the present invention may contain a surfactant as an additive. The surfactant is added as needed for purposes such as preventing pits, preventing abnormal deposition, improving the plating appearance, and improving stability, caused by nitrogen gas and hydrogen gas generated during the plating reaction. The type of surfactant applied in the present invention is not particularly limited, and various anionic, nonionic, cationic, and amphoteric surfactants can be applied. Specific components include sodium alkyl sulfonate, sodium alkyl naphthalene sulfonate, sodium dialkyl sulfosuccinate, polyethylene glycol, polyethylene oxide-polypropylene oxide block copolymer (Pluronic (registered trademark)), polyacetylene glycol (Surfynol), polyethyleneimine, butynediol, propargyl alcohol, and the like.
[0049] The electroless nickel plating solution according to the present invention described above can be produced by mixing and dissolving a nickel salt, a reducing agent (hydrazine or a hydrazine compound), a complexing agent, and any of the above additives in a solvent. There is no particular limitation on the order of mixing at this time, and the state of the compound (solid state or solution state) during mixing is also not limited.
[0050] (C) Plating method using the electroless nickel plating solution according to the present invention In the method for forming a nickel plating film using the electroless nickel plating solution according to the present invention, a nickel plating film is formed by bringing a substrate into contact with the nickel plating solution, similar to the ordinary electroless plating method.
[0051] The electroless nickel plating method of the present invention can be applied to the above-mentioned metal substrates. The electroless nickel plating solution of the present invention exhibits direct deposition properties on various metal substrates, and can directly form a nickel plating film on copper substrates or copper alloy substrates. Therefore, the application of catalysts such as palladium (pretreatment), which is essential when applying the conventional hypophosphorous acid bath to a copper substrate, is unnecessary in the present invention. However, the electroless nickel plating solution of the present invention does not discourage the application of catalysts, but rather does not discourage them. Therefore, catalysts may be applied to the substrate in the electroless nickel plating method of the present invention. For example, catalysts can be applied as needed to improve the deposition rate or adhesion strength.
[0052] In the electroless nickel plating method of the present invention, a nickel plating film can be formed at a suitable deposition rate by setting the pH and bath temperature of the electroless nickel plating solution within a suitable range. The pH of the electroless nickel plating solution is preferably 10.0 or higher. Below a pH of 10, the reducing ability is weak, making it difficult to obtain a suitable deposition rate. Furthermore, if the pH exceeds 13.5, the stability of the plating solution decreases, making it easier for nickel to deposit in unwanted areas; therefore, a pH of 13.5 or lower is preferable. A more preferable pH for the electroless nickel plating solution is 10.5 to 12.5. A pH adjusting agent can be added to adjust the pH. Sodium hydroxide is commonly used as a pH adjusting agent, but potassium hydroxide, lithium hydroxide, tetramethylammonium hydroxide, tetraethylammonium hydroxide, etc., can be used as needed.
[0053] Furthermore, the bath temperature of the electroless nickel plating solution is preferably between 40°C and 95°C. Below 40°C, the deposition of the nickel plating film is difficult. On the other hand, if the bath temperature is too high, the stability of the plating solution decreases, and nickel may be deposited in unwanted areas. A bath temperature of 70°C to 90°C is more preferable.
[0054] Furthermore, after forming an electroless nickel plating film on copper as described above, various metal thin films may be formed according to conventional methods. Examples include palladium thin films, gold thin films, and silver thin films. By combining these metal thin films, multilayer structures such as nickel / gold, nickel / silver, nickel / palladium / gold, and nickel / palladium / silver can be obtained.
[0055] This section describes specific embodiments of the present invention. The following description will sequentially clarify the relationship between the problems related to electroless nickel plating films and electroless nickel plating solutions described in the background art section and the present invention. Specifically, in the first embodiment, the feasibility of forming a nickel plating film on a copper substrate, the purity of the nickel plating film (nickel content), and the bath stability of the plating solution were examined by comparing the embodiment of the present invention with conventional electroless nickel plating solutions, namely hypophosphorous acid baths and boron compound baths. In the second embodiment, the smoothness of a nickel plating film formed from a hydrazine bath was the main focus of the examination. Furthermore, in the third embodiment, the deposition performance and the effect of additives (deposition accelerators) were confirmed when the thickness of the nickel plating film was set to an extremely thin 1 μm or less.
[0056] First Embodiment: An electroless nickel plating solution was prepared consisting of a nickel salt, a reducing agent, a complexing agent, and an additive (pH buffer). Hydrazine (Example 1, Comparative Example 1) and hypophosphorous acid (Conventional Example 1) and DMAB (Conventional Example 2) were used as the reducing agent. Electroless plating was then performed on copper and nickel substrates using the prepared electroless nickel plating solutions. The bath stability of each electroless nickel plating solution and the feasibility and thickness of nickel plating film formation on each substrate were confirmed. Furthermore, the nickel content, hydrogen storage capacity, and appearance of the formed nickel plating films were measured and evaluated.
[0057] <Manufacturing of Electroless Nickel Plating Solution> The composition of the electroless nickel plating solution manufactured in this embodiment is shown in Table 1, which will be described later. The solution compositions for Conventional Examples 1 and 2 were determined by referring to Non-Patent Documents 1 and 2. The nickel salt, reducing agent, complexing agent, and pH buffer used were all commercially available products, and these were dissolved in pure water to prepare the electroless nickel plating solution. After manufacturing each electroless nickel plating solution, the bath stability was evaluated, and then electroless nickel plating treatment was performed and each evaluation was carried out.
[0058] [Evaluation of Bath Stability of Electroless Nickel Plating Solutions] The bath stability of electroless nickel plating solutions was evaluated by placing the plating solution in a beaker, maintaining the temperature at 85°C, and measuring the time until nickel deposition. Plating solutions in which no nickel deposition occurred after 24 hours were evaluated as having "Good (○)" bath stability. On the other hand, plating solutions in which nickel deposition occurred between 12 and 24 hours were evaluated as having "Poor (△)" bath stability, and plating solutions in which nickel deposition occurred in less than 12 hours were evaluated as having "Poor (×)" bath stability.
[0059] <Electroless Nickel Plating Treatment> Electroless nickel plating treatment was performed using various electroless nickel plating solutions manufactured. In this embodiment, commercially available copper plates (dimensions: 20 mm x 100 mm, thickness: 0.3 mm, copper purity 99.96% by mass) and nickel plates (dimensions: 20 mm x 100 mm, thickness: 0.3 mm, nickel purity 99% by mass) were used as substrates. The copper substrate and nickel substrate underwent the following pretreatment prior to electroless nickel plating treatment. ・Copper substrate: Degreased with a commercially available alkaline degreasing solution (85°C, 5 minutes), then soft etching treatment with a mixed solution of sodium persulfate and concentrated sulfuric acid (25°C, 2 minutes), followed by sulfuric acid activation treatment with 10% by volume sulfuric acid (25°C, 2 minutes). ・Nickel substrate: Degreased with a commercially available alkaline degreasing solution (85°C, 5 minutes), then hydrochloric acid activation treatment with 50% by volume hydrochloric acid (25°C, 5 minutes).
[0060] In the electroless nickel plating process, the copper substrate was immersed in an electroless nickel plating solution at a set bath temperature, and the electroless nickel plating was performed. The plating time was set to 30 minutes, with the goal of achieving a film thickness of approximately 2 μm or less. In addition, to investigate the feasibility of direct film formation on the copper substrate, the copper substrate was also plated using a hypophosphorous acid bath (Conventional Example 1) without the addition of a catalyst. After the plating process, the substrate was removed, washed, and then various measurements and evaluations were performed.
[0061] [Determination of Nickel Plating Feasibility and Measurement of Film Thickness] The mass of each substrate was measured before and after the electroless nickel plating process described above. Based on the mass difference, the feasibility of nickel plating and the film thickness were determined. The film thickness was calculated as the average film thickness based on the mass difference and the surface area of the substrate. In addition, the color of the plated film surface was confirmed by visual inspection.
[0062] [Evaluation of Nickel Plating Film] As described later, since nickel plating films were not formed in the hypophosphorous acid bath (Conventional Example 1), the investigation of nickel plating films was conducted on nickel plating films formed on nickel substrates. The nickel content of the nickel plating film was measured using an X-ray fluorescence analyzer (XRF: ZSX Primus IV, manufactured by Rigaku Corporation) to measure the content of nickel and other elements.
[0063] Furthermore, the hydrogen storage capacity of the nickel plating film was measured using a temperature-controlled desorption gas analyzer (TDS1200II, manufactured by Denshi Kagaku Co., Ltd.). The analysis conditions for hydrogen storage capacity were a temperature range of room temperature to 700°C and a heating rate of 10°C / min. Based on the measured amount of hydrogen gas, the number of hydrogen atoms in the nickel plating film was calculated. Then, referring to the results of the nickel purity test performed on the nickel plating film, the number of nickel atoms in the plating film was calculated. From these, the ratio of the number of hydrogen atoms to the number of nickel atoms (H / Ni) in the nickel plating film was calculated.
[0064] Table 1 summarizes the composition of the various electroless nickel plating solutions manufactured in this embodiment, the evaluation results of the bath stability of the plating solutions, the feasibility of forming nickel plating films on copper and nickel substrates, and the evaluation results of the nickel plating films.
[0065]
[0066] Referring to Table 1, we first examine the results for conventional electroless nickel plating solutions: a hypophosphorous acid bath (Conventional Example 1) and a boric acid compound bath (Conventional Example 2). It was confirmed that a nickel plating film could not be directly formed on the copper substrate using the hypophosphorous acid bath (Conventional Example 1). This can be attributed to the fact that no catalyst was applied to the substrate during the electroless nickel plating process in this embodiment. In contrast, it was confirmed that a nickel plating film could be directly formed on the copper substrate using the boric acid compound bath (Conventional Example 2).
[0067] However, the boron compound bath (Conventional Example 2) exhibited poor bath stability, and decomposition due to nickel deposition occurred in less than 12 hours (measured at 2 hours). Furthermore, it was confirmed that the nickel plating film produced by Conventional Example 2 was Ni-B containing 4.5% boron, and also had a high hydrogen storage capacity (H / Ni) of 4.6%. Regarding the nickel content of the nickel plating film, the nickel plating film produced by the hypophosphorous acid bath (Conventional Example 1) also contained 8.8% phosphorus.
[0068] From these results, it can be confirmed that conventional technologies, such as hypophosphorous acid baths and boron compound baths, cannot simultaneously address the issues of bath stability, direct deposition onto copper substrates, and nickel content in the nickel plating film.
[0069] On the other hand, electroless nickel plating using hydrazine as a reducing agent has the potential to address all of the above challenges. The electroless nickel plating in Example 1 (hydrazine 0.04 mol / L) showed good bath stability, and a nickel plating film could be directly deposited on the copper substrate. The deposited nickel plating film had a nickel content exceeding 99% by mass and contained very few impurities. Furthermore, the hydrogen storage capacity (H / Ni) was less than 1%. In other words, it was confirmed that the hydrazine bath can address bath stability, direct deposition on the copper substrate, and high purity of the nickel plating film.
[0070] However, even with hydrazine baths, the bath stability decreases when the reducing agent concentration becomes too high (Comparative Example 1). Also, unlike other electroless nickel plating solutions, the appearance of the deposited nickel plating film was black. From these findings, it can be said that even hydrazine baths may not be suitable depending on the hydrazine concentration. This suggests that precise adjustment of the hydrazine concentration is necessary for electroless nickel plating solutions that use hydrazine as a reducing agent.
[0071] Second Embodiment: Based on the results of the first embodiment, in this embodiment, multiple electroless nickel plating solutions with different hydrazine concentrations were prepared using hydrazine as a reducing agent. The same evaluation as in the first embodiment was performed, and the surface smoothness and spectral reflectance of the deposited nickel plating film were evaluated.
[0072] The manufacturing of the electroless nickel plating solution and the evaluation of its bath stability were carried out in the same manner as in the first embodiment. Then, the same substrates (copper substrate and nickel substrate) as in the first embodiment were prepared, and electroless nickel plating was performed using the same process. In this embodiment, while aiming for a nickel plating film thickness of 2 μm or less, the plating time was adjusted according to the reducing agent concentration, taking into consideration the results of the first embodiment. Furthermore, the feasibility of film formation and measurement of film thickness of the nickel plating film, as well as the measurement of nickel content and hydrogen storage capacity, were carried out in the same manner as in the first embodiment. The evaluation of the nickel plating film was performed on the plating film on the nickel substrate, as in the first embodiment.
[0073] [Evaluation of Surface Smoothness (Crystal Shape)] In this embodiment, emphasis was placed on evaluating the surface smoothness of the nickel plating film. Surface smoothness was evaluated by observing the surface of the deposited nickel plating film with an SEM and assessing the surface morphology based on the crystal shape. In this case, if the surface morphology of the nickel plating film was smooth, it was evaluated as "good (○)" smoothness, and if needle-like crystals were growing on the surface, it was evaluated as "poor (×)" smoothness.
[0074] [Measurement of Reflectance of Nickel Plating Film] In order to examine the appearance of the nickel plating film in more detail, the reflectance was measured using a spectrophotometer (CM-5 manufactured by Konica Minolta, Inc.: SCI (including specular reflection) method, measurement wavelength range 360 nm to 740 nm). The appearance was then evaluated based on the reflectance at a wavelength of 650 nm.
[0075] Table 2 shows the composition of the electroless nickel plating solution (hydrazine bath) produced in this embodiment, the evaluation results of the bath stability of the plating solution, and various evaluation results regarding the nickel plating film. Table 2 also shows the results of Example 1 and Comparative Example 1 of the first embodiment.
[0076]
[0077] Table 2 confirms that even in electroless nickel plating solutions using hydrazine as a reducing agent, optimizing the hydrazine concentration is essential to achieve good bath stability and smoothness of the nickel plating film. Specifically, when the hydrazine content exceeds 0.1 mol / L, the bath stability decreases (Comparative Examples 1 and 2). Furthermore, when the hydrazine content is excessive, the nickel plating film is composed of needle-like crystals, resulting in an undesirable surface morphology (Comparative Examples 1 and 2). Figure 1 shows SEM images of the surfaces of nickel plating films of the same thickness deposited using the electroless nickel plating films (hydrazine bath) of Example 1 and Comparative Example 1. These SEM images also show that the hydrazine concentration has a significant impact on the surface morphology of the nickel plating film. From these results, it can be said that for the electroless nickel plating solution using hydrazine (hydrazine compound) as a reducing agent of the present invention, it is necessary to keep the reducing agent content low.
[0078] Figure 2 shows photographs of the electroless nickel plating films of Examples 1 to 5 and Comparative Examples 1 and 2. It can be seen that the nickel plating films of Examples 1 to 5, with optimized hydrazine content, can be distinguished by their color (gray and black) and reflectance. Furthermore, it can be confirmed that the suitable nickel plating film produced by the suitable plating solution is gray in color and has a spectral reflectance of 10% or more.
[0079] Third Embodiment: In this embodiment, electroless nickel plating was performed using hydrazine as a reducing agent, with various deposition accelerators added as additives, and the thickness of the nickel plating film was set to 1 μm or less.
[0080] The electroless nickel plating solution was based on the composition of the electroless nickel plating solution (hydrazine concentration 0.04 mol / L) of Example 1 of the first embodiment, with metal compounds such as bismuth compounds and organic compounds such as EDA added as deposition accelerators. The method for manufacturing the plating solution was the same as in the first embodiment. The method for electroless nickel plating the substrate (copper substrate, nickel substrate) was also the same as in the first embodiment. The thickness of the nickel plating film (1 μm or less) was achieved by setting the plating time to 30 minutes. In this embodiment as well, the bath stability of the electroless nickel plating solution, the thickness of the nickel plating film, the nickel content, the hydrogen storage capacity, and the smoothness were evaluated. These evaluation methods were the same as in the first and second embodiments.
[0081] Furthermore, in this embodiment, in order to confirm the effect of the deposition accelerator when the nickel plating film was made extremely thin, the presence or absence of undeposited nickel in the nickel plating film was evaluated. Undeposited nickel was evaluated by observing the nickel plating film with an SEM (×50,000x magnification), and if the substrate material was visible due to undeposited nickel in the whole or in part, it was marked as undeposited (×). Undeposited nickel was evaluated for both copper substrates and nickel substrates. The composition and evaluation results of the electroless nickel plating solution manufactured and evaluated in this embodiment are shown in Tables 3 and 4.
[0082]
[0083]
[0084] In the electroless nickel plating solution of Example 1, which did not contain a deposition accelerator, while overall nickel plating film formation was confirmed on the nickel substrate, some areas were found to be undeposited. No undeposited areas were observed on the copper substrate. From Table 3, it was found that the undeposited areas on the nickel substrate could be resolved by adding a metal compound or organic compound, which is a deposition accelerator, to the electroless nickel plating solution. Various metal compounds and organic compounds were found to be effective as deposition accelerators that could suppress undeposited areas. However, it was found that some compounds (potassium thiocyanate and ammonium chloride) had no effect as deposition accelerators, or caused aggregation or refinement of crystal grains, which could lead to expansion of grain boundaries and the formation of grooves. Regarding the evaluation results of undeposited nickel plating films, Figure 3 shows SEM images of the nickel plating film of Example 1 (without deposition accelerator) (copper substrate, nickel substrate). Furthermore, Figure 4 shows an SEM image of the nickel plating film of Example 6 (precipitation accelerator: bismuth compound), and Figure 5 shows an SEM image of the nickel plating film of Reference Example 1 (potassium thiocyanate added).
[0085] Furthermore, regarding evaluation results other than those for undegraded surfaces, such as bath stability, nickel content of the nickel plating film, and smoothness, all of the electroless nickel plating solutions examined in this embodiment performed well.
[0086] From the results of the study in this embodiment, it can be said that the addition of a deposition accelerator is effective when the thickness of the nickel plating film is extremely thin, less than 1 μm. In the electroless nickel plating solution of Example 1, which did not contain a deposition accelerator, there was concern about the occurrence of undeposited film depending on the substrate. Examples 6 to 16 were electroless nickel plating solutions to which bismuth compounds, lead compounds, thallium compounds, tellurium compounds, antimony compounds, tin compounds, germanium compounds, ethylenediamine, diethylenetriamine, triethylenetetramine, and bipyridine compounds were added as deposition accelerators, and the occurrence of undeposited film was suppressed in nickel plating films with a thickness of 1 μm or less produced by these solutions.
[0087] However, results from the second embodiment confirm that even without the addition of a deposition accelerator, a nickel plating film with good smoothness can be formed regardless of the substrate, as long as the film thickness is 1 μm or more (Examples 1 to 5). In this respect, the present invention places importance on improving the smoothness of the nickel plating film due to the surface morphology, regardless of the film thickness. Therefore, it is preferable to add the deposition accelerator as appropriate according to the target film thickness.
[0088] The electroless nickel plating solution according to the present invention applies a low concentration of hydrazine or a hydrazine compound as a reducing agent optimized for forming thin nickel plating films. According to the present invention, a high-purity nickel plating film can be directly formed on a metal substrate containing a copper substrate. Furthermore, the electroless nickel plating solution according to the present invention also exhibits excellent bath stability. The present invention is suitably applicable to forming nickel plating films that serve as undercoats during surface treatment of bonding areas such as wire bonding in copper wiring and electrodes on printed circuit boards.
Claims
1. A nickel-plated film formed on a metal substrate by electroless plating, characterized in that the nickel content is 98% by mass or more, and the spectral reflectance on the surface of the plated film for light with a wavelength of 650 nm is 20% or more and 80% or less.
2. The nickel-plated film according to claim 1, wherein the hydrogen storage capacity, as indicated by the ratio of the number of hydrogen atoms to the number of nickel atoms (H / Ni) in the nickel-plated film, is 1% by mass or less.
3. An electroless nickel plating solution for forming a nickel plating film on a metal substrate by electroless plating, wherein the solution contains a nickel salt, a reducing agent, and a complexing agent as essential components, and the reducing agent contains 0.001 mol / L or more and 0.1 mol / L or less of hydrazine or a hydrazine compound.
4. The electroless nickel plating solution according to claim 3, comprising glycine, citric acid, tartaric acid, malic acid, succinic acid, malonic acid, gluconic acid, nitrilotriacetic acid, ethylenediaminetetraacetic acid, and at least one of these compounds as a complexing agent.
5. The electroless nickel plating solution according to claim 3 or claim 4, comprising at least one of boric acid, carbonic acid, phosphoric acid, and compounds thereof as a pH buffer.
6. The electroless nickel plating solution according to claim 3 or claim 4, comprising at least one of the following as a deposition accelerator: bismuth compounds, lead compounds, thallium compounds, tellurium compounds, antimony compounds, tin compounds, germanium compounds, selenium compounds, ethylenediamine and its compounds, diethylenetriamine and its compounds, triethylenetetramine and its compounds, tetraethylenepentamine and its compounds, bipyridine and its compounds.
7. An electroless plating method for forming a nickel plating film on a metal substrate using an electroless nickel plating solution, characterized in that the electroless nickel plating solution used is the electroless nickel plating solution described in claim 3 or claim 4.
8. The electroless plating method according to claim 7, wherein the pH of the electroless nickel plating solution is set to 10 or higher to form a nickel plating film.