Electroless au alloy plating solution and au alloy plated film

Abstract

The electroless Au alloy plating solution of the present invention contains: a water-soluble Au compound with an Au concentration of 0.05 g / L or more and 5.0 g / L or less; a water-soluble Sn compound with a Sn concentration of 0.05 g / L or more and 1.5 g / L or less; and an aldehyde compound as a reducing agent with an aldehyde concentration of 6.6 × 10⁻³ mol / L or more and 4.0 × 10⁻¹ mol / L or less.

Description

Technical Field

[0001] This invention relates to an electroless Au alloy plating solution. More specifically, it relates to a displacement-reduction type electroless Au alloy plating solution capable of forming an Au alloy coating that exhibits solder wettability and corrosion resistance comparable to or better than conventional electroless Au coatings. Background Technology

[0002] To ensure corrosion resistance to solder and other materials, Au plating is typically formed on metal electrodes such as Cu in printed circuit board patterns, wire bonding pads of chips, and at the junctions of wiring. Electroless plating has gained attention as a manufacturing process for such Au plating. This is because the miniaturization and high integration of wiring in circuit boards and semiconductor devices has continued unabated in recent years, and electroless plating is useful in addressing this trend.

[0003] As for structures used to protect Cu wiring and the like with Au coatings, besides the structure where the Au coating is directly formed on Cu wiring as the base metal (DIG), it is also known to form a single or multiple barrier layer such as a Ni coating or a Pd coating on Cu wiring, and then use this as the base layer to form the Au coating (ENIG, ENEPIG). Furthermore, as Au plating solutions for forming Au coatings by electroless plating, displacement-reduction type electroless Au plating solutions have been known in recent years (Patent Documents 1 and 2). In displacement-reduction type electroless Au plating solutions, Au is precipitated through a displacement reaction between Au ions in the Au plating solution and the metal of the base layer, and the Au coating is grown by the action of a reducing agent contained in the Au plating solution. Displacement-reduction type electroless Au plating solutions are suitable for achieving a balance between the high adhesion of Au generated in the above displacement reaction and the good precipitation efficiency of the Au coating resulting from the reduction reaction. Moreover, as described in the aforementioned patent documents 1 and 2, by selecting appropriate additives or reducing agents, a preferred electroless Au coating can be formed efficiently.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent No. 4831710 Specification

[0007] Patent Document 2: Japanese Patent No. 3994279 Specification

[0008] Patent Document 3: Japanese Patent No. 7449411 Specification Summary of the Invention

[0009] The technical problem to be solved by the present invention

[0010] However, the price of Au has recently surged to astonishing levels, continuously setting new highs in recent days. Au is a metal that is not only important as an industrial material as mentioned above, but also circulates due to its rarity and monetary / asset value, making its price prone to fluctuations with market conditions. Therefore, in the aforementioned fields of electronic components and semiconductor devices, there is a strong desire to curb the use of Au.

[0011] However, Au possesses excellent electrical conductivity and high chemical stability, making it arguably the most suitable metal for use as a protective layer for the aforementioned electrodes and wiring. Furthermore, Au exhibits good solder wettability and moderate hardness, resulting in excellent wire bonding. Therefore, it is currently difficult to find a metal film that can replace Au.

[0012] The present invention was made against the background described above, providing a metal coating and an electroless plating solution for forming the metal coating, which is useful as a protective layer for electrodes and wiring of printed circuit boards, etc., and can replace Au coatings. In addressing this technical problem, the inventors of the present invention clarified that it is an Au alloy coating containing an Au alloy, and an electroless Au alloy plating solution having solder wettability, corrosion resistance, etc., that are equal to or better than those of Au coatings.

[0013] Technical means for solving technical problems

[0014] In replacing Au coatings with Au alloy coatings, increasing the amount of other alloying metals leads to a decrease in Au content, which is not preferable from a performance perspective. This is because it is foreseeable that a decrease in Au concentration in the Au alloy will deteriorate the aforementioned suitable properties of Au, such as corrosion resistance. In other words, the application of Au alloy coatings does not directly result in a reduction in Au usage. Therefore, the reduction in Au usage should be achieved by adjusting the film thickness of the Au alloy coating. However, even with Au coatings, a certain film thickness is required to ensure solder wettability and corrosion resistance. To achieve a reduction in Au usage, an Au alloy coating is needed that can achieve the same or better of the aforementioned properties as Au coatings with a film thickness thinner than that required for Au coatings.

[0015] In addition, when forming an Au alloy coating using an electroless Au alloy plating solution, other metals that are alloyed together with Au need to be deposited (eutectoidally) on the base metal.

[0016] In order to clarify the composition of an Au alloy coating that can replace Au coating and an electroless Au alloy plating solution capable of forming an Au alloy coating, the inventors of this invention studied Au alloy coatings containing various metals. As a result, it was found that the aforementioned prerequisites could be met in a coating containing an Au alloy (AuSn alloy) comprising Au and trace amounts of Sn. Furthermore, the composition of an electroless Au alloy plating solution capable of forming this Au alloy coating in a suitable state was discovered, thereby realizing the present invention.

[0017] That is, the present invention is an electroless Au alloy plating solution containing: a water-soluble Au compound at a concentration of 0.05 g / L or more and 5.0 g / L or less based on the Au concentration; a water-soluble Sn compound at a concentration of 0.05 g / L or more and 1.5 g / L or less based on the Sn concentration; and 6.6 × 10⁻⁶ ppm as a reducing agent. -3 mol / L or higher and 4.0 × 10 -1 Aldehyde compounds with a concentration of less than mol / L. The structure and effects of the electroless Au alloy plating solution of the present invention will be described below, and the Au alloy coating formed by the electroless Au alloy plating solution will also be described.

[0018] (A) The composition of the electroless Au alloy plating solution of the present invention

[0019] The electroless Au alloy plating solution of the present invention comprises, as essential components, water-soluble compounds of Au and Sn as metal sources, and aldehyde compounds as reducing agents.

[0020] (A-1) Water-soluble Au compounds (Au source)

[0021] In the electroless Au alloy plating bath of the present invention, a water-soluble Au compound is an essential component as the Au source constituting the Au alloy coating. Examples of water-soluble Au compounds in the present invention include gold cyanide compounds, gold sulfite compounds, gold citrate compounds, hydantoin gold compounds, and propane sulfonate gold compounds. More specifically, examples include potassium gold cyanide, potassium gold cyanide, ammonium gold cyanide, potassium gold chloride, potassium gold chloride, sodium gold chloride, sodium gold chloride, potassium gold thiosulfate, sodium gold thiosulfate, potassium gold sulfite, sodium gold sulfite, potassium gold citrate, potassium hydantoin gold, sodium hydantoin gold, and sodium methanepropane sulfonate; one or more of these water-soluble Au compounds are applicable. Potassium gold cyanide is preferably used as the gold cyanide compound. This is because plating baths using gold cyanide compounds as the gold source have good bath stability, making it difficult for gold to precipitate onto the inner wall of the plating tank during the plating process.

[0022] The content of the water-soluble Au compound, used as the Au source, is 0.05 g / L or more and 5.0 g / L or less (converted to Au). If it is less than 0.05 g / L, the amount of Au precipitated will be insufficient, making it difficult to obtain the desired precipitation rate. On the other hand, an excessive amount of water-soluble Au compound exceeding 5.0 g / L may reduce the stability of the plating solution. The content of the water-soluble Au compound is more preferably 0.10 g / L or more and 4.0 g / L or less, and particularly preferably 0.25 g / L or more and 2.0 g / L or less.

[0023] (A-2) Water-soluble Sn compounds (Sn source)

[0024] This invention relates to an electroless Au alloy plating solution for manufacturing an Au alloy coating containing Sn, characterized by the presence of a water-soluble Sn compound along with an Au source (a water-soluble Au compound). The water-soluble Sn compound serves as the Sn source for the Au alloy coating. The water-soluble Sn compound can be either divalent or tetravalent Sn compounds. Specific examples of water-soluble Sn compounds include tin halides such as tin chloride, stannates such as sodium stannate, potassium stannate, and sodium hexahydroxystannate, as well as tin compounds such as tin oxide, tin acetate, tin sulfate, and tin methanesulfonate. Tin chloride, potassium stannate, and sodium stannate are particularly preferred. They are mostly used in hydrate form (tin chloride dihydrate, potassium stannate trihydrate, sodium stannate trihydrate, etc.).

[0025] The content of the water-soluble Sn compound, which serves as the Sn source, is 0.05 g / L or more and 1.5 g / L or less when converted to Sn. If it is less than 0.05 g / L, the amount of Sn co-deposition into the coating is insufficient, making it difficult to impart the desired properties to the Au alloy coating. On the other hand, when an electroless Au alloy plating bath containing an excess of water-soluble Sn compound exceeding 1.5 g / L is used, the Sn content in the Au alloy coating tends to increase. In this case, the Au alloy coating has low wettability to the solder, and is not preferred as a substitute for conventional Au coatings. The content of the water-soluble Sn compound is more preferably 0.10 g / L or more and 1.2 g / L or less, and particularly preferably 0.15 g / L or more and 1.0 g / L or less.

[0026] (A-3) Reducing agent (aldehyde compound)

[0027] This invention relates to a displacement-reduction type electroless Au alloy plating bath, in which a reducing agent is an essential component. In this invention's electroless Au alloy plating bath, an aldehyde compound is used as the reducing agent. The aldehyde compound exerts moderate reducing power, ensuring bath stability during the plating process while allowing for appropriate precipitation of the Au alloy coating. Various compounds are known as reducing agents for electroless plating baths, but reducing agents other than aldehyde compounds have excessively high reducing power, making stable plating difficult.

[0028] Specific examples of aldehyde compounds used as reducing agents include aliphatic saturated aldehydes such as formaldehyde, acetaldehyde, propionaldehyde, n-butyraldehyde, and isobutyraldehyde; aliphatic dialdehydes such as glyoxal and succinaldehyde; aliphatic unsaturated aldehydes such as crotonaldehyde; and aromatic aldehydes such as benzaldehyde, nitrobenzaldehyde, and hydroxybenzaldehyde. Formaldehyde is particularly preferred as a reducing agent. In the present invention, which is a displacement-reduction type electroless Au alloy plating solution, a displacement reaction between Au, which has a low ionization tendency, and the base metal readily occurs. Formaldehyde, which has high reducing power in suppressing the corrosion of the base metal caused by the displacement reaction and in improving the reduction and precipitation of Au, is preferred. Furthermore, hexamethylenetetramine, which generates formaldehyde through hydrolysis, can also be added as a reducing agent.

[0029] The content of the aldehyde compound used as a reducing agent was set at 6.6 × 10⁻⁶. -3 mol / L or higher and 4.0 × 10 -1 The range below mol / L. When below 6.6 × 10⁻⁶ -3 At concentrations of mol / L, the electroless Au alloy plating bath, acting as a displacement-reduction agent, performs poorly, making it difficult to achieve film growth through reduction precipitation. On the other hand, when the aldehyde compound content exceeds 4.0 × 10⁻⁶ mol / L... -1 At concentrations of mol / L, the plating bath is prone to instability. A more preferable concentration of aldehyde compounds is 1.0 × 10⁻⁶. -2 mol / L or higher and 3.5 × 10 -1 Below mol / L, particularly preferably 1.6 × 10⁻⁶. -2 mol / L or higher and 2.0 × 10 -1 Below mol / L. Furthermore, as mentioned above, when hexamethylenetetramine is added for the purpose of generating formaldehyde in the electroless Au alloy plating bath, its concentration is preferably set to 2.1 × 10⁻⁶ mol / L. -3 mol / L or higher and 6.4 × 10 -2 Below mol / L, preferably set to 2.8 × 10⁻⁶. -3 mol / L or higher and 2.8 × 10 -2 Below mol / L.

[0030] (A-4) Any additives

[0031] The water-soluble Au compound, water-soluble Sn compound, and reducing agent described above are essential components of the electroless Au alloy plating solution of the present invention. The electroless Au alloy plating solution of the present invention may contain any of the following additives along with these essential components.

[0032] (A-4-1) Tl compound

[0033] In electroless Au alloy plating solutions, thallium (Tl) acts as a Sn precipitation promoter, making it a suitable additive to ensure the precipitation rate of Au alloy films. Tl is preferably added in the form of a water-soluble Tl compound. Examples of Tl compounds include organic acid salts such as thallium formate and inorganic acid salts such as thallium sulfate.

[0034] The concentration of the water-soluble thallium compound in the electroless Au alloy plating solution of the present invention, converted to Tl, is preferably 0.05 mg / L or more and 25 mg / L or less, more preferably 0.075 mg / L or more and 20 mg / L or less, and particularly preferably 0.1 mg / L or more and 10 mg / L or less. Furthermore, the addition of the Tl compound is arbitrary, and it is preferable to determine whether the addition of the Tl compound is necessary based on the intended use, the composition of the base metal, etc. This is because even without the addition of the Tl compound, an appropriate amount of Sn can be precipitated by adjusting other components such as the water-soluble Sn compound and optimizing the electroless plating conditions.

[0035] (A-4-2) Complexing agents

[0036] Complexing agents are additives used to ensure bath stability by forming Au ions in the plating solution into Au complexes. In the electroless Au alloy plating bath of this invention, potassium cyanide, hydantoin compounds, ethylenediaminetetraacetic acid (EDTA), sulfites, thiosulfates, etc., can be used as complexing agents. As hydantoin compounds, hydantoin, 5-methylhydantoin, 5,5-dimethylhydantoin, 3-methyl-5-ethylhydantoin, 3-methyl-5,5-diethylhydantoin, 5-propylhydantoin, 5-isopropylhydantoin, 1-(hydroxymethyl)-5,5-dimethylhydantoin, etc., can be used.

[0037] The electroless Au alloy plating solution of the present invention may contain one or more of the above-mentioned compounds as complexing agents. The total content of the compound serving as a complexing agent is preferably set at 1.5 × 10⁻⁶. -4 mol / L or higher and 3.2 × 10 -1 Below mol / L. Furthermore, the preferred concentrations of the compounds used as the complexing agents are as follows. The concentration of potassium cyanide is preferably set to 1.5 × 10⁻⁶. -4 mol / L or higher and 3.1×10 -2 Below mol / L. The addition of excess potassium cyanide significantly reduces the amount of Sn precipitated. The preferred concentration of the hydantoin compound is 7.8 × 10⁻⁶ mol / L. -3 mol / L or higher and 2.4 × 10 -1 Below mol / L, a more preferred value is 3.9 × 10⁻⁶. -3 mol / L or higher and 1.2×10 -1 Below mol / L. The preferred concentration of ethylenediaminetetraacetic acid (EDTA) is 2.7 × 10⁻⁶ mol / L.-3 mol / L or higher and 8.0 × 10 -2 Below mol / L. Furthermore, the concentration of sulfite is preferably set to 6.3 × 10⁻⁶. -4 mol / L or higher and 1.9 × 10 -1 Below mol / L. The preferred concentration of thiosulfate is 4.0 × 10⁻⁶ mol / L. -4 mol / L or higher and 4.0 × 10 -2 Below mol / L.

[0038] (A-4-3) Amine compound (reducing aid)

[0039] Amine compounds are added as reduction aids to balance the displacement and reduction reactions in electroless Au alloy plating solutions. While aldehyde compounds, which are the optimal reducing agents in this invention, exhibit moderate reducing power, using only aldehyde compounds as reducing agents sometimes makes it difficult to generate a displacement reaction, resulting in the precipitation of Au as a catalyst from the substrate metal. By using amine compounds as auxiliary reducing agents together with aldehyde compounds, the balance between the displacement reaction for Au precipitation (as a catalyst) and the reduction reaction for Au alloy film growth is optimized.

[0040] The amine compounds added as auxiliary reducing agents in this invention can be primary, secondary, and tertiary water-soluble amines, including aliphatic amines, aromatic amines, heterocyclic amines, polyamines, and alkanolamines. Specifically, N,N'-bis(2-hydroxyethyl)ethylenediamine, triethylenetetramine, N,N'-diisopropylethylenediamine, N,N,N',N'-tetra(2-hydroxypropyl)ethylenediamine, N,N,N',N'-tetra(2-hydroxyethyl)ethylenediamine, 3,3'-diamino-N-methyldipropylamine, and piperazine hexahydrate are preferred. Furthermore, the content of the amine compound in the electroless Au alloy plating bath is preferably set at 7.0 × 10⁻⁶. -4 mol / L or higher and 1.4 × 10 -1 Below mol / L. If it exceeds 1.4 × 10⁻⁶. -1 A concentration of mol / L can easily cause instability in the plating bath. The preferred content of amine compounds is 1.7 × 10⁻⁶ mol / L. -3 mol / L or higher and 7.0 × 10 -2 Below mol / L.

[0041] However, the use of amine compounds is not essential in the electroless Au alloy plating bath of the present invention. This is because the formation of byproducts of amine and aldehyde compounds can sometimes affect the stability of the bath. Regarding amine compounds, they are preferably added arbitrarily when there is a particular need to increase the deposition rate of the Au alloy coating.

[0042] (A-4-4) Other additives

[0043] In addition to the additives mentioned above, the electroless Au alloy plating bath of the present invention may also contain additives contained in the electroless Au plating bath. Examples of such additives include pH buffers. To stabilize the pH of the plating bath, a pH buffer may be added, for example, in the range of 0.1 g / L or more and 100 g / L, phosphoric acid or a phosphoric acid compound, or boric acid or a boric acid compound.

[0044] The electroless Au alloy plating solution of the present invention described above can be manufactured by mixing and dissolving the aforementioned Au source, Sn source, reducing agent, and additives selected as needed in a solvent. The mixing order is not particularly limited, nor is the state of the compounds during mixing (solid or solution state). However, it is preferable to mix the compounds in solution state. Furthermore, as the mixing order, it is preferable to start mixing from the alkaline side, taking into account the pH of each compound solution. Moreover, water, such as pure water, is preferred as the solvent.

[0045] The components and concentrations of the electroless Au plating solution of the present invention can be determined by high-frequency inductively coupled plasma optical emission spectrometry (ICP), ion chromatography (IC), high-performance liquid chromatography (HPLC), capillary electrophoresis (CE), chelate titration, and ultraviolet-visible spectrophotometry (UVVIS).

[0046] (B) Plating method using the electroless Au alloy plating solution of the present invention

[0047] Next, the Au plating method using the electroless Au alloy plating solution of the present invention will be described. The formation of an Au alloy film using the electroless Au alloy plating solution of the present invention is the same as in conventional electroless plating methods. That is, an Au alloy film is formed by contacting the substrate with the electroless Au alloy plating solution. At this time, by setting the pH and bath temperature of the electroless Au alloy plating solution to appropriate ranges, an Au film is formed at an appropriate deposition rate.

[0048] (B-1) Substrate (Base Metal)

[0049] In this invention, there are no particular limitations on the composition of the substrate for forming the Au alloy coating. The substrate only needs to have a base metal at the site where the Au alloy coating is formed; the main body of the substrate can be either a conductive material or an insulating material. The electroless Au alloy plating solution of this invention can form Au alloy coatings using a wide range of metals as base metals, including Au, Ag, Al and Al alloys (Al-Cu, Al-Si, etc.), Cu and Cu alloys (Cu-Sn, Cu-Zn, etc.), Ni and Ni alloys (Ni-P, Ni-B, etc.), and Pd and Pd alloys (Pd-P, etc.). Furthermore, the base metal can be a single layer or formed from multiple layers. Specific examples of substrates include circuit boards (printed circuit boards) and silicon substrates (silicon wafers), but are not limited to these.

[0050] The preferred pH of the electroless Au alloy plating bath of the present invention is 5.0 or higher and 9.0 or lower. When the pH is less than 5.0, the reducing power weakens, making it difficult to obtain a suitable precipitation rate. Furthermore, depending on the type of Au complex, the complexation cannot be maintained, easily leading to instability of the plating bath. On the other hand, when the pH exceeds 9.0, the reducing power becomes too strong, easily causing instability of the plating bath. Additionally, when the pH exceeds 9.0, it easily causes damage to photoresist and dry film resist, resulting in poor plating and contamination of the plating bath. A more preferred pH for the Au alloy plating bath is 6.0 or higher and 8.0 or lower.

[0051] The bath temperature of the electroless Au alloy plating bath during plating is preferably set to 40°C or higher and 90°C or lower. Below 40°C, it is difficult for Au alloy film to precipitate. On the other hand, if the bath temperature is too high, the stability of the electroless Au alloy plating bath decreases, and gold may precipitate in unwanted areas. A plating bath temperature more preferably is 60°C or higher and 85°C or lower.

[0052] According to the electroless Au alloy plating solution of the present invention, by performing plating treatment under the above plating conditions, a displacement reaction with the substrate metal can be appropriately carried out, and an Au alloy coating can be appropriately precipitated through a reduction reaction.

[0053] (C) Au alloy coating of the present invention

[0054] This invention provides an Au alloy coating that serves as a protective film for the junctions of metal electrodes, wiring, etc., and can replace the Au coatings (pure Au coatings) currently in use. This Au alloy coating is formed from the electroless Au alloy plating solution described above. The Au alloy coating of this invention is formed from an Au alloy containing trace amounts of Sn. Specifically, it is an Au alloy coating containing Sn in the range of 200 ppm to 5000 ppm.

[0055] In the Au alloy coating of the present invention, the Sn concentration is set to 200 ppm or more and 5000 ppm or less because when the Sn concentration is less than 200 ppm, the wettability and other properties with the solder are poor relative to the Au coating. Furthermore, since an excessively high Sn concentration also leads to a decrease in wettability with the solder, the upper limit for the Sn concentration is 5000 ppm or less.

[0056] The Au alloy coating of the present invention exhibits superior wettability and corrosion resistance with solder compared to conventional Au coatings, making it suitable as a protective film for joints such as metal electrodes and wiring. To achieve these properties, the Au alloy coating of the present invention achieves at least the same level of performance with a film thickness thinner than that required for conventional Au coatings. Specifically, even with a film thickness of approximately 60% compared to conventional Au coatings, the Au alloy coating of the present invention is effective. That is, in terms of obtaining the same level of performance, the Au alloy coating of the present invention can reduce the amount of Au used by 40%. Furthermore, when using the Au alloy coating of the present invention as a protective film for metal electrodes, wiring, etc., its film thickness is preferably 0.02 μm or more, more preferably 0.03 μm or more. There is no upper limit to the film thickness, but from the viewpoint of reducing the amount of Au used, 0.1 μm or less is preferred.

[0057] [The effects of the invention]

[0058] As explained above, this invention proposes a novel Au alloy coating containing Sn, addressing the conventional use of Au coatings as protective films for joints in metal electrodes, wiring, etc. This Au alloy coating can be formed using the electroless Au alloy plating solution of this invention, enabling the formation of a coating with an appropriate Sn concentration and high efficiency. The Au alloy coating of this invention exhibits superior wettability and protective performance compared to existing Au coatings with solder, achieving the same level of performance as prior art while reducing film thickness. This allows for a reduction in the amount of Au used. Detailed Implementation

[0059] First Embodiment: Hereinafter, embodiments of the present invention will be described. In this embodiment, an electroless Au alloy plating solution containing various additive metals along with an Au source was prepared and a plating process was performed. Furthermore, the wettability and other properties of the obtained Au coating and Au alloy coating were evaluated, and Sn was identified as the optimal additive metal.

[0060] Table 1 below shows the composition of the electroless Au plating solution and the electroless Au alloy plating solution manufactured in this embodiment. All of these compounds are commercially available products.

[0061] [Table 1]

[0062] Various electroless Au alloy plating solutions were used for plating treatment, and the characteristics of Au coatings and Au alloy coatings were evaluated. In this embodiment, a Ni-plated plate that had undergone electroless Ni plating treatment on a Ni plate was used as the substrate. This Ni-plated plate was formed by electroless Ni plating treatment (electroless Ni plating solution: AC MEISTER NP7600, 80°C, 14 minutes) on a Ni plate (size: 20mm × 20mm, thickness: 0.1mm) to form a Ni coating with a target thickness of 4μm. Then, electroless plating treatment was performed while maintaining a bath temperature of 78°C and pH 7.0. In addition, the target film thickness was set to 0.03μm, and the plating treatment time was set to 3 to 5 minutes. For the electroless Au plating solution, a sample was also prepared with a target film thickness of 0.05μm and electroless plating treatment for 5 minutes. After electroless plating, the formed Au coating and the Au alloy coating and the substrate metal (Ni substrate) were evaluated by the following tests.

[0063] [Coating Thickness Measurement]

[0064] For substrates recovered after electroless plating, the film thickness of Au coating and Au alloy coating was measured using a fluorescence X-ray film thickness meter (Hitachi High Technology Co., Ltd.: FT-150h), confirming that the set film thickness was achieved.

[0065] [Determination of the Eutectoid Amount of Added Metal]

[0066] In this embodiment, an electroless Au alloy plating solution suitable for Sn, Cu, In, Bi, Ag, Pt, and Ni was prepared as the added metal for the Au alloy coating, and a plating treatment was performed. For the Au alloy coating thus formed, the amount of each added metal contained in the film (eutectoid amount) was measured to confirm whether eutectoid formation of the added metal had occurred. The determination of the eutectoid amount in the coating was performed in the following order.

[0067] (1) The quality of the coating was determined based on the quality of the substrate before and after the coating treatment.

[0068] (2) The substrate after plating was immersed in 5 mL of stripping solution (EEJA HAKUREX GS-N) to dissolve and strip the coating.

[0069] (3) The volume of the treatment solution was adjusted with pure water (10 mL), and the solution was analyzed using an ICP analyzer (Shimadzu Corporation: ICPS-8100CL).

[0070] (4) Based on the ICP analysis results and the quality of the coating, calculate the concentration of the added metal in the coating and use it as the co-deposition amount.

[0071] [Solder wettability evaluation]

[0072] For each Au alloy coating and Au coating, a solder wetting spread test was conducted to evaluate solder wettability. The solder wetting spread test was performed in the following order.

[0073] (1) The substrate sample with Au alloy coating was placed on a hot plate at 280°C for 30 seconds (preheating).

[0074] (2) To 760μm solder balls (SAC305 (96.5% mass Sn - 3.0% mass Au - 0.5% mass Cu)) were coated with flux (RMA-367EN, manufactured by ALPHA Metals).

[0075] (3) Place the nine solder balls one by one on the Au alloy coating of the sample.

[0076] (4) Place the sample on a hot plate at 250°C for 15 seconds to melt the solder balls (reflow).

[0077] (5) The solder wetting spread area was determined using the measurement mode of a microscope. Nine solders were measured, and the average value was recorded.

[0078] [Visual inspection and corrosion evaluation of the base metal]

[0079] The appearance of the Au alloy coatings formed by each electroless Au alloy plating solution and whether they corroded the substrate metal were confirmed. Whether the substrate metal corroded was evaluated by determining the eutectoid amount of the added metal and observing the substrate after the coating was removed with a stripping solution. These observations were conducted using visual inspection, a semiconductor inspection microscope (Olympus Corporation: MX61L), and a scanning electron microscope (Hitachi High Technology Corporation: SU8020). The presence or absence of corrosion marks was used to identify samples with corrosion.

[0080] Table 2 shows the evaluation results of the coatings obtained from various electroless Au alloy plating solutions of this embodiment.

[0081] [Table 2]

[0082] Regarding the types of metals (Sn, Cu, In, Bi, Ag, Pt, Ni) added to Au to form the Au alloy coating, when Sn is added, the Sn concentration (eutectoid amount) in the coating is approximately 200 ppm. On the other hand, for metals other than Sn, it has been confirmed that the metal concentrations in the coating for In, Bi, and Pt are extremely low, less than 50 ppm or less than 10 ppm. For these metals with low eutectoid amounts, it is expected that their effect on improving the properties of the Au coating will be minimal.

[0083] Therefore, the evaluation results of solder wettability for various Au alloy coatings confirmed that the Sn-containing Au alloy coating (AuSn alloy coating) was particularly good. For the same film thickness (0.03 μm), the AuSn alloy coating (F1) had a wettability spread area approximately 1.5 times that of the unalloyed Au coating (F8). Furthermore, the 0.03 μm thick AuSn alloy coating (F1) had a larger wettability spread area than the 0.05 μm thick Au coating (F9), indicating excellent solder wettability. This demonstrates that the AuSn alloy coating achieves solder wettability of equal or greater than that of the Au coating with a thinner film thickness. In other words, it can be seen that the amount of Au used can be suppressed by reducing the film thickness using the AuSn alloy coating. Moreover, the AuSn alloy coating did not exhibit corrosion of the base metal, thus confirming its ability to replace the Au coating.

[0084] Observations of the characteristics of Au alloy coatings formed by adding metals other than Sn reveal that Au alloy coatings with added In, Bi, and Pt (F3, F4, F6) exhibit worse solder wettability than AuSn alloy coatings, and corrosion of the base metal was observed. For In, Bi, Ag, and Pt, these additive metals are not preferred as alternatives to conventional Au coatings. Furthermore, regarding Ni, although the eutectoid content is relatively high (200 ppm), the solder wettability of the Ni-containing Au alloy coating (F7) is similar to that of the Au coating (F8), making it difficult to say that it can replace Au coatings.

[0085] Furthermore, the Au alloy coating containing Ag (F5) has a high eutectoid content (1000 ppm) and exhibits similarly good solder wettability to the AuSn alloy coating (F1). However, corrosion of the base metal was observed in the Ag alloy coating containing Ag (F5). Therefore, it is difficult to say that it can replace the Au alloy coating.

[0086] Furthermore, the Cu-containing Au alloy coating (F2) exhibits a higher eutectoid content (1000 ppm) and superior solder wettability compared to the Au coating (F8). Additionally, this Au alloy coating does not corrode the base metal. However, the solder wettability of the Cu-containing Au alloy coating is worse than that of the Sn-containing Au alloy coating (F1). Therefore, it can be said that the reduction in Au usage based on film thickness is insufficient.

[0087] The above research results confirm that, from the viewpoint of solder wettability and inhibition of substrate corrosion, AuSn alloy coating is the most preferred Au alloy coating as an alternative to conventional Au coating, and an electroless Au alloy plating solution suitable for forming such coating is the most preferred.

[0088] [Evaluation of wire bonding]

[0089] Therefore, the wire bonding properties of the films coated with electroless Au alloy plating solution No. 1 and electroless Au plating solution No. 8 were evaluated. In this evaluation test, a copper plate (size: 20mm × 20mm, thickness: 0.1mm) was used as the substrate. Moreover, multilayer films with the following three patterns were formed.

[0090] (i) Direct formation of Au alloy coating (DIG) on Cu substrate

[0091] (ii) A Ni film is formed on a Cu substrate as the base metal to form an Au alloy coating (ENIG).

[0092] (iii) A Ni film and a Pd film are formed on a Cu substrate as the base metal to form an Au alloy coating (ENEPIG).

[0093] In the preparation of the aforementioned substrate, the copper plate underwent acid degreasing (EETOREX 15, 25°C, 1 min), soft etching (MICROFAB 74, 25°C, 1 min), and then sulfuric acid activation treatment with 10% sulfuric acid. Next, catalyst-attributing treatment (IM FAB Pd-AC2, 25°C, 1 min) and electroless Ni treatment (AC MEISTERNP7600, 80°C, 14 min) were performed to form a 4 μm thick Ni film (ENIG). Then, an electroless Pd plating solution (AC FAB Pd2000S, 52°C, 10 min) was applied to this Ni film to form a 0.1 μm thick Pd film.

[0094] For substrates with coatings (i) to (iii), electroless plating was performed using electroless Au alloy plating solution No. 1 and electroless Au plating solution No. 8, respectively. The plating conditions were maintained at a bath temperature of 78°C and a pH of 7.0. The plating time was adjusted to achieve a film thickness of 0.15 μm, thereby forming an Au alloy coating.

[0095] In the evaluation of wire bonding performance, a wire tensile test was performed. In the wire tensile test, Au wire (25 μm diameter) was bonded to the aforementioned Au alloy coating using a wire bonding machine (a manual bonding machine (MicroPoint Pro iBond 5000) at a stage temperature of 120°C). Then, a 10-point evaluation of the tensile strength at a tensile speed of 200 μm / sec was performed using a bond tester (device name: Pull Tester (Nordson Advanced Technology STELLAR4000)).

[0096] The results of the above-mentioned wire bonding evaluation tests are shown in Table 3, which presents the tensile strength of each substrate with configurations (i) to (iii).

[0097] [Table 3]

[0098] As shown in Table 3, the tensile strength of the AuSn alloy coating is equal to or better than that of the Au coating. This confirms that the AuSn alloy coating offers superior wire bonding compared to conventional Au coatings. Furthermore, it was confirmed that the preferred properties of the AuSn alloy coating can be utilized on substrates with various substrate structures (DIG, ENIG, ENEPIG) with different base film structures.

[0099] [Hardness Measurement of Au Alloy Coating]

[0100] Generally, it is known that there is a correlation between the hardness of a coating and wire bonding; higher hardness tends to reduce wire bonding. Therefore, the hardness of coatings formed from electroless Au alloy plating solutions No. 1 and No. 8 was measured. In the hardness measurement, Au alloy coatings and Au coatings with a thickness of 1.5 μm were formed on a substrate (ENIG) constructed as described in (ii) above. The hardness of each coating (indentation strength 5000 μN) was then measured using a nanoindenter.

[0101] As a result, the Au coating using the electroless Au alloy plating solution No. 8 has a Vickers hardness of 146.1 Hv. The Au alloy coating using the electroless Au alloy plating solution No. 1 has a hardness of 142.9 Hv. This confirms that both have equivalent hardness. Based on the hardness test results, it is also confirmed that the Au alloy coating of the present invention has the same level of wire bonding as the Au coating and can replace the Au coating.

[0102] Second Implementation Method Based on the results of the first embodiment, it was confirmed that an AuSn alloy coating is the most preferred alternative to an Au coating. Therefore, in this embodiment, an electroless Au alloy plating solution was manufactured with variations in the type and concentration of the Sn source (water-soluble Sn compound), as well as the reducing agent and additives, for an electroless Au alloy plating solution containing Sn as the added metal. Furthermore, the Au alloy coatings formed from each electroless Au alloy plating solution were evaluated in the same manner as in the first embodiment. The structure of the electroless Au alloy plating solution manufactured in this embodiment is shown in Table 4 below.

[0103] [Table 4] The electroless plating solution in Example 1 is No. 1 of the first embodiment.

[0104] Furthermore, using the same Ni-plated plate as in the first embodiment as the substrate, an Au alloy coating (AuSn alloy coating) was formed. In the electroless plating process of this embodiment, the bath temperature was maintained at 78°C and the pH at 7.0. The target film thickness was set to 0.03 μm, and the plating treatment time was adjusted. During the plating treatment, the presence or absence of precipitation was observed to evaluate bath stability. The absence of precipitation was then evaluated as "good" bath stability (◎), and the presence of precipitation was judged as "poor" bath stability (×). After the plating treatment, film thickness measurement, solder wettability evaluation, and substrate metal corrosion evaluation were performed using the same methods / conditions as in the first embodiment. These results are shown in Table 5.

[0105] [Table 5]

[0106] Table 5 confirms that an electroless Au alloy plating bath with appropriate Sn source concentration, reducing agent, and additives can form a preferred Au alloy coating (Examples 1-13). Furthermore, the bath exhibits good stability.

[0107] Regarding the concentration and type of Sn source in the electroless Au alloy plating solution, there is a tendency to increase the Sn concentration of the Au alloy coating by increasing the concentration of the Sn source in the electroless Au alloy plating solution (Examples 2 and 4). Furthermore, in addition to tin chloride dihydrate, potassium stannate (IV) trihydrate can also be used as a water-soluble Sn source. In this case, the concentration of the Sn source is also correlated with the Sn concentration of the Au alloy coating (Examples 9 and 10). Moreover, if the concentration of the Sn source in the plating solution is too high, the Sn concentration of the Au alloy coating exceeds 4000 ppm (Comparative Example 3); if the concentration of the Sn source in the plating solution is too low, the Sn concentration of the Au alloy coating is below 200 ppm (Comparative Example 4). The AuSn alloy coatings of Comparative Examples 3 and 4 have small solder spread areas and poor solder wettability. Therefore, regarding Sn compounds in electroless Au alloy plating solutions, if they are water-soluble, various types of compounds can be selected, and the concentration of the compounds needs to be set within an appropriate range.

[0108] In addition to the Au and Sn sources, the essential components of the electroless Au alloy plating bath of the present invention also include a reducing agent. In an electroless Au alloy plating bath without a reducing agent, corrosion of the base metal occurs, the amount of Sn co-deposition is low, and the solder wettability is poor (Comparative Example 1). This is because the electroless Au alloy plating bath without a reducing agent is a plating bath that only performs a displacement-type precipitation function, so the displacement reaction between Au ions and the base metal in the bath becomes the main process. Furthermore, when a reducing agent is added to the electroless Au alloy plating bath, formaldehyde or hexamethylenetetramine can be added to generate formaldehyde in the plating bath (Example 7).

[0109] Furthermore, the concentration of formaldehyde, as a reducing agent, can be adjusted in the same way as the other components (Example 13). However, electroless Au alloy plating baths containing excessive reducing agent precipitate during plating, resulting in poor bath stability (Comparative Example 2). Appropriate adjustment of the reducing agent concentration is required for stable plating. However, even with the electroless Au alloy plating bath of Comparative Example 2, the resulting Au alloy coating exhibits suitable solder wettability and does not cause substrate corrosion. This is because the Au alloy coating itself contains an appropriate amount of Sn.

[0110] Regarding additives for electroless Au alloy plating solutions, the amount of Sn co-deposition increased by adding a Tl compound (comparison between Example 2 and Example 8). However, even with an electroless Au alloy plating solution without a Tl compound, the necessary amount of Sn co-deposition was confirmed, and the AuSn alloy coating exhibited good solder wettability and no corrosion of the base metal (Example 8). Therefore, a Tl compound is a preferred additive, but not essential. Furthermore, in addition to organic acid salts (thallium formate), inorganic acid salts (thallium sulfate) can also be used as the Tl compound (Example 3).

[0111] Regarding the complexing agent, it can be seen that the co-eutectoid amount of Sn tends to increase as the content of potassium gold cyanide decreases (Example 1). Furthermore, regarding the amine compound as a reduction aid, its addition tends to increase the co-eutectoid amount of Sn, but even without its addition, the solder wettability of the Au alloy coating is good, so it can be considered an arbitrary additive (Example 5). In addition to N,N'-bis(2-hydroxyethyl)ethylenediamine, triethylenetetramine can also be used as the amine compound (Example 6). Furthermore, the concentration of the amine compound can be adjusted (Example 12).

[0112] Based on the above research results, it has been confirmed that a suitable AuSn alloy coating can be formed by appropriately configuring the electroless Au alloy plating solution. Regarding the AuSn alloy coating, if the Sn concentration (eutectoid content) is 200 ppm or higher, the solder wettability is good, and there is no corrosion of the base metal. Furthermore, the AuSn alloy coatings of each embodiment exhibit solder wettability equal to or greater than that of an Au coating with a thickness of 0.05 μm (F9 of the first embodiment), thus confirming that a reduction in the amount of Au used can be achieved.

[0113] [Industry availability]

[0114] As explained above, this invention provides an Au alloy coating containing Sn, which can replace Au coatings currently used for electrode protection. Even with a thinner film thickness compared to existing Au coatings, the Au alloy coating of this invention exhibits solder wettability, corrosion resistance, wire bonding, and other properties of equal or higher. By reducing the film thickness, cost reduction due to the reduced amount of Au used can be achieved. The Au alloy coating of this invention can be manufactured using an electroless Au alloy plating bath containing a suitable Au source, a Sn source, and a reducing agent as essential components.

[0115] The electroless Au alloy plating solution of the present invention is useful for forming Au alloy coatings on circuit boards and the like in various electronic and semiconductor devices, and is also applicable to the miniaturization and high integration of wiring in recent years.

Claims

1. An electroless Au alloy plating solution, comprising: Water-soluble Au compounds with an Au concentration of 0.05 g / L or higher and 5.0 g / L or lower; Water-soluble Sn compounds with a Sn concentration of 0.05 g / L or higher and 1.5 g / L or lower; and 6.6 × 10⁻⁶ as a reducing agent -3 mol / L or higher and 4.0 × 10 -1 Aldehyde compounds below mol / L.

2. The electroless Au alloy plating solution according to claim 1, wherein it contains a water-soluble Ti compound at a concentration of 0.05 mg / L or more and 25 mg / L or less.

3. The electroless Au alloy plating solution according to claim 1 or 2, wherein, As a complexing agent, it contains a total of 1.5 × 10 -4 mol / L or higher and 3.2 × 10 -1 At least one of the following: potassium cyanide, hydantoin compounds, ethylenediaminetetraacetic acid salt, sulfite, thiosulfate, thiomalate, and thiocyanate, at a concentration of less than mol / L.

4. The electroless Au alloy plating solution according to claim 1 or 2, wherein, As a reducing aid, it contains 7.0 × 10 - 4 mol / L or higher and 1.4 × 10 -1 Amine compounds with concentrations below mol / L.

5. The electroless Au alloy plating solution according to claim 1 or 2, wherein, The water-soluble Sn compound includes at least one of tin halides, stannates, tin oxides, tin acetate, tin sulfate, and tin methanesulfonate.

6. The electroless Au alloy plating solution according to claim 1 or 2, wherein, The water-soluble Sn compound includes at least one of tin chloride, potassium stannate, and sodium stannate.

7. An Au alloy coating formed on a metal film formed on a substrate, containing Sn at a concentration of 200 ppm or more and 5000 ppm or less.

8. An Au alloy coating, formed from the electroless Au alloy plating solution according to any one of claims 1 to 6.

9. A device having an Au alloy coating as described in claim 7 or 8.

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