Gold plating bath and final gold plating finish

Abstract

An autocatalytic gold bath capable of depositing gold from a solution onto a substrate having one or more metallic layers. The autocatalytic gold bath includes: (a) a chelating agent; (b) a gold salt; and (c) a reducing agent comprising an organic molecule with more than one carbon atom. A gold coating process is also included for applying a final finish to the surface of the metallic layers of the substrate. The metallic layers can be formed by ENIG, ENEPIG, EPAG, direct gold-on-copper, or gold-on-silver processes.

Description

Gold plating bath and final gold plating finish Field of invention The present invention relates in general to a final gold surface treatment to increase the solderability of a circuit board or integrated circuit (IC) substrate. Background of the invention Surface treatments are used to improve connectivity. One example of a surface treatment involves gold plating, which is best suited for the final surface treatment of printed circuit boards (PCBs). It has excellent physical properties such as electrical conductivity, chemical resistance, and oxidation resistance, as well as ensuring reliable solder joints when mounting electronic components. Nickel plating is typically used as the base metal for the electrolytic gold plating. The surface finish / treatment protects or forms the connection between the board and a device. Methods for applying electrolytic gold plating after electrolytic nickel plating on the copper wiring of a PCB include, but are not limited to, the following: a) Ni anelectrolytic / Au immersion (ENIG); b) Anelectrolytic Ni / autocatalytic Au (ENAG); c) Ni anelectrolytic / Au immersion / Au autocatalytic (ENIGAG); d) Ni anelectrolytic / Pd anelectrolytic / Au immersion (ENEPIG); e) Au direct on Cu; and f) Au on Ag. As the manufacturing and development of electronic components and semiconductor parts continue to advance, improvements in metal plating techniques are also required. For example, a metal plating technique can be used to form a circuit pattern on a substrate using a metal such as copper, which has low electrical resistance, when fabricating a semiconductor package, followed by nickel plating, palladium plating, and a gold plating to form a bonded part. Electrolytic nickel plating and gold immersion plating are frequently used for surface treatment in applications requiring high reliability in the assembly of printed circuit boards or electronic components. For example, in electrolytic nickel / gold immersion plating, the gold immersion layer protects the underlying electrolytic nickel plating from oxidation. Furthermore, a nickel-plated coating film is often used as a barrier film to prevent erosion of a copper circuit caused by soldering. Subsequently, a palladium-plated film can be used as a barrier film to prevent diffusion from the nickel-plated coating film to the gold-plated metal coating film. Since the gold-plated metal coating film has low electrical resistance and good solder wettability, it can be applied as a final finish to produce a bonded part with excellent bonding properties, including solderability and / or wire bonding. This metal coating film comprises a metal coating film made of an underlying metal, such as nickel and / or palladium, and the gold-plated metal coating film. Furthermore, an underlying metal, such as palladium, can be subjected to gold immersion plating to ensure adhesion between the plating film and the underlying metal. However, since gold immersion plating stops the reaction when the underlying metal is fully substituted, it can limit the thickness of the gold plating layer that forms. On the other hand, the formation of a thick gold plating film may be necessary on certain parts joined by wire soldering.To form the thick gold-coated film, a two-stage gold coating process is performed: subjecting the underlying metal to an immersion gold coating process to ensure adhesion, and then further subjecting the underlying metal to an electrolytic reduction-type gold coating. The plating process itself can be complicated by subjecting the underlying metal, such as palladium, to an immersion gold plating process, followed by a reduction-type electrolytic gold plating process. Immersion gold plating deposits gold using the redox potential difference between the plating film and the underlying metal. Immersion gold plating can partially cause severe corrosion of the underlying metal. Electrolytic gold plating baths can suppress corrosion of the underlying metal; however, problems related to the stability of the electrolytic gold plating bath can arise, resulting in plating deficiencies and an undesirable appearance of the gold plating. In gold-plated dip coating, gold is deposited using the redox potential difference between an underlying nickel layer and a coating bath, whereby the gold dissolves and corrodes the nickel. Additionally, nickel diffusion occurs over the gold film, reducing the wire's solderability. To mitigate this, a reduction gold coating can be applied over the dip-plated gold or electroless nickel coating films to form a thicker gold film, thus preventing the reduction in wire solderability, but this comes at a cost. Due in part to the convenience of using lead-free solders, there has been a trend toward the use of Sn-Ag-Cu solders. However, a higher thermal load is required at the solder joint compared to conventional tin-lead eutectic solders, resulting in inferior solder characteristics. To overcome this problem, a method has been developed that interposes a palladium film between the electroless nickel plating layer and the gold-in-dip metallic coating layer using an electroless palladium-coated metal coating. Autocatalytic and electrolytic gold plating baths tend to be unstable, leading to the precipitation of costly gold salts and metallic gold in the plating solution. Therefore, it is desirable to avoid this instability through careful selection of the additives used to stabilize the gold plating solution. US patent 8,124,174 to Kurosaka et al. describes an electrolytic gold plating bath comprising a water-soluble gold compound, a complexing agent, formaldehyde and / or a formaldehyde bisulfite adduct, and an amine compound. However, the use of formaldehyde and / or a formaldehyde bisulfite adduct has been found to inherently cause instability in the gold plating bath. Therefore, it would be desirable to discover other reducing agents that do not cause instability in the gold-plated metallic coating bath. In other words, it would be desirable to provide a gold-plated metallic coating bath that exhibits improved stability and reduced corrosion, and that can preserve the weldability of the deposit. US patent 2008 / 0138507 A1 describes an electrolytic gold-plated coating bath that includes a water-soluble gold compound, a complexing agent, an aldehyde compound, and an amine compound. US patent 5,803,957 describes an electrolytic gold plating bath comprising a water-soluble gold compound, a complexing agent, a reducing agent, and polyvinylpyrrodidone. US patent 2007 / 0095249 A1 describes an electrolytic gold plating liquid comprising a cyanide-free gold salt, an alkali metal salt or ammonium salt of sulfurous acid and thiosulfuric acid as a metal complexing agent, a hydroxyalkylsulfonic acid or a salt thereof as a reducing agent, and an amine compound. Summary of the invention An object of the present invention is to provide an electrolytic gold plating bath capable of providing a metallic gold coating layer or film on an underlying substrate. Another object of the present invention is to provide an electrolytic gold plating bath capable of providing a metallic gold coating layer over an underlying nickel-plated or palladium layer. Another object of the present invention is to provide a metallic coating layer with gold that exhibits greater final solderability of a circuit board. Another object of the present invention is to provide a metallic coating layer with gold that exhibits good conductivity of an electrical contact interface. Another object of the present invention is to provide a gold-plated metal coating bath that provides good stability. Another object of the present invention is to provide a metallic coating bath with gold that does not precipitate gold or gold salts. To that end, in one aspect, the present invention provides a self-catalytic gold metallic coating bath according to claim 1. In another aspect, the present invention provides a method for providing a gold finish on a substrate by self-catalytic gold metallic coating, according to claim 7. Some more specific aspects of the invention are set out in the dependent claims. Detailed description The inventors of the present invention have unexpectedly discovered that using higher molecular weight reducing agents containing additional carbon and / or oxygen atoms bonded to the reducing agent can produce a gold-plated metal coating that exhibits greater stability against plating removal and precipitation of gold and gold salts than the formaldehyde and formaldehyde adducts described above. In one embodiment, the gold-plated metal coating described herein can enhance the final solderability of a circuit board. Therefore, when using the gold-plated metal coating described herein, a final finish coating can be applied to an underlying metallic layer (such as electroless gold or electroless palladium) to preserve solderability. As used in this report, "a", "one" and "the" refer to both singular and plural referents, unless the context clearly indicates otherwise. As used herein, the term "approximately" refers to a measurable value such as a parameter, quantity, duration, and the like, and is intended to include variations of + / -15% or less, preferably variations of + / -10% or less, more preferably variations of + / -5% or less, even more preferably variations of + / -1% or less, and still more preferably variations of + / -0.1% or less of, and with respect to, the value specifically mentioned, insofar as such variations are suitable for implementation in the invention described herein. Furthermore, it should also be understood that the value to which the modifier "approximately" refers is specifically described herein. As used herein, spatially relative terms such as "below," "below," "lower," "above," "superior," and the like are used to facilitate description and to describe the relationship of one element or feature to another element(s) or feature(s), as illustrated in the figures. It is further understood that the terms "previous" and "subsequent" are not intended to be limiting and are intended to be interchangeable where appropriate. As used herein, the terms "comprises" and / or "comprising" specify the presence of established features, integers, stages, operations, elements, and / or components, but do not exclude the presence or addition of one or more additional features, integers, stages, operations, elements, components, and / or groups thereof. As used herein, the term "substantially free" or "essentially free," unless otherwise defined herein for a particular element or compound, means that a given element or compound is not detectable by ordinary analytical means well known to those skilled in the art of metal coating for bath analysis. Such methods typically include atomic absorption spectrometry, titration, UV-Vis analysis, secondary ion mass spectrometry, and other commonly available analytical methods. One aspect of the present invention relates to the provision of a self-catalytic gold bath according to claim 1, for depositing gold from the solution onto a surface by immersion deposition and / or non-electrolytic deposition. The gold-plated metal coating described herein generally comprises: A) a chelating agent; B) a golden salt; and C) a reducing agent, wherein the reducing agent comprises an organic molecule having more than one carbon atom in the organic molecule. The chelating agent, the gold salt, and the reducing agent are more specifically defined in claim 1. The chelating or complexing agent is selected from phosphoric acid, boric acid, Rochelle salt, citric acid, gluconic acid, tartaric acid, lactic acid, malic acid, ethylenediamine, triethanolamine, ethylenediaminetetraacetic acid, nitrilotriacetic acid, diethylenetriaminepentaacetic acid, hydroxyethylethylenediaminetriacetic acid, triethylentetraminohexaacetic acid, 1,3-propanediaminetetraacetic acid, 1,3-diamino-2-hydroxypropanetetraacetic acid, hydroxyethylimiminodiacetic acid, dihydroxyglycine, glycoletherdiaminetetraacetic acid, dicarboxymethylglutamic acid, hydroxyethylidenediphosphoric acid, ethylenediaminetetra(methylenephosphoric) acid, alkali metal salts (e.g., sodium or potassium), alkaline earth metal salts, and ammonium salts thereof. These complexing agents can be used alone or in combination. In a preferred embodiment, the chelating or complexing agent is ethylenediaminetetraacetic acid or a salt thereof.In another preferred embodiment, the complexing agent comprises one or more of: ethylenediaminetetraacetic acid, malic acid or alkaline earth metal or ammonium salts thereof; even more preferably, the complexing agent may comprise a combination of ethylenediaminetetraacetic acid, sodium salt and sodium malate. The chelating or complexing agent can be used in the gold metal coating bath at a concentration of between approximately 2.0 and approximately 100 g / l, more preferably from approximately 5 to approximately 75 g / l, most preferably from approximately 10 to approximately 50 g / l. When the concentration is below this range, the deposition rate may decrease due to metal dissolution. Furthermore, concentrations above this range do not provide any additional benefit to the function of the metal coating bath. Gold salt is a water-soluble gold salt, more specifically a gold cyanide, such as potassium gold cyanide, sodium gold cyanide, ammonium gold cyanide, and the like. Other water-soluble gold salts include sulfites, sulfates, thiosulfates, thiocyanates, nitrates, methanesulfonates, tetraamine complexes, chlorides, bromides, iodides, hydroxides, oxides, and the like. It is also noted that these water-soluble gold salts can be used alone or in combination with one another. In a preferred embodiment, the gold salt is potassium gold cyanide. Water-soluble gold salt can be used in the composition at a concentration of between approximately 0.5 and approximately 5.0 g / l, more preferably from approximately 0.2 to approximately 3.0 g / l, and most preferably at a concentration of between approximately 0.5 and approximately 1.5 g / l. The pH of the electroless gold plating bath of the present invention is preferably in the range of approximately 5 to approximately 10, more preferably from approximately 6 to approximately 9, and most preferably approximately 8.0. When the pH is below the aforementioned range, the deposition rate may decrease, while above the aforementioned range, the plating bath may become unstable. Based on the foregoing, if necessary, a pH adjuster may be added to the gold plating solution. Suitable pH adjusters include, but are not limited to, sodium hydroxide, potassium hydroxide, ammonia, sulfuric acid, phosphoric acid, boric acid, and other similar compounds that can be used in autocatalytic or electroless gold plating baths. The electrolytic gold plating bath is normally maintained at an elevated temperature, such as a temperature within the range of approximately 60 to approximately 100 °C, more preferably in the range of approximately 70 to approximately 95 °C, most preferably at a temperature between approximately 80 and approximately 88 °C. The reducing agent usable in the gold metallic coating bath of the invention is selected from the sodium bisulfite and 2-methoxybenzaldehyde addition compound, the sodium bisulfite and acetone addition compound, the sodium bisulfite and p-anisaldehyde addition compound, the bisulfite and glutaraldehyde addition compound, and combinations of one or more of the foregoing. The inventors of the present invention have also discovered that formaldehyde or bisulfite-formaldehyde addition compounds do not provide a stable bath composition and, therefore, formaldehyde and bisulfite-formaldehyde addition compounds cannot be used in the baths of the present invention and are specifically excluded from the compositions and processes described herein. That is, the aldehydes usable in the practice of the present invention must contain two or more carbon atoms. The bisulfite and aldehyde addition compounds usable in the present invention include the sodium bisulfite salt of p-anisaldehyde and the sodium bisulfite salt of 2-methoxybenzaldehyde. The bisulfite and aldehyde addition compounds usable in the present invention may be the sodium salts of glutaraldehyde. The bisulfite and ketone addition compound usable in the practice of the present invention is the sodium bisulfite salt of acetone. Although the addition compounds described above comprise sodium salts, potassium salts and ammonium salts of bisulfite addition compounds could also be used. The concentration of the reducing agent is normally in the range of approximately 0.5 to approximately 25 g / l, more preferably from approximately 1 to approximately 20 g / l, most preferably in the range of approximately 3 to approximately 10 g / l. In one embodiment, the gold-plated metal coating bath also comprises an amine. Examples of suitable amines include, but are not limited to, alkylamines containing an amino group, such as butylamine, enthylamine, hexylamine, heptylamine, octylamine, nonylamine, decylamine, undecylamine, dodecylamine, tridecylamine, tetradecylamine, pentadecylamine, hexadecylamine, heptadecylamine, octadecylamine, nonadecylamine, or eicodecylamine. Furthermore, the compound having an amino group may have a branched structure. Other suitable amines are described, for example, in US patent 8,124,174 to Kurosaka et al. and US patent 8,771,409 to Asakawa et al. In one embodiment, the amine is selected from the group consisting of ethylenediamine, diethylenetriamine, triethylenetetramine, pentaethylenehexamine, and combinations of one or more of the above. If used, the amine is used in the metal coating bath at a concentration of between approximately 0.1 and approximately 100 g / l, more preferably at a concentration of between approximately 0.5 and approximately 10 g / l. In one embodiment, the molar ratio between the reducing agent and the amine compound is preferably in the range of approximately 1:30 to approximately 3:1, more preferably from 1:10 to 1:1. If desired, the gold metallic coating bath may also contain other suitable additives including, but not limited to, surfactants, crystallization modifiers, buffers, flattening agents, thickness control agents, antifoaming agents, and other similar compounds. In a preferred embodiment, the gold-plated metal coating bath is subjected to gentle stirring, such as by stirring with a rod. Additionally, the bath may be subjected to periodic or continuous filtration. The gold electroplating bath can be monitored periodically or continuously to maintain the concentration of the constituents within the desired range. As described herein, in one embodiment, the gold metallic coating bath described herein is used to produce a final finish on an underlying metallic layer, such as in ENIG, ENEPIG, EPAG processes, direct gold coating on copper or gold or on silver. Therefore, the present invention can be used in the printed circuit board industry as a final coating for circuit board metals to preserve solderability. This final coating allows circuit boards to be properly soldered with components, especially surface-mount components. In one embodiment, the present invention is used as a final finish in an electrolytic gold immersion process on nickel (ENIG) or in an electrolytic gold immersion process on nickel and platinum (ENEPIG). The invention described herein provides an immersion and / or autocatalytic gold bath that exhibits improved stability and reduced plaque formation compared to prior art gold metal coating baths. In another aspect, the present invention also relates in general to a method as defined in claim 7 for providing a final gold finish on a substrate. As described herein, the gold-based metal coating described herein can be used as a final finish over a variety of underlying metals. Accordingly, in one embodiment, the substrate comprises a metallic layer, preferably a metal coating film, deposited onto the substrate. In another embodiment, the substrate comprises a circuit board or an integrated circuit (IC) substrate with a metallic film deposited thereon. The deposited metallic film can be EN, ENEP, or ENIP, by way of example and not as a limitation. The substrate with the metallic layer on it is preferably brought into contact with the gold plating bath by immersing the substrate in the gold plating bath for a period of time sufficient to achieve the desired thickness. In one embodiment, the desired thickness is sufficient to increase the solderability of the metal film stack. In one embodiment, the desired thickness is in the range of approximately 0.001 to approximately 40 µm, more preferably from approximately 0.01 to approximately 10 µm, and most preferably from approximately 0.05 to approximately 1 µm. The plating time is simply the time required to achieve the desired thickness. In an ENIG metal coating process, a catalytic metal coating film with anelectrolytic nickel is deposited onto an underlying substrate using known anelectrolytic nickel processes. When an electroless nickel plating film is deposited onto a surface to be coated (e.g., a copper substrate) using a catalyst, the catalyst metal includes nickel, cobalt, iron, silver, gold, ruthenium, palladium, platinum, or similar metals, with palladium being preferred. The amount of catalyst deposition may be sufficient to activate the process to the point where an electroless nickel plating film is deposited onto the surface to be coated. In one embodiment, the anelectrolytic nickel plating bath comprises a water-soluble nickel salt, a reducing agent, and a complexing agent. Suitable water-soluble nickel salts include nickel sulfate and nickel chloride. Suitable reducing agents include hypophosphorous acid such as hypophosphite or sodium hypophosphite, dimethylamine borane, trimethylamine borane, hydrazine, or similar compounds. Complexing agents include carboxylic acids such as malic acid, succinic acid, lactic acid, or citric acid, sodium salts thereof, and amino acids such as glycine, alanine, iminodiacetic acid, arginine, or glutamic acid. In another embodiment, the anelectrolytic nickel plating bath also contains a sulfur compound. The formed anelectrolytic nickel metallic coating film should preferably have a thickness of 0.1 to 20 µm, more preferably 1 to 15 µm. When the thickness is less than 0.1 µm, there is concern that the cable's bonding capacity will decrease. Above 20 µm, the metallic coating process is time-consuming, potentially worsening productivity, which represents a cost disadvantage. In an ENEPIG coating process, the catalytic nickel anelectrolytic metallic coating film is deposited onto it by an anelectrolytic palladium metallic coating film. The anelectrolytic palladium metallic coating film can be deposited from various baths, including an immersion type, a reduction type (a formic acid bath, a hypophosphite bath, or a phosphite bath), or another similar bath type. In one embodiment, it is preferred to form a metallic coating film in an anelectrolytic palladium metallic coating bath, which is characterized by including, for example, a palladium compound, at least one compound selected from ammonia and amine compounds for use as a complexing agent, at least one hypophosphorous acid compound selected from hypophosphorous acid and hypophosphites for use as a reducing agent, and at least one unsaturated carboxylic acid compound selected from unsaturated carboxylic acids, unsaturated carboxylic anhydrides, salts of unsaturated carboxylic acids, and derivatives of unsaturated carboxylic acids. The palladium compound may be any of the following water-soluble compounds, including, for example, palladium chloride, palladium sulfate, palladium acetate, palladium nitrate, palladium tetraamine chloride, and the like. The palladium bath may also contain at least one ingredient selected from hypophosphorous acid and hypophosphites as a reducing agent. In addition, at least one ammonia or amine compound may be included in the composition as a complexing agent. The anelectrolytic palladium metal plating bath may also include at least one unsaturated carboxylic acid compound selected from unsaturated carboxylic acids, unsaturated carboxylic anhydrides, salts of unsaturated carboxylic acids, and derivatives of unsaturated carboxylic acids. The anelectrolytic palladium metallic coating bath preferably has a pH of 4 to 10, more preferably 6 to 8. The thickness of the anelectrolytic palladium metallic coating film is preferably in the range of 0.001 to 1.0 µm, more preferably 0.01 to 0.3 µm. The pH of the solution can be adjusted by adding a suitable pH adjuster. Unlike a conventional gold immersion plating bath, the anelectrolytic gold plating bath of the invention is a substitution-reduction type, where both the substitution and reduction reactions take place within the same plating bath. Since the aldehyde and / or aldehyde bisulfite adduct and the amine compound represented by general formula (1) or (2), which have a specific type of structure, are contained within the gold plating bath, the anelectrolytic gold plating bath of the invention allows not only the deposition of gold onto an underlying metal via the substitution reaction, but also the further deposition of gold by means of a reducing agent through the initially deposited gold as a catalyst. The anelectrolytic gold plating bath of the invention allows a palladium surface to be activated and gold to be deposited by means of a reducing agent while using the palladium as a catalyst. The gold can be further deposited using the gold itself as a catalyst, so that the gold-coated metallic film can be thickened onto the palladium. When the gold anelectrolytic plating bath of the invention is brought into contact with a palladium metallized coating film, the surface of the palladium metallized coating film can undergo an electrolytic gold plating treatment. In this case, a gold metallized coating film 0.01 to 2 µm thick can be formed in a contact time of, for example, 5 to 60 minutes. The gold metallized coating film can be formed at a deposition rate of, for example, 0.002 to 0.03 µm / minute. The present invention will now be illustrated by reference to the following non-limiting examples: Example 1: Circuit board test pieces containing a 25 µm copper layer were coated in an electrolytic nickel bath (Affinity 1.0, from MacDermid, Inc.) to deposit Ni / P to a thickness of approximately 5 µm. The pieces were rinsed and immersed in the following gold-plated metal coating bath operated at 80 °C until a deposit thickness of approximately 0.1 µm was achieved: Gold and potassium cyanide 1.2 g / l Sodium salt of ethylenediaminetetraacetic acid 20 g / l Glutaraldehyde bisulfite, sodium salt 5 g / l Sodium malate 20 g / l Equilibrium with deionized water The pH of the solution was adjusted to pH 8 with sulfuric acid or potassium hydroxide. The resulting gold deposit was bright and uniform. Inspection of the pieces for Ni / P layer hypercorrosion revealed no corrosion on any of the features. The gold plating remained stable, and no gold was deposited out of solution. The amount of nickel dissolved in the plating after prolonged use demonstrated that at least some of the surface gold was deposited by autocatalytic reduction from solution, and only some of the deposited gold resulted from nickel immersion displacement. Example 2: Circuit board test pieces containing 25 µm of copper were coated in an electrolytic nickel bath (Affinity 1.0, MacDermid, Inc.) to deposit Ni / P to a thickness of approximately 5 µm, and a second deposit was deposited over the Ni / P from an electrolytic palladium bath (Affinity Pd, MacDermid, Inc.) to a thickness of 0.05 µm. The pieces were rinsed and immersed in the subsequent gold plating bath until a deposit thickness of approximately 0.1 µm was achieved. Gold and potassium cyanide 1.2 g / l Sodium salt of ethylenediaminetetraacetic acid 20 g / l Glutaraldehyde bisulfite, sodium salt 5 g / l Sodium malate 20 g / l Equilibrium with deionized water The pH of the solution was adjusted to pH 8 with sulfuric acid or potassium hydroxide. The resulting gold deposit was bright and uniform. Inspection of the pieces for Ni / P layer hypercorrosion revealed no corrosion on any of the features. The gold plating remained stable, and no gold was deposited out of solution. Example 3: To the formulation of Example 1, 10 g / L of hydroxyethylethylenediamine was added to the solution. The same pieces as in Example 1 were recoated with gold at 80 °C until a deposit of approximately 0.1 µm was achieved. The resulting gold deposit was bright and lustrous. Again, hypercorrosion of the Ni / P layer was not evident. The gold plating remained stable, and no gold was deposited out of solution. Comparative example 1: The same tests were performed using a gold plating bath that did not contain an aldehyde reducing agent. The bath was a gold immersion bath (MacDermid, Inc. Affinity 1.0) that does not have any autocatalytic reduction of gold. The same pieces as in Example 1 were coated at 80 °C until a deposit of approximately 0.1 µm of gold was achieved. Upon inspection of the pieces, hypercorrosion of the Ni / P layer was observed due to the gold immersion reaction corroding the Ni / P layer. Comparative example 2: A gold bath was prepared with the following formulation and its stability was tested. Gold and potassium cyanide 1.2 g / l Sodium salt of ethylenediaminetetraacetic acid 20 g / l Bisulfite and formaldehyde adduct 5 g / l Sodium malate 20 g / l Equilibrium with deionized water The pH of the solution was adjusted to pH 8 with sulfuric acid or potassium hydroxide. The bath was maintained at 80 °C for 5 hours / day for 5 days. The bath was not stable and showed a gold deposit at the bottom of the tank. As shown in the examples, metallurgical coating baths using the reducing agents described herein can be used to produce gold deposits that are more corrosion-resistant and have greater weldability compared to prior art metallurgical coating baths. The metallurgical coating baths described herein also exhibit improved stability and do not form plaques. The electrolytic gold plating method described in this document can be used for the metallic gold coating treatment of, for example, wiring circuit assembly parts or terminal parts of printed circuit boards, ceramic substrates, semiconductor substrates, and integrated circuit (IC) packages.

Claims

1. A self-catalytic gold bath capable of depositing gold from solution onto a surface, the self-catalytic gold bath comprising: a. a chelating agent, wherein the chelating agent is selected from phosphoric acid, boric acid, Rochelle salt, citric acid, gluconic acid, tartaric acid, lactic acid, malic acid, ethylenediamine, triethanolamine, ethylenediaminetetraacetic acid, nitrilotriacetic acid, diethylenetriaminepentaacetic acid, hydroxyethylethylenediaminetriacetic acid, triethylenetetraminohexaacetic acid, 1,3-propanediaminetetraacetic acid, 1,3-diamino-2-hydroxypropanetetraacetic acid, hydroxyethylimidodiacetic acid, dihydroxyglycine, glycoletherdiaminetetraacetic acid, dicarboxymethylglutamic acid, hydroxyethylidenediphosphoric acid, ethylenediaminetetra(methylenephosphoric acid) and alkali metal salts, alkaline earth metal salts and ammonium salts of any of the foregoing, and combinations thereof; b. a golden salt,wherein the gold salt is a gold cyanide salt; and c. a reducing agent, characterized in that the reducing agent is selected from the sodium bisulfite and 2-methoxybenzaldehyde addition compound, the sodium bisulfite and acetone addition compound, the sodium bisulfite and p-anisaldehyde addition compound, the bisulfite and glutaraldehyde addition compound, and combinations of one or more of the foregoing.

2. The self-catalytic gold bath according to claim 1, wherein the bath is maintained at a pH in the range of approximately 6 to approximately 9.

3. The self-catalytic gold bath according to claim 1, wherein the bath is maintained at a temperature in the range of approximately 75 to approximately 95 °C.

4. The self-catalytic gold bath according to claim 1, wherein the self-catalytic bath further comprises an amine selected from ethylethylenediamine, diethylenetriamine, triethylenetetramine,Pentaethylenehexamine and combinations thereof.

5. The self-catalytic bath according to claim 4, wherein the molar ratio between the reducing agent and the amine is between 1:10 and 1:

1.

6. The self-catalytic gold bath according to claim 1, wherein the chelating agent comprises a sodium salt of ethylenediaminetetraacetic acid and sodium malate.

7. A method for providing a final gold finish on a substrate by metal coating with self-catalytic gold, the method comprising the steps of: a) providing a substrate with one or more metallic layers thereon; (b) contacting one or more metallic layers with a gold-bearing metallic coating bath to deposit metallic gold thereon, the gold-bearing metallic coating bath comprising: (i) a chelating agent, wherein the chelating agent is selected from phosphoric acid, boric acid, Rochelle salt, citric acid, gluconic acid, tartaric acid, lactic acid, malic acid, ethylenediamine,triethanolamine, ethylenediaminetetraacetic acid, nitrilotriacetic acid, diethylenetriaminepentaacetic acid, hydroxyethylethylenediaminetriacetic acid, triethylenetetraminohexaacetic acid, 1,3-propanediaminetetraacetic acid, 1,3-diamino-2-hydroxypropanetetraacetic acid, hydroxyethylimidodiacetic acid, dihydroxyglycine, glycoletherdiaminetetraacetic acid, dicarboxymethylglutamic acid, hydroxyethylidenediphosphoric acid, ethylenediaminetetra(methylenephosphoric acid), alkali salts, alkaline earth metal salts and ammonium salts of any of the foregoing, and combinations thereof; ii) a gold salt, wherein the gold salt is a gold cyanide salt; and iii) a reducing agent, characterized in that the reducing agent is selected from a sodium bisulfite and 2-methoxybenzaldehyde addition compound, a sodium bisulfite and acetone addition compound, a sodium bisulfite and p-anisaldehyde addition compound,A bisulfite and glutaraldehyde addition compound and combinations thereof; wherein metallic gold is deposited onto one or more metal layers, and the solderability of the one or more metal layers is increased.

8. The method of claim 7, wherein the one or more metal layers comprise palladium and / or nickel.

9. The method of claim 7, wherein a gold coating is applied over an electroless nickel-plated layer on the copper features of a semiconductor substrate or printed circuit board, preferably wherein the electroless nickel layer comprises a palladium-containing metallic coating film on top of the nickel layer to prevent diffusion from the nickel layer into the gold deposit.

10. The method of claim 7, wherein the gold-containing metallic coating bath deposits a final finish on the surface of the one or more metal layers.wherein the one or more metal layers are formed in an electrolytic nickel / immersion gold process; electrolytic nickel, electrolytic palladium / immersion gold; autocatalytic gold on electrolytic nickel; direct gold on copper; or gold on silver.

11. The process according to claim 7, wherein the substrate is a circuit board and the gold is deposited as a final finish on the one or more metal layers of the circuit board, wherein the deposited gold increases the solderability of the one or more metal layers.