TWI688672B - Cyanide-free electroless gold plating bath and electroless gold plating method - Google Patents

Abstract

The object of the present invention is to provide a gold plating bath that can be used for the gold plating formation of both the ENIG process and the ENEPIG process.

The solution of the present invention is a cyanide-free electroless gold plating bath, which is a cyanide-free electroless gold plating bath containing a water-soluble gold salt, a reducing agent and a complexing agent, characterized in that the aforementioned reducing agent contains formic acid or Salts and hydrazines.

Description

Cyanide-free electroless gold plating bath and electroless gold plating method

The invention relates to a cyanide-free electroless gold plating bath and an electroless gold plating method.

Conventionally, in the mounting process of printed wiring boards or electronic parts, as the final surface treatment, there is an ENIG (Electroless Nickel Immersion Gold) process for forming replacement type gold plating on electroless nickel plating. This process can be used for welding. Furthermore, by applying thickening gold after the ENIG process, it can also be used for wire bonding.

On the other hand, the ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) process in which electroless palladium plating is formed on the substrate electroless nickel plating via gold plating is also adopted as the final surface treatment. This process system is most suitable for lead-free soldering. It is also suitable for wire bonding.

The above-mentioned ENIG process and ENEPIG process are selected according to the purpose of the object to be plated, and either process is finally applied to gold plating. However, when forming the gold plating, the difference is that the former is based on nickel with a large ionization tendency (low oxidation-reduction potential), while the latter is based on ions Palladium with a low tendency to change (high oxidation-reduction potential) is used as a substrate. Therefore, gold plating baths corresponding to each process have been used so far.

In addition, as the above-mentioned gold plating bath, a cyano gold plating bath has been widely used in the past, but in view of the toxicity of cyanide, a non-cyanide type gold plating bath is required.

For example, as a non-cyanide gold plating bath used in the ENEPIG process described above, Patent Document 1 discloses that an electroless nickel plating film, an electroless palladium plating film, and an electroless gold plating film are sequentially formed on the conductor portion of a printed wiring board, The gold plating solution used in the formation of the aforementioned electroless gold plating film is composed of an aqueous solution containing a water-soluble gold compound, a reducing agent and a complexing agent. The reducing agent is selected from the group consisting of formaldehyde bisulfite, insurance powder and hydrazine. At least one of the group. Also, in Patent Document 2, as an electroless gold plating solution that can directly precipitate a gold film on an electroless palladium plating film by a self-catalyst reduction reaction, it is revealed that a non-cyanide gold sulfite and sulfurous acid are contained at a specified concentration Plating solution for salts, thiosulfates, water-soluble polyaminocarboxylic acids, benzotriazole compounds, sulfur-containing amino acid compounds, and hydroquinone.

However, preparing a plating bath for each process or exchanging a plating bath according to the use of the object to be plated consumes the number of steps and costs, and is not practical from the viewpoint of workability or economy. Therefore, a gold plating bath that can be used in any gold plating process of the ENIG process and the ENEPIG process is desired.

In addition, compared with the cyano gold plating bath, the above-mentioned non-cyanide type gold plating bath system tends to cause a decrease in bath stability or plating reactivity. Therefore, in non-cyanide type gold plating baths, it is required to have both bath stability and better characteristics of plating reactivity.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent No. 5526440

[Patent Document 2] Japanese Patent Laid-Open No. 2010-180467

The present invention has been completed with the above circumstances in mind, and its purpose is to realize a cyanide-free electroless gold plating bath that can be used in both the ENIG process and the ENEPIG process, and the electroless gold plating using the cyanide-free electroless gold plating bath method.

The cyanide-free electroless gold plating bath of the present invention that can solve the above problems is a cyanide-free electroless gold plating bath containing a water-soluble gold salt, a reducing agent and a complexing agent, and is characterized in that the reducing agent includes formic acid or Salts and hydrazines.

The aforementioned cyanide-free electroless gold plating bath preferably further contains a compound having a nitro group.

The present invention also includes a method of electroless gold plating, which is characterized in that electroless gold plating is applied to the surface of the object to be plated using the above-mentioned cyanide-free electroless gold plating bath. The electroless gold plating method may be nickel or nickel alloy on the surface of the aforementioned plating object, ie ENIG process, or It can be palladium or palladium alloy, ie ENEPIG process.

According to the present invention, a cyanide-free electroless gold plating bath that can be used in both the ENIG process and the ENEPIG process can be provided. Specifically, in the ENIG process, the corrosion of nickel plating when forming gold plating on nickel plating is suppressed. In addition, in both the ENIG process and the ENEPIG process, the precipitation reactivity of gold is increased, and a thick film of gold plating is possible. For example, in the ENIG process, over 0.07 μm can be precipitated on nickel plating in about 20 minutes, and in the ENEPIG process, over 0.05 μm can be precipitated on palladium plating in about 30 minutes.

FIG. 1 is an SEM (Scanning Electron Microscope) observation photograph showing whether or not the nickel-plated surface in the examples is corroded.

[Forms for carrying out the invention]

In order to solve the aforementioned problems, the present inventors have repeatedly concentrated on research. As a result, it was found that if it is a substitution-reduction type cyanide-free electroless gold plating bath containing formic acid or its salts and hydrazines as reducing agents, excessive corrosion of nickel plating or the like as a base is not caused, and gold precipitation reactivity can be improved , Can be used for both ENIG process and ENEPIG process, and complete the present Bright. Hereinafter, the cyanide-free electroless gold plating bath of the present invention is also referred to as "electroless gold plating bath" or "gold plating bath". In the following, each compound contained in the cyanide-free electroless gold plating bath of the present invention will be described.

(A) Hydrazine

The hydrazine series is particularly useful for compounds that improve the precipitation of the plating on palladium, and promotes the formation of gold plating on the palladium plating in the ENEPIG process. In addition, hydrazines also contribute to ensuring a good plating appearance. As a result, they also contribute to ensuring good weldability or wire bondability (W/B bondability). On the other hand, the hydrazine series is considered to promote the substitution reaction on nickel plating beyond what is needed in the ENIG process, incurring excessive corrosion of nickel plating. However, as will be described later, by using formic acid or its salts together, this excessive corrosion of nickel plating can be suppressed.

As the aforementioned hydrazines, hydrazine; hydrazine hydrate such as hydrazine ‧ hydrate; hydrazine carbonate, hydrazine sulfate, neutral hydrazine sulfate, hydrazine hydrochloride and other hydrazine salts; pyrazoles, triazoles, hydrazines, etc. can be used Organic derivatives of hydrazine, etc. As the aforementioned pyrazoles, in addition to pyrazole, pyrazole derivatives such as 3,5-dimethylpyrazole and 3-methyl-5-pyrazole may be used. As the aforementioned triazoles, 4-amino-1,2,4-triazole, 1,2,3-triazole and the like can be used. As the hydrazides, adipic acid dihydrazide, maleic acid hydrazide, carbohydrazine and the like can be used. Preferred are hydrazine hydrate and hydrazine sulfate such as hydrazine and hydrate. These can be used alone or in combination of two or more.

The total concentration of the hydrazines is preferably 0.1 to 5 g/L, and more preferably 0.3 to 3 g/L.

(B) Formic acid or its salt

Formic acid or its salt system is considered to have an effect of suppressing the excessive replacement reaction due to the above hydrazines. Hereinafter, "formic acid or its salt" is also collectively referred to as formic acid. In particular, during gold plating in the ENIG process, excessive corrosion of the Ni film as a base can be suppressed. On the other hand, when only formic acid is used in combination with the above hydrazines, especially in the ENEPIG process, the reactivity of gold precipitation tends to decrease. Specifically, the precipitation reactivity of gold on palladium plating as a base is poor, and it is difficult to ensure the film thickness of gold plating. Therefore, it must be used in combination with the above hydrazines. By the combined use of hydrazines and formic acid, the excessive corrosion of the nickel plating described above is also suppressed, which also contributes to ensuring the weldability or W/B bondability.

Examples of the salt of formic acid include alkali metal salts of formic acid such as potassium formate and sodium formate; alkaline earth metal salts of formic acid such as magnesium formate and calcium formate; ammonium salts of formic acid, quaternary ammonium salts, including primary to tertiary Amine salts of grade amines. In the present invention, formic acid or a salt thereof may be used alone or in combination of two or more.

The total concentration of the aforementioned formic acids is preferably contained in the range of 1 to 100 g/L. In order to fully exert the above effects, it is preferably 1 g/L or more, more preferably 5 g/L or more, and particularly preferably 10 g/L or more. On the other hand, if it is contained excessively, the bath tends to become unstable, so as described above, it is preferably 100 g/L or less.

That is, in the present invention, by using hydrazines and formic acid as a reducing agent together, in the gold plating process of the ENIG process, the formic acid can suppress the nickel corrosion caused by hydrazines (displacement reaction), which can be suppressed as gold plating Corrosion of nickel plating on the substrate. On the other hand, gold plating in the ENEPIG process In the middle, due to the high reactivity of hydrazines on palladium plating, the formation of gold plating is promoted more than when formic acid alone is used, and thickening of gold plating becomes possible. This is because the reduction reaction is promoted.

In addition to the hydrazines and formic acid, the gold-plating bath of the present invention also requires a cyano group-free electroless gold-plating bath of water-soluble gold salts and complexing agents. In addition, as described later, reducing agents other than hydrazines and formic acids may also be used. Hereinafter, the water-soluble gold salt will be described in order.

First, the electroless gold plating bath of the present invention contains a water-soluble gold salt as a gold source. As mentioned above, the water-soluble gold salt system is cyano-free, and specific examples include gold sulfite, thiosulfate, thiocyanate, sulfate, nitrate, methanesulfonate, and tetramine complex. , Chloride, bromide, iodide, hydroxide, oxide, etc. These can be used alone or in combination of two or more. The total concentration of the water-soluble gold salts in the plating bath is preferably 0.3 to 5 g/L in terms of gold (Au) concentration, and particularly preferably 0.5 to 4 g/L. When it is less than 0.3g/L, the precipitation rate may become slow. On the other hand, if it exceeds 5 g/L, the stability may decrease, and the effect hardly changes even if it is increased, and the cost becomes high.

As the reducing agent, the electroless gold plating bath system of the present invention may further have the following reducing agents in addition to the hydrazines and formic acids. That is, ascorbic acid compounds such as ascorbic acid, isoascorbic acid (isovitamin C) or their salts (sodium salt, potassium salt, ammonium salt, etc.); hydroquinone or methylhydroquinone and other hydroquinone or its derivatives; pyrogallol, Pyrogallol monomethyl ether, pyrogallol-4-carboxylic acid, pyrogallol-4,6-dicarboxylic acid, gallic acid etc. pyrogallol or derivatives thereof. These can be used alone or in combination of two or more. In the present invention, as Reducing agent, the combination of the above hydrazines and formic acid is necessary, No. 10 of the embodiment described later shows that even if the reducing agent other than the above hydrazines and formic acid, such as ascorbic acid and formic acid, are used together, the desired characteristic.

The total concentration of the above reducing agents other than hydrazines and formic acids in the gold plating bath is preferably 0.5 to 50 g/L, and particularly preferably 1 to 10 g/L.

The electroless gold plating bath of the present invention contains a complexing agent. The complexing agent is preferably a complexing agent having a complexation of eluted metals (such as nickel, palladium, etc.) and a complexing agent having a complexation of gold. Suitable as a complexing agent having the above-mentioned dissolution of metal complexes, there may be mentioned hydroxycarboxylic acids such as glycolic acid, diglycolic acid, lactic acid, malic acid, citric acid, gluconic acid, heptagluconic acid or their salts (Sodium salt, potassium salt, ammonium salt, etc.); glycine, aminodicarboxylic acid, nitrotriacetic acid, EDTA, hydroxyethylethylenediaminetriacetic acid, diethylenetriaminepentaacetic acid, polyamino Aminocarboxylic acids such as carboxylic acids or their salts (sodium, potassium, ammonium, hydrochloride, sulfate, etc.); HEDP (hydroxyethane-1,1-diphosphonic acid), amino tris Phosphorous acid chelating agents such as methanesulfonic acid and ethylenediaminetetramethylsulfonic acid, or their salts (sodium salt, potassium salt, ammonium salt, hydrochloride, sulfate, etc.); ethylenediamine, dioxane Amine chelating agents such as ethyl triamine and triethylidene amine, and their salts (hydrochloride, sulfate, etc.), etc. These can be used alone or in combination of two or more.

Also, suitable as a complexing agent having a gold complexation include sodium sulfite, potassium sulfite, ammonium sulfite, sodium bisulfite, potassium bisulfite, ammonium bisulfite, sodium disulfite, potassium disulfite , Ammonium disulfite, sodium thiosulfate, potassium thiosulfate, ammonium thiosulfate, hydantoin compound, amide imine compound, etc. Can be alone or combined 2 More than one species use this. More preferably, sodium sulfite, ammonium sulfite, etc. are used.

The total concentration of the aforementioned complexing agent with eluted metal and the aforementioned complexing agent with gold complexing in the gold plating bath is preferably 1 to 200 g/L, particularly preferably 10 to 150 g/L.

The electroless gold plating bath of the present invention preferably further contains a compound having a nitro group. By including this compound having a nitro group, even if it does not contain a cyano group, the plating reactivity can be sufficiently ensured, that is, the precipitation reactivity of gold can not be impaired, and the bath stability can be sufficiently ensured. As this mechanism of action, I think that it is a nitro trapped and stable. On the other hand, it does not contain a nitro group. For example, only catechol or benzoic acid shows a stable effect.

Examples of the compound having a nitro group include an aromatic compound having a nitro group. Examples of the aromatic compound having a nitro group include nitrobenzene; aromatic compounds having a nitro group and a hydroxyl group such as nitrophenol and 4-nitrocatechol; nitrotoluene, nitroxylene, Aromatic compounds with nitro and alkyl groups such as nitrostyrene; Aromatic compounds with nitro and amino groups such as nitroaniline and 4-nitro-1,2-phenylenediamine; Nitrothiophenol , 2,4-dinitrobenzene sulfonic acid and other nitrobenzene sulfonic acid and other aromatic compounds with nitro and sulfur-containing groups. Furthermore, examples of the nitrobenzoic acid having a nitro group and a carboxyl group include 2-nitrobenzoic acid, 3,5-dinitrobenzoic acid, 3,4-dinitrobenzoic acid, and more amino groups. 5-amino-2-nitrobenzoic acid, etc. In addition, aromatic compounds having a nitro group together with a halogen group, an ester group, an ether group, a carbonyl group, an aldehyde group, and the like are mentioned. Alternatively, as the salts of these aromatic compounds having a nitro group, ammonium salts, sodium salts, potassium salts and the like can also be used.

For example, when it is the above nitrobenzoic acid, the nitro group is adjacent to At the position of the carboxyl group, the stability effect becomes larger and better. That is, the magnitude of the stability effect is in the order of 2-nitrobenzoic acid>3-nitrobenzoic acid>4-nitrobenzoic acid.

As the compound having a nitro group, in addition to the aromatic compound having a nitro group, an aliphatic compound having a nitro group can also be used.

More preferred is a compound having a nitro group and an electron-donating group, especially an aromatic compound having a nitro group and an electron-donating group. If it has an electron-donating group, the effect of stabilizing the nitro group becomes greater. In the case where the nitro group is two adjacent dinitro groups, it is believed that the formation of the trapping shape by the two nitro groups 2 has a greater effect on stability. Examples of the electron-donating group include a hydroxyl group, an alkyl group, an amine group, a sulfur-containing group, a carboxyl group, an ester group, a halogen group, and an ether group. It is preferable to have one or more of these.

The above-mentioned compound having a nitro group can be used alone or in combination of two or more kinds as described above. The total concentration of the compounds having nitro group is preferably in the range of 0.0010 to 5 g/L, for example. This is because if the total concentration is less than 0.0010 g/L, it is difficult to obtain the above effect. The above-mentioned total concentration is more preferably 0.005 g/L or more, and particularly preferably 0.010 g/L or more. On the other hand, if the concentration of the compound having a nitro group is too high, the nickel-plated surface as a base is easily corroded. Therefore, as mentioned above, the total concentration is preferably 5 g/L or less, more preferably 4 g/L or less, and particularly preferably 3 g/L or less.

The pH of the electroless gold plating bath of the present invention is preferably 5-10. This is because if it falls below this range, the rate of gold precipitation tends to decrease, and the other On the other hand, if it exceeds the above range, the bath will easily become unstable. The above pH is more preferably 6-9.

唑等)、含氮雜環化合物(苯并三唑、N-羥基苯并三唑等)等。 In the electroless gold plating bath of the present invention, well-known pH adjusters, pH buffers, and other additives may also be suitably contained within the range that does not impair the object of the present invention. Examples of the pH adjuster include hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and carboxylic acid among acids, and sodium hydroxide, potassium hydroxide, and ammonia water among alkalis. In addition, examples of the pH buffering agent include carboxylic acids such as citric acid, tartaric acid, malic acid, and phthalic acid; phosphoric acid such as orthophosphoric acid, phosphorous acid, hypophosphorous acid, and pyrophosphoric acid, or their potassium salts and sodium Phosphates such as salts and ammonium salts; boric acid, tetraboric acid, etc. In addition, examples of the metal ion concealing agent include azoles such as benzotriazole and methylbenzotriazole, morpholine, bipyridine, and salicylate. In addition, as the auxiliary complexing agent, amine carboxylic acid such as EDTA, EDTMP, ammonium salt, chloride, etc. may be mentioned. Moreover, as a stabilizer, for example, a sulfur-containing heterocyclic compound (2-mercaptobenzothiazole, 2-mercaptobenzo)

Azole, etc.), nitrogen-containing heterocyclic compounds (benzotriazole, N-hydroxybenzotriazole, etc.), etc.

In the electroless gold plating bath of the present invention, one or more kinds of thallium compound, arsenic compound and lead compound may be further added. These compounds act as an increase in gold plating rate or as a crystal modifier. Specific examples of the compound include carbonates, acetates, nitrates, sulfates, and hydrochlorides of metals (arsenic, thallium, and lead) constituting the compound. The concentration of the crystallization modifier in the gold plating bath is expressed in terms of metal concentration. For example, the total amount is preferably 0.1 to 100 mg/L, the total amount is more preferably 0.2 to 50 mg/L, and the total amount is particularly preferably 0.2 to 20 mg/L.

The present invention also stipulates the method of electroless gold plating using the above-mentioned cyanide-free electroless gold plating bath. The surface of the object to be plated to be gold-plated may be nickel or nickel alloy. In the above ENIG process, the case where the surface of the object to be plated is electroless nickel plating or electroless nickel alloy gold plating (hereinafter referred to as "electroless nickel plating"). Examples of the aforementioned nickel alloys include nickel-phosphorus alloys and nickel-boron alloys.

The surface of the plated object to be gold-plated may be palladium or palladium alloy. In the above ENEPIG process, the case where the surface of the object to be plated is electroless palladium plating or electroless palladium alloy plating (hereinafter referred to as "electroless palladium plating"). Examples of the aforementioned palladium alloy include palladium-phosphorus alloys.

In the ENIG process, for example, on the Al or Al-based alloy, Cu or Cu-based alloy that forms the electrode, an electroless nickel-based plating is formed, and then electroless gold plating is formed thereon. In the ENEPIG process, for example, in forming the electrode On the Al or Al-based alloy, Cu or Cu-based alloy, electroless nickel-based plating is formed, followed by electroless palladium-based plating, and then electroless gold plating is formed thereon. The above electroless nickel-based plating or no The formation of electrolytic palladium-based plating may be carried out by a commonly used method.

In both the ENIG process and the ENEPIG process, the electroless gold plating is formed by using the cyanide-free electroless gold plating bath of the present invention, and other than the usual conditions. For example, contacting the electroless gold plating bath of the present invention for about 3 to 20 minutes can be mentioned. As this contact, a conventional method such as dipping can be used. The use temperature of electroless gold plating bath is preferably 40~90℃. If it does not reach the above range, the precipitation rate may be reduced, on the other hand, if it exceeds In the above range, the bath may become unstable. The above use temperature is preferably 50 to 80°C.

The electroless gold plating bath of the present invention and the electroless gold plating method using the same are suitable for the case where the wiring circuit mounting portion or terminal portion of electronic components such as printed wiring boards, ceramic substrates, semiconductor substrates, IC packages, etc. are treated with gold plating. In particular, it is suitable for UBM (Under Barrier Metal) formation technology for the purpose of welding and wire bonding (W/B) bonding of Al electrodes or Cu electrodes on a wafer. By using the gold plating bath of the present invention, the formation of electroless gold plating as part of the UBM formation technology can be performed stably, and as a result, stable coating characteristics can be achieved.

This case is based on Japanese Invention Patent Application No. 2015-148523 filed on July 28, 2015, and claims the benefits of priority. The entire contents of the specification of Japanese Invention Patent Application No. 2015-148523 filed on July 28, 2015 are incorporated for reference in this case.

[Examples]

The present invention will be described in more detail with the following examples. However, the present invention is of course not limited by the following examples, and it is of course possible to implement appropriate changes within the scope of the purposes described above and below. Etc. are included in the technical scope of the present invention

[Measurement of film thickness of gold plating, confirmation of corrosion of nickel plating, and preparation of samples for observation of appearance of gold plating] [Sample of ENIG process]

The samples of the ENIG process used for the measurement of the thickness of the gold plating described above are obtained as follows. That is, the prepared electrode is a TEG wafer composed of Al-Cu of an Al-based alloy, and an electroless nickel plating bath (NPR-18 manufactured by Uemura Industry Co., Ltd.) is used on this electrode, which is formed by an electroless plating method Nickel plating with a thickness of 5.0 μm was obtained by applying electroless gold plating using the electroless gold plating bath shown in Table 1. This sample is also referred to as "sample I of the ENIG process" below.

[Sample of ENEPIG process]

Samples of the ENEPIG process used in the film thickness measurement of the above-mentioned gold plating are obtained as follows. That is, the prepared electrode is a TEG wafer composed of Al-Cu of an Al-based alloy, and an electroless nickel plating bath (NPR-18 manufactured by Uemura Industry Co., Ltd.) is used on this electrode, which is formed by an electroless plating method Nickel plating with a thickness of 5.0 μm, and then on this nickel plating, using an electroless palladium plating bath (TFP-30 manufactured by Shangcun Industry Co., Ltd.), palladium plating with a thickness of 0.05 μm is formed by electroless plating, and more The electroless gold plating bath shown in Table 1 was obtained by applying electroless gold plating. This sample is also referred to as "ENEPIG process sample I" below.

The further conditions of the electroless gold plating performed in the preparation of the samples of the ENIG process and the ENEPIG process are as follows. That is, in the electroless gold plating bath, a gold sodium sulfite solution (Au concentration=100 g/L) is used as a gold source, and Table 1 below shows the Au concentration in the electroless gold plating bath. In addition, using thallium carbonate as the thallium (Tl) compound, there is no T1 concentration in electrolytic gold plating bath. The temperature of the electroless gold plating bath was 75° C. The immersion in the electroless gold plating bath was performed for sample I in the ENIG process for 20 minutes and sample I in the ENEPIG process for 30 minutes to form gold plating. In addition, in the sample I of the ENEPIG process of No. 1, 2, 4, and 6 of Table 1 mentioned later, in order to ensure the film thickness of gold plating early, the immersion time was set to 20 minutes.

[Determination of gold plating thickness in ENIG process or ENEPIG process]

The thickness of the gold plating film formed by the sample I of the ENIG process and the sample I of the ENEPIG process was measured with a fluorescent X-ray film thickness meter. Furthermore, in particular, the case where the thickness of the gold plating film on the palladium plating in the ENEPIG process is 0.05 μm or more is evaluated as being applicable to the gold plating bath in the ENEPIG process.

[Whether the corrosion of nickel plating observed by SEM]

The nickel-plated surface that appeared by removing the gold plating of Sample I of the ENIG process with a gold stripping solution was observed with a SEM at a magnification of 5000 times to confirm the presence of corrosion marks. Furthermore, those who saw corrosion marks were evaluated as having "corrosion" of nickel-plated corrosion, and those who did not see corrosion marks were evaluated as "no" corrosion of nickel-plating. For reference only, the SEM observation photos are shown in Figure 1. FIG. 1(a) is a photograph of an example of the present invention in which nickel-plated corrosion marks are not seen, and FIG. 1(b) is a photograph of a comparative example in which nickel-plated corrosion marks are seen.

〔Gold-plated appearance observation〕

Visually observe the sample I and ENEPIG process of the above ENIG process The gold-plated surface of sample I. Furthermore, those who exhibited a uniform golden plating appearance were evaluated as "good", and those who were not golden but reddish were evaluated as "bad".

[Production of samples for evaluation of weldability and wire bondability (W/B)] [Sample of ENIG process]

The samples of the ENIG process used in the evaluation of solderability and W/B properties are prepared for the BGA substrate (pad diameter Φ0.5mm) made by Uemura Industrial Co., Ltd. On this substrate, the same as the sample I of the ENIG process described above , Using an electroless nickel plating bath (NPR-18 manufactured by Uemura Industrial Co., Ltd.), a nickel plating of 5.0 μm thickness was formed by electroless plating, and then using the electroless gold plating bath shown in Table 1 by applying Derived from electroless gold plating. This sample is also referred to below as "sample II of the ENIG process".

[Sample of ENEPIG process]

The samples of the ENEPIG process used in the evaluation of solderability and W/B properties are prepared for the BGA substrate (pad diameter Φ0.5mm) made by Uemura Industrial Co., Ltd. On this substrate, the same as the sample I of the ENEPIG process described above , Using an electroless nickel plating bath (NPR-18 manufactured by Uemura Industrial Co., Ltd.), a nickel plating of 5.0 μm thickness was formed by electroless plating, followed by an electroless palladium plating bath (TFP- 30) On the nickel plating, palladium plating with a thickness of 0.05 μm is formed by electroless plating, and then electroless gold plating is performed by using the electroless gold plating bath shown in Table 1. This sample is also referred to below as the "ENEPIG process test" Material II".

[Evaluation of weldability]

Using the above-mentioned sample II of the ENIG process and sample II of the ENEPIG process, a joining tester SERIES4000 manufactured by Dage Corporation was used, and 20 points were evaluated for each condition. The details are for each No. in Table 1, measured using the ENIG process sample II, when the following reflow times are 1; using the ENEPIG process sample II, when the following reflow times are 1; using the ENIG process samples II, when the following number of reflows is 5; and when using the sample II of the ENEPIG process, when the following number of reflows is 5; a total of 4 conditions × 20 = 80 points of welding strength. As this welding strength, the welding fracture rate of the failure mode was obtained. The conditions for welding formation and welding strength measurement are as follows. In this example, the case where the welding fracture rate was 85% or more was evaluated as “good”, and the case where the welding fracture rate was less than 85% was evaluated as “bad”.

[Conditions for welding formation and welding strength measurement]

Measurement method: Pull ball test

Solder ball: Senju Metal Φ0.6mm Sn-3.0Ag-0.5Cu

Reflow device: TMR-15-22LH manufactured by TAMURA

Reflow conditions: Top 240℃

Reflow environment: Air

Reflow times: 1 or 5 times

Flux: Senju Metal 529D-1 (RMA type)

Test rate: 5000μm/s

Mature after welding: 1 hour

[Evaluation of wire bonding (W/B) properties]

Using the above-mentioned sample II of the ENIG process and sample II of the ENEPIG process, wire bonding was performed by a semi-automatic multi-wire bonding machine HB16 manufactured by TPT Corporation, and a joint testing machine SERIES4000 manufactured by Dage Corporation was evaluated at 20 points per condition. In detail, for each No. of Table 1, the total of 2 conditions × 20 = 40 points of wire bonding strength (W/B strength) when the sample II using the ENIG process and the sample II using the ENEPIG process were measured, and the average value was calculated W/B average intensity and standard deviation. Furthermore, based on them, the coefficient of variation (=standard deviation÷average value×100) was obtained. The conditions of wire bonding formation and the conditions of wire bondability evaluation are as follows. Furthermore, the case where the average W/B strength is 8 gf or more and the coefficient of variation is 15% or less is evaluated as “good” in wire bondability, and at least one of the above average W/B strength and the coefficient of variation is outside the above range The wire bondability was evaluated as "poor".

[Conditions for wire bonding formation and wire bondability evaluation]

Capillary: B1014-51-18-12 (PECO)

Line: 1Mil-Gold

Stage temperature: 150℃

Ultrasonic wave (mW): 250 (1st), 250 (2nd)

Joining time (ms): 200 (1st), 50 (2nd)

Tensile force (gf): 25 (1st), 50 (2nd)

Step (length of 1st to 2nd): 0.700mm

Measurement method: pull wire test

Test rate: 170μm/sec

Furthermore, in the present embodiment, when both the weldability and the wire bondability are "good", the weldability and the W/B property are "good", and at least one of the weldability and the wire bondability is evaluated. Evaluation of weldability and W/B "bad" for the "bad" situation.

[Evaluation of bath stability]

The electroless gold-plated bath composed of each bath shown in Table 1 was left at a temperature of 70°C for 1 month to evaluate the stability of the bath. Those who did not decompose even after being left for 1 month were evaluated as "stable", and those who decomposed within 1 month were evaluated as "unstable".

Table 1 shows these results together.

Table 1 shows the following. No.1~6 Because the gold plating baths with formic acid and hydrazine are used for gold plating, the thickness of the gold plating can be fully ensured in the case of the ENEPIG process, and the corrosion of nickel plating is also suppressed in the ENIG process To get a good plating appearance. In addition, in these examples, welding and wire bonding can be simultaneously performed satisfactorily. Furthermore, from the comparison between No. 1 to No. 3 and No. 4, from the viewpoint of ensuring sufficient bath stability, it is preferable to further include a gold-plated bath containing a compound having a nitro group. In addition, as can be seen from the comparison between No. 3 and No. 5, it is preferable to set the content of formic acid below the recommended upper limit (100 g/L or less).

In contrast, Nos. 7 to 10 did not contain any of the prescribed formic acids and hydrazines, resulting in unfavorable conditions. In detail, since No. 7 and No. 8 do not contain formic acid, corrosion of nickel plating occurs. As a result, weldability and W/B properties are poor. In addition, No. 9 is an example that does not contain hydrazines. In this example, the thickness of the gold plating becomes thinner during the ENEPIG process. In addition, No. 10 is an example of using ascorbic acid and formic acid as reducing agents instead of hydrazines. In this example, the thickness of the gold plating thinned during the ENEPIG process. Also, the appearance of gold plating in Nos. 9 and 10 is red and discolored. Due to the occurrence of such red discoloration defects, the weldability and W/B properties are poor.

Claims (5)

A cyanide-free electroless gold plating bath, which is a cyanide-free electroless gold plating bath containing a water-soluble gold salt, a reducing agent and a complexing agent, is characterized in that the reducing agent includes formic acid or its salts and hydrazines. The cyanide-free electroless gold plating bath of claim 1 further contains a compound having a nitro group. A method of electroless gold plating, characterized in that electroless gold plating is applied to the surface of the object to be plated using a cyanide-free electroless gold plating bath as in claim 1 or 2. The electroless gold plating method according to claim 3, wherein the surface of the aforementioned plating target is nickel or nickel alloy. The electroless gold plating method according to claim 3, wherein the surface of the aforementioned plating target is palladium or a palladium alloy.

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