Patent Application
20230151493
The present invention relates to a metal displacement solution and a method for surface treatment of aluminum or an aluminum alloy.
Aluminum readily forms an oxide film in the air or water. It is known that when aluminum or an aluminum alloy is subjected to plating, the plating film has low adhesion due to such an oxide film. Hence, zinc displacement (zincate treatment) has been performed prior to plating in order to remove the oxide film on the aluminum or aluminum alloy surface and thereby ensure adhesion to a plating film to be formed on the aluminum (e.g., Patent Literatures 1 to 3 and Non-Patent Literatures 1 to 3).
As a result of extensive studies by the present inventors, they have found that the conventional techniques have room for improvement in adhesion to a plating film.
The present invention aims to solve the problem newly found by the present inventors and provide a metal displacement solution that can provide good adhesion to a plating film (metal film), and a method for surface treatment of aluminum or an aluminum alloy using the metal displacement solution.
As a result of extensive studies by the present inventors, they have found that the use of a metal displacement solution having a specific composition can provide good adhesion to a plating film (metal film). This finding has led to the completion of the present invention.
That is, the present invention relates to a metal displacement solution, containing:
Preferably, the metal displacement solution contains the zinc compound in an amount corresponding to a zinc concentration of 0.2 to 5.0 g/L.
Preferably, the metal displacement solution contains the nickel compound in an amount corresponding to a nickel concentration of 0.2 to 10 g/L.
Preferably, the metal displacement solution contains the germanium compound in an amount corresponding to a germanium concentration of 0.2 to 5.0 g/L.
Preferably, the metal displacement solution contains the fluorine compound in an amount corresponding to a fluorine concentration of 5.0 to 50 g/L.
Preferably, the metal displacement solution has a ratio of zinc concentration to germanium concentration of 1:5 to 5:1.
Preferably, the metal displacement solution has a pH of 4.0 to 6.5.
Preferably, the metal displacement solution is for treating aluminum or an aluminum alloy.
The present invention also relates to a method for surface treatment of aluminum or an aluminum alloy, the method including:
bringing a workpiece having aluminum or an aluminum alloy on its surface into contact with the metal displacement solution to remove an oxide film on the aluminum or aluminum alloy and displace the aluminum with the metals in the metal displacement solution to form a displacement metal film containing the metals on the surface of the workpiece.
Preferably, the method includes, after forming the displacement metal film, forming a plating film on a surface of the displacement metal film.
The metal displacement solution according to the present invention contains a zinc compound, a nickel compound, a germanium compound, and a fluorine compound and thus can provide good adhesion to a plating film (metal film).
The metal displacement solution of the present invention contains a zinc compound, a nickel compound, a germanium compound, and a fluorine compound. This can provide good adhesion to a plating film (metal film).
The reason why the metal displacement solution can provide the aforementioned effect is believed to be as follows.
As a workpiece having aluminum or an aluminum alloy on its surface is brought into contact with the metal displacement solution to remove the oxide film on the aluminum or aluminum alloy and displace the aluminum with the metals in the metal displacement solution, zinc (Zn) may co-deposit with nickel (Ni) and germanium (Ge) to form a displacement metal film containing Zn, Ni, and Ge on a surface of the aluminum or aluminum alloy.
When such an aluminum or aluminum alloy having a displacement metal film containing Zn, Ni, and Ge on its surface is subjected to plating to form a plating film (metal film, e.g., nickel film), the Zn, Ni, and Ge present in the displacement metal film may act synergistically between the aluminum or aluminum alloy and the plating film (metal film, e.g., nickel film), thereby providing good adhesion to the plating film (metal film) to the aluminum or aluminum alloy.
The synergistic action of Zn, Ni, and Ge is evident from the fact that good adhesion to the plating film (metal film) cannot be provided when Zn, Ni, or Ge is present alone, when a combination of Zn and Ni is present (without Ge), when a combination of Zn and Ge is present (without Ni), and when a combination of Ni and Ge is present (without Zn).
The metal displacement solution of the present invention contains a zinc compound, a nickel compound, a germanium compound, and a fluorine compound.
The zinc compound is not limited as long as it is a water-soluble zinc compound. Specific examples include zinc sulfate, zinc nitrate, zinc chloride, zinc acetate, zinc oxide, and zinc gluconate. These may be used alone or in combinations of two or more. Preferred among these is zinc sulfate.
The metal displacement solution preferably contains at least one zinc compound in an amount corresponding to a zinc (metallic zinc (Zn)) concentration of 0.1 to 7.0 g/L, more preferably 0.2 to 5.0 g/L, still more preferably 0.2 to 4.0 g/L. With a concentration of lower than 0.1 g/L, the deposition of Zn tends to be small, failing to ensure sufficient adhesion. With a concentration of higher than 7.0 g/L, the deposition of Zn tends to be excessive, failing to ensure sufficient adhesion.
The nickel compound is not limited as long as it is a water-soluble nickel compound. Specific examples include nickel sulfate, nickel nitrate, nickel chloride, nickel acetate, and nickel gluconate. These may be used alone or in combinations of two or more. Preferred among these is nickel sulfate.
The metal displacement solution preferably contains at least one nickel compound in an amount corresponding to a nickel (metallic nickel (Ni)) concentration of 0.1 to 12 g/L, more preferably 0.2 to 10 g/L. With a concentration of lower than 0.1 g/L, the co-deposition with Zn tends to be reduced, failing to ensure sufficient adhesion. With a concentration of higher than 12 g/L, the co-deposition with Zn tends to be excessive, failing to ensure sufficient adhesion.
The germanium compound is not limited as long as it is a water-soluble germanium compound. Specific examples include germanium dioxide, germanium sulfate, germanium sulfide, germanium fluoride, germanium chloride, and germanium iodide. These may be used alone or in combinations of two or more. Preferred among these is germanium dioxide.
Herein, a compound that corresponds to both a germanium compound and a fluorine compound, such as germanium fluoride, is regarded as a germanium compound. Similarly, a zinc compound and a nickel compound, each of which is as described above, are also regarded as a zinc compound and a nickel compound, respectively.
The metal displacement solution preferably contains at least one germanium compound in an amount corresponding to a germanium (metallic germanium (Ge)) concentration of 0.1 to 7.0 g/L, more preferably 0.2 to 5.0 g/L. With a concentration of lower than 0.1 g/L, the co-deposition with Zn tends to be reduced, failing to ensure sufficient adhesion. With a concentration of higher than 7.0 g/L, the co-deposition with Zn tends to be excessive, failing to ensure sufficient adhesion.
The ratio of the zinc concentration to the germanium concentration (zinc concentration:germanium concentration) is preferably 1:5 to 5:1.
The fluorine compound dissolves aluminum from the oxide film on the aluminum or aluminum alloy surface to allow it to be smoothly displaced with the metals such as zinc.
Specific examples of the fluorine compound include hydrofluoboric acid, sodium fluoride, potassium fluoride, ammonium hydrogen fluoride, ammonium fluoride, hydrogen fluoride, and lithium fluoride. These may be used alone or in combinations of two or more. Preferred among these are hydrofluoboric acid, sodium fluoride, potassium fluoride, ammonium hydrogen fluoride, ammonium fluoride, and hydrogen fluoride, with hydrofluoboric acid, sodium fluoride, potassium fluoride, ammonium hydrogen fluoride, and ammonium fluoride being more preferred.
The metal displacement solution preferably contains at least one fluorine compound in an amount corresponding to a fluorine (F) concentration of 1.0 to 100 g/L, more preferably 5.0 to 50 g/L. With a concentration of lower than 1.0 g/L, the aluminum-dissolving action tends to be lowered, failing to ensure sufficient adhesion. With a concentration of higher than 100 g/L, excess aluminum tends to be dissolved, failing to ensure sufficient adhesion.
Herein, the zinc (metallic zinc (Zn)) concentration, nickel (metallic nickel (Ni)) concentration, and germanium (metallic germanium (Ge)) concentration in the metal displacement solution are measured by ICP (HORIBA, Ltd.).
Also, herein, the fluorine (F) concentration in the metal displacement solution is measured using a fluorine ion electrode.
The pH of the metal displacement solution is preferably 1.0 to 12.0, more preferably 2.0 to 10.0. In other words, the metal displacement solution of the present invention may be used under either alkaline or acidic conditions. Here, in the usual zincate treatment, excess aluminum may be eluted under alkaline conditions (e.g.,
In contrast, the metal displacement solution of the present invention, even under acidic conditions, can ensure sufficient adhesion, and a more significant effect on improving adhesion is obtained when it is used under acidic conditions. Further, as no elution of excess aluminum occurs under acidic conditions, it is also possible to reduce aluminum spikes. However, when the pH is lower than 3.5, excess aluminum may be eluted.
Thus, the pH of the metal displacement solution is still more preferably 3.5 to 6.5, particularly preferably 4.0 to 6.5, most preferably 4.5 to 6.5. In this case, a more significant effect on improving adhesion is obtained as described above, and it is also possible to reduce aluminum spikes as no elution of excess aluminum occurs. Here, if excess aluminum is eluted to form aluminum spikes, many wedge-shaped depressions are formed on the aluminum surface, and in the subsequent plating film formation step, for example, a nickel plating may enter the depressions so that a plating film having poor smoothness is formed, which also affects the conductivity and greatly impairs the appearance. Thus, the reduction of aluminum spikes enables the formation of a plating film having high smoothness and excellent plating appearance.
Herein, the pH of the metal displacement solution is measured at 25° C.
The pH of the metal displacement solution may be adjusted by selecting the type of the zinc compound, nickel compound, germanium compound, or fluorine compound. An alkaline component or an acid component may also be added as necessary.
Non-limiting examples of the alkaline component include sodium hydroxide and ammonium. Non-limiting examples of the acid component include sulfuric acid and phosphoric acid. These alkaline or acid components may be used alone or in combinations of two or more.
The metal displacement solution may contain a buffer to enhance the pH buffering capacity.
The buffer may be any compound having a buffering capacity, and examples of compounds having a buffering capacity around a pH of 4.0 to 6.5 include acetic acid, malic acid, succinic acid, citric acid, malonic acid, lactic acid, oxalic acid, glutaric acid, adipic acid, and formic acid. These may be used alone or in combinations of two or more.
The buffer concentration in the metal displacement solution is preferably 1.0 to 50 g/L, more preferably 5.0 to 30 g/L.
The metal displacement solution may contain, in addition to the above-described components, components that are generally used in metal displacement solutions, such as surfactants and brightening agents. The metal displacement solution may also contain water-soluble salts of metals other than the above-described metals, such as iron, copper, silver, palladium, lead, bismuth, and thallium. These may be used alone or in combinations of two or more.
The metal displacement solution can be prepared by appropriately mixing the components using a solvent, preferably water. Although the metal displacement solution is preferably prepared as an aqueous solution from the standpoint of operational safety, other solvents such as methanol, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, glycerol, or IPA may be used, or they may be used as a solvent mixture with water. Here, these solvents may be used alone or in combinations of two or more.
The metal displacement solution can be suitably used as a metal displacement solution for treating aluminum or an aluminum alloy.
The following describes a method for surface treatment of aluminum or an aluminum alloy of the present invention using the metal displacement solution of the present invention.
The method for surface treatment of aluminum or an aluminum alloy of the present invention includes bringing a workpiece having aluminum or an aluminum alloy on its surface into contact with the metal displacement solution of the present invention to remove the oxide film on the aluminum or aluminum alloy and displace the aluminum with the metals in the metal displacement solution to form a displacement metal film containing the metals on the surface of the workpiece.
The surface treatment method includes a pretreatment method prior to forming a plating film such as a nickel or palladium plating film on a workpiece. This method brings a workpiece having aluminum or an aluminum alloy at least on its surface into contact with the metal displacement solution of the present invention to remove the oxide film adhered to the surface and form a displacement metal film, thereby increasing adhesion to a nickel plating film or other plating film to be formed in the subsequent treatment.
In the method for surface treatment of aluminum or an aluminum alloy of the present invention, the metal displacement solution of the present invention removes the oxide film adhered to a workpiece (hereinafter also referred to as “aluminum substrate”) having aluminum or an aluminum alloy at least on its surface, and allows the zinc, nickel, and germanium particles to deposit on the surface of the workpiece via a displacement reaction between the metals such as zinc and the aluminum which have different electrode potentials.
In general, in the pre-plating treatment of an aluminum substrate using a zincate treatment solution, a double zincate process is performed in which zinc displacement is performed twice. Specifically, the process includes: (1) a first zinc displacement of an aluminum substrate, (2) pickling, and then (3) a second zinc displacement. The double zincate treatment is followed by (4) plating such as electroless nickel plating.
In contrast, the method for surface treatment of aluminum or an aluminum alloy of the present invention using the metal displacement solution of the present invention, which can provide very good adhesion, requires no double zincate treatment and can provide good adhesion by a single zincate treatment. Thus, in the method for surface treatment of aluminum or an aluminum alloy of the present invention, preferably, (1) metal displacement of an aluminum substrate is performed, and this single zincate treatment is followed by (4) plating such as electroless nickel plating. In other words, preferably, (2) pickling followed by (3) a second metal displacement are not performed between the metal displacement and the plating.
The aluminum substrate, which is a workpiece to be plated, may be any substrate that has aluminum or an aluminum alloy at least on its surface. Examples of such aluminum substrates include various articles made of aluminum or aluminum alloys, articles in which an aluminum or aluminum alloy film is formed on a non-aluminum material (e.g., various substrates such as ceramic substrates and wafers), hot-dip aluminized articles, castings, and die castings. The aluminum substrate may also have any shape and may be in the form of a typical plate (including a film, a sheet, or other thin film) or in the form of any formed article of any of various shapes. Moreover, the plate is not limited to a plate made of aluminum or an aluminum alloy alone, and may include, for example, an aluminum film that is formed on (integrated with) a substrate such as a ceramic substrate or a wafer by sputtering, vacuum deposition, ion plating, or other conventional techniques.
The aluminum alloy may be, but is not limited to, for example, any of various alloys containing aluminum as a main metal component. Examples of applicable alloys include A1000 series quasi-aluminum, A2000 series aluminum alloys containing copper and manganese, A3000 series aluminum-manganese alloys, A4000 series aluminum-silicon alloys, A5000 series aluminum-magnesium alloys, A6000 series aluminum-magnesium-silicon alloys, A7000 series aluminum-zinc-magnesium alloys, and A8000 series aluminum-lithium alloys.
The aluminum purity of the aluminum or aluminum alloy is preferably 98% or higher, more preferably 98.5% or higher, still more preferably 99% or higher, from the standpoint of plating smoothness.
The aluminum substrate, which is a workpiece to be plated, can be prepared by coating a non-aluminum material such as a silicon plate with an aluminum layer using well-known techniques such as sputtering. The aluminum layer may fully or partially coat the non-aluminum material, and the coated aluminum layer usually has a thickness of 0.5 µm or more, preferably 1 µm or more. Moreover, the method for preparing the aluminum substrate is not limited to sputtering and may include vacuum deposition, ion plating, or other techniques.
First, the thus-prepared aluminum substrate may be cleaned in a well-known manner, for example, by degreasing, rinsed with water as appropriate, and then subjected to a well-known etching process using alkali or acid. Specifically, the degreasing process may be carried out by immersion in a degreasing solution for aluminum or by electrolytic degreasing. Moreover, the etching process may be carried out, for example, by immersion in an about 1-10% alkaline solution or an about 1-20% acidic solution having a liquid temperature of about 25 to 75° C. for about 1 to 15 minutes.
Next, the aluminum substrate may be immersed in an acidic solution for a predetermined period of time in order to remove the residues (smuts) produced by etching using alkali or acid. Specifically, for example, the aluminum substrate after etching may be immersed in an aqueous nitric acid solution having a concentration in the range of about 10 to 800 ml/L, preferably about 100 to 600 ml/L and a liquid temperature of about 15 to 35° C. for about 30 seconds to 2 minutes to remove the smuts.
Then, the thus-treated (e.g., desmutted) aluminum substrate may be rinsed with water and then immersed in the metal displacement solution of the present invention (zincate treatment solution) to perform metal displacement. Specifically, for example, the aluminum substrate may be immersed in a zincate treatment solution having the composition described above and a liquid temperature of 10 to 50° C., preferably 15 to 30° C. This liquid temperature is preferred because when the temperature of the zincate treatment solution is 10° C. or higher, the displacement reaction does not become too slow and a metal film without irregularities can be formed, while when the temperature is 50° C. or lower, the displacement reaction is not excessively enhanced and the surface of the displacement metal film can be prevented from becoming rough.
The conditions of the immersion period are not limited and can be set appropriately in consideration of, for example, the thickness of the aluminum oxide film to be removed. For example, the immersion period is usually about five seconds or longer, preferably 10 seconds or longer, and the upper limit thereof is five minutes or shorter. With too short an immersion period, the displacement does not proceed enough to cause sufficient removal of the oxide film, while with too long an immersion period, the treatment solution may penetrate through the small holes in the displacement metal layer to cause elution of the aluminum or aluminum alloy. Thus, the conditions need to be set in consideration of these points.
Such immersion of the aluminum substrate in the zincate treatment solution makes it possible to remove the oxide film adhered to the substrate surface and also to further coat the surface with a displacement metal film containing Zn, Ni, and Ge to activate the aluminum surface, thereby enabling the formation of a plating film having good adhesion to the workpiece.
The metal displacement process is not limited as long as it is an embodiment in which the surface of the aluminum substrate can be brought into contact with the metal displacement solution of the present invention. Examples of such contact methods include, in addition to immersion, application and spraying.
The plating process may be carried out by electroless plating or electrolytic plating on the zincated aluminum substrate. For example, an appropriate metal plating solution such as an electroless nickel plating, electroless palladium plating, or copper plating bath may be used to form a plating having a desired final film thickness.
The following specifically describes an example of electroless nickel plating. For example, an electroless nickel plating solution contains a water-soluble nickel salt such as nickel sulfate, nickel chloride, or nickel acetate, which provides nickel ions at a concentration of, for example, about 1 to 10 g/L. The electroless nickel plating solution may further contain, for example: a nickel complexing agent such as an organic acid salt (e.g., acetate, succinate, or citrate), an ammonium salt, or an amine salt at a concentration in the range of about 20 to 80 g/L; and hypophosphorous acid or a hypophosphite such as sodium hypophosphite as a reducing agent at a concentration in the range of about 10 to 40 g/L. The presence of a hypophosphite or the like as a reducing agent increases the stability of the plating solution and enables the formation of a low-cost nickel-phosphorus alloy film. Then, the plating solution containing these compounds may be prepared to have a pH of about 4 to 7 before use. Further, this plating solution may be prepared to have a liquid temperature of 60 to 95° C., and the aluminum substrate may be immersed in the plating solution for about 15 seconds to 120 minutes to perform plating. Moreover, the thickness of the plating film can be varied by changing this plating period as appropriate.
Here, the plating process is not limited to electroless plating and may be carried out by electrolytic plating, as described above. Besides the above-mentioned types of metals, other plating metals such as Cu and Au may also be used. Moreover, the plating process may be performed by displacement plating or other techniques to form two or more layers.
The conditions and concentration settings of the zincate treatment and plating processes described above are not limited to those described above. It goes without saying that these can be appropriately changed depending on, for example, the thickness of the film to be formed.
In the method for surface treatment of aluminum or an aluminum alloy of the present invention, a workpiece having aluminum or an aluminum alloy on its surface is brought into contact with the metal displacement solution of the present invention to remove the oxide film on the aluminum or aluminum alloy and displace the aluminum with the metals in the metal displacement solution. Thus, Zn may co-deposit with Ni and Ge to form a displacement metal film containing Zn, Ni, and Ge on a surface of the aluminum or aluminum alloy.
When such an aluminum or aluminum alloy having a displacement metal film containing Zn, Ni, and Ge on its surface is subjected to plating to form a plating film (metal film, e.g., nickel film), the Zn, Ni, and Ge present in the displacement metal film may act synergistically between the aluminum or aluminum alloy and the plating film (metal film, e.g., nickel film), thereby providing good adhesion to the plating film (metal film) to the aluminum or aluminum alloy.
The aluminum or aluminum alloy provided with a plating film (metal film) according to the present invention can be used in various electronic components. Examples of the electronic components include electronic components used in home appliances, in-vehicle equipment, power transmission systems, transportation equipment, and communication equipment. Specific examples include power modules such as power control units for air conditioners, elevators, electric vehicles, hybrid vehicles, trains, and power generation equipment, general home appliances, and personal computers.
In the present invention, when the metal displacement solution has a pH of 4.0 to 6.5, pre-plating treatment can be performed to also reduce aluminum spikes and form a plating film having high smoothness and excellent plating appearance. Thus, the present invention is suitable for semiconductor applications, preferably for wafer applications. In particular, the present invention is suitable as a metal displacement solution for treating aluminum or an aluminum alloy which is effective for pretreatment prior to forming an under-bump metal or bump on a wafer, or as a method for surface treatment of aluminum or an aluminum alloy using the metal displacement solution.
The present invention will be specifically described with reference to examples, but the invention is not limited to these examples.
An aluminum substrate was subjected to the treatments according to the conditions shown in Tables 1 to 3 to form a plating film. Here, a 1 cm × 2 cm Al—Cu TEG wafer was used as the aluminum substrate. The thus-prepared plating films and substrates provided with the plating films were evaluated as described below. The evaluation results are shown in Tables 2 and 3.
In Tables 2 and 3, the numerical values (concentrations) were calculated as fluorine (F) or each of the metal elements (g/L), except for succinic acid.
Each of the substrates provided with the plating films was dried by air-blowing, and cellophane tape was put on the plated surface. Then, the center of the wafer with the tape was cut and broken into two pieces. The tape was peeled off from the broken center, and the amount of removal between the Al substrate and the Ni film was determined in percentage. Here, the cutting and breaking test is schematically illustrated in
“0%” means that the plating film was not removed at all when the tape was peeled off, and “100%” means that the plating film was fully removed when the tape was peeled off.
The plating films were subjected to focused ion beam (FIB) cross-section observation using XVision 210DB (Hitachi High-Technologies Corporation).
1
2
3
4
Tables 2 and 3 show that the metal displacement solutions of the examples containing a zinc compound, a nickel compound, a germanium compound, and a fluorine compound provided good adhesion to a plating film (metal film). It is also shown that the metal displacement solutions having a pH of 3.5 to 6.5 also reduced aluminum spikes. Here, although Tables 2 and 3 show the results when an Al-Cu TEG wafer was used as the aluminum substrate, similar results were also obtained when an Al-Si TEG wafer was used as the aluminum substrate.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-182009 | Nov 2021 | JP | national |
