CRYSTAL GRAIN SIZE REDUCTION METHOD FOR PLATING FILM

Abstract

In a crystal grain size reduction method for a plating film, electroplating is performed in a condition where ions of a plating metal, a nanocarbon, and an anion based surfactant as a dispersion agent for dispersing the nanocarbon have been blended in a plating solution.

Claims

1. A crystal grain size reduction method for a plating film comprising: performing electroplating in a condition where ions of a plating metal, a nanocarbon, and an anion based surfactant as a dispersion agent for dispersing the nanocarbon have been blended in a plating solution.

2. The crystal grain size reduction method for the plating film according to claim 1, wherein the nanocarbon is positively charged when in the blended state with the plating solution.

3. The crystal grain size reduction method for the plating film according to claim 1, wherein the plating metal is one of silver, nickel, tin, or gold.

4. The crystal grain size reduction method for the plating film according to claim 2, wherein the plating metal is one of silver, nickel, tin, or gold.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 is a diagram that generally illustrates a crystal grain size reduction method for a plating film according to an exemplary embodiment of the invention.

[0017] FIGS. 2A and 2B show microscopic photographs that exhibit a plating film obtained as illustrated in FIG. 1 and a plating film of a comparative example, respectively.

[0018] FIGS. 3A and 3B are schematic diagrams of the plating films exhibited in FIGS. 2A and 2B, respectively.

[0019] FIGS. 4A and 4B are graphs representing the durabilities and the contact resistances of the plating films exhibited in FIGS. 2A and 2B, respectively.

[0020] FIGS. 5A and 5B show microscopic photographs that exhibit plating films according to another exemplary embodiment of the invention and another comparative example.

DETAILED DESCRIPTION

[0021] Exemplary embodiments of the present invention will be described in detail hereinafter with reference to the accompanying drawings. The dimensions, materials, and other concrete numerical values mentioned in conjunction with the following exemplary embodiments are merely illustrative for the sake of easy understanding of the invention and not intended to limit the scope of the invention unless otherwise indicated. In the description and drawings, elements substantially the same in function and construction are indicated by the same reference characters and are not redundantly described. Furthermore, elements and the like that are not directly relevant to the present invention are omitted from graphical representation.

[0022] FIG. 1 illustrates a crystal grain size reduction method for a plating film according to an exemplary embodiment. The size reduction method of this exemplary embodiment may be carried out, for example, by using a plating apparatus 100. The plating apparatus 100 is an apparatus for carrying out electroplating and includes a container 102, a plating solution 104 in the container 102, a negative electrode 106 and a positive electrode 108 immersed in the plating solution 104, and an electricity source 110 that applies voltage between the two electrodes.

[0023] Blended in the plating solution 104 are ions of a plating metal 112, a nanocarbon 114, and a dispersion agent 116. The plating metal 112, as shown in this example is a monovalent cation of silver (Ag).

[0024] The dispersion agent 116 used in this example is an anion based surfactant. As illustrated in FIG. 1, when molecules of the surfactant are adsorbed to a nanocarbon particle 114, the liphophilic group 118b of each surfactant molecule becomes attached to the nanocarbon particle 114, with the hydrophilic group 118a of each surfactant molecule positioned outward. Therefore, the nanocarbon particles 114 do not aggregate but are dispersed in the plating solution 104 due to the dispersion agent 116.

[0025] As for the nanocarbon 114, for example, the amount added to the plating solution 104 was set to 0.2 g/L, and the particle diameter of the nanocarbon 114 was set to 2.6±0.5 nm. Furthermore, the nanocarbon particles 114 in a mixture with the plating solution 104 are positively charged. The plating solution 104 is neutral because the plating metal 112 is silver (Ag).

[0026] When a plating process is started in the plating apparatus 100 by applying, from the electricity source 110, a voltage between the negative electrode 106 and the positive electrode 108, then epitaxial growth of the plating metal 112 progresses on the surface of a plating object 120 connected to the negative electrode 106, so that crystal grains of the plating metal 112 form. As a result, the surface of the plating object 120 has on its surface a plating film 122 as indicated by hatching in FIG. 1.

[0027] FIGS. 2A and 2B show microscopic photographs of a plating film 122 formed as illustrated in FIG. 1 and a plating film 122A formed as a comparative example. The plating film 122 shown in FIG. 2A was obtained by adding the nanocarbon 114 into the plating solution 104 according to the crystal grain size reduction method of the exemplary embodiment. The plating film 122A of the comparative example shown in FIG. 2B was obtained without adding the nanocarbon 114 into the plating solution 104.

[0028] Observation of the microscopic photographs of the plating films 122 and 122A reveals that the crystal grains of the plating film 122 are clearly smaller in size than the crystal grains of the plating film 122A. Therefore, it is clear that the crystal grain size reduction method of this exemplary embodiment is capable of reducing the size of the crystal grains (forming nanocrystal grains) of the plating film 122. Table 1, presented below, compares the carbon contents of the plating films 122 and 122A.

TABLE-US-00001 TABLE 1 Carbon content of plating Addition of nanocarbon film (mass %) No 0.00182 Yes 0.00178

[0029] As depicted in Table 1, the carbon content of the plating film 122 according to this exemplary embodiment in which the nanocarbon 114 was added was substantially the same as the carbon content of the plating film 122A of the comparative example in which the nanocarbon 114 was not added. Thus, it is clear that the plating film 122 formed by the crystal grain size reduction method of the exemplary embodiment did not substantially incorporate the nanocarbon 114.

[0030] Thus, in the crystal grain size reduction method according to the exemplary embodiment, size reduction of the crystal grains of the plating film 122 is achieved by the nanocarbon 114 functioning as if it was a catalyst, without substantial incorporation of the nanocarbon into the plating film 122. This phenomenon will be discussed below.

[0031] First of all, the nanocarbon 114 dispersed in the plating solution 104 is not readily incorporated into the plating film 122 on the surface of the plating object 120 that is connected to the negative electrode 106 because an anion based surfactant is used as the dispersion agent 116. In addition, since the amount of the nanocarbon 114 added is as small as 0.2 g/L, incorporation of the nanocarbon 114 into the plating film 122 does not easily occur in the first place. Thus, in the conditions as indicated above, the nanocarbon 114 was, actually, negligibly incorporated into the plating film 122.

[0032] Next, it is hypothesized that, because the nanocarbon particles 114 are positively charged in the plating solution 104, the molecules of the anion based surfactant adsorbed to the nanocarbon particles 114 do not prevent the nanocarbon particles 114 from being attracted to the surface of the plating object 120 connected to the negative electrode 106, so that the nanocarbon particles 114 can affect the epitaxial growth of the plating metal 112.

[0033] Although the behavior of the nanocarbon particles 114 during this process is not entirely known, it can be hypothesized that, due to the Brownian motion in the plating solution 104, the nanocarbon particles 114 come into contact with and exert forces to crystal grains, thereby achieving size reduction of the crystal grains. Specifically, it can be hypothesized that the positively charged nanocarbon 114 in the plating solution 104 is attracted to the surface of the plating object 120 thereby definitely coming into contact with, and exerting forces on, crystal grains of the plating film, so that the crystal grains of the plating film can be definitively reduced in size.

[0034] Furthermore, since the particle diameter of the nanocarbon 114 is set within the range of 2.6±0.5 nm, the particles of the nanocarbon 114 in the plating solution 104 certainly undergo Brownian motion, so that as nanocarbon particles 114 come into contact with crystal grains of the plating film, the nanocarbon particles 114 exert to the crystal grains appropriate forces that reduce the size of the crystal grains. It is hypothesized that size reduction of the crystal grains becomes insufficient if the particle diameter of the nanocarbon 114 is above the aforementioned range because the Brownian motion of the nanocarbon particles is not sufficient and therefore cannot exert appropriate forces on the crystal grains. On the other hand, it is further hypothesized that crystal grain size reduction becomes insufficient when the particle diameter of the nanocarbon 114 is below the range is speculated so that, despite occurrence of the Brownian motion, the small masses of the nanocarbon particles cannot exert, on the crystal grains, a force sufficient to reduce the size of the crystal grains.

[0035] Plating object 120 may be provided with plating film 122 that can be used as an electrical contact. Therefore, it is desirable for the plating film 122 to have a low electric resistivity (contact resistance). Furthermore, since the plating object 120 may be repeatedly inserted into a socket or the like, it is also desirable that the plating film 122 have high durability (i.e., greater anti-abrasion properties that may be associated with sliding).

[0036] A crystal structure of a metal will be described with reference to FIGS. 3A and 3B. FIGS. 3A and 3B are schematic diagrams that correspond to the plating films 122 and 122A shown in FIGS. 2A and 2B.

[0037] The metal can be viewed as a crystal grain aggregate which includes crystal grains and grain boundaries that surround the crystal grains (defects of crystals or impurities) and in which crystal grains are bound to each other at grain boundaries. The abrasion of metal caused by sliding occurs in two different ways: crystal grains themselves undergo transgranular fracture; and boundaries fracture and the metal chips and erodes in block units of crystal grains. In this exemplary embodiment, it is an object to restrain the grain boundary fracture in which the metal chips off grain by grain and therefore increase the durability. In metal, when grain boundary fracture involves erosion of large crystal grains, the chipped-off volume, that is, the amount of erosion, is large; on the other hand, erosion of a small crystal grain means a small amount of erosion. Furthermore, in metal, since crystal grains are bound together at grain boundaries, it is hypothesized that the greater the strength of grain boundaries and the binding force thereof, the less easily erosion occurs. Therefore, crystal structure features of a metal that facilitate realization of a highly durable plating film are that the crystal grains are small and that the binding force at grain boundaries that binds crystal grains together is strong.

[0038] In the plating film 122 illustrated in FIG. 3A, the crystal grains 124 are smaller than the crystal grains 124A of the plating film 122A illustrated in FIG. 3B and the grain boundaries 126 where crystal grains 124 are bound together outnumber the grain boundaries 126A of the plating film 122A. Therefore, the plating film 122 is less easily erodable to sliding and therefore more durable than the plating film 122A.

[0039] Moreover, it is generally considered that metal becomes harder as the crystal grains are made smaller. In this regard, the plating film 122A of the comparative example, in which the crystal grains 124A were not reduced in size, had a Vickers hardness of 90 to 110 Hv. On the other hand, the plating film 122 of the exemplary embodiment, in which the crystal grains 124 were reduced in size, had a Vickers hardness of 100 to 110 Hv, making it clear that size reduction of the crystal grains 124 did not make the plating film 122 harder. Therefore, good running-in property (lubricity) of the contact surface (which is a characteristic of silver (Ag)) of the plating metal 112 can be maintained, so that the surface of the plating film 122 (the contact surface at the time of sliding) is smooth and the friction coefficient does not considerably change even after repeated sliding. Thus, the durability of the plating film 122 can be increased.

[0040] Next, the electrical contact resistance of the plating film 122 will be described. It is considered that when the crystal grains of metal are made smaller, grain boundaries generally increase, so that the electrical contact resistance increases. However, in the plating film 122 according to the exemplary embodiment, although the crystal grains 124 are small, the contact resistance is not high but about 3×10.sup.−6 to about 3.5×10.sup.−6 Ω.Math.cm. Incidentally, the contact resistance of a super hard silver plating that has substantially the same crystal grain diameter is as high as greater than or equal to 8×10.sup.−6 Ω.Math.cm. A reason for this is speculated to be that because the size reduction of the crystal grains 124 of the plating film 122 does not involve the alloying with a different metal, such as antimony (Sb), or because no adsorbent organic luster is used in the plating film 122, the grain boundaries 126 contain only a small amount of impurities.

[0041] FIGS. 4A and 4B are graphs indicating the durability and the contact resistance of the plating films 122 and 122A illustrated in FIGS. 2A and 2B, respectively. In each one of the graphs in FIGS. 4A and 4B, the horizontal axis represents the number of back-and-forth movements (number of sliding movement cycles) and the vertical axis represents the friction force (N) or the resistance value (me). Note that high friction forces, meaning high friction coefficient, are considered to mean that abrasion of the surface easily progresses and the anti-abrasion property thereof, that is, the durability thereof, is low.

[0042] The plating film 122 described in FIG. 4A produces a smaller friction force as a whole than the plating film 122A described in FIG. 4B, and therefore is higher in anti-abrasion property. In fact, the plating film 122 remained unfractured even when the number of back-and-force movements reached 1000. On the other hand, the plating film 122A, being low in anti-abrasion property, was destroyed as exhibited in FIG. 4B when the number of back-and-force movements was about 600. Furthermore, the plating film 122 exhibited stable electrical resistance at low resistance values, compared with the plating film 122A. On the other hand, the plating film 122A exhibited unstable electrical resistance values as a whole. Furthermore, the resistance value of the plating film 122A rapidly increased as the plating film 122A was fractured at the time of about 600 back-and-forth movements.

[0043] Thus, it was made clear that the plating film 122 formed by the crystal grain size reduction method according to the exemplary embodiment was lower in contact resistance and higher in durability than the plating film 122A of the comparative example, in which nanocarbon 114 was not added into the plating solution 104. That is, in the crystal grain size reduction method according to the exemplary embodiment, reform of the surface of the plating film 122 is realized by reducing the size of the crystal grains of the plating film 122 without substantial incorporation of the nanocarbon 114 into the plating film 122 although the nanocarbon 114 is added into the plating solution 104.

[0044] Examples and comparative examples in which different amounts of the nanocarbon 114 were added will be described below. Table 2 describes Examples 1 and 2 and Comparative Examples 1 and 2. In Examples 1 and 2, the amounts of the nanocarbon 114 added were 0.1 g/L and 0.2 g/L, respectively. In Comparative Example 1, the amount of the nanocarbon 114 added was zero, that is, no nanocarbon 114 was added. In Comparative Example 2, the amount of the nanocarbon 114 added was 0.3 g/L.

TABLE-US-00002 TABLE 2 The amount of Evaluations nanocarbon Size of Anti- added crystal abrasion Volume (g/L) grains property resistance Comparative 0 Large No good Low Example 1 and soft Example 1 0.1 Quite Acceptable Low small Example 2 0.2 Quite Acceptable Low small Comparative 0.3 or Intermediate Acceptable High Example 2 larger

[0045] As mentioned in Table 2, in Comparative Example 1 with no nanocarbon 114 added into the plating solution 104, the size of the crystal grains of the plating film was “Large”, the anti-abrasion property (durability) thereof was “No good”, and the volume resistance (electrical resistance) thereof was “Low”. In Comparative Example 2 with the amount of the nanocarbon 114 added being greater than or equal to 0.3 g/L, the size of the crystal grains of the plating film was “Intermediate”, the anti-abrasion property thereof was “Acceptable”, and the volume resistance thereof was “High”. In contrast, in both Examples 1 and 2 with the amount of the nanocarbon 114 added being less than or equal to 0.2 g/L, the size of the crystal grains of the plating film was “Quite small”, the anti-abrasion property thereof was “Acceptable”, and the volume resistance thereof was “Low”. Therefore, it is clear that if the amount of the nanocarbon 114 added is less than or equal to 0.2 g/L, the size of the crystal grains of the plating film 122 can be made quite small and the anti-abrasion property thereof can be made high and, furthermore, the volume resistance thereof does not increase but remains low despite the quite small size of the crystal grains.

[0046] FIGS. 5A and 5B show microscopic photographs exhibiting a plating film 128 according to another exemplary embodiment and a plating film 128A of another comparative example. The plating film 128 according to the another exemplary embodiment exhibited in FIG. 5A is different from the above-described plating film 122 in that the plating metal 112 of the plating film 128 was nickel (Ni) instead of silver (Ag). The plating film 128A of the another comparative example exhibited in FIG. 5B was obtained by using as the plating metal 112 nickel (Ni) instead of silver (Ag) and by omitting addition of the nanocarbon 114 into the plating solution 104. Note that, due to the use of nickel (Ni) as a plating metal, the plating solution 104 was weakly acidic.

[0047] Observation of the microscopic photographs of the plating films 128 and 128A reveals that the crystal grains of the plating film 128 are clearly smaller than the crystal grains of the plating film 128A. Therefore, it is clear that the crystal grain size reduction method according to the another exemplary embodiment is able to reduce the size of the crystal grains of the plating film 128. Table 3, presented below, indicates results of a sliding test of the plating films 128 and 128A.

TABLE-US-00003 TABLE 3 Number of slidings Load (g) Nickel sulfamate +Nanocarbon 50 434.35 520.55 50 426.91 513.61 50 423.91 526.31 50 426.9 523.30 50 423.88 529.15 50 416.68 526.33 Average 425.4 523.2

[0048] The plating film 128A of the comparative example was formed by blending nickel sulfamate in the plating solution 104 and omitting addition of the nanocarbon 114 into the plating solution 104. As indicated in Table 3, the plating film 128A was subjected to repeated sliding with a load of 50 g and was destroyed when the number of sliding cycles reached, averagely, 425.4.

[0049] On the other hand, the plating film 128 according to the another exemplary embodiment was formed by blending nickel sulfamate in the plating solution 104 and adding the nanocarbon 114 into the plating solution 104. As indicated in Table 3, the plating film 128 was destroyed when the number of sliding movements reached, averagely, 523.2. This clarifies that the plating film 128 was more durable than the plating film 128A of the another comparative example.

[0050] Therefore, by the size reduction method according to this exemplary embodiment, reform of the surfaces of the plating films 122 and 128 can be realized by making the crystal grains of the plating films 122 and 128 quite small without incorporation of the nanocarbon 114 into the plating films 122 and 128, respectively.

[0051] Incidentally, although in the foregoing exemplary embodiments of the invention, the plating metal 112 is silver (Ag) or nickel (Ni) as an example, these examples are non-limiting. The plating metal 112 may also be tin (Sn) or gold (Au). In such cases, it is hypothesized that the crystal grains of the plating film can also be made quite small to reform the surface of the plating film by causing the nanocarbon 114 to function as if the nanocarbon 114 was a catalyst, while avoiding incorporation of the nanocarbon 114 into the plating film.

[0052] While the exemplary embodiments of the invention have been described with reference to the drawings, it should be apparent that the invention is not limited by the foregoing examples or the like. It should be understood that a person having ordinary skill in the art can conceive various changes and modifications within the scope described in the appended claims and that such changes and modifications belong to the technical scope of the present invention.

[0053] The invention can be utilized as a method for forming reduced-size crystal grains of a plating film.

FIG. 4A PARAMETERS

FRICTION FORCE (N)

[0054] RESISTANCE VALUE (mΩ)

NUMBER OF BACK-AND-FORTH MOVEMENT CYCLES

FIG. 4B PARAMETERS

FRICTION FORCE (N)

[0055] RESISTANCE VALUE (mΩ)

NUMBER OF BACK-AND-FORTH MOVEMENT CYCLES