Palladium plating solution

Abstract

This invention provides a palladium plating solution for forming a palladium plating film using multilayer coatings such as nickel plating films, palladium plating films, and gold plating films, which offers superior heat resistance and barrier properties compared to conventional solutions. [Solution] The palladium plating solution of the present invention comprises a soluble palladium salt as a palladium source, a water-soluble germanium compound, and a conductive salt as essential components, and further includes an organic additive consisting of a monocyclic aromatic hydrocarbon or polycyclic aromatic hydrocarbon (aromatic hydrocarbons include heterocyclic aromatic hydrocarbons) or a salt thereof having a sulfo group or a carboxyl group as a diffusion inhibitor. The palladium plating film formed by the present invention has excellent heat resistance and can maintain its barrier layer even after being subjected to long-term thermal history, for example, in ceramic packages.

Description

[Technical Field]

[0001] This invention relates to an electrolytic palladium plating solution. More specifically, it relates to a palladium plating solution for forming a palladium plating film on joints made by solder, bonding wires, etc., in the mounting process of electronic component packages and the like. [Background technology]

[0002] In the manufacturing processes of various electronic and semiconductor devices, when semiconductor elements are mounted onto substrates, they are joined to connection terminals using bonding materials such as solder or bonding wires. This joining process provides electrical connection and fixes the semiconductor elements to the substrate. To improve the bonding properties, such as the bonding strength of the joint, a multilayer coating consisting of nickel plating, palladium plating, and gold plating is often formed on the joints of connection terminals.

[0003] In such a multilayer coating, the bottom nickel plating layer ensures adhesion to the constituent metal (usually copper) of terminals, etc. The outermost gold plating layer ensures adhesion to bonding materials containing gold, such as bonding wires. The palladium plating layer in between these layers inhibits the diffusion of nickel and copper from the underlying nickel plating layer or terminals into the outer gold plating layer. This is to prevent a decrease in adhesion to the bonding material due to the diffusion of nickel and copper into the outer gold plating layer, which would reduce the purity of the gold. Therefore, the palladium plating layer plays an important role in the above-described multilayer coating.

[0004] To date, multilayer coatings using palladium plating have been widely applied to lead frames used in IC packages, particularly Pd-PPF (Pre It is well known as a (Plated Frame). And, the applicant of the present application has disclosed a palladium plating solution capable of improving the properties of a palladium plating film in a multilayer film and an electronic component using the same (Patent Documents 1 and 2). This palladium plating solution is characterized in that it contains a water-soluble germanium compound together with a soluble palladium salt which is a palladium source.

[0005] The background for the development of the above germanium-added palladium plating solution by the applicant of the present application is based on the further improvement of the properties of the multilayer film and the demand for cost reduction due to the soaring prices of precious metals such as palladium and gold. In the palladium plating film formed by the palladium plating solution of the applicant of the present application, germanium as an additive eutectifies in the plating film, and this acts as a crystal regulator to form a crystal state with excellent heat resistance. This palladium plating film with improved film quality exhibits a more suitable barrier function than the conventional palladium plating film and can suppress the diffusion of metal from the substrate. And, this barrier function can act effectively even when the palladium plating film is thinned.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] The germanium-added palladium plating solution by the applicant of the present application has been widely used in the formation of multilayer films in the mounting process of lead frame packages (Pd-PPF) and has also received good evaluations in the market. However, according to the studies by the present inventors, it has been confirmed that there may be limitations in the properties of the current palladium plating film considering the expansion of the uses of the palladium plating film.

[0008] Specifically, when considering ceramic packages as an application target for palladium plating films, it is anticipated that conventional germanium-doped palladium plating films will not provide sufficient barrier function during the mounting process.

[0009] One possible contributing factor is the difference in thermal history after the formation of the multilayer coating and the bonding area. In lead frame packages, the substrate is subjected to relatively short heating periods of 30 seconds or less at temperatures ranging from approximately 300°C to 400°C. On the other hand, in ceramic packages, where the way heat is applied differs from that of lead frames, the heating temperature range is similar to that of lead frames, but the heating time is often long, exceeding 60 minutes. According to the inventors' research, there is a concern that in conventional germanium-doped palladium plating films, prolonged thermal exposure causes palladium to diffuse into the surface gold plating film, reducing the bonding properties. In this case, increasing the thickness of the gold plating film can suppress the arrival of palladium to the outermost surface, but this is not a practical solution given the continuously rising price of gold. Therefore, further improvement in heat resistance, taking into account palladium diffusion, is required for the germanium-doped palladium plating solution developed by the present applicant.

[0010] Therefore, the present invention aims to provide a palladium plating solution for forming a palladium plating film as a barrier layer in a multilayer coating that has superior heat resistance and barrier properties compared to conventional solutions. [Means for solving the problem]

[0011] The present inventors, building upon the germanium-added palladium plating solution developed by the applicant, investigated further improvements through the addition of additional additives. As a result, they discovered that a specific organic additive is effective in solving the aforementioned problem, leading to the present invention.

[0012] In other words, the present invention relates to a palladium plating solution containing a soluble palladium salt as a palladium source, a water-soluble germanium compound, and a conductive salt as essential components, characterized in that it contains an organic additive consisting of a monocyclic aromatic hydrocarbon or polycyclic aromatic hydrocarbon (aromatic hydrocarbons include heterocyclic aromatic hydrocarbons) or a salt thereof having a sulfo group or a carboxyl group as a diffusion inhibitor. The composition of the palladium plating solution according to the present invention will be described below, along with a method for forming a palladium plating film.

[0013] (A) Composition of the palladium plating solution according to the present invention The palladium plating solution according to the present invention contains a palladium plating solution with soluble palladium salt, water-soluble germanium compound, and conductive salt as essential components, and further contains a predetermined organic additive that acts as a diffusion inhibitor. In addition, the palladium plating solution according to the present invention optionally contains other additives. These components will be described below.

[0014] (A-1) Soluble palladium salt Soluble palladium salts are essential components that serve as the metal source (palladium source) for palladium plating films. The concentration of soluble palladium salts in the palladium plating solution is preferably between 0.1 g / L and 50 g / L based on palladium. If the soluble palladium concentration is less than 0.1 g / L, the deposition rate of the palladium plating film cannot be sufficiently achieved. While the deposition rate increases with increasing soluble palladium salt concentration, there is no significant difference in deposition rate above 50 g / L, leading to increased costs for the palladium plating solution. The concentration of soluble palladium salts is more preferably between 0.5 g / L and 50 g / L, and even more preferably between 1.0 g / L and 20 g / L.

[0015] For efficient deposition of the palladium plating film, amino-based palladium complexes or ammonia-based palladium complexes are preferred as soluble palladium salts. Specifically, examples include dichlorodiamine palladium, palladium chloride, diamine palladium nitrite, tetraamine palladium nitrate, diamine palladium sulfate, diamine palladium oxalate, tetraamine palladium oxalate, dichloroethylenediamine palladium(II), palladium chloride, dichlorodiaminepalladium(II), dinitrodiaminepalladium(II), tetraamminepalladium(II) nitrate, tetraamminepalladium(II) sulfate, oxalatodiaminepalladium(II), tetraamminepalladium(II) oxalate, and tetraamminepalladium(II) chloride. The palladium plating solution according to the present invention contains at least one of the above-mentioned soluble palladium salts.

[0016] (A-2) Water-soluble germanium compounds The germanium derived from the water-soluble germanium compound in the palladium plating solution co-deposits with palladium during the plating process. This process regulates the crystal growth of palladium, smoothing the grain boundaries of the palladium plating film and creating a smooth surface. In this invention, the germanium derived from the water-soluble germanium compound imparts basic barrier properties and heat resistance to the palladium plating film and suppresses metal diffusion from the substrate.

[0017] The germanium concentration in the palladium plating solution according to the present invention is preferably 0.05 g / L or more and 0.5 g / L or less. If the germanium concentration is less than 0.05 g / L, it becomes difficult to obtain the above-mentioned improvement in heat resistance, etc. Furthermore, germanium itself does not have the effect of suppressing the diffusion of metal from the substrate. Even if the germanium concentration in the palladium plating solution is excessive, the barrier performance of the palladium plating film does not improve. Therefore, the germanium concentration is preferably 0.5 g / L or less. The germanium concentration in the palladium plating solution is more preferably 0.1 g / L or more and 0.4 g / L or less.

[0018] Examples of water-soluble germanium compounds in the palladium plating solution of the present invention include germanium dioxide, germanium halides, tetraalkoxygermanium, germanium sulfide, and synthetic organic germanium such as Ge-132. The palladium plating solution may contain at least one or more water-soluble germanium compounds.

[0019] Furthermore, as mentioned above, since germanium acts on the crystal growth of palladium, it is preferable to set the germanium concentration in the palladium plating solution in consideration of the balance with the palladium concentration. Specifically, it is preferable to set the ratio of germanium concentration to palladium concentration in the plating solution to 0.2 or higher.

[0020] (A-3) Diffusion inhibitor (organic additive) The palladium plating solution according to the present invention contains an organic additive that acts as a diffusion inhibitor to suppress the diffusion of palladium into a metal plating film such as gold formed on the upper layer of the palladium plating film. This organic additive is a monocyclic aromatic hydrocarbon or polycyclic aromatic hydrocarbon (aromatic hydrocarbons include heterocyclic aromatic hydrocarbons) having a sulfo group (-SO3H) or a carboxyl group (-COOH), or a salt thereof.

[0021] Palladium plating films formed by palladium plating solutions containing these organic additives provide further heat resistance to palladium plating films that already possess basic barrier properties and heat resistance due to germanium addition, and can suppress palladium diffusion even during prolonged heat treatment. The factors by which the diffusion inhibitors achieve the above effects are not entirely clear. Palladium plating films with organic additives acting as diffusion inhibitors do not show any clear differences in surface condition or crystal state compared to palladium plating films without such additives. The inventors speculate that the effect of the diffusion inhibitors is due to a change in the distribution of germanium elements in the palladium plating film, and that this slight change affects the diffusivity of palladium.

[0022] Monocyclic aromatic hydrocarbons or polycyclic aromatic hydrocarbons (aromatic hydrocarbons include heterocyclic aromatic hydrocarbons) having a sulfo group or carboxyl group that act as a diffusion inhibitor, or salts thereof, specifically include pyridinesulfonic acid or pyridinecarboxylic acid, or salts thereof, in which a sulfo group or carboxyl group is introduced into pyridine, which is a heterocyclic aromatic hydrocarbon. More specifically, nicotinic acid, picolinic acid, isonicotinic acid, 2-pyridinesulfonic acid, 3-pyridinesulfonic acid, 4-pyridinesulfonic acid, etc., and salts thereof can be suitably used.

[0023] Furthermore, monocyclic aromatic hydrocarbons or polycyclic aromatic hydrocarbons having a sulfo group or carboxyl group that act as a diffusion inhibitor, or salts thereof, can also be used, such as benzenecarboxylic acid, benzenesulfonic acid, naphthalenecarboxylic acid, naphthalenesulfonic acid, or salts thereof, in which a sulfo group or carboxyl group is introduced into benzene or naphthalene, which are monocyclic aromatic hydrocarbons or polycyclic aromatic hydrocarbons.

[0024] The organic additive acting as a diffusion inhibitor may be a single type or a combination of multiple types. The concentration of the diffusion inhibitor is preferably between 0.01 g / L and 50 g / L. Below 0.01 g / L, its effect is not realized, and prolonged heat treatment may cause palladium to diffuse from the palladium plating film to the upper layer. On the other hand, adding an excessive amount of the organic additive does not cause any particular change, so its concentration is preferably 50 g / L or less. The concentration of the organic additive acting as a diffusion inhibitor is more preferably between 0.05 g / L and 50 g / L, and even more preferably between 0.1 g / L and 40 g / L.

[0025] Furthermore, as mentioned above, since organic additives acting as diffusion inhibitors are thought to affect the distribution of germanium in the palladium plating film, the concentration of the organic additive may be adjusted considering the germanium concentration in the plating solution. Specifically, it is preferable that the ratio of the organic additive concentration (g / L) to the germanium concentration (g / L) in the solution be between 0.1 and 200, more preferably between 0.5 and 150, and even more preferably between 1 and 150.

[0026] (A-4) Conducting Salt The palladium plating solution of the present invention deposits a palladium plating film by electroplating. Therefore, the palladium plating solution of the present invention must contain a conductive salt.

[0027] Examples of conductive salts include inorganic salts (ammonium salts, sodium salts, potassium salts) of hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, and citric acid. More specifically, ammonium chloride, ammonium nitrate, ammonium sulfate, ammonium phosphate, and ammonium citrate are preferably used. Conductive salts are added to the plating solution one or more of these. To ensure the conductivity of the plating solution while preventing salting out during the plating process, the concentration of the conductive salt is preferably 10 g / L to 500 g / L.

[0028] (A-5) Additives The palladium plating solution according to the present invention may contain additives other than organic additives that act as diffusion inhibitors. Other additives may include, for example, pH buffers, surfactants, complexing agents, stabilizers, etc. Compounds commonly used in metal plating solutions can be applied to these additives. However, the palladium plating solution according to the present invention preferably contains only palladium and germanium as metal components, and preferably does not contain any other metal components. Here, "metal components" refers to transition metals (groups 3 to 12). Alkali metals such as sodium and potassium, and alkaline earth metals, are not included in the metal components of the plating solution.

[0029] Furthermore, the concentrations of palladium, germanium, and organic additives (diffusion inhibitors) in the palladium plating solution can be determined using various analytical methods while the plating solution is in its current state. Examples of analytical methods include inductively coupled plasma emission spectroscopy (ICP), ion chromatography (IC), high-performance liquid chromatography (HPLC), high-performance liquid chromatogram-mass spectrometry (LC-MS, LC-MS / MS), Fourier transform infrared spectroscopy (FT-IR), and nuclear magnetic resonance (NMR). One or a combination of these methods can be used for analysis.

[0030] (B) Method for forming a palladium plating film using the palladium plating solution according to the present invention The palladium plating solution according to the present invention is an electroplating solution, and when forming a palladium plating film, the bath temperature, bath pH, ​​and current density are set as plating conditions.

[0031] The bath temperature is preferably between 25°C and 70°C. Since bath temperature is related to deposition efficiency, deposition efficiency will be poor below 25°C. On the other hand, operation at high temperatures exceeding 70°C raises concerns about deterioration of components of the plating equipment, such as jigs. The bath pH is preferably neutral to weakly alkaline, preferably between pH 5 and pH 10. Bath stability is a concern below pH 5, and deterioration of the plating equipment and the object to be plated (substrate, etc.) is a concern above pH 10.

[0032] The current density is 0.05 A / dm 2 More than 5A / dm 2 The following is preferable: 0.05 A / dm 2 Below 5 A / dm², it is inefficient to deposit a plating film of the required thickness. 2 If the temperature exceeds a certain level, the palladium plating may discolor or burn.

[0033] Under the plating conditions described above, a palladium plating film is formed from the palladium plating solution according to the present invention. The object to which the palladium plating solution according to the present invention is applied is not particularly limited. However, as mentioned at the beginning, the present invention aims to improve heat resistance to heat treatment in the mounting process of ceramic packages and the like, so its application is primarily to connection parts such as connection terminals and wiring of various packages. In this case, the palladium plating solution according to the present invention is useful for forming a multilayer palladium plating film (nickel plating film / palladium plating film / gold plating film) at the connection part.

[0034] Furthermore, although this invention was conceived with the mounting process of ceramic packages in mind, it can also be applied to lead frame packages to which conventional germanium-doped palladium plating solutions have been used. This is because the palladium plating film with improved heat resistance according to this invention is naturally effective even for multilayer films with relatively mild heat treatment conditions. In addition, the palladium plating film with improved heat resistance can contribute to lead frame packages by reducing the film thickness.

[0035] There are no particular restrictions on the thickness of the palladium plating film formed by the palladium plating solution according to the present invention; it should be in accordance with the specifications of the applicable package. In the multilayer coating application of the package connection portion described above, the thickness of the palladium plating film is often set to 0.001 μm or more and 5 μm or less, and heat resistance is ensured within this range.

[0036] Furthermore, the palladium plating film formed by the present invention contains germanium, similar to the plating film formed by conventional germanium-added palladium plating solutions. In the case of the present invention, the germanium content in the palladium plating film is 1 ppm to 5000 ppm. [Effects of the Invention]

[0037] As described above, the palladium plating solution according to the present invention can form a palladium plating film with superior heat resistance compared to conventional germanium-added palladium plating solutions. The palladium plating film with improved heat resistance can suppress the diffusion of palladium into the gold plating film or the like on top during the package mounting process, which is subjected to a greater thermal history than before, thereby maintaining the bonding properties between the gold plating film or the like and the bonding material. [Brief explanation of the drawing]

[0038] [Figure 1a] This figure shows the results of GD-OES analysis of multilayer thin films produced using the germanium-doped palladium plating solution of Example 1. [Figure 1b] This figure shows the results of GD-OES analysis of multilayer thin films produced using the germanium-doped palladium plating solution of Comparative Example 1. [Modes for carrying out the invention]

[0039] First Embodiment Embodiments of the present invention will now be described. In this embodiment, a conventional germanium-added palladium plating solution and a palladium plating solution prepared by adding an appropriate amount of 3-pyridinesulfonic acid as a diffusion inhibitor to the germanium-added palladium plating solution were prepared as baths. Then, using each palladium plating solution, a multilayer film (nickel plating film / palladium plating film / gold plating film) was formed on a Cu substrate, and the heat resistance and barrier performance of the palladium plating film against heat treatment were evaluated.

[0040] [Formation of nickel plating film] In this embodiment, a commercially available copper plate (dimensions: 20 mm x 20 mm) was used as the substrate. As a substrate pretreatment, acid degreasing (EEJA Corporation: E-Trex 15, 25°C, 1 minute) was performed, followed by pickling with 10% sulfuric acid by volume (25°C, 1 minute). Furthermore, the substrate surface was soft-etched (EEJA Corporation: Microfab 74, 25°C, 1 minute), followed by pickling with 10% sulfuric acid by volume (25°C, 1 minute) and washing with pure water.

[0041] A nickel plating film was formed on the copper substrate that had undergone the above pretreatment. The nickel plating solution was prepared using a microfabric. Using Ni100 (manufactured by EEJA Corporation), with a bath temperature of 55°C and a current density of 3A / dm² 2 Electroplating was performed for 10 minutes (target film thickness: 0.7 μm).

[0042] [Formation of palladium plating film] A palladium plating film was formed on a copper substrate that had a nickel plating film on it, using a palladium plating solution. As described above, in this embodiment, two types of plating solutions were prepared and used for plating: a conventional germanium-added palladium plating solution (Comparative Example) and a palladium plating solution (Example 1) to which 3-pyridine sulfonic acid, a diffusion inhibitor, was added.

[0043] The composition of the palladium plating solution prepared in this embodiment is as follows. Palladium plating solution composition • Dichlorodiamminepalladium (palladium equivalent): 3g / L Germanium oxide (in germanium equivalent): 0.1 g / L • 3-Pyridinesulfonic acid: No addition (Comparative Example 1), 0.05 g / L (Example 1) • Ammonium sulfate (conducting salt): 160 g / L • Ammonia solution (pH adjuster): 20 mL / L (appropriate amount) Plating solution pH: 7.0

[0044] The palladium plating process was performed with a bath temperature of 50°C and a current density of 0.75 A / dm². 2 The procedure was performed under the condition of 29 seconds (target film thickness 0.05 μm).

[0045] [Formation of gold plating film] After palladium plating, a gold plating film was formed. The gold plating solution was Precious Fab. Using an Au8500 (manufactured by EEJA Corporation), with a bath temperature of 65°C and a current density of 0.2 A / dm² 2 Electroplating was performed for 3.3 minutes (target film thickness: 0.3 μm).

[0046] [Heat Resistance Evaluation Test] The multilayer films (Example 1, Comparative Example 1) manufactured above were heat-treated, and the presence or absence of palladium diffusion from the palladium plating film to the gold plating film was examined. The heat treatment was carried out under two heat treatment conditions with heat treatment temperatures of 300 °C, 350 °C, and 400 °C in the air. The heating time was 60 minutes for all cases.

[0047] The evaluation of the presence or absence of palladium diffusion into the gold plating film by heat treatment was performed by using a spectrophotometer (manufactured by Konica Minolta: SPECTRO PHOTOMETER CM-36dG) for the surface of the multilayer film (gold plating film surface) before and after heat treatment, and measuring the color tone based on the CIE color system (L * a * b * color system) (JIS Z8729). Then, the reduction rate of the L * value ((L * value before and after heat treatment - L * value before heat treatment) / L * value before heat treatment × 100 (%)) was calculated.

[0048] Also, for the multilayer film after heat treatment at 350 °C for 60 minutes, analysis was performed by GD-OES (glow discharge optical emission spectroscopy), and compositional analysis in the depth direction from the outermost layer (gold plating film) to the bottom layer (nickel plating film) of the multilayer film was carried out.

[0049] Regarding the multilayer films of Example 1 and Comparative Example 1 of this embodiment, the measured values of the L * value, a * value, b * value, and the reduction rate of the L * value are shown in Table 1. Also, the analysis results by GD-OES before and after heat treatment at 350 °C for the multilayer films of Example 1 and Comparative Example 1 are shown in Fig. 1 (Fig. 1(a): Example 1, Fig. 1(b): Comparative Example 1).

[0050]

Table 1

[0051] Regarding the evaluation results of the multilayer coating in this embodiment, first referring to the analysis results by GD-OES, it can be seen that in Comparative Example 1 in Figure 1(b), a multilayer coating with germanium-added palladium plating without a diffusion inhibitor (after heat treatment at 350°C for 60 minutes), palladium is present within the gold plating film. This palladium is thought to be due to diffusion from the palladium plating film directly beneath the gold plating film. In contrast, in Example 1 in Figure 1(a), a multilayer coating with germanium-added palladium plating with a diffusion inhibitor, no palladium is present within the gold plating film even after the same heat treatment. From this, it can be confirmed that the heat resistance of the palladium plating film has been improved by the diffusion inhibitor.

[0052] Based on the results in Figures 1(a) and 1(b), the results of the color tone measurement of the multilayer film surface before and after heat treatment in Table 1 are confirmed. Based on the results of Comparative Example 1, the heat treatment temperature and L * Looking at the relationship with the rate of decrease of the value, L increases with increasing heat treatment temperature. * The rate of decrease in the value increases exponentially. In particular, by changing the heat treatment temperature from 350°C to 400°C, L * The rate of decrease in the value has more than quadrupled.

[0053] In contrast, referring to the results of Example 1, at all heat treatment temperatures, Example 1 was L compared to Comparative Example 1. * The rate of decrease in value is small. Also, as the heat treatment temperature increases, the L of Example 1 and Comparative Example 1 * The difference in the rate of decrease of the values ​​is large. The GD-OES analysis in Figures 1(a) and (b) is the result at a heat treatment temperature of 350°C, and the L of Example 1 and Comparative Example 1 at this time is large. * The difference in the rate of decrease of the value is clear, and the presence or absence of palladium diffusion in the multilayer coating and L * A correlation can be confirmed between the difference in the rate of decrease of the values ​​and the observed correlation.

[0054] Furthermore, when the heat treatment temperature was set to 400°C, the L of Example 1 and Comparative Example 1 was * The difference in the rate of decrease of the value is extremely large. By setting the heat treatment temperature to 400°C, L was also obtained in Example 1. * The rate of decrease in value is increasing. However, at this time L* The rate of decrease in value is L at a heat treatment temperature of 350°C. * The rate of decrease in the value is less than twice.

[0055] From the above findings, it was confirmed that applying the germanium-added palladium plating solution containing the diffusion inhibitor of Example 1 improves the heat resistance of the palladium plating film and suppresses palladium diffusion to the multilayer film surface (gold plating film).

[0056] Second Embodiment In this embodiment, the effects of applying various organic additives as diffusion inhibitors to germanium-added palladium plating solutions were confirmed.

[0057] After forming a nickel plating film on a copper substrate in the same manner as in the first embodiment, a germanium-added palladium plating solution containing various organic additives was prepared, and a palladium plating film was formed. The composition of the palladium plating solution in this embodiment is as follows. The conditions for the palladium plating treatment were the same as in the first embodiment.

[0058] Palladium plating solution composition • Dichlorodiamminepalladium (palladium equivalent): 3g / L Germanium oxide (in germanium equivalent): 0.1 g / L • Diffusion inhibitors: Nicotinic acid (Example 2), sodium naphthalene sulfonate (Example 3) • Diffusion inhibitor concentration: 0.05 g / L • Ammonium sulfate (conducting salt): 160 g / L • Ammonia solution (pH adjuster): 20 mL / L (appropriate amount) Plating solution pH: 7.0

[0059] Furthermore, in this embodiment, as a reference example, polyoxyethylene alkyl ether, a known organic additive, was added as a leveling agent to a germanium-added palladium plating solution (the polyoxyethylene alkyl ether concentration was the same as in Examples 2 and 3), and a sample was prepared by forming a palladium plating film with this palladium plating solution.

[0060] After the palladium plating treatment described above, a gold plating film was formed. The gold plating solution and plating conditions were the same as in the first embodiment.

[0061] Then, the multilayer coatings of Examples 2 and 3 and the Reference Example were subjected to heat treatment (350°C for 60 minutes) in the same manner as in the First Embodiment. The color tone of the surface of the multilayer coating after heat treatment was measured using a spectrophotometer, and L * value, a * value, b * Measure the value and L * The rate of decrease in the value was calculated. Furthermore, from the comparison results with the GD-OES analysis in the first embodiment, it was confirmed that the change due to palladium diffusion into the gold plating film can be estimated by color measurement, so in this embodiment, L * The heat resistance of the palladium plating film was evaluated based on the rate of decrease in the value. The measurement results are shown in Table 2. For comparison, Table 2 also shows the results for Example 1 of the first embodiment (diffusion inhibitor: 3-pyridine sulfonic acid) and Comparative Example 1 (no diffusion inhibitor added).

[0062] [Table 2]

[0063] Table 2 shows that even with palladium plating films prepared using germanium-added palladium plating solutions to which nicotinic acid (Example 2) and naphthalene sulfonate (Example 3) were added as diffusion inhibitors, the L after heat treatment (350°C for 60 minutes) was also * It was confirmed that the rate of decrease in the value could be suppressed. Therefore, it can be said that these organic additives also have the effect of diffusion inhibitors. As a reference example, the palladium plating film obtained by germanium-added palladium plating solution with polyoxyethylene alkyl ether added was L * The rate of decrease in the value was greater than in Comparative Example 1 (no additive). Leveling agents are additives added with the intention of improving the smoothness of the plating film. It can be said that such general additives are not effective in improving the heat resistance of palladium plating films.

[0064] Third Embodiment In this embodiment, compared to Example 1 of the first embodiment, a palladium plating solution with an increased concentration of diffusion inhibitor (Example 4) and a palladium plating solution with increased concentrations of palladium and germanium (Example 5) were prepared, and palladium plating films were formed using these solutions. The composition of the palladium plating solution in this embodiment is as follows. The conditions for the palladium plating treatment were the same as in the first embodiment.

[0065] Palladium plating solution composition • Dichlorodiamminepalladium (palladium equivalent): 3 g / L (Example 4), 16 g / L (Example 5) Germanium oxide (in germanium equivalent): 0.1 g / L, 0.2 g / L • Diffusion inhibitor (3-pyridinesulfonic acid) concentration: 5 g / L (Example 5), 10 g / L (Example 4) • Ammonium sulfate (conducting salt): 160 g / L • Ammonia solution (pH adjuster): 20 mL / L (appropriate amount) Plating solution pH: 7.0

[0066] After palladium plating, a gold plating film was formed. The gold plating solution and plating conditions were the same as in the first embodiment. Then, the multilayer films of each embodiment were heat-treated (350°C for 60 minutes) in the same manner as in the first embodiment, and the color tone was measured using a spectrophotometer. The measurement results are shown in Table 3.

[0067] [Table 3]

[0068] Table 3 shows that in the palladium plating solution of Example 4, where the concentration of the diffusion inhibitor 3-pyridinesulfonic acid was 10 g / L, which is 20 times that of Example 1, the L of the gold plating film after heat treatment (350°C for 60 minutes) was *It was confirmed that the rate of decrease in the value could be further reduced, thereby improving the heat resistance of the palladium plating film. Furthermore, from the results of Example 5, it was confirmed that the effect of the diffusion inhibitor on improving the heat resistance of the palladium plating film is maintained even when the palladium concentration and germanium concentration of the palladium plating solution are increased.

[0069] Fourth Embodiment In this embodiment, multiple germanium-added palladium plating solutions with different concentrations of diffusion inhibitor were prepared to form palladium plating films, and their heat resistance was evaluated.

[0070] In this embodiment, the substrate configuration for the palladium plating film is different from that of the first to third embodiments. Specifically, instead of the nickel plating film and copper substrate, a silicon wafer on which a ruthenium film has been formed is used as the substrate.

[0071] The reason for changing the substrate configuration was to evaluate the heat resistance of the palladium plating film while eliminating the influence of the nickel plating film and copper substrate. Under the heat treatment conditions of the first embodiment, etc. (350°C for 60 minutes), it cannot be ruled out that the copper substrate and nickel plating film may discolor, affecting the color tone of the gold plating film on top. * a * b * L is measured according to the color system. * The value indicates the brightness of the sample. L * The values ​​are displayed on a scale of 0 to 100, L * As the value decreases and approaches 0, it turns black. It is predicted that the underlying nickel or copper will turn black when oxidized by heat treatment, and this is L * This may affect the value. In the evaluation tests of the first and second embodiments, the same heat treatment was performed on samples with the same substrate configuration, so the L of the gold plating film *Even if the values ​​are affected by the substrate, this does not mean that the evaluation results will be incorrect. However, in this embodiment, the concentration of the diffusion inhibitor in the palladium plating solution is set to a very small amount (0.01 g / L), and even subtle changes in the color tone of the gold plating film should be captured. Therefore, ruthenium, which has oxidation resistance and is less likely to discolor due to heat treatment, was applied as the substrate.

[0072] In this embodiment, a palladium plating film was formed on a silicon wafer (dimensions 2.5 × 2.5 cm) on which a ruthenium film (thickness 0.01-0.02 μm) had been formed as a substrate. The palladium plating solution had the following composition. The palladium plating conditions were a bath temperature of 50°C and a current density of 0.75 A / dm². 2 The test was performed for 60 seconds (target film thickness 0.1 μm).

[0073] Palladium plating solution composition • Dichlorodiamminepalladium (palladium equivalent): 3g / L Germanium oxide (in germanium equivalent): 0.1 g / L • Diffusion inhibitor: 3-pyridine sulfonic acid • Diffusion inhibitor concentration: No additive (Comparative Example 2), 0.01 g / L (Example 6), 0.1 g / L (Example 7), 1.0 g / L (Example 8), 10.0 g / L (Example 9), 20.0 g / L (Example 10) • Ammonium sulfate (conducting salt): 160 g / L • Ammonia solution (pH adjuster): 20 mL / L (appropriate amount) Plating solution pH: 7.0

[0074] After palladium plating, a gold thin film was formed by sputtering. In this embodiment, the gold thin film was made by sputtering because it can be made denser than a plated film. Furthermore, even though it is a sputtered film, it is considered that it will be affected by the palladium plating film in the same way as a plated film. The gold thin film was formed by sputtering a gold sputtering target using a magnetron sputtering apparatus to a film thickness of 0.1 μm.

[0075] The multilayer coatings (ruthenium / palladium plating film / gold sputtered film) formed in Examples 6 to 10 and Comparative Example 2 in this embodiment were subjected to the same heat treatment (350°C for 60 minutes) as in the first embodiment. The color tone of the multilayer coating surface after heat treatment was measured using a spectrophotometer, and L * value, a * value, b * The value was measured.

[0076] Furthermore, in this embodiment, L * In addition to the rate of decrease in the value, b * The rate of decrease in the value was also calculated. L * a * b * In a color system, a * Value and b * The value indicates the color of the object being measured. * The value is a parameter that shows blue to yellow on a scale of -60 to 60. Since the gold thin film is being evaluated in this embodiment, L * While focusing on the rate of decrease in value, b * The evaluation also took into account the rate of decrease in the value. The measurement results in this embodiment are shown in Table 4.

[0077] [Table 4]

[0078] Table 4 shows that even when the amount of diffusion inhibitor (3-pyridine sulfonic acid) added in Example 6 was minimized (0.01 g / L), the L was still greater than in Comparative Example 2 (no diffusion inhibitor). * The rate of decrease in the value can be reduced. However, at the concentration of 0.01 g / L in Example 6, b * Considering that the rate of decrease in the value is almost the same as in Comparative Example 2, it is preferable to increase the diffusion inhibitor further. Specifically, it is preferable to use a 3-pyridinesulfonic acid concentration of 0.05 g / L or higher than that of Example 1 in the First Embodiment, or a 3-pyridinesulfonic acid concentration of 0.1 g / L in Example 7. Furthermore, from the results of Examples 7 to 10, if the germanium concentration is the same, as the concentration of the diffusion inhibitor increases, L *The rate of decrease in the value tends to decrease. In the palladium-plated films of these examples, palladium diffusion due to heat treatment is suppressed, and it can be said that there is an improvement in heat resistance. [Industrial applicability]

[0079] The palladium plating solution according to the present invention can improve the heat resistance of the palladium plating film compared to conventional germanium-added palladium plating solutions. This improved heat resistance suppresses the diffusion of palladium into the metal thin film formed in the upper layer during the heat treatment process. Therefore, the present invention is effective for forming multilayer coatings applied to the mounting process of packages subjected to various thermal histories. In particular, the present invention is useful for the mounting process of ceramic packages subjected to more severe thermal histories than lead frame packages.

Claims

1. In a palladium plating solution containing a soluble palladium salt as a palladium source, a water-soluble germanium compound, and a conductive salt as essential components, The organic additive includes a diffusion inhibitor consisting of monocyclic aromatic hydrocarbons or polycyclic aromatic hydrocarbons (aromatic hydrocarbons include heterocyclic aromatic hydrocarbons) or salts thereof having a sulfo group or a carboxyl group. The palladium plating solution is characterized in that the organic additive, which is a diffusion inhibitor, is at least one of pyridinesulfonic acid, pyridinecarboxylic acid, or a salt thereof, or at least one of benzenecarboxylic acid, benzenesulfonic acid, naphthalenecarboxylic acid, naphthalenesulfonic acid, or a salt thereof.

2. The palladium plating solution according to claim 1, wherein the concentration of the organic additive acting as a diffusion inhibitor is 0.01 g / L or more and 50 g / L or less.

3. The palladium plating solution according to claim 1 or claim 2, wherein the concentration of the soluble palladium salt is 0.1 g / L or more and 50 g / L or less based on palladium.

4. The palladium plating solution according to claim 1 or claim 2, wherein the concentration of the water-soluble germanium compound is 0.05 g / L or more and 0.5 g / L or less based on germanium.

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