KR100404369B1 - Electroless Nickel Plating Solution and Method - Google Patents

Abstract

To an electroless nickel plating solution consisting of a water soluble nickel salt, a reducing agent and a complexing agent is added a compound having an S-S bond such as thiosulfate, dithiate, polythiate and addiate. The present invention also provides a method of chemically attaching a nickel film onto a workpiece by immersing the workpiece in an electroless nickel plating bath and immersing the nickel plated workpiece in the electroless gold plating bath, thereby chemically coating the gold film on the workpiece. It provides a high build electroless gold plating method consisting of attaching.

Description

Electroless Nickel Plating Solution and Method

Background of the Invention

Field of invention

The present invention relates to an electroless nickel-plated gold solution with improved fine patterning capability and a method for chemically depositing a nickel film on a workpiece. The present invention also relates to a high-build gold plating method capable of chemically attaching a thick gold film on a chemically nickel-plated workpiece. The method is industrially advantageous for forming a gold film on printed wiring boards and electronic components. There is this.

Prior art

Electroless or chemical nickel plating is used in you field because of its beneficial properties. For example, electroless nickel plating has been widely applied to electronic products. However, electroless nickel plating technology has not kept up with the urgent demands in the electronics sector.

The demand for weight reduction in electronic products promotes an increase in the density of constituent circuits, resulting in even finer circuit patterns. Several problems arise when conventional electroless plating solutions are used for plating on such fine patterns. Reduced line widths create a problem with plating having thin shoulders. Narrow pattern pitches create problems of reduced resistance between lines due to plating protrusions or outgrowth and shorting by the plating bridges. The term "thin shoulder" means that the plating is not sufficiently adhered on the shoulder of the circuit runner as seen in cross section and the plating area at the shoulder is significantly thinner than the rest of the plating.

This is probably because the stabilizer is excessively attached to the shoulders to limit metal adhesion. The term "plated out-gross" means that the plating protrudes from metallic copper or circuit runners and the coating is attached around the circuit runners. This is probably because palladium ions, which remain attached to the circuit runner after palladium treatment, are reduced to metallic palladium that catalyzes with the electroless nickel plating solution to aid the deposition of nickel thereon.

In addition, electroless gold plating has many advantages such as physical properties such as electrical conductivity, thermocompression bonding ability, oxidation resistance, and chemical resistance, so that electronics such as printed wiring boards, ceramic IC packages, ITO substrates, and IC cards Mainly used in the field of industrial parts. In the printed wiring board industry, it is an important task to chemically attach a thick gold film in an efficient manner.

Summary of the Invention

An object of the present invention is to improve the fine pattern sharpness by providing an electroless nickel plating solution and method that overcomes the problems of thin shoulders on the pattern line and nickel coating outgrowth.

Another object of the present invention is to provide an industrially advantageous high build electroless gold plating method, that is, a method of chemically attaching a thick gold film in a short time.

The present inventors have unexpectedly overcomed the problem of thin shoulders and nickel plated outgrowths by adding compounds with SS bonds, in particular thiosulfate, dithionate, polythiate or adithionate, to the electroless nickel plating solution. It has been found that the problem of short circuit caused by the bridge is eliminated. The inventors have found that when a workpiece provides additional chemical gold plating to a chemical nickel plated and nickel plated workpiece in an electroless nickel plating bath containing a compound having an SS bond, the gold plating can be simply attached to a substantial thickness. Was found further.

The inventors first immersed the workpiece in an electroless nickel plating bath without a compound having an SS bond to chemically attach a nickel undercoat onto the workpiece, and then the electroless nickel plating bath containing a compound having an SS bond. It was further discovered that when the nickel film is chemically attached to the nickel undercoat by immersion, and the nickel plated workpiece is additionally provided to the chemical gold plating, the gold film can be simply attached to a substantial thickness. The gold film does not discolor over time and has an excellent appearance. The present invention is based on these findings.

According to the first aspect of the present invention, there is provided an electroless nickel plating solution composed of a water-soluble nickel salt, a reducing agent, a complexing agent and a compound having an S-S bond.

According to a second aspect of the present invention, there is provided an electroless nickel plating method comprising immersing a workpiece in an electroless nickel plating solution as defined above and chemically attaching a nickel film on the workpiece.

According to a third aspect of the invention, a workpiece is immersed in an electroless nickel plating solution as defined above, chemically attaching a nickel coating on the workpiece, and immersing the nickel plated workpiece in an electroless gold plating bath. There is provided a high build electroless gold plating method comprising the steps of chemically attaching a gold film on a workpiece.

In a further aspect, the present invention provides a method for chemically attaching a nickel undercoat to a workpiece by immersing the workpiece in an electroless nickel plating bath free of compounds having S-S bonds; Immersing the workpiece in an electroless nickel plating bath containing a compound having an S-S bond to chemically attach the nickel film on the nickel undercoat; It provides a high build electroless gold plating method comprising electroless gold plating on a double nickel plated workpiece.

Detailed description of the invention

Generally, electroless nickel plating solutions contain water-soluble nickel salts, reducing agents and complexing agents.

Nickel sulfate and nickel chloride are typical water soluble nickel salts. The amount of nickel salt used is preferably 0.01 to 1 mol / liter, more preferably 0.05 to 0.2 mol / liter.

Examples of reducing agents include hypophosphorous acid, hypophosphite such as sodium hypophosphite, dimethylamineborane, trimethylamineborane and hydrazine. The amount of reducing agent used is preferably 0.01 to 1 mol / liter, more preferably 0.05 to 0.5 mol / liter.

Examples of complexing agents include carboxylic acids such as malic acid, succinic acid, lactic acid and citric acid, sodium salts of carboxylic acids and amino acids such as glycine, alanine, iminodiisacetic acid, arginine and glutamic acid. The amount of complexing agent used is preferably 0.01 to 2 mol / liter, more preferably 0.05 to 1 mol / liter.

Usually stabilizers are added to the electroless nickel plating solution. Examples of stabilizers are water soluble lead salts such as lead acetate and sulfur compounds such as thiodiglycolic acid. It is preferred to use stabilizers in amounts of 0.1 to 100 mg / liter.

According to the present invention, a compound having an S-S bond is added to the electroless nickel plating solution. When plating on the fine pattern with the addition of this compound, a solution that chemically attaches the nickel film is allowed without the problem of thin shoulders and the outer gross of the nickel film.

Compounds having an SS bond are also organic sulfur compounds, but inorganic sulfur compounds such as thiosulfate, dithiate, polythiate and addiate are preferred. Polythionate is a structural formula of O ₃ SS _n -SO ₃ , wherein n is from 1 to 4.

Water-soluble salts, typically alkali metal salts, are often used. The compound having an S-S bond is preferably added in an amount of 0.01 to 100 mg / liter, especially 0.05 to 50 mg / liter. Compounds of 0.01 mg / liter or less are ineffective for the purposes of the present invention, while compounds of 100 mg / liter or more interfere with the adhesion of the nickel coating.

The electroless nickel plating solution of the present invention is pH 4-7, especially pH 4-6.

Using the electroless nickel plating solution of the above-described composition, a nickel film can be chemically formed in the fine pattern or the workpiece by conventional art, that is, simply immersing the workpiece in the plating solution. The workpiece to be plated is a metal capable of promoting catalytic reduction adhesion of the electroless nickel film such as iron, cobalt, nickel, palladium and alloys thereof. Non-catalytic metals can be used as long as the workpiece is energized until the reduction starts. Alternatively, electroless plating is performed on the non-catalytic metal workpiece after the coating of the catalyst metal as described above is preplated. Moreover, a catalytic metal nucleus, such as a palladium nucleus, may be applied by the prior art to conduct electroless plating on workpieces of glass, ceramic, plastic or noncatalytic metals. The plating temperature is preferably 40 to 95 ° C, particularly 60 to 95 ° C. If desired, the plating solution is stirred during plating.

When the nickel film is deposited on the fine pattern in the electroless nickel plating bath according to the present invention, since the overgrowth of the nickel film is overcome, flakes hardly occur in the short circuit problem by the pattern line shoulder and the bridge.

Workpieces with chemically attached nickel films are susceptible to electroless gold plating. More specifically, when electroless gold plating is carried out on a nickel film chemically deposited from an electroless nickel plating solution, characterized by containing a compound having an SS bond, the chemically adhered chemically in a conventional electroless nickel plating solution. Compared to the electroless gold plating on the nickel film, a thick gold film can be attached in a short time.

Electroless gold plating baths used herein contain gold ingredients, complexing agents and other ingredients. The gold raw material may be selected from gold cyanide, gold sulfite and gold thiosulfate, which are used in conventional gold plating baths. Water-soluble salts of gold cyanide, such as potassium cyanide, are particularly useful. The amount of gold material added is not critical, but the gold concentration in the bath is preferably 0.5 to 10 g / liter, especially 1 to 5 g / liter. The deposition rate increases substantially in proportion to the amount of gold source added, ie the concentration of gold ions in the bath. Gold concentrations above 10 g / liter increase the adhesion rate, but bath stability is poor. Gold concentrations below 0.5 g / liter will result in very low adhesion rates.

Any known complexing agent may be used for the electroless gold plating bath. For example, ammonium sulfate, aminocarboxylate, carboxylate and hydroxycarboxylate are preferred. The complexing agent is preferably added in an amount of 5 to 300 g / liter, especially 10 to 200 g / liter. Complexing agents below 5 g / liter are less effective and have a detrimental effect on solution stability. Complexing agents of 300 g / liter or more are uneconomical as there are no further effects.

In addition, thiosulfate, hydrazine and ascorbate are mixed as reducing agents. Examples of thiosulfate are ammonium thiosulfate, sodium thiosulfate and potassium thiosulfate. The reducing agent may be used alone or in combination of two or more. The amount of reducing agent added is not critical, but a concentration of 0 to 10 g / liter, in particular 0 to 5 g / liter, is preferred. The adhesion rate increases substantially in proportion to the concentration of the reducing agent. When 10 g / liter or more of reducing agent is added, the adhesion rate no longer increases and the stability of the bath is lowered. If no reducing agent is added, gold adhesion will occur through substitution with nickel.

In addition to the above compounds, the electroless gold plating bath may further contain pH regulators such as phosphates, hypophosphites and carboxylates, crystal regulators such as Tl, AS and Pb and various other additives.

Electroless gold plating baths are preferably used in the vicinity of neutrals, usually at pH 3.5-9, especially pH 4-9.

Electroless gold plated baths are used herein as high build systems. The electroless gold plating method according to the present invention can be carried out by a conventional method except using the electroless gold plating bath described above. Using the electroless gold plating bath described above, the gold coating can be chemically attached directly onto a workpiece having a nickel coating chemically attached according to the present invention. In particular, in order to form a thick gold film, it is preferable to perform high-build electroless gold plating after strike electroless gold plating. The preceding strike electroless gold plating acts to modify the surface of the nickel-plated workpiece to accommodate the subsequent thick gold plating. As a result, the subsequent thick gold film adheres to the underlying workpiece and has a uniform thickness.

Strike electroless gold plating used herein comprises gold raw materials as described above at concentrations of 0.5 to 10 g / liter, in particular 1 to 5 g / liter of gold and EDTA at concentrations of 5 to 300 g / liter, especially 10 to 200 g / liter, its It has a composition containing an alkali metal salt and a complexing agent such as a reagent as described above. The bath is adjusted to pH 3.5-9.

When the gold plating is carried out using the electroless gold plating solution described above, for a strike electroless gold plating bath, a temperature of 20 to 95 ° C., in particular 30 to 90 ° C. and a time of 1/2 to 30 minutes, in particular 1 to 15 minutes, is applied. Conditions are preferred, and for high build electroless plating baths, plating conditions of 20 to 95 ° C., especially 50 to 90 ° C. and a time of 1 to 60 minutes, in particular 5 to 40 minutes, are preferred. If the temperature of the high-build electroless gold plating bath is lower than 20 ° C., the deposition rate is slow, and the productivity is low for the thick plating, which is uneconomical. At temperatures above 95 ° C., decomposition of the plating bath may occur.

When high build electroless gold plating is carried out directly on a nickel plated workpiece, the bath temperature is preferably from 50 to 95 ° C, in particular from 70 to 90 ° C. At bath temperatures below 50 ° C, the deposition rate is lower, whereas at bath temperatures above 95 ° C, the adhesion rate increases, but the stability of the resulting gold film is lowered.

According to the present invention, the thick gold film can be attached by performing electroless gold plating on the nickel film chemically attached from the electroless nickel plating solution characterized by containing a compound having an S-S bond as described above. In this regard, electroless nickel plating is performed on the workpiece in a bath without a compound having an SS bond to chemically attach the lower nickel coating, and then the SS bonding is used to chemically attach the nickel coating onto the lower nickel coating. It is recommended to carry out electroless nickel plating in a bath containing a compound having a compound having a compound having a compound, and finally to conduct electroless gold plating.

The reason is described below. Regardless of the presence of reducing agents in electroless gold-plated baths, especially when the gold material of an electroless gold-plated bath is a salt of gold cyanide, the chemical plating of gold is essentially an electroless nickel coating (a bath containing a compound with an SS bond). from occurring through the substitution reaction with the generated), and that is, the nickel coating is a mechanism which at the same time, electroless gold Au ⁺ ion reduction by being dissolved in a gold plating bath.

Ni ⁰ → Ni ²⁺ 2e

2Au ⁺ + 2e → 2Au ⁰

Referring to FIG. 2, the workpiece 1 includes a nickel film 2 attached thereto from an electroless nickel plating bath containing a compound having an SS bond and a gold film attached thereto from an electroless gold plating bath. Have 3). The above mechanism suggests that during chemical plating of gold, the nickel film 2 can be locally dissolved to form a pinhole 4 reaching the workpiece 1. Under the situation that the pinhole 4 extends deeply into the workpiece, if the workpiece base material is a corrosive metal such as copper, the corrosive metal may dissolve.

Once dissolved, corrosive metal migrates through the pinholes to contaminate the electroless gold plating bath and adhere to the gold film to discolor it.

3 shows the structure of the preferred embodiment, in which the nickel lower coating 5 is sandwiched between the workpiece 1 and the nickel coating 2. More specifically, the nickel undercoat 5 is attached onto the workpiece 1 from an electroless nickel plating bath without a compound having an SS bond, and the nickel film 2 contains an electroless containing a compound having an SS bond. It is attached thereon from a nickel plating bath. As for the rate of dissolution of the electroless nickel film in the electroless gold plating bath (speed of conversion of gold ions to metallic gold), the nickel film 2 produced from the electroless nickel plating bath containing a compound having an SS bond is The SS bond is much faster than the nickel undercoat 5 produced from the electroless nickel plating bath without the compound having it. That is, the nickel undercoat 5 produced from the electroless nickel plating bath without the compound having S-S bonds has a very low dissolution rate. Then, the nickel plating (2) produced from the electroless nickel plating bath containing the compound having SS bond is locally dissolved to pass through the coating (2) as shown in FIG. 3 to form the pinhole (4). However, these pinholes 4 terminate at the surface of the nickel undercoat 5. There is no pinhole further extending to the nickel undercoat 5. The next state is that new pinholes are formed at different positions in the nickel film 2 or the pinholes 4 already formed are widened sideways.

Therefore, the electroless nickel plating bath containing the compound having the SS bond was attached to the nickel film 2 on the nickel undercoat 5 produced from the electroless nickel plating bath without the compound having the SS bond. When performing gold plating, the electroless gold plating bath can be contaminated with metal ions in which the workpiece base material is dissolved, and a gold film of substantial thickness can be attached in a short time without the problem that the gold film can discolor with it.

The composition of the electroless nickel plated bath without the compound having S-S bond may be the same as the composition of the electroless nickel plated bath containing the compound having the S-S bond, except that the compound having the S-S bond is omitted. In addition, the plating conditions may be the same.

Thus, a preferred embodiment using a nickel undercoat is advantageously acceptable when the base metal of the workpiece is a corrosive metal such as copper, for example when the workpiece is a printed wiring board.

It is preferable that the nickel undercoat produced from the electroless nickel plating bath without a compound having an S-S bond has a thickness of 0.5 to 5 mu m, especially 1 to 3 mu m. On this nickel undercoat, the nickel coating is deposited from an electroless nickel plating bath containing a compound having an S-S bond, preferably at a thickness of 0.5 to 5 mu m, especially 1 to 5 mu m.

If the workpiece substrate is not a corrosive metal, the nickel coating can be deposited directly on the workpiece in an electroless nickel plating bath containing a compound having an S-S bond. In this embodiment, the nickel coating preferably has a thickness of 0.5 to 10 mu m, especially 1 to 8 mu m. Preferably, the nickel coating is attached to a sufficient thickness to prevent pinholes extending through the coating or to reduce the pinholes.

The thickness of the electroless gold film is not critical, but is generally 0.1 to 2 mu m, preferably 0.3 to 0.8 mu m.

Example

Embodiments of the invention are given below for illustrative purposes and are not intended to be limiting.

Using each plating solution at the indicated temperature, nickel was chemically deposited on a test pattern of copper having a thickness of 18 μm, a line width of 50 μm, and a slit width of 50 μm to form a nickel film having a thickness of 5.0 μm. Nickel coatings were visually observed for the outgrowth and bridge of nickel on the circuit lines by stereoscopic microscope. The pattern was cut and the cut surface of the circuit line was observed for shoulder flaking with a stereomicroscope. The results are shown in Table 1.

Example 4

Using the electroless nickel plating solution of Example 1, a 5 μm thick nickel film was chemically attached onto the copper plate. Next, the strike plating was carried out on a nickel plated copper plate in a strike electroless gold plating solution of the following composition under the following conditions, and then a thick gold film was chemically attached thereto in the high build electroless gold plating solution of the following composition under the following conditions. . The thickness of the gold film was measured over time. The results are shown in the graph of FIG.

Comparative Example 3

Example 4 was repeated except that the electroless nickel plating solution of Comparative Example 1 was used. The results are also shown in the graph of FIG.

When electroless gold plating (Example 4) was carried out on the nickel film chemically attached to the electroless nickel plating solution of Example 1, the electroless on the nickel film chemically attached to the electroless nickel plating solution of Comparative Example 1 Compared with gold plating (Comparative Example 3), it can be seen from FIG. 1 that a significantly thicker gold film can be attached per unit time.

A gold film having a thickness of 0.5 µm or more can be attached in a short time.

Example 5

Using an electroless nickel plating solution having the composition shown below, chemical nickel plating was performed for 15 minutes under the conditions shown below to attach a 2.5 μm thick nickel undercoat to the copper plate.

Using an electroless nickel plating solution having the composition shown below, chemical nickel plating was performed for 15 minutes under the conditions shown below to attach a 3.0 μm thick nickel film on the nickel undercoat.

Subsequently, after a 7-minute strike plating on the double nickel plated copper plate in the strike electroless gold plating solution of the same composition under the same conditions as in Example 1, the high build electroless of the same composition under the same conditions as in Example 1 Gold plating was carried out for 20 minutes in the plating solution, and a thick gold film having a thickness of 0.5 µm was attached.

The plated plate was kept at 150 ° C. in air for 4 hours before the appearance test. No discoloration was found and the appearance remained the same immediately after plating.

After the same test as described above, a similar sample without nickel undercoat was slightly discolored but sufficiently acceptable in practical use.

While some preferred embodiments have been described, many modifications and variations may be made in light of the above description. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

FIG. 1 is a graph showing the thickness of the gold film as a function of plating time when the gold film is chemically attached onto the chemically deposited nickel film.

FIG. 2 is a schematic cross-sectional view of a coating structure on a workpiece showing a pinhole extending through the nickel film and including the nickel film and the gold film.

3 is a schematic cross-sectional view of a coating structure on a workpiece showing pinholes extending through the nickel coating and including a nickel undercoat, a nickel coating and a gold coating.

Claims (6)

An electroless nickel plating solution, comprising a water-soluble nickel salt, a reducing agent, a complexing agent, and a compound having an S-S bond. The electroless nickel plating solution according to claim 1, wherein the compound having an S-S bond is selected from the group consisting of thiosulfate, dithiate, polythiate and addiate. Electroless nickel plating, comprising immersing a workpiece in an electroless nickel plating bath made of a water-soluble nickel salt, a reducing agent, a complexing agent, and a compound having an SS bond, and chemically attaching the nickel film on the workpiece. Way. Immersing the workpiece in an electroless nickel plating bath consisting of a water-soluble nickel salt, a reducing agent, a complexing agent, and a compound having an S-S bond, thereby chemically attaching a nickel film on the workpiece, and A high build electroless gold plating method, comprising immersing a nickel plated workpiece in an electroless gold plating bath to chemically attach a gold film on the workpiece. 5. The workpiece according to claim 4, wherein the workpiece is immersed in an electroless nickel plating bath free of compounds having SS bonds prior to the step of immersing the workpiece in an electroless nickel plating bath containing a compound having SS bonds. A high build electroless gold plating method, further comprising the step of chemically attaching the nickel sub-coat to the. The high build electroless gold plating method according to claim 4 or 5, wherein the electroless gold plating bath contains a gold cyanide salt as a gold raw material, and has a pH of 4 to 9.