JP2023075905A - Metallic body, formation method of metallic body, and mating type connection terminal comprising metallic body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a metallic body, a formation method of the metallic body, and a mating type connection terminal comprising the metallic body, in which the metallic body can be formed in a short time while occurrence of a whisker due to an external stress is suppressed.

SOLUTION: A formation method of a metallic body is a method of forming a metallic body in which a base metal is laminated with a barrier layer and a metal plating layer in this order. The method includes: a barrier layer lamination step in which the base metal is laminated with the barrier layer whose primary component is Ni; and a metal plating layer lamination step in which the barrier layer-laminated base metal is further laminated with the metal plating layer in a DC plating treatment with a current density of 1-50 A/dm², after a PR plating treatment is performed on the barrier layer, in which the current density is 1-50 A/dm², Duty ratio is more than 0.8 and less than 1, and the ratio between the current density of forward current and the current density of backward current denotes forward-current density : backward-current density=1:0.5-1:3.

SELECTED DRAWING: Figure 2

COPYRIGHT: (C)2023,JPO&INPIT

Description

Applied for application of Article 30, Paragraph 2 of the Patent Act Published on September 6, 2021 in the collection of abstracts of the 144th Surface Finishing Technology Association Conference [Publications] On September 1, 2021, Published in the journal of the Institute of Electronics Packaging

The present invention relates to a method for forming a metal body formed by electroplating, the metal body, and a fitting-type connection terminal provided with the metal body.

In recent years, as electronic components have been miniaturized, there has been a tendency for the electrode area of fitting-type connection terminals such as connectors to become smaller as the pitch interval becomes narrower. For example, in connectors used for FPCs (Flexible Printed Circuits) and FFCs (Flexible Flat Cables), the smaller the electrode area, the greater the pressure applied to the contact points with the contacts.

By the way, conventionally, electrodes used in connectors and the like are plated with a Sn plating layer containing Sn as a main component from the viewpoint of suppressing oxidation. When the male connector is fitted to the female connector, pressure is applied to the Sn-plated layer due to contact with the contact portion, and whiskers may be generated from places where the stress concentrates in the Sn-plated layer. Whiskers generated in the Sn plating layer are needle-like crystals of Sn, and cause short circuits in FPC/FFC connectors having a narrow pitch. In addition to whiskers generated by pressure from the outside as described above, there are various other causes of whiskers. For example, when an intermetallic compound grows during the formation of the Sn plating layer, the volume expands, and compressive stress generated inside the Sn plating layer may cause whiskers.

Therefore, when an external stress is applied to the Sn plating layer, it is considered that whiskers are generated from the places where the compressive stress concentrates. In order to prevent the stress from concentrating inside the Sn plating layer, for example, the growth of the intermetallic compound may be suppressed inside the Sn plating layer.

Patent Literature 1 discusses the suppression of the growth of intermetallic compounds in the Sn plating layer. In the same document, in order to suppress diffusion of Cu and improve heat resistance, an intermediate layer having a Ni layer and a Cu—Sn layer, and a Sn A conductive material is disclosed in which plated layers are formed in this order. Since the conductive material described in the document does not have a work-affected layer of the base material, the Ni layer can be epitaxially grown on the base material, and the average crystal grain size of the Ni layer is as large as 1 μm or more. In addition, in paragraph 0008 of the same document, Cu diffuses through the grain boundary of the Ni layer as a diffusion path. is described. Furthermore, considering the plating treatment conditions described in the document, it is considered that each layer laminated on the base material is formed using a direct current plating method.

On the other hand, investigations are underway to suppress external stress whiskers by changing the conventional plating formation method. Patent Document 2 discloses a technique for suppressing whiskers using a pulse plating method. In the same document, by adjusting the ratio of the energization time and the stop time in the pulse plating method, a discontinuous surface is formed in the Sn plating layer, and the discontinuous surface inhibits the movement of Sn atoms and suppresses the growth of whiskers. It is stated that Furthermore, the same document describes that a discontinuous surface is formed by performing DC plating after pulse plating.

Further, Patent Document 3 discloses a technique for suppressing the generation of whiskers using a PR plating method in which the direction of current flow is periodically reversed. This document describes that the generation of whiskers is suppressed by adjusting the current densities and the current energization times of the positive and reverse currents.

Patent Document 4 describes a technology that can prevent needle-like or thread-like abnormal deposition occurring on the surface of the plating film when the reverse current is applied for 20% or more of the positive current in the PR plating method. is disclosed. The document also states that the plating current density is 5 A/dm ² or less, and the recommendation is 4.5 A/dm ² .

JP 2014-122403 A Japanese Patent Application Laid-Open No. 2006-307328 JP-A-63-118093 Japanese Unexamined Patent Application Publication No. 2004-204308

In the invention described in Patent Document 1, the grain size of the Ni layer is increased to improve the effect of suppressing the diffusion of Cu from the substrate. However, even if the crystal grain size of the Ni layer increases, the crystal grain boundary remains, so the Cu diffusion path is not lost. Further investigation is required to suppress the diffusion of Cu. Furthermore, in order to manufacture the conductive material described in Patent Document 1, it is necessary to laminate a Cu plating layer between the Ni plating layer and the Sn plating layer as described above, and further perform reflow treatment. The manufacturing process becomes complicated. Cost reduction by simplification of the manufacturing process must always be pursued.

In the invention described in Patent Document 2, as described above, the generation of whiskers is suppressed by forming a discontinuous surface in the Sn plating layer by pulse plating. Paragraph 0021 and FIG. 2 of the same document describe a columnar joint structure in which the columnar structure is divided in the plane direction. However, although the pulse current periodically flows, the polarity of the current is the same. Therefore, even if the migration of Sn can be suppressed, Cu diffuses from the Cu base material into the Sn plating layer formed by the pulse current, and an intermetallic compound grows, resulting in the generation of whiskers.

Furthermore, in the invention described in Patent Document 2, the Sn plating layer is formed by performing DC plating after pulse plating. However, in the same document, even if DC plating is performed after pulse plating, from the viewpoint that the movement of Sn is hindered by being divided in the plane direction, it is not divided in the plane direction in the DC plating. For this reason, considering that whiskers cannot be sufficiently suppressed by the pulse plating method, it is considered that whisker formation is difficult to suppress even if a direct-current plating layer is laminated.

Patent Document 3 describes that the growth of whiskers is suppressed by keeping the current density low in the PR plating method. Patent Literature 4 describes that an electrolytic double layer deposited when electrolytic deposition is continued is eliminated to prevent local concentration of plating deposition. Further, in the invention described in Patent Document 4, it is recommended to lower the current density. However, even if the concentration of plating deposition is prevented, if the current density is low, an intermetallic compound will grow in the Sn plating layer, and there is a concern that whiskers will grow due to external stress. Furthermore, the PR plating method requires a long time to form the Sn plating layer because a reverse current is applied for a certain period of time.

An object of the present invention is to provide a method for forming a metal body, a metal body, and a fitting-type connection terminal comprising the metal body, which can form a metal body in which the generation of whiskers caused by external stress is suppressed in a short time. to provide.

In view of the fact that it is difficult to avoid the external pressure applied to the Sn plating layer in a situation where the external pressure is applied to the connector, etc., the present inventors investigated the cause of the generation of whiskers in the conductive material described in Patent Document 1. reconsidered. One of the reasons for this is that the invention described in Patent Document 1 must form a Cu plating layer in spite of the fact that the purpose is to suppress the diffusion of Cu. Another example is that the Sn plating layer is formed by a direct current plating method. However, in order to shorten the film formation time, it is desirable to form the Sn plating layer using a direct current plating method.

The present inventors have investigated the cause of whisker formation in the conductive material described in Patent Document 1 when a Sn plating layer is formed by direct current plating without forming a Cu plating layer. When the X-ray diffraction spectrum of this Sn-plated layer was confirmed, it was found that the angles formed by the c-axis and the film thickness direction were relatively uniform in each crystal orientation of βSn constituting the Sn-plated layer. Therefore, the present inventors have focused on the possibility that the generation of whiskers may be suppressed if the angles formed by the c-axis and the film thickness direction are not uniform.

The present inventors replaced the DC plating method with the pulse plating method described in Patent Document 2, the AC plating method, and the PR plating method described in Patent Documents 3 and 4, respectively, to form Sn plating layers. In both the pulse plating method and the AC plating method, it was found that the angles formed by the c-axis and the film thickness direction are relatively uniform. In addition, although the PR plating method described in Patent Documents 3 and 4 suppresses the growth of whiskers to some extent as compared with the DC plating method, it was found that it is necessary to further reduce the growth of whiskers for practical use. . In addition, it was also found that productivity was inferior due to the long energization time.

Therefore, the present inventors tried to form a Sn plating layer by forming a film by a direct current plating method after forming a film by the PR plating method, and happened to find that the growth of whiskers is suppressed. got In addition, since the DC plating method is used to form the film, the film formation time is shorter than when the PR plating method is used alone. The knowledge that it is possible to manufacture was also obtained.
The present invention completed based on these findings is as follows.

(1) A method for forming a metal body in which a Ni layer and a Sn plating layer are laminated in this order on a metal substrate, comprising a Ni layer lamination step of laminating the Ni layer on the metal substrate; The current density of the positive current and the reverse current is each 1 to 50 A / dm ² , the Duty ratio is more than 0.8 and less than 1, and the ratio of the current density of the positive current and the current density of the reverse current is the current density _{positive current} : current density _{reverse current} = 1: 0.5 to 1: 3 after PR plating treatment, current density is 1 to 50 A / dm ² Sn plating layer laminating step. A method of forming a metal body.

(2) The method for forming a metal body according to (1) above, wherein the metal substrate is made of a metal containing Cu as a main component.

(3) A metal body in which a Ni layer and a Sn-plated layer are laminated in this order on a metal substrate, and the Sn-plated layer has an average dispersed stress ratio of 8.4%/seed, which is represented by the following formula. A metal body characterized by:
Average distributed stress ratio (%/species) = principal stress ratio (%)/number of crystal orientations propagated (species)

In the above formula, the principal stress ratio (%) is the peak intensity ratio of the crystal orientation showing the maximum peak intensity in the X-ray diffraction spectrum of the Sn plating layer, and the c-axis of the crystal orientation showing the maximum peak intensity and the Sn plating The maximum peak tilt angle (A), which is the angle formed with the film thickness direction of the layer, and the non-maximum angle formed between the c-axis of the crystal orientation showing the peak strength other than the maximum peak strength and the film thickness direction of the Sn plating layer Represents the total of the peak intensity ratio of the crystal orientations whose angle difference is within ±6 °, and the number of propagating crystal orientations (seeds) is the principal stress ratio in the X-ray diffraction spectrum of the Sn plating layer. represents the number of peaks not used in the calculation of

(4) The metal body according to (3) above, wherein the metal substrate is made of a metal containing Cu as a main component.

(5) A fitting-type connection terminal comprising the metal body according to any one of (3) to (4) above.

FIG. 1 is a schematic diagram showing a whisker growth mechanism when external stress is applied when the c-axes of the crystal orientations constituting βSn are relatively aligned. FIG. 2 is a schematic diagram showing the growth mechanism of whiskers when external stress is applied when the c-axes of the crystal orientations constituting βSn are relatively misaligned. FIG. 3 is a reference diagram for calculating the tilt angle, FIG. 3(a) is a reference diagram showing the a-axis, b-axis, and c-axis of a tetragonal crystal, and FIG. 3(b) is a crystal of βSn. FIG. 4 is a reference diagram for calculating the tilt angle θ between the Z-axis and the c-axis of the crystal plane when the plane intersects the XYZ-axes. FIG. 4 is a reference diagram for calculating the tilt angle θ between the Z-axis and the c-axis of the crystal plane when the crystal plane of βSn intersects the XYZ-axis by another method. FIG. 5 is a diagram showing an X-ray diffraction spectrum, FIG. 5(a) is an X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Example 1, and FIG. It is an X-ray diffraction spectrum of the Sn plating layer formed by the formation method, FIG. 5(c) is an X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Example 3, and FIG. 5(e) is an X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Example 4. FIG. FIG. 6 is a diagram showing an X-ray diffraction spectrum, FIG. 6(a) is an X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Comparative Example 1, and FIG. It is the X-ray diffraction spectrum of the Sn plating layer formed by the formation method, FIG. 6(c) is the X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Comparative Example 3, and FIG. It is an X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Comparative Example 4, and FIG. 6E is an X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Comparative Example 5. (f) is the X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Comparative Example 7, and FIG. 6 (g) is the X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Comparative Example 8. 6(h) is the X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Comparative Example 9. As shown in FIG. FIG. 7 is a diagram showing the relationship between the principal stress ratio and the maximum whisker length. FIG. 8 is a diagram showing the relationship between the average dispersed stress ratio and the maximum whisker length.

Although the present invention will be described in detail below, it is not limited to the following forms.
1. Method for Forming Metal Body (1) Barrier Layer Lamination Step In the method for forming a metal body according to the present invention, first, a barrier layer containing Ni as a main component is formed on a metal substrate.

Although the material of the metal substrate is not particularly limited, it is preferably made of a metal containing Cu as a main component. The metal substrate containing Cu as a main component represents that the Cu content is 50% by mass or more of the metal substrate, preferably 100% by mass. Cu alloys and pure Cu are included. The remainder may contain unavoidable impurities. The metal substrate used in the present invention includes, for example, a metal substrate forming terminal connection portions (bonding regions) of FFCs and FPCs, and a metal substrate forming electrodes. The thickness of the metal substrate is not particularly limited, but it may be 0.05 to 0.5 mm from the viewpoint of ensuring the strength of the metal body and reducing its thickness.

From the viewpoint of suppressing the diffusion of elements constituting the metal substrate, the material of the barrier layer is preferably made of a metal containing Ni as a main component. When the metal substrate contains Cu as a main component, the diffusion of Cu can be particularly suppressed. A barrier layer whose main component is Ni means that the Ni content is 50 mass % or more of the barrier layer. A preferable Ni content is 100% by mass. Included are Ni alloys and pure Ni. The remainder may contain unavoidable impurities. The film thickness and crystal grain size of the barrier layer are not particularly limited, but the film thickness should be 0.1 to 5 μm and the crystal grain size should be 0.1 to 2.0 μm.
Means for forming the barrier layer is not particularly limited, and can be performed by a known plating method using an electroplating apparatus.

(2) Metal Plating Layer Lamination Step Next, a metal plating layer is formed on the barrier layer. In the present invention, first, a portion of the metal plating layer is formed by PR plating. The conditions of the PR plating process are a current density of 1 to 50 A/dm ² for forward current and reverse current, and a duty ratio of more than 0.8 and less than 1. If the current density of the positive current is less than 1 A/dm ² , it takes a long time to form a film and affects productivity, and if the current density exceeds 50 A/dm ² , scorching occurs on the surface. It is preferably more than 5 A/dm ² and not more than 50 A/dm ² , more preferably 8 to 30 A/dm ² , particularly preferably 8 to 15 A/dm ² . If the current density of the reverse current is less than 1 A/dm ² , the effect of applying the reverse current to suppress the growth of whiskers due to the presence of many crystal planes is not exhibited, and the current density is less than 50 A/dm ² . If it exceeds, it takes a long time to form a film, which affects productivity. It is preferably more than 5 A/dm ² and 50 A/dm ² or less, more preferably 5 to 30 A/dm ² , still more preferably 12 to 20 A/dm ² , particularly preferably 16.7 to 18.5 A/dm 2 ^dm2 , most preferably between 17.7 and 18.5 A/ ^dm2 .

Further, when the duty ratio is within the above range, the ratio of the current density of the positive current and the current density of the reverse current is current density _{positive current} :current density _{reverse current} =1:0.5 to 1:3. When the ratio of the current density of the positive current and the current density of the reverse current exceeds 1:3, the amount of dissolution due to the reverse current becomes larger than the amount of plating formed by the positive current, so a plating film is produced. Can not do it. On the other hand, when the ratio is less than 1:0.5, the effect of reverse current is insufficient, so whiskers cannot be suppressed. Within this range, the elution time of the plated layer during reverse current flow is short, and the film formation time can be suppressed even if the PR plating method is employed. The ratio is preferably 1:1 to 1:2, more preferably 1:1.77 to 1:1.86, even more preferably 1:1.77 to 1:1.85.

If the duty ratio is 0.8 or less, a metal plating layer cannot be formed in the first place, and if the duty ratio is 1, a direct current will be generated and whiskers will grow. It is preferably 0.85 to 0.99. The energization time is not particularly limited, and is appropriately adjusted so as to obtain a required film thickness. Note that the duty ratio represents the ratio of the positive current energization time to the total energization time of the PR plating method.

The energization time is appropriately adjusted so as to obtain a required film thickness. When forming a PR plating layer with a thickness of about 0.5 to 3.0 μm, the positive current application time may be 30 to 120 seconds, and may be 40.5 to 117.0 seconds. It may be 63.0 to 100.8 seconds. The reverse current application time may be about 1 to 80 seconds, may be 4.5 to 13.0 seconds, or may be 7.0 to 11.2 seconds. It may be obtained from the duty ratio. The total energization time for the PR plating process may be 10 to 200 seconds, may be 45 to 130 seconds, or may be 70 to 112 seconds. Although the frequency is not particularly limited, it is preferably 0.004 Hz to 3 kHz, more preferably 0.01 to 100 Hz, and particularly preferably 0.05 to 9 Hz.

After that, a direct-current plating process is performed at a current density of 1 to 50 A/dm ² to complete the formation of the metal plating layer. If the current density is within this range, scorching of the surface can be suppressed. From the viewpoint of shortening the time, it is preferably 5 to 50 A/dm ² , more preferably 8 to 30 A/dm ² . In addition, the energization time of the DC plating process may be appropriately adjusted according to the film thickness of the plated layer by the PR plating process so as to obtain a desired final film thickness. For example, when forming a direct-current plating layer having a thickness of about 2.0 to 5.0 μm, the time may be 30 to 70 seconds, or 40 to 65 seconds. From the viewpoint of simplification of the working process, the plating solution used in the direct-current plating treatment is preferably the same plating solution as the plating solution used in the PR plating treatment.

The film formation time of the PR plating process and the film formation time of the DC plating process are not particularly limited, but may be adjusted so that the ratio of the respective film thicknesses falls within a predetermined range. The thickness ratio of the layers deposited in each treatment is preferably 1:1 to 1:5, more preferably 1:1 to 1:4. Within this range, the film formation time does not become too long, and the growth of whiskers can be suppressed.

In addition, the energization time of the DC plating process may be appropriately adjusted according to the film thickness of the plated layer by the PR plating process so as to obtain a desired final film thickness. When forming a direct-current plating layer of about 2.0 to 5.0 μm, the time may be 17 to 70 seconds, and may be 40 to 65 seconds. In addition, since the effect of reducing whiskers by PR plating is not diminished by DC plating, it is difficult for whiskers to grow even if DC plating is performed.

Even if the metal plating layers are formed by two different methods, the two layers cannot be distinguished. Therefore, the film thickness for each method is obtained as follows. First, plating is performed in advance for a certain period of time by the PR plating method and the DC plating method. The film formation rate in each method is calculated by processing the cross-section of the metal plating layer produced by FIB and measuring each film thickness from the cross-section SEM photograph. Then, the plating process time to obtain the desired film thickness is calculated from each calculated film formation rate, and the plating process is performed for the calculated plating process time to determine the film thickness of the layer formed by each method.

The metal plating layer formed by the method for forming a metal body according to the present invention has the effect of suppressing oxidation of the metal substrate. A metal plating layer composed of a metal containing Sn as a main component means that the metal has a Sn content of 50% by mass or more in the metal plating layer. A preferred Sn content is 100% by mass. Sn alloys and pure Sn are included. The remainder may contain unavoidable impurities.

When the metal plating layer is a Sn alloy, it contains at least one of Ag, Bi, Cu, In, Ni, Co, Ge, Ga, Sb and P as an optional element within a range that does not impair the effects of the present invention. good too. The content of these elements is preferably 5% by mass or less of the total mass of the metal plating layer. The film thickness of the metal plating layer is preferably 1 to 7 μm in consideration of manufacturing cost and manufacturing time.

The plating solution used in the method for forming a metal body according to the present invention is not particularly limited, and a commercially available metal plating solution may be used. For example, as the metal plating solution, an acidic bath metal plating solution made of Sn alloy containing 95% by mass or more of Sn or pure Sn is used. When a Sn-based alloy is contained in the metal plating solution, in addition to Sn, one or more elements selected from Ag, Bi, Cu, In, Ni, Co, Ge, Ga, Sb, P, etc. mixed.

As described above, in the method for forming a metal body according to the present invention, the DC plating method, which is known to promote the growth of whiskers, is purposely selected from among various plating methods, and the PR plating method is followed by the DC plating method. A metal plating layer was formed by the method. As a result, a metal plating layer in which growth of whiskers is suppressed can be laminated in a short time.

2. Metal Body The metal body according to the present invention comprises a metal substrate, a barrier layer and a metal plating layer laminated in this order. Each layer will be described in detail.
(1) Metal base The material of the metal base constituting the metal body according to the present invention is not particularly limited as described above, but it is preferably made of a metal containing Cu as a main component. The metal substrate containing Cu as a main component represents that the Cu content is 50% by mass or more of the metal substrate, preferably 100% by mass. Cu alloys and pure Cu are included. The remainder may contain unavoidable impurities. The metal substrate used in the present invention includes, for example, a metal substrate forming terminal connection portions (bonding regions) of FFCs and FPCs, and a metal substrate forming electrodes. The thickness of the metal substrate is not particularly limited, but it may be 0.05 to 0.5 mm from the viewpoint of ensuring the strength of the metal body and reducing its thickness.

(2) Barrier layer The material of the barrier layer that constitutes the metal body according to the present invention is, as described above, from the viewpoint of suppressing the diffusion of the elements that constitute the metal base material, that the main component is Ni. is preferred. When the metal substrate contains Cu as a main component, the diffusion of Cu can be particularly suppressed. A barrier layer whose main component is Ni means that the Ni content is 50 mass % or more of the barrier layer. A preferable Ni content is 100% by mass. Included are Ni alloys and pure Ni. The remainder may contain unavoidable impurities. The film thickness and crystal grain size of the barrier layer are not particularly limited, but the film thickness should be 0.1 to 5 μm and the crystal grain size should be 0.1 to 2.0 μm.

(3) Metal Plating Layer The metal plating layer forming the metal body according to the present invention has the effect of suppressing oxidation of the metal substrate as described above. A metal plating layer composed of a metal containing Sn as a main component means that the metal has a Sn content of 50% by mass or more in the metal plating layer. A preferred Sn content is 100% by mass. Sn alloys and pure Sn are included. The remainder may contain unavoidable impurities.

When the metal plating layer is a Sn alloy, it contains at least one of Ag, Bi, Cu, In, Ni, Co, Ge, Ga, Sb and P as an optional element within a range that does not impair the effects of the present invention. good too. The content of these elements is preferably 5% by mass or less of the total mass of the metal plating layer. The film thickness of the metal plating layer is preferably 1 to 7 μm in consideration of manufacturing cost and manufacturing time.

Moreover, the metal plating layer constituting the metal body according to the present invention has an average dispersed stress ratio of 8.4% or less. The average distributed stress ratio is described in detail below.

In the present invention, as described above, after the metal plating layer is laminated by the PR plating method, the metal plating layer is further laminated by the conventional DC plating method. When the metal plating layer is formed by adopting the direct current plating method after the PR plating method, various crystals having different crystal orientations are deposited in the Sn plating layer. It is presumed that this disperses the external stress and suppresses the growth of whiskers. More details are inferred as follows.

The external stress is basically a compressive stress applied to each crystal. At this time, the crystal orientation is aligned, that is, when the film thickness direction of the Sn plating layer is taken as the Z axis, for example, the crystal orientation c axis and the Z axis are aligned. The compressive stress propagates in the surface direction of the Sn plating layer as it is. Then, for example, when a crystal plane with a crystal orientation in which the angle formed by the c-axis and the Z-axis deviates greatly, it is assumed that the propagated stress concentrates on the crystal plane and whiskers grow.

According to this speculation, in the present invention, in order to reduce the whisker length, in the X-ray diffraction spectrum, the peak intensity ratio (%) of the crystal orientation (A) showing the maximum peak intensity, and the crystal orientation showing the maximum peak intensity ( A) the maximum peak tilt angle (a°), which is the angle between the c-axis and the film thickness direction, and the angle between the c-axis of the crystal orientation showing the peak intensity other than the maximum peak intensity and the film thickness direction, It is necessary to consider the sum of the peak intensity ratios (%) of the crystal orientations (B) in which the angle difference (a°-b°) of the maximum peak tilt angle (b°) is within ±6°. In other words, the sum of the intensity ratios of the crystal orientations in which the tilt angles of the c-axis are relatively uniform corresponds to the main stress for the generation of whiskers. There is a need. In the present invention, the sum of these strength ratios is called the principal stress ratio (%).
In the present invention, the peak intensity ratio represents the value (%) obtained by dividing the peak intensity by the sum of all peak intensities and multiplying the result by 100.

In the forming method according to the present invention, the metal body is formed by adopting the PR plating method before the DC plating method, and the existence probability of the crystal plane that causes the principal stress in the PR plating method is reduced. . Even if the direct-current plating method is applied after the PR plating method, the principal stress ratio does not increase, and it is presumed that the growth of whiskers in the Sn plating layer is suppressed. Along with this, a Sn plating layer is formed in a short time.

Here, for example, in the present invention, the evaluation of the whisker length can be measured by the ball indentation method. In this measurement, the ball indenter is pressed in the plate thickness direction of the surface, but the external stress due to the ball indenter is not limited to the plate thickness direction, and is thought to propagate as compressive stress in the surface direction of the Sn plating layer. That is, it is considered that external compressive stress propagates in the Sn plating layer from various directions. In view of these, the above whiskers are further speculated as follows.

It is assumed that when the compressive stress described above propagates to a crystal plane having an inclination angle greatly deviating from the inclination angle of the c-axis of the crystal plane that causes the principal stress, the stress concentrates there and whiskers grow. Assuming that this assumption is correct, it is inferred that the fewer the crystal planes with large differences in angle, the more likely the compressive stress will concentrate on the crystal planes and the whiskers will grow. Conversely, it is presumed that the more the number of crystals with different crystal orientations, the more the compressive stress is dispersed, and the more difficult it is for whiskers to grow. In other words, it is presumed that as the principal stress is reduced and the number of propagated crystal orientations, which is the number of crystal orientations to which the compressive stress propagates, increases, the stress is dispersed and whiskers are less likely to grow. In the present invention, a large number of propagating crystal orientations is called polyhedral.

The extent to which the stress is dispersed by this multifaceted surface can be represented by the average dispersed stress ratio shown below.
Average distributed stress ratio (%/species) = principal stress ratio (%)/number of crystal orientations propagated (species)

In the present invention, whisker growth is suppressed when the average dispersion stress ratio is 8.4%/seed or less, and whisker growth occurs when it exceeds 8.4%/seed. It is preferably 7.8%/seed or less, more preferably 6.1%/seed or less, and still more preferably 5.1%/seed or less. Although the lower limit is not particularly limited, it may be 2%/seed or more, and may be 5.0%/seed.

If the principal stress ratio is very high or the number of propagating crystal orientations is small, the stress is not sufficiently dispersed, so the average dispersed stress ratio does not decrease, and the effect of multifaceted crystal orientation cannot be fully exhibited. Conceivable. In the present invention, if the number of crystal orientations to be propagated is two or more, it is considered that such an adverse effect can be suppressed. The number is preferably 4 or more, more preferably 5 or more, still more preferably 6 or more, and particularly preferably 7 or more. Propagation of stress in the Sn plating layer will be described in more detail with reference to FIGS. 1 and 2. FIG.

FIG. 1 is a schematic diagram showing the growth mechanism of whiskers when external stress is applied when the crystal orientations of the crystals forming the Sn plating layer are relatively uniform. As shown in FIG. 1, external compressive stress is applied to the Sn plating layer at all angles. At this time, since the compressive stress directly reaches the adjacent crystals, it is considered that the regions with aligned crystal orientations have a high stress propagation capability. Then, when a region with a different crystal orientation happens to exist in a narrow range due to the presence of crystals with different crystal orientations to the destination to which the compressive stress is propagated, it is presumed that the stress concentrates in this region and whiskers grow. .

FIG. 2 is a schematic diagram showing the growth mechanism of whiskers when external stress is applied when the crystal orientations of the crystals forming the Sn plating layer are relatively uneven. As shown in FIG. 2, even if there is a region with uniform crystal orientation, if a large number of crystals with different crystal orientations exist and a wide range of regions with different crystal orientations exist, the compressive stress will not propagate in that region. It is presumed that the whiskers are dispersed and relaxed, and the growth of whiskers is suppressed.

Here, in view of the above-mentioned average dispersed stress ratio, even if the number of propagating crystal orientations is one, if the principal stress ratio is small, the average dispersed stress ratio shows a low value. However, even if regions with different crystal orientations exist widely, if the principal stress ratio is large and the number of propagating crystal orientations is one, the compressive stress will propagate in the regions with different crystal orientations and whiskers will grow. On the other hand, if the number of propagating crystal orientations is two or more in the regions of different crystal orientations, even if the principal stress ratio is large, the principal stress ratio is at least halved, so the average dispersed stress ratio becomes low and whisker growth. is thought to be suppressed.

Since the formation method according to the present invention adopts the direct current plating method after the PR plating method, the Sn plating layer is formed in a short time compared to the case of using only the PR plating method. In addition, it is presumed that the presence of many crystal planes reduces the average dispersed stress ratio, and even if the principal stress ratio is large, the growth of whiskers in the Sn plating layer is suppressed.

The reason why it is preferable to disperse the crystal orientation in order to suppress the growth of whiskers is speculated as follows using the Young's modulus of βSn. Since Sn has a tetragonal crystal structure (βSn) at normal temperature and normal pressure, its properties vary greatly depending on the crystal orientation. Since the βSn crystal has a higher Young's modulus in the c-axis direction than in the a-axis direction, it is difficult to deform in the c-axis direction. Therefore, when an external stress is applied to the surface of the metal plating layer, the external stress tends to propagate without dispersing if the inclination angles of the crystal orientations of βSn are uniform as shown in FIG. If a crystal with a large difference in tilt angle exists ahead of it, the propagation of the compressive stress is cut off there, and the compressive stress concentrates on that portion, facilitating the growth of whiskers. Therefore, in a metal plating layer having many crystal orientations with greatly different tilt angles, which is the angle between the c-axis and the film thickness direction, the compressive stress acting on adjacent crystals is relaxed, further suppressing the growth of whiskers. can be done.

An example of how to obtain the tilt angle in the present invention will be described with reference to FIG. FIG. 3 is a reference diagram for calculating the tilt angle, FIG. 3(a) is a reference diagram showing the a-axis, b-axis, and c-axis of a tetragonal crystal, and FIG. 3(b) is a crystal of βSn. FIG. 4 is a reference diagram for calculating the tilt angle θ between the Z-axis and the c-axis of the crystal plane when the plane intersects the XYZ-axes. The c-axis in FIG. 3(a) corresponds to the c-axis in FIG. 3(b).
In the present invention, the film thickness direction of the metal plating layer is defined as the Z-axis.

Assuming that the length of the unit cell of tetragonal βSn is (a, b, c), the crystal planes are, as shown in FIG.
x ₁ = α・a
y ₁ =β·b
z ₁ =γ·c
cross at The Miller index at this time is represented by an integer ratio of (1/α:1/β:1/γ)=(hkl).

At this time, L2, θ2, L1, tan θ, and θ shown in FIG. 3B are expressed as follows.

However, when the crystal plane is parallel to the Z-axis, θ=0°, and when the crystal plane is perpendicular to the Z-axis, θ=90°.

とする。 If it does not cross the Y-axis like (101),

and

とする。 Also, when it does not cross the X-axis like (011),

and

Here, the lengths of the sides constituting the tetragonal unit lattice are a=b=0.5831 nm and c=0.3181 nm. Using these values and the above formula, the tilt angle θ of the c-axis at each Miller index is given in Table 1.

Another example of how to obtain the tilt angle in the present invention will be described with reference to FIG. FIG. 4 is a reference diagram for calculating the tilt angle θ between the Z-axis and the c-axis of the crystal plane when the crystal plane of βSn intersects the XYZ-axis by another method.

As shown in FIG. 4, the intersection point H The (x, y, z) coordinates are calculated as follows.

であり、

となる。 Using the coordinates (x, y, z) of the intersection point H,

and

becomes.

From Equation 2: y=ax/b Equation 4
is obtained. Also, from Equation 3, z=ax/c Equation 5
is obtained.

Substituting equations 4 and 5 into equation 1, we get
^{x2 -} ax+ ^a2x2 / ^b2 + ^a2x2 / ^{c2 =} 0
x ² (1+a ² /b ² +a ² /c ² )−ax=0
x((1+a ² /b ² +a ² /c ² )x−a)=0
becomes,
x=a/(1+a ² /b ² +a ² /c ² ) Equation 6
y=ax/b Expression 7
z=ax/c Equation 8
is obtained.

Using these, the tilt angle θ, which is the angle between the c-axis and the Z-axis of each mirror index shown in FIG. 4, is derived. An example of the derivation method when the Miller index is the (3,2,1) plane will be described.
In the (3,2,1) plane, the intercept of the XYZ axes is (2,3,6), and each side of the tetragonal unit lattice is a = b = 0.5831 nm, c = 0 .3181 nm. Considering these, the length of each segment is a = 2 x 0.5831 = 1.1662
b = 3 x 0.5831 = 1.7493
c = 6 x 0.3181 = 1.9086
Then, the point H (x, y, z) obtained from the above formulas 6 to 8 is
(x, y, z) = (0.6415, 0.4277, 0.3920).

ＯＨ＝０．８６５０
となる。よって、傾斜角度θは以下のように算出される。
ｓｉｎθ＝ＯＨ／ＯＣ＝０．８６／１．９０８６＝０．４５３１
θ＝ＡＲＣＳＩＮθ＝２６．９５°
他のミラー指数におけるｃ軸の傾斜角度θは表２に示す値になる。 The distance OH from the origin to point H is

OH = 0.8650
becomes. Therefore, the tilt angle θ is calculated as follows.
sin θ=OH/OC=0.86/1.9086=0.4531
θ=ARCSIN θ=26.95°
The tilt angle θ of the c-axis at other Miller indices is the value shown in Table 2.

In either method, θ has the same value, and the tilt angle θ, which is the angle between the c-axis of the crystal orientation of βSn (tetragonal) and the Z-axis, can be obtained. The method of obtaining as shown in Table 1 is preferable in that the calculation is easier than the method of obtaining as shown in Table 2. Note that the crystal planes (220), (440), (420), and (200) in Table 2 are parallel to the z-axis and cannot be calculated because the values of x, y, and z cannot be calculated.

Using the tilt angle θ obtained in this way, the average dispersed stress ratio (%/seed) of the present invention is determined by the following formula.
Average distributed stress ratio (%/species) = principal stress ratio (%)/number of crystal orientations propagated (species)
As described above, the principal stress ratio (%) of the present invention is the peak intensity ratio (%) of the crystal orientation (A) showing the maximum peak intensity in the X-ray diffraction spectrum of the metal plating layer, and the maximum peak intensity. The maximum peak tilt angle (a°), which is the angle between the c-axis of the crystal orientation (A) and the film thickness direction of the metal plating layer, and the c-axis of the crystal orientation (B) that indicates the peak intensity other than the maximum peak intensity The peak intensity ratio (%) of the crystal orientation in which the angle difference (a°-b°) of the non-maximum peak tilt angle (b°), which is the angle formed with the film thickness direction of the metal plating layer, is within ±6° , is the sum (%) of Also, the propagating crystal orientation number (seed) is the number of peaks not used for calculating the principal stress ratio (%) in the X-ray diffraction spectrum of the metal plating layer.

For example, in the X-ray diffraction spectrum, the peak intensity ratio of the (321) plane is 40%, the peak intensity ratio of the (411) plane is 5%, and the peak intensity ratio of the (521) plane is 35%, and the (332) plane have a peak intensity ratio of 20%. The peak intensity ratio (A) of the (321) plane showing the maximum peak intensity is 40%, and the maximum peak tilt angle (a°) of the (321) plane (A) is 26.95°. There are three crystal orientations showing peak intensities other than the maximum peak intensity: the (411) plane, the (521) plane, and the (332) plane. Among these, the angle difference (a°-b°) between the maximum peak tilt angle (a°) and the non-maximum peak tilt angle (b°) is within ±6° (20.95° to 32.95°) The crystal orientation (B) is the (411) plane with an inclination angle θ of 23.97°. The peak intensity ratio (%) of the (411) plane (B) is 5%. Therefore, the principal stress ratio is 40%+5%=45%.
In addition, the number of propagating crystal orientations (seeds) is two kinds, the (521) plane and the (332) plane, which are not used in the calculation of the principal stress ratio.
Therefore, the average dispersed stress ratio (%/seed) is 45%/2seeds=22.5%/seeds.

Although the present invention employs the PR plating method and the DC plating method in which the positive current and the reverse current are alternately applied, even if the pulse plating method or the AC plating method is employed instead of the PR plating method, the growth of whiskers is prevented. It cannot be suppressed. It is presumed that this is because only a positive current flows in either case, so that it does not contribute to multifaceted crystal planes.

Moreover, it is considered that the Sn-plated layer formed by the forming method according to the present invention tends to be multifaceted to the same extent as the Sn-plated layer formed by the PR plating method. For this reason, even if direct-current plating is performed after PR plating, the multifaceted crystals constituting the Sn-plated layer are realized, and the growth of whiskers is suppressed. On the other hand, when only the pulse plating method, the AC plating method, or the DC plating method is used, only a positive current is applied, so multi-facets cannot be realized.

In the metal plating layer formed by the forming method according to the present invention, even if the cross section is observed with an SEM, the boundaries between the processing methods cannot be recognized. In addition, since the layers formed by each processing method are thin, they are difficult to identify with X-rays. Therefore, even if the plating layer is formed by two methods like the forming method according to the present invention, the metal plating layer according to the present invention constitutes one layer.

3. Fitting type connection terminal Since the metal body formed by the metal body forming method according to the present invention can sufficiently suppress the generation of whiskers, it can be used as an electrical contact that conducts by mechanical bonding. It can be suitably used for terminals. Specifically, it is preferable to use the metal body according to the present invention for connector pins (metal terminals) of connectors, terminal connection portions (bonding regions) of FFCs and FCPs that are fitted with connectors, and press-fit pins.

Specific examples according to the present invention will be described below, but the present invention is not limited thereto.
(1) Preparation of evaluation sample In order to prove the effect of the present invention, a Ni-plated Cu plate (size: 30 mm × 30 mm × 0.3 mm, Ni plating thickness: 3 μm) and a Sn plate used as an anode were metal-plated. A metal plating layer is formed on the Ni plating layer by immersing it in a beaker containing a liquid and passing a current under the conditions shown in Table 5 at room temperature, and the metal plating layer having the thickness shown in Table 5. formed.

In Table 5, "current density (of positive current)" represents current density of positive current in the PR plating method, or current density in the DC plating method, pulse plating method, and AC plating method. "Current density of reverse current" represents the current density of reverse current in the PR plating method. “Positive current application time” and “reverse current application time” are the application times obtained from the PR plating method application time and the duty ratio. The duty ratio is the ratio of the positive current energization time to the total energization time.
The following plating solutions were used in each plating method.
Made by Uemura Industry Co., Ltd.: Model number GTC
Manufactured by Ishihara Chemical Co., Ltd.: Model number PF-095S
In Comparative Example 4, after the metal plating layer was formed under the conditions shown in Table 5, the surface temperature of the base material was raised to 270° C., held for 6 seconds, and then air-cooled.

(2) Film thickness of barrier layer and metal plating layer The metal plating layer of the evaluation sample prepared as described above was cross-sectionally processed by FIB, and the cross section was magnified 30,000 times on the SEM monitor, and an arbitrary The average value of the film thickness of each layer was calculated for 10 locations.

Further, the film thickness of each of the PR plating layer and the DC plating layer in Examples 1 to 5 was calculated as follows. First, plating was performed for a certain period of time by each method in advance. The prepared metal plating layer was subjected to cross-sectional processing by FIB, the film thickness of the metal plating layer was measured from the cross-sectional SEM photograph, and the film formation rate in each method was calculated. Then, the plating process time to obtain the desired film thickness was calculated from each of the calculated film formation rates, and the plating process was performed for the calculated plating process time to determine the film thickness of the layer formed by each method.

(3) Average Dispersion Stress Ratio The X-ray diffraction spectrum of the metal plating layer produced as described above was measured using an X-ray diffractometer under the following conditions.
・ Analyzer: MiniFlex 600 (manufactured by Rigaku)
・X-ray tube: Co (40kV/15mA)
・Scan range: 3° to 140°
・Scan speed: 10°/min

The X-ray diffraction spectra of Examples 1 to 5 and Comparative Examples 1 to 5 and 7 to 9 are shown in FIGS. 5 and 6. FIG. FIG. 5 is a diagram showing an X-ray diffraction spectrum, FIG. 5(a) is an X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Example 1, and FIG. It is an X-ray diffraction spectrum of the Sn plating layer formed by the formation method, FIG. 5(c) is an X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Example 3, and FIG. 5(e) is an X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Example 4. FIG. FIG. 6 is a diagram showing an X-ray diffraction spectrum, FIG. 6(a) is an X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Comparative Example 1, and FIG. It is the X-ray diffraction spectrum of the Sn plating layer formed by the formation method, FIG. 6(c) is the X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Comparative Example 3, and FIG. It is an X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Comparative Example 4, and FIG. 6E is an X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Comparative Example 5. (f) is the X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Comparative Example 7, and FIG. 6 (g) is the X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Comparative Example 8. 6(h) is the X-ray diffraction spectrum of the Sn plating layer formed by the formation method of Comparative Example 9. As shown in FIG.

Examples 1 to 5 shown in FIGS. 5A to 5E have more peaks than Comparative Examples 1 to 4 shown in FIGS. 6A to 6D, and are multifaceted. It turns out there is. For this reason, it was found that even if DC plating is performed after PR plating, the multi-faceted crystal orientation constituting the Sn plating layer is realized, and the growth of whiskers is suppressed even if DC plating is adopted. On the other hand, in Comparative Examples 1 to 4 shown in FIGS. 6(a) to 6(d), films are formed only by the pulse plating method, the AC plating method, and the DC plating method, so that all of them are multifaceted. it wasn't. Moreover, even if the direct current plating method was adopted after the pulse plating method or the alternating current plating method, multi-facets were not realized. Comparative Examples 5 and 7 to 9 shown in FIGS. 6(e) to 6(h) were formed by the PR plating method, and it was found that the number of peaks was large and multifaceted.

The tilt angle (°) of each plane orientation was calculated from the obtained X-ray diffraction spectrum. Also, the total value of each peak intensity was calculated, and the spectral intensity ratio (%) of each peak was calculated by dividing each peak intensity by the total value and multiplying by 100.

The average dispersed stress ratio (%/seed) was calculated by: average dispersed stress ratio=principal stress ratio (%)/number of crystal orientations propagated (seed). In this example, the crystal orientation showing the maximum peak intensity ratio in the X-ray diffraction spectrum was defined as (A), and the maximum peak tilt angle was defined as (a). In addition, among the non-maximum peak tilt angles (b) that are the tilt angles of the c-axis of the crystal orientation that does not show the maximum peak intensity ratio, the angle with the tilt angle (a) of the c-axis of the crystal orientation that shows the maximum peak intensity The crystal orientation with a difference (ab) of ±6° was defined as (B). The values shown in Tables 1 and 2 were used as the tilt angles. Then, the principal stress ratio (%), which is the X-ray diffraction spectrum intensity ratio of the dominant crystal orientation, was obtained.

Finally, among the crystal orientations, the number of propagating crystal orientations, which is the number of crystal orientations not used for calculating the principal stress ratio, is obtained, and the principal stress ratio (%) is divided by the number of propagating crystal orientations (seeds). , the average distributed stress ratio (%/species) was calculated. In Comparative Example 6, since film formation was not possible, the average dispersed stress ratio (%/species) could not be calculated.

For example, in Example 1, it is as follows. In the X-ray diffraction spectrum, the peak intensity ratio of the crystal orientation (220) having the maximum peak intensity is 32.3%. The maximum peak tilt angle (a), which is the angle between the c-axis of the crystal orientation and the film thickness direction, was 0°, and this crystal orientation was called "A". Further, in crystal orientations other than (220), the difference (ab) between the non-maximum peak tilt angle (b), which is the angle between the c-axis and the film thickness direction, and the maximum peak tilt angle (a) is within ±6° are (420) and (440), and these crystal orientations were referred to as "B". These peak intensity ratios were 1.3% and 5.6%, respectively. The "X-ray diffraction spectrum intensity ratio of dominant crystal orientation", which is the sum of this intensity ratio and the maximum peak intensity ratio, is 39.2%, which is the value of the principal stress ratio.
Among the crystal orientations of Example 1, there were five types of crystal orientation numbers that were not used for calculating the principal stress ratio. Therefore, the average dispersed stress ratio (%/seed) was 39.2 (%)/5 (seeds)≈7.8 (%/seed).
Tables 3 and 4 show the values of the average dispersed stress ratio (%/species) of Examples 1-5, Comparative Examples 1-5, and Comparative Examples 7-9.

As shown in Table 4, Comparative Examples 5 and 7 to 9, which were formed by the PR plating method, are multifaceted and have a large number of propagating crystal orientations. The stress ratio showed a large value.

(4) Whisker Length The whisker length was measured on a Ni-plated Cu plate on which a metal plating layer was formed, by a ball indenter method based on JEITA RC-5241 "Whisker Test Method for Connectors for Electronic Devices". In this measurement, three samples were prepared under the same conditions, the maximum whisker length of each sample was measured, and the average was calculated as the whisker length.
The test equipment and conditions used for the test are as follows.

(test equipment)
A load tester that satisfies the specifications stipulated in JEITA RC-5241 “4.4 Load tester” (diameter of zirconia ball indenter: 1mm)
(Test condition)
・Load: 300g
・Test period: 10 days (240 hours)
(Measurement equipment/conditions)
・FE-SEM: Quanta FEG250 (manufactured by FEI)
・Acceleration voltage: 10 kV

As a result of the measurement, when the whisker length is less than 15 μm, the occurrence of whiskers is evaluated as “○”, and when the whisker length is 15 μm or more and 20 μm or less, it is evaluated as “Δ”. Those having a thickness of more than 20 μm were evaluated as “×” because the generation of whiskers could not be suppressed.
The evaluation results are shown below.

In Examples 1 to 5, since the formation conditions of the PR plating method and the direct current plating method are within the predetermined range, the average dispersion stress ratio of the Sn plating layer is 8.4%/type or less, and the growth of whiskers is suppressed. was done. Moreover, the formation time was within 200 seconds, and the formation time could be shortened. Furthermore, in the metal plating layer formed by the formation method according to the present invention, even when the cross section was observed with an SEM, the boundaries between the treatment methods could not be recognized. In addition, since the layers formed by each treatment method were thin, they could not be identified by X-rays. Therefore, although the Sn plating layers of Examples 1 to 5 were formed by two methods, it was found to be a single layer.

On the other hand, in Comparative Examples 1 and 4, since the metal plating layers were laminated by the DC plating method, the average dispersion stress ratio was large and the whisker length could not be reduced. In Comparative Example 2, since the metal plating layer was laminated by the pulse plating method, the average dispersion stress ratio was large and the whisker length could not be sufficiently reduced. In Comparative Example 3, since the metal plating layer was laminated by the AC plating method, the average dispersion stress ratio was large and the whisker length was inferior to that of the Examples.

In Comparative Examples 5, 7 and 8, metal plating layers were laminated by the PR plating method, so that the surfaces were multifaceted and the growth of whiskers was suppressed to some extent. Moreover, the formation time greatly exceeded 200 seconds, and could not contribute to shortening the manufacturing time. In Comparative Example 6, the PR plating method was adopted, but the duty ratio was small and a metal plating layer could not be formed. In Comparative Example 9, the PR plating method was adopted, but the average dispersed stress was large and the whisker length was inferior to that of the Example because the ratio of the current densities was small. In Comparative Example 10, the PR plating method was adopted, but the metal plating layer could not be formed due to the high current density ratio.

Next, based on the results in Table 5, the relationship between the principal stress ratio (%) and the whisker length and the relationship between the average dispersed stress ratio (%/seed) and the whisker length are shown. FIG. 7 is a diagram showing the relationship between the principal stress ratio and the maximum whisker length. As shown in FIG. 7, no clear relationship between the principal stress ratio and whiskers was found, and it was found that the number of propagating crystal orientations must be considered in addition to the principal stress ratio.

FIG. 8 is a diagram showing the relationship between the average dispersed stress ratio and the maximum whisker length. The average distributed stress ratio is the value obtained by dividing the principal stress ratio by the number of propagating crystal orientations. As shown in FIG. 8, it was found that whisker growth was suppressed when the average dispersed stress ratio was 8.4%/seed or less, and whisker growth was observed when it exceeded 8.4%/seed. That is, it was found that the growth of whiskers was not suppressed only by reducing the principal stress ratio, but was suppressed by reducing the average dispersed stress ratio. Moreover, as is clear from the approximation curve in FIG. 8, it was also found that the number of whiskers increases exponentially as the average dispersion stress ratio increases. The approximate curve shown in FIG. 8 is
Maximum whisker length = 8.4e ^{(0.0621 x average dispersion stress ratio)}
Met.

Claims (5)

A method for forming a metal body in which a Ni layer and a Sn plating layer are laminated in this order on a metal substrate,
A Ni layer lamination step of laminating a Ni layer on the metal substrate;
The Ni layer has a current density of a positive current and a reverse current of 1 to 50 A / dm ² , a duty ratio of more than 0.8 and less than 1, and the current density of the positive current and the current density of the reverse current. The ratio of current density _{positive current} : current density _{reverse current} = 1: 0.5 to 1: 3 is applied, and then DC plating is applied with a current density of 1 to 50 A / dm ² . and a Sn-plated layer lamination step of laminating the Sn-plated layer. 2. The method of forming a metal body according to claim 1, wherein said metal substrate is made of a metal containing Cu as a main component. A metal body in which a Ni layer and a Sn plating layer are laminated in this order on a metal substrate,
The metal body, wherein the Sn plating layer has an average dispersion stress ratio represented by the following formula of 8.4%/species or less.
Average distributed stress ratio (%/species) = principal stress ratio (%)/number of crystal orientations propagated (species)
In the above formula, the principal stress ratio (%) is the peak intensity ratio of the crystal orientation showing the maximum peak intensity in the X-ray diffraction spectrum of the Sn plating layer, and the c-axis of the crystal orientation showing the maximum peak intensity. The maximum peak tilt angle, which is the angle formed with the film thickness direction of the Sn plating layer, and the angle formed between the c-axis of the crystal orientation showing the peak intensity other than the maximum peak intensity and the film thickness direction of the Sn plating layer. The non-maximum peak tilt angle, the peak intensity ratio of the crystal orientations whose angle difference is within ± 6 °, and the number of propagating crystal orientations (seeds) is the X-ray diffraction spectrum of the Sn plating layer. , represents the number of peaks not used in the calculation of the principal stress ratio. 4. The metal body according to claim 3, wherein said metal substrate is made of a metal containing Cu as a main component. A fitting-type connection terminal comprising the metal body according to claim 3 or 4.

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