JP5288362B2 - Multilayer plating film and printed wiring board - Google Patents

Description

  The present invention relates to a multilayer plating film that can provide excellent solder joint strength when applied as a surface treatment of a conductor portion of a printed wiring board, for example, and a printed wiring board on which the multilayer plating film is formed.

  When bonding or soldering gold wires, aluminum wires, etc. on electronic parts, especially printed wiring boards, after forming an electroless nickel plating film as a surface treatment for the conductor part of the joint, form a gold plating film There are things to do. This treatment is intended to protect the surface of the nickel plating film with the gold plating film to exhibit high solder joint strength and to ensure good solder wettability. In the above surface treatment, an electroless nickel plating film of about 1 to 10 μm is usually formed, and then a gold plating film of about 0.03 to 0.1 μm is formed by a displacement plating method. For the purpose of imparting excellent heat resistance, a gold plating film having a thickness of about 0.2 to 1 μm is sometimes formed by a displacement plating method or a reduction plating method.

  However, the above-described method has a problem in that when the gold plating film is formed by the displacement plating method, the surface of the nickel plating film is eroded because gold plating is deposited due to a potential difference from the underlying nickel plating film. is there. Such erosion of the nickel plating film causes deterioration of solderability and bonding property, and further causes contamination of the liquid.

For this reason, a method of forming an electroless palladium plating film as an intermediate layer between the electroless nickel plating film and the displacement gold plating film has been proposed for the purpose of protecting the nickel plating film (Patent Document 1 below). reference). This method is intended to suppress the erosion of the nickel plating film by the displacement gold plating solution by forming a palladium plating film as an intermediate layer, but the electroless palladium plating film has a diffusibility into solder. Since it is inferior, it may remain on the surface of the electroless nickel plating film during soldering and cause a decrease in solder joint strength. Further, when the electroless palladium plating film is thin or the shape of the object to be plated is complicated, the deposition of the palladium plating film may be insufficient and an exposed portion of the nickel plating film may occur. In this case, since nickel is a base metal than palladium, if even a part of the nickel plating film is exposed, the nickel in this part is preferentially melted and the nickel surface is eroded, causing a decrease in solderability. .
Japanese Patent Laid-Open No. 10-168578

  The present invention has been made in view of the current state of the prior art described above, and its main object is to apply it to electronic components, particularly conductor circuit portions of printed wiring boards, etc. It is an object of the present invention to provide a novel processing method capable of imparting bonding properties and the like.

  The present inventor has intensively studied to achieve the above-described object. As a result, after forming a specific electroless nickel plating film having a higher phosphorus content than the commonly used nickel plating film on the conductor portion of the printed wiring board, a very thin film thickness of 0.01 to 1 μm is obtained. When forming an electrolytic palladium plating film, the underlying nickel plating film is hardly eroded when performing substitutional gold plating, and the palladium is rapidly put into the solder during soldering. It has been found that good solder bondability is ensured by diffusion. The present invention has been completed as a result of further research based on such novel findings.

That is, the present invention provides the following multilayer plating film and a printed wiring board on which the multilayer plating film is formed.
1. A multilayer plating film obtained by sequentially laminating an electroless nickel plating film having a phosphorus content of 10 to 13% by weight, an electroless palladium plating film having a thickness of 0.01 to 1 μm, and a displacement type gold plating film.
2. 2. The multilayer plating film according to item 1, wherein a reduced electroless gold plating film is further formed on the substitutional gold plating film. 3. A printed wiring board, wherein the multilayer plating film according to item 1 or 2 is formed on a conductor portion of the printed wiring board.

The multilayer plating film of the present invention is formed by sequentially laminating an electroless nickel plating film having a phosphorus content of 10 to 13% by weight, an electroless palladium plating film having a thickness of 0.01 to 1 μm, and a displacement type gold plating film. It is.

  In the multilayered plating film in which the plating films satisfying such specific conditions are laminated, the presence of the electroless palladium plating film can prevent erosion of the nickel plating film when performing substitutional gold plating. In addition, since the electroless nickel plating film having a high phosphorus content of 10 to 13% by weight and excellent corrosion resistance is formed, the nickel plating film is not completely covered by the palladium plating film and is partially exposed. Even if it is a case, when performing substitution type | mold gold plating, erosion of a nickel plating film | membrane can be suppressed. For this reason, the protective effect of a sufficient nickel plating film is exhibited only by forming a very thin electroless palladium plating film having a thickness of 0.01 to 1 μm. Furthermore, since the electroless palladium plating film is very thin, when soldering, palladium is sufficiently diffused into the solder, and high solder joint strength can be secured.

  As described above, by using the plating film having the specific multilayer structure described above, a plating film having high solder bonding strength can be formed on the conductor portion of the printed wiring board. Hereinafter, the multilayer plating film of the present invention will be described more specifically.

(1) Electroless nickel plating film In the present invention, an electroless nickel plating film having a phosphorus content of about 10 to 13% by weight is formed as the nickel plating film. Such an electroless nickel plating film with a high phosphorus content has good corrosion resistance, and even when a very thin electroless palladium plating film is formed thereon, a substitutional gold plating solution Erosion due to is hardly generated.

  The electroless nickel plating film described above can be formed using an electroless nickel plating solution made of an aqueous solution containing a water-soluble nickel compound, a reducing agent, and a complexing agent as essential components.

  The water-soluble nickel compound is not particularly limited as long as it is soluble in the plating solution and can obtain an aqueous solution having a predetermined concentration. For example, nickel sulfate, nickel chloride, nickel sulfamate, nickel hypophosphite, or the like can be used. A water-soluble nickel compound can be used individually by 1 type or in mixture of 2 or more types. In particular, nickel sulfate is preferable in terms of good solubility.

  The content of the water-soluble nickel compound is preferably about 0.5 to 50 g / L, and more preferably about 2 to 10 g / L.

  As the reducing agent, at least one selected from hypophosphorous acid and hypophosphite can be used. As hypophosphite, for example, sodium salt, potassium salt, ammonium salt and the like can be used. The content of the reducing agent is preferably about 5 to 100 g / L, and more preferably 10 to 50 g / L.

  A known complexing agent can be used as the complexing agent. For example, monocarboxylic acids such as acetic acid and formic acid, ammonium salts, potassium salts, sodium salts thereof; dicarboxylic acids such as malonic acid, succinic acid, adipic acid, maleic acid, fumaric acid, ammonium salts, potassium salts thereof, Sodium salts, etc .; hydroxycarboxylic acids such as malic acid, lactic acid, glycolic acid, gluconic acid, citric acid, ammonium salts, potassium salts, sodium salts thereof; ethylenediaminediacetic acid, 1-hydroxyethylidene-1,1-diphosphonic acid These ammonium salts, potassium salts, sodium salts and the like can be used singly or in combination of two or more.

  The complexing agent content is preferably about 0.5 to 100 g / L, more preferably 5 to 50 g / L.

  Aminopolycarboxylic acids such as ethylenediaminetetraacetic acid and diethylenetriaminepentaacetic acid and their sodium, potassium and ammonium salts; glycine, alanine, iminodiacetic acid, nitrilotriacetic acid, L-glutamic acid, L-glutamic acid diacetic acid, L- Amino acids such as aspartic acid and taurine, aminotrimethylenephosphonic acid, ethylenediaminetetramethylenephosphonic acid, compounds having amino groups such as ammonium salt, potassium salt and sodium salt, and amine compounds such as ethylenediamine and diethylenetriamine are, for example, If it is blended in an amount of about 0.5 g / L or more, it is difficult to obtain a nickel-plated film having a target phosphorus content.

  In addition to the electroless nickel plating solution, one or more of lead salts such as lead nitrate and lead acetate, bismuth salts such as bismuth nitrate and bismuth acetate, which are generally used, are added as stabilizers. Can do. The addition amount is preferably about 0.01 to 100 mg / L. However, for thiodiglycolic acid, sodium thiosulfate, and other known sulfur compounds, a nickel plating film with the desired phosphorus content can be obtained by adding a concentration that is effective as a stabilizer, for example 0.05 mg / L or more. It is desirable not to blend because it is difficult to be blended.

  Furthermore, in order to improve the permeability of the plating solution, a surfactant can be blended. As the surfactant, nonionic, cationic, anionic, and amphoteric surfactants can be added singly or in combination of two or more. The addition amount is preferably about 0.1 to 100 mg / L.

  The pH of the electroless nickel plating solution is preferably about 2 to 9, particularly about 3 to 8. For pH adjustment, inorganic acids such as sulfuric acid and phosphoric acid, sodium hydroxide, aqueous ammonia and the like can be used.

  By setting the pH low in the above pH range, a nickel plating film having a high phosphorus content can be formed.

(2) Electroless palladium plating film An electroless palladium plating film can be formed using an electroless palladium plating solution comprising an aqueous solution containing a palladium compound, a reducing agent and a complexing agent as essential components.

  The palladium compound blended in the electroless palladium plating solution is not particularly limited as long as it is soluble in the plating solution and can obtain an aqueous solution having a predetermined concentration. For example, water-soluble palladium compounds such as palladium sulfate, palladium chloride, palladium acetate, dichlorodiethine rediamine palladium, and tetraammine palladium dichloride can be used. Further, as the palladium compound, a so-called palladium solution in which palladium is made into a solution can also be used. As the palladium solution, for example, a dichlorodiethylenediamine palladium solution or a tetraammine palladium dichloride solution can also be used. A palladium compound can be used individually by 1 type or in mixture of 2 or more types.

  The content of the palladium compound is preferably about 0.1 to 30 g / L as palladium, and more preferably about 0.3 to 10 g / L.

  As a reducing agent for electroless palladium plating, formic acid, hypophosphorous acid, phosphorous acid, salts thereof (sodium salt, potassium salt, ammonium salt, etc.) and the like can be used. These reducing agents can be used singly or in combination of two or more.

  The content of the reducing agent is preferably about 0.1 to 100 g / L, more preferably 1 to 50 g / L.

  As the reducing agent, formic acid or a salt thereof is particularly preferable. By using this, it becomes possible to form a palladium plating film having a purity of 99% or more, and higher solder joint strength can be obtained.

  A known complexing agent can be used as the complexing agent. For example, amines such as ethylenediamine and diethylenetriamine; aminopolycarboxylic acids such as ethylenediaminediacetic acid, ethylenediaminetetraacetic acid and diethylenetriaminepentaacetic acid, sodium salts, potassium salts and ammonium salts thereof; glycine, alanine, iminodiacetic acid, nitrilotriacetic acid, L Amino acids such as glutamic acid, L-glutamic acid diacetic acid, L-aspartic acid, taurine, sodium salts, potassium salts, ammonium salts thereof; aminotrimethylenephosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, ethylenediamine Tetramethylene phosphonic acid, ammonium salts thereof, potassium salts, sodium salts and the like can be blended. Complexing agents can be used alone or in combination of two or more. The compounding amount of the complexing agent is preferably about 0.5 to 100 g / L, more preferably about 5 to 50 g / L.

  The pH of the electroless palladium plating solution is preferably about 2 to 9, and particularly preferably about 3 to 8. For pH adjustment, inorganic acids such as sulfuric acid and phosphoric acid, sodium hydroxide, aqueous ammonia and the like can be used.

(3) Substitutional gold plating film A substitutional gold plating film can be formed using a substitutional gold plating solution comprising an aqueous solution containing a water-soluble gold salt and a complexing agent as essential components.

  As the replacement-type gold plating solution, those generally used in printed wiring board applications can be used as they are. As the substitutional gold plating solution, there are a cyan-containing plating bath and a cyan-free plating bath, and any plating bath may be used.

  The water-soluble gold salt to be blended in the substitutional gold plating solution is not particularly limited as long as it is soluble in the plating solution and an aqueous solution having a predetermined concentration can be obtained. For example, potassium gold sulfite, sodium gold sulfite, gold ammonium sulfite, potassium gold cyanide, sodium gold cyanide, ammonium gold cyanide and the like can be used. The water-soluble gold salt can be used alone or in combination of two or more. The content of the water-soluble gold salt is preferably about 0.1 to 20 g / L, and more preferably about 0.3 to 5 g / L.

  A known complexing agent can be used as the complexing agent. For example, amines such as ethylenediamine and diethylenetriamine; aminopolycarboxylic acids such as ethylenediaminediacetic acid, ethylenediaminetetraacetic acid and diethylenetriaminepentaacetic acid, sodium salts, potassium salts and ammonium salts thereof; glycine, alanine, iminodiacetic acid, nitrilotriacetic acid, L Amino acids such as glutamic acid, L-glutamic acid diacetic acid, L-aspartic acid, taurine, ammonium salts, potassium salts, sodium salts thereof; aminotrimethylenephosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, Alkylsulfonic acids such as ethylenediaminetetramethylenephosphonic acid, methanesulfonic acid, ethanesulfonic acid, ammonium salts, potassium salts, sodium salts thereof; hydroxymethanesulfonic acid Alkanol sulfonic acids such as hydroxyethanesulfonic acid, ammonium salts, potassium salts, sodium salts thereof; aromatic sulfonic acids such as benzenesulfonic acid, p-phenolsulfonic acid, ammonium salts, potassium salts, sodium salts, etc. Can be used. Complexing agents can be used alone or in combination of two or more. The complexing agent content is preferably about 0.5 to 100 g / L, more preferably about 5 to 50 g / L.

  The pH of the displacement type gold plating solution is preferably about 2 to 9, particularly about 3 to 8. For pH adjustment, inorganic acids such as sulfuric acid and phosphoric acid, sodium hydroxide, aqueous ammonia and the like can be used.

(4) Formation method of multilayer plating film:
In order to form the multilayer plating film of the present invention, first, an electroless nickel plating film having a phosphorus content of about 10 to 13% by weight is formed on an object to be plated.

  There is no particular limitation on the object to be plated, but in particular, the present invention provides solderability, wire bonding property, etc., when a conductor part such as a pad part, a circuit part, or a terminal part of a printed wiring board is to be plated. It is possible to form an excellent film.

  As a printed wiring board, for example, a substrate in which a conductor portion made of copper or the like is formed using epoxy resin, polyimide resin or the like as a base material; a conductor portion made of copper, silver, tungsten, molybdenum or the like is made of glass or ceramics as a base material A formed substrate or the like can be used.

  The electroless nickel plating film can be formed according to a conventional method. Usually, the object to be plated may be immersed in an electroless nickel plating solution.

  Prior to electroless nickel plating, pretreatment such as degreasing and activation can be performed according to a conventional method. When the object to be plated does not have catalytic activity for electroless nickel plating, electroless nickel plating may be performed after attaching a metal catalyst nucleus such as a palladium nucleus according to a conventional method. During electroless nickel plating, the plating solution can be agitated and the object to be plated can be swung as necessary.

  The treatment temperature of the electroless nickel plating solution is preferably about 50 ° C to 95 ° C, particularly about 60 ° C to 90 ° C.

  The film thickness of the electroless nickel plating film is not particularly limited, but is preferably about 0.1 to 15 μm, and more preferably about 0.2 μm to 10 μm. If the thickness of the nickel plating solution is too thin, it is difficult to obtain the effect of nickel plating, and if it is too thick, an effect of a certain level cannot be obtained, which is not economical.

  Next, the electroless palladium plating film can be formed by immersing the object on which the electroless nickel plating film is formed by the above-described method in an electroless palladium plating solution.

  The treatment temperature of the electroless palladium plating solution is preferably about 30 ° C to 90 ° C, particularly about 40 ° C to 80 ° C.

  The thickness of the electroless palladium plating film needs to be about 0.01 to 1 μm, and preferably about 0.05 to 0.5 μm. By forming such a very thin electroless palladium plating film on an electroless nickel plating film having a phosphorus content of about 10 to 13% by weight, the erosion of the nickel plating film can be prevented when forming a substitutional gold plating film. In addition, when soldering, palladium is sufficiently diffused in the solder without remaining, and high solder joint strength can be obtained.

  Next, the gold plating film can be formed by immersing the object on which the electroless palladium plating film is formed in a substitutional gold plating solution.

  The processing temperature of the substitutional gold plating solution is preferably about 50 ° C. to 95 ° C., particularly about 60 ° C. to 90 ° C.

  The film thickness of the displacement-type gold plating film is not particularly limited, and may be appropriately determined according to the purpose of use of the object to be plated. Usually, the range may be about 0.001 to 0.3 μm.

  According to the present invention, substitutional gold plating is performed by forming an electroless palladium plating film having a thickness of about 0.01 to 1 μm on an electroless nickel plating film having a phosphorus content of about 10 to 13% by weight. In this case, erosion of the electroless nickel plating film can be suppressed. As a result, a good state of the electroless nickel plating film can be maintained, and the film thickness of the palladium plating film is thin, and satisfactory solder bonding can be obtained by sufficiently diffusing into the solder.

  Furthermore, if necessary, thick gold plating can be performed using an electroless gold plating solution containing a reducing agent. By performing thick gold plating, a nickel / palladium / gold plating film with better heat resistance is obtained, and in particular, the heat resistance in wire bonding performance is improved. The thickness of the thick gold plating film is not particularly limited, but is usually a film thickness of up to about 1 μm as the total film thickness of the gold plating film.

  For example, when the multilayer plating film of the present invention is applied as a surface treatment for a conductor portion or the like of a printed wiring board, it can impart excellent solder bonding properties, wire bonding properties, and the like.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1
Hereinafter, the composition of the plating solution used in the examples and comparative examples of the present invention is shown.
(I) Electroless nickel plating solution (1) Nickel baths A1 to A3:
Nickel sulfate hexahydrate 22.5g / L
Sodium hypophosphite 20 g / L
Malic acid 10 g / L
Succinic acid 10 g / L
The aqueous solution having the above composition was adjusted to pH with sodium hydroxide, adjusted to pH 4.2 with nickel bath A1, adjusted to pH 4.6 with nickel bath A2, adjusted to pH 5.0 with nickel. Let it be bath A3.

(2) Nickel baths B1 and B2:
Nickel sulfate hexahydrate 22.5g / L
Sodium hypophosphite 20 g / L
Malic acid 10 g / L
Succinic acid 10 g / L
Thiodiglycolic acid 20 mg / L
About the aqueous solution of the said composition, pH adjustment is performed using sodium hydroxide, what was adjusted to pH 4.2 is nickel bath B1, and what was adjusted to pH 4.6 is nickel bath B2.

(II) Electroless palladium plating solution (1) Palladium bath A
Dichlorodiethylenediamine palladium solution 2g / L (as palladium)
Ethylenediamine 10g / L
Sodium formate 10g / L
pH 6.0
(2) Palladium bath B
Dichlorodiethylenediamine palladium solution 2g / L (as palladium)
Ethylenediamine 10g / L
Sodium hypophosphite 10g / L
pH 6.0
(3) Palladium bath C:
Dichlorodiethylenediamine palladium solution 2g / L (as palladium)
Ethylenediamine 10g / L
Sodium phosphite 10g / L
pH 6.0

(III) Replacement gold plating solution (1) Gold plating bath A
Ethylenediaminetetraacetic acid disodium 20g / L
Potassium cyanide 2g / L (as gold)
pH 5.0

  A printed circuit board in which a copper circuit and a resist were formed on a resin substrate was used as an object to be plated. The printed circuit board is obtained by forming a large number of pad portions having a pad diameter of 0.5 mmφ on a 6 cm × 6 cm resin substrate using a copper foil having a thickness of 18 μm.

  First, after the degreasing treatment was performed on the object to be plated, etching of about 0.5 μm was performed with a sodium persulfate solution, and a catalyst application solution (trade name: ICP Axela, manufactured by Okuno Pharmaceutical Co., Ltd.) in 200 ml / L And a catalyst for electroless plating was applied by immersion for 1 minute at room temperature.

  Thereafter, using the above nickel baths A1 to A3 or B1 to B2 as electroless nickel plating solutions, the object to be plated is immersed at a bath temperature of 80 ° C. with air stirring, and the nickel plating has a thickness of about 5 μm. A film was formed. The plating time in each nickel bath is as shown in Table 1 below.

  Thereafter, an electroless palladium plating film was formed by immersing an object to be plated in a plating solution at 60 ° C. using any of the palladium baths A to C as an electroless palladium plating solution. In Examples 1 to 5 and Comparative Examples 1 to 4, a palladium plating film having a thickness of about 0.2 μm was formed with an immersion time of 10 minutes. In Example 6, a palladium plating film having a thickness of about 0.8 μm was formed with an immersion time of 40 minutes. For Comparative Example 5 and Comparative Example 6, palladium plating films with thicknesses of 1.3 μm and 2 μm were formed with immersion times of 65 minutes and 100 minutes, respectively.

  Next, the sample on which the electroless palladium plating film was formed by the above-described method was immersed in a gold plating bath A at 80 ° C. for 15 minutes to form a substitutional gold plating film.

  In addition, each film thickness of electroless nickel, palladium, and gold plating was measured with the fluorescent X-ray film thickness measuring apparatus.

  As for the purity of the palladium plating film, an electroless nickel plating is formed on a copper-clad resin substrate, and a sample on which 3 μm electroless palladium plating is formed is filled with resin and polished. (Dispersive X-ray spectroscopy).

  Moreover, about the phosphorus content rate in a nickel plating film | membrane, gold | metal | money and palladium were peeled using the peeling liquid containing cyan | cyanogen, the nickel plating surface was exposed, and it measured by EDS (energy dispersive X ray spectroscopy).

  Table 2 below lists the film thickness of each plating film, the phosphorus content of the nickel plating film, and the purity of the palladium plating film.

Solder Bondability Test The solder bondability of each sample on which the plating film was formed by the above-described method was evaluated by a solder ball pull test.

  In the solder ball pull test, Sn-3.5% Ag-0.75% Cu lead-free solder balls were used, and the substrate pad diameter was 0.5 mm.

  First, a flux was applied to a test substrate, a solder ball was placed thereon, and a reflow process was performed. The reflow conditions were a preheat of 160 ° C. and a peak temperature of 240 ° C. (230 ° C. or more and 10 seconds). Thereafter, the solder pull strength was measured using a solder bump pull strength measuring device (Digi Corporation # 4000). The measurement was performed at 20 points, and the average value of the solder ball pull strength was obtained. The results are shown in Table 3 below.

  In addition, as a result of having observed the surface and cross section of the junction part about the sample after the said pull strength measurement, about each sample of Examples 1-6 and Comparative Examples 1-4, an electroless nickel plating film and a solder ball It was confirmed that fracture at the interface was observed, and the bonding strength between the electroless nickel plating film and the solder could be measured. In addition, in the samples of Comparative Examples 5 and 6, the palladium film remained at the fracture site, and the insufficient diffusion of palladium into the solder is considered to be a cause of the drop in pull strength. .

  Moreover, about the sample which peeled gold | metal | money and palladium by the method mentioned above, exposed the nickel plating surface, and measured the phosphorus content rate in a nickel plating film, the presence or absence of pitting corrosion in a nickel plating interface was confirmed by SEM (scanning electron microscope). Observed. The results are shown in Table 4 below.

  As is apparent from the results shown in Tables 2 to 4, in Examples 1 and 4 to 6, very good solder joint strength was obtained. Also, the gold and palladium plating peeled surface was free from pitting corrosion and intergranular corrosion, and no damage was observed in substitutional gold plating.

  In Examples 2 and 3, the strength was slightly lower compared to these Examples, but a significant improvement in bonding strength was observed compared to Comparative Examples 1-6. Moreover, there was no pitting corrosion, intergranular corrosion, etc. like Example 1 etc., and the damage by substitution gold plating was not recognized.

  In Examples 1 to 6, the surface and cross section of the bonding interface between the plating film and the solder were observed. However, the detection elements other than the solder component were only nickel and phosphorus, and palladium and gold were completely diffused in the solder. It is thought that.

  On the other hand, in Comparative Examples 1-4, the solder joint strength was a low value compared with the Example. Moreover, although the time of gold plating is the same, the gold plating is formed thicker than in each of the examples, and it is considered that a portion of nickel with low potential is dissolved. This can be confirmed from the fact that pitting corrosion and intergranular corrosion were observed on the nickel plating surfaces of Comparative Examples 1 to 4, as described in Table 3.

  In Comparative Examples 5 and 6, pitting corrosion and intergranular corrosion were not observed in the surface observation after peeling off the gold plating and palladium plating, but the solder joint strength was particularly low. About this point, palladium was detected from the board | substrate interface after a solder pull test, As a result of performing cross-sectional observation, it became clear that about 0.5 micrometer of palladium plating layers remain | survived on the nickel plating film. From this, it was clarified that when the palladium plating is too thick, palladium cannot be completely diffused into the solder during soldering, and the solder joint strength is lowered.

Claims (3)

Phosphorus content from 10.3 to 13% by weight of electroless nickel plating film, an electroless palladium plating film having a thickness of 0.01 to 1 [mu] m, and Ri are Na are sequentially stacked substitute gold plating film,
The electroless palladium plating film is formed using an electroless palladium plating solution composed of an aqueous solution containing a palladium compound, a reducing agent and a complexing agent,
The reducing agent is formic acid or a multilayer plating film Ru salt thereof der.   The multilayer plating film according to claim 1, wherein a reduced electroless gold plating film is further formed on the substitutional gold plating film.   A printed wiring board, wherein the multilayer plating film according to claim 1 or 2 is formed on a conductor portion of the printed wiring board.