JP4797708B2 - Electroless nickel plating method - Google Patents

Description

  The present invention relates to an electroless nickel plating method using hydrazine as a reducing agent.

  Conventionally, in general, an electroless nickel plating bath uses a nickel metal salt for supplying nickel ions and sodium diphosphate, sodium boron, and hydrazine as reducing agents. Here, the plating bath using hydrazine as a reducing agent is mainly used as a magnetic thin film.

  In recent years, film characteristics obtained by an electroless nickel plating method using hydrazine as a reducing agent have been reviewed. For example, Non-Patent Document 1 proposes electroless nickel plating made only of nickel ions and hydrazine as a reducing agent.

Patent Document 1 proposes an electroless nickel plating bath for forming a needle-like film that contains nickel ions, hydrazine, and glycine or a derivative thereof and does not contain phosphoric acid or a carbonate buffer.
Surface technology, vol. 51, no. 10, 2000, p. 1035 Japanese Patent No. 3554741

  However, in the thing of the said nonpatent literature 1, the plating rate is slow, and in the thing of the said patent document 1, it is trying to maintain PH high by adding boric acid etc. as a buffering agent. However, according to the study of the present inventor, the problem that the plating speed is slow has not been solved.

  The present invention has been made in view of the above problems, and an object thereof is to improve the plating rate in an electroless nickel plating method using hydrazine as a reducing agent.

In order to achieve the above object, the present invention uses an electroless nickel plating solution that contains nickel ions and hydrazine as a reducing material and does not contain phosphorus with PH as a weakly basic region. An electroless nickel plating film is formed on the object to be plated by applying a minus potential of the mixed potential specific to the plating bath to the object to be plated. When the minus potential is applied , a minus potential of 75 to 250 mV is applied to 10 to 180. It is characterized by adding seconds .

  According to this, by adding a minus potential of the mixed potential unique to the plating bath, the object to be plated can be quickly brought to the mixed potential, so that the plating rate can be improved.

  In addition to nickel ions and hydrazine, the electroless nickel plating solution contains other complexing agents and buffering agents, that is, without adding an additive for controlling the surface state of the electroless nickel plating film. Although the surface state of the electroless nickel plating film can be controlled by adjusting the pH, nickel ion concentration, hydrazine concentration, bath temperature, etc., the effect of this plating method is sufficient even when the above additives are added. Can demonstrate.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a schematic cross-sectional structure of an electroless nickel plating film 10 according to the present embodiment. FIG. 2 is a photograph obtained by SEM observation of the surface state of the electroless nickel plating film 10. It is the figure typically expressed based on.

  An electroless nickel plating film 10 is formed on the surface of the workpiece 20. Here, the surface of the electroless nickel plating film 10 is composed of a large number of needle-like protrusions, and such a film state is also shown in Patent Document 1 described above.

  Thus, the surface becomes a film made of acicular protrusions, and the surface becomes roughened, and the degree is a ratio obtained using an atomic force microscope capable of measuring surface irregularities of several nm. It is represented by the surface area Sa. Specifically, the specific surface area Sa of the electroless nickel plating film 10 of the present embodiment can be about 1.1 to 3.8.

  Such an electroless nickel plating film 10 is formed by the electroless nickel plating method according to this embodiment as described below. The method will be described.

  FIG. 3 is a diagram showing a schematic configuration of a plating bath 100 for performing the present plating method. Moreover, FIG. 4 is a figure which shows typically the production | generation mechanism of the electroless nickel plating film 10 by this plating method.

  In FIG. 3, the plating bath 100 in the plating method of the present embodiment uses an electroless nickel plating solution containing nickel ions and hydrazine as a reducing material and having PH as a weakly basic region.

  Although nickel ion is not specifically limited, it is supplied using aliphatic hydroxycarboxylate. Specifically, when nickel acetate is employed, the nickel acetate concentration is about 10 to 30 g / liter. The hydrazine concentration is about 5 to 30 g / liter. The bath temperature is about 70 to 90 ° C., and the pH is weakly basic in the range of 7.5 to 9.0.

  In the plating bath 100 adjusted to such conditions of composition, temperature, and PH, the relationship between the bath temperature and the mixed potential is as shown in FIG. This mixed potential is unique to the plating bath. When the mixed potential is lower than this mixed potential, a film is deposited on the object to be plated in the plating bath. As shown in FIG. 5, in the present plating bath 100, it takes time to reach a mixed potential.

  Therefore, in the plating bath 100 adjusted to such conditions, in this embodiment, by adding a minus potential of the mixed potential inherent to the plating bath 100 to the object 20 to be plated, the plating rate is improved, An electroless nickel plating film 10 is formed on the plated object 20.

  In this plating method, the mixed potential of the plating bath 100 to be used is measured, and while monitoring the potential of the object 20 to be plated, the electrode plate 30 and the rectifier 40 are used so that the mixed potential becomes a negative potential. The current to be plated 20 is energized.

  FIG. 6 shows the time change of the potential of the workpiece 20 when a negative potential of this mixed potential is added. As shown in FIG. 6, even if the temperature is as low as about 50 ° C., by adding the negative potential, the object to be plated 20 can be quickly brought to the mixed potential, so that the plating speed can be greatly improved.

  This negative potential can be applied by setting the potential amount according to the plating solution regardless of the material of the workpiece 20. Further, in order to apply a potential by energization in this way, the object to be plated 20 may be a metal or a nonmetal, and may be ceramic, plastic, or the like.

  In addition to the method of energizing, when the object to be plated 20 is a metal, the negative potential can be applied to the object to be plated 20 by bringing the dissimilar metal into contact with the object to be plated 20.

  In other words, by applying the negative potential, a compulsory potential that does not depend on the local potential on the surface of the object to be plated 20 can be obtained, so that the potential necessary for plating is applied to the entire surface of the object to be plated 20. Then, as shown in FIG. 4A, the plating nuclei 10a can be uniformly formed on the entire surface.

  Therefore, the initial rate of plating deposition can be increased and plating deposition can be greatly improved. Further, the nuclei 10a are uniform, and precipitation from the nuclei 10a can easily obtain a state in which the film formation is uniform on any surface.

  In this manner, the plating film 10 starts to be deposited, and a roughened electroless nickel plating film 10 is formed as shown in FIGS. In this plating method, the main matter that the surface of the electroless nickel plating film 10 is roughened is required for hydrazine as a reducing agent.

  The reducing power of hydrazine is weaker than that of sodium diphosphate, which is a reducing agent for conventional electroless nickel-phosphorous plating. Therefore, as shown in FIGS. 4B and 4C, the reducing force only at the tip of the portion serving as the plating nucleus 10a works effectively, and the growth of the plating grows in the direction perpendicular to the plating surface.

  The film formation process in the plating bath having a weak reducing power is obtained by obtaining a plating nucleus 10a at the local potential of the surface of the object 20 to be plated, forming a mountain range / skeleton by the continuity of the nucleus 10a, and growing in the vertical direction. Priority is given to the valley of the mountain range. Thus, the needle-like protrusions constituting the surface of the film 10 grow with the lapse of the plating time, and become roughened. This state is confirmed by SEM observation.

  Moreover, although hydrazine is used as the reducing agent, hydrazine is easily decomposed, so it is necessary to consider the stability of the plating bath 100. In the plating method of this embodiment, the instability of the plating bath 100 can be eliminated by adopting an automatic analysis automatic replenishment device that measures the pH of the plating bath 100 and monitors the hydrazine concentration from this PH to automatically replenish hydrazine. .

  In such a plating method, the negative additional potential is in the range of 75 to 250 mV, the negative additional potential time is in the range of 10 to 180 seconds, and the plating time is in the range of 10 to 180 minutes. The thickness of the electroless nickel plating film 10 is about 0.4 to 2 μm.

  As described above, according to the electroless nickel plating method of the present embodiment, by adding the minus potential of the mixed potential unique to the plating bath 100, the object to be plated 20 can be quickly brought to the mixed potential, thereby improving the plating speed. Can be made.

  Further, in addition to nickel ions and hydrazine, the electroless nickel plating solution does not need to contain other complexing agents and buffering agents, that is, additives for controlling the surface state of the electroless nickel plating film 10. The surface state of the plating film 10 can be controlled by adjusting PH, nickel ion concentration, hydrazine concentration, bath temperature, and the like.

  However, according to the present plating method, even when the additive is added, the effect of the present plating method can be sufficiently exerted, and the surface unevenness of the film 10 can be controlled. For example, even if a small amount of citric acid is added, a desired surface unevenness can be formed without significantly reducing the plating rate. Here, the present inventor has confirmed by SEM observation that plating does not precipitate in a plating bath 100 to which a small amount of citric acid is added without applying a negative potential.

  Next, the present invention will be described more specifically by the following examples, but the present invention is not limited to these examples.

  As mentioned above, the surface state of the electroless nickel plating film can be controlled by adjusting the pH, nickel ion concentration, hydrazine concentration, bath temperature, etc. The adhesion strength of the resin to the specific surface area Sa was investigated.

  FIG. 7 is a diagram showing the relationship between the specific surface area Sa of the film and the adhesive force (unit: MPa) of the resin when the film is sealed with resin. Here, the specific surface area Sa is measured using an atomic force microscope capable of measuring surface irregularities of several nm.

  As shown in FIG. 7, the resin adhesion strength is improved as the specific surface area Sa is increased. Thus, by employing the plating method shown in the above embodiment, the electroless nickel plating film has a fine roughened plating surface, and the specific surface area is controlled from 1.1 to 3.8. Could be possible.

  Thus, when specific surface area Sa becomes large, the improvement of the resin adhesive strength by an anchor effect is anticipated. Further, when the specific surface area Sa is increased, the solder wettability is expected to be improved by the capillary phenomenon. Therefore, the present plating method is effective for the portion where the Pb-free solder having poor wettability is provided.

  If the specific surface area Sa is too large, the solder may get too wet beyond the desired region, or the wire bondability may be hindered due to bleed-out due to capillary phenomenon when a conductive adhesive is used for connecting the components, or due to large surface irregularities. There's a problem. Therefore, a plating film forming condition with a necessary specific surface area Sa may be used depending on the configuration of the semiconductor device to be applied.

  Using an object to be plated made of a copper alloy, as a plating bath, nickel acetate concentration: 18 g / liter (0.07 mol / liter), hydrazine concentration: 13.5 g / liter (0.27 mol / liter), bath temperature: 80 ° C. , PH: 8.5, negative potential applied is −100 mV × 60 seconds, and electroless nickel plating was performed, and an electroless nickel plating film having a thickness of 2.0 μm was formed in a plating time of 20 minutes. It was.

(Comparative Example 1)
Using an object to be plated made of a copper alloy, as a plating bath, nickel chloride concentration: 0.05 mol / liter, hydrazine concentration: 0.4 mol / liter, glycine: 0.3 mol / liter, boric acid: 0.3 mol / liter, When electroless nickel plating was performed at a bath temperature of 85 ° C. and a pH of 12, an electroless nickel plating film having a thickness of 0.68 μm was formed in 20 minutes.

(Comparative Example 2)
Using an object to be plated made of a copper alloy, as a plating bath, nickel acetate concentration: 18 g / liter (0.07 mol / liter), hydrazine concentration: 13.5 g / liter (0.27 mol / liter), bath temperature: 85 ° C. When electroless nickel plating was performed at pH 8.5, an electroless nickel plating film having a thickness of 0.556 μm was formed in 20 minutes.

  Thus, compared with these comparative examples 1 and 2, with the plating method according to the present Example 2, the plating rate could be significantly improved.

  Using an object to be plated made of a copper alloy, as a plating bath, nickel acetate concentration: 18 g / liter, hydrazine concentration: 13.5 g / liter, citric acid concentration: 1 g / liter, bath temperature: 80 ° C., PH: 8.3 When the electroless nickel plating was performed at −100 mV × 60 seconds with a negative potential applied, an electroless nickel plating film having a thickness of 1.5 μm was formed in a plating time of 18 minutes.

It is a figure which shows the typical cross-section of the electroless nickel plating film | membrane which concerns on embodiment of this invention. It is the figure which represented typically the surface state of the electroless nickel plating membrane | film | coat of the said embodiment. It is a figure which shows the typical structure of the plating bath for performing the electroless nickel plating method which concerns on the said embodiment. It is a figure which shows typically the production | generation mechanism of the membrane | film | coat by the electroless nickel plating which concerns on the said embodiment. It is a figure which shows the relationship between the bath temperature and mixed potential in the plating bath of the electroless nickel plating method which concerns on the said embodiment. It is a figure which shows the time change of the electric potential of a to-be-plated object when the minus electric potential of mixed potential is added. It is a figure which shows the relationship between the specific surface area Sa of an electroless nickel plating film | membrane, and resin adhesive force.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Electroless nickel plating film | membrane, 20 ... To-be-plated object, 100 ... Plating bath.

Claims (2)

Using an electroless nickel plating solution that contains nickel ions and hydrazine as a reducing material and does not contain phosphorus with PH as a weakly basic region,
In the plating bath, an electroless nickel plating film is formed on the plating object by adding a minus potential of the mixed potential inherent to the plating bath to the plating object ,
In the addition of the negative potential, a negative potential of 75 to 250 mV is applied for 10 to 180 seconds . 2. The electroless nickel plating method according to claim 1, wherein the electroless nickel plating solution is further added with an additive for controlling the surface state of the electroless nickel plating film.

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