Conductive material and manufacturing method therefor

Abstract

A conductive material having a base material formed from copper or a copper alloy, one or more base layers composed of one or more elements selected from the group consisting of Ni, Co, and Fe, a Cu-Sn alloy layer, and a Sn layer in the order given, wherein the skewness evaluated using a cutoff value of 25 μm in a 250 μm square region of the Sn layer–side surface of the conductive material that contains 50 area% or more of the Sn layer is -3.50 to 3.50.

Description

Conductive material and method for producing the same

[0001] The present disclosure relates to conductive materials and methods for making the same.

[0002] The increasing use and sophistication of electronic controls in automobiles has led to an increase in the number of poles in automotive terminals. This increase in the number of poles also increases the mounting density of terminals, creating an environment in which heat tends to build up (and is difficult to dissipate). Furthermore, since terminals are likely to be used in such environments for long periods of time, the conductive materials used in the terminals must have sufficient heat resistance.

[0003] Patent Document 1 discloses a material in which the arithmetic mean roughness of the material surface is set within a predetermined range when the cutoff value is set to 0.8 mm, and the area ratio of the area where the Cu—Sn alloy layer is exposed to the area where it is not exposed is set within a predetermined range. Patent Document 1 also describes that the increase in contact resistance of the material is suppressed after long-term use at high temperatures.

[0004] Japanese Patent Application Laid-Open No. 2017-115210

[0005] With the recent trend toward autonomous driving and electrification of automobiles, on-board terminals have become increasingly smaller. As terminals become smaller, their spring structures become smaller, resulting in a decrease in terminal contact pressure. Therefore, there is a demand for conductive materials that exhibit high conductivity (i.e., low contact resistance) even at low contact pressure. However, conventional technologies such as those described in Patent Document 1 do not address contact resistance at low contact pressures (<2N), and it has been found that there is room for further improvement.

[0006] The present disclosure has been made in view of the above circumstances, and one of its objectives is to provide a conductive material that has sufficient heat resistance and can sufficiently reduce contact resistance under low contact pressure (<2 N), and a method for manufacturing the same.

[0007] A first aspect of the present invention is a conductive material having, in this order, a base material made of copper or a copper alloy, one or more underlayers composed of at least one type selected from the group consisting of Ni, Co, and Fe, a Cu—Sn alloy layer, and a Sn layer, wherein a skewness evaluated with a cutoff value of 25 μm is −3.50 to 3.50 in a 250 μm square region containing at least 50 area% of the Sn layer on the surface of the conductive material on the Sn layer side.

[0008] A second aspect of the present invention is the conductive material according to the first aspect, wherein an arithmetic mean height, evaluated with a cutoff value of 25 μm, of a 250 μm square region of the surface of the conductive material on the Sn layer side, which region contains the Sn layer at 50 area % or more, is 0.03 μm or more.

[0009] A third aspect of the present invention is the conductive material according to the first or second aspect, wherein a part of the Cu—Sn alloy layer is exposed on a surface of the conductive material on the Sn layer side.

[0010] A fourth aspect of the present invention is the conductive material according to any one of the first to third aspects, wherein the surface of the base material facing the undercoat layer has an arithmetic mean roughness of 0.15 μm or more in at least one direction and an arithmetic mean roughness of 3.0 μm or less in all directions.

[0011] A fifth aspect of the present invention is a method for producing a conductive material, comprising: forming, on a base material made of copper or a copper alloy, one or more underlayers composed of at least one type selected from the group consisting of Ni, Co, and Fe; forming a Cu layer and a Sn layer on the underlayer in this order, and then performing a reflow treatment to obtain a Cu-Sn alloy layer; cooling the reflow treatment to 50°C or less; and performing a matte Sn plating treatment after the cooling to form a matte Sn plating layer having a plating thickness of 0.01 to 0.50 μm, wherein the matte plating treatment is performed within 30 seconds after cooling to 50°C, or is performed after performing an acid pickling treatment using sulfuric acid at a temperature of 40 to 60°C and a concentration of 30 mass% or more.

[0012] A sixth aspect of the present invention is the manufacturing method according to the fifth aspect, further comprising exposing a part of the Cu—Sn alloy layer on the surface of the matte Sn plating layer.

[0013] A seventh aspect of the present invention is the manufacturing method according to the fifth or sixth aspect, wherein the formation of the underlayer is carried out by forming the underlayer on a surface of the base material that has been roughened so that the arithmetic mean roughness in at least one direction is 0.15 μm or more and the arithmetic mean roughness in all directions is 3.0 μm or less.

[0014] According to an embodiment of the present invention, it is possible to provide a conductive material that has sufficient heat resistance and can sufficiently reduce contact resistance under low contact pressure (<2N), and a method for manufacturing the same.

[0015] FIG. 1A is an optical microscope image of the surface of the Sn layer side of the conductive material of Test No. 1. FIG. 1B is an optical microscope image of the surface of the Sn layer side of the conductive material of Test No. 2. FIG. 1C is an optical microscope image of the surface of the Sn layer side of the conductive material of Test No. 3. FIG. 1D is an optical microscope image of the surface of the Sn layer side of the conductive material of Test No. 7. FIG. 2A is a secondary electron (SE) image of the surface of the Sn layer side of the conductive material of Test No. 1 taken by an SEM. FIG. 2B is a backscattered electron (BSE) image of the surface of the Sn layer side of the conductive material of Test No. 1 taken by an SEM. FIG. 3A is a cross-sectional SEM image of the conductive material of Test No. 1 taken parallel to the stacking direction of each layer. FIG. 3B is a cross-sectional SEM image of the conductive material of Test No. 7 taken parallel to the stacking direction of each layer, including a portion where a portion of the Cu—Sn alloy layer is exposed from the surface of the Sn layer side. Figure 3C is a cross-sectional SEM image parallel to the lamination direction of each layer of a portion of the conductive material of Test No. 7 where the Cu—Sn alloy layer is not exposed from the surface of the Sn layer side. Figure 4 is a schematic diagram of the device used to evaluate the dynamic friction coefficient.

[0016] The present inventors have conducted research from various angles in order to realize a conductive material that has sufficient heat resistance and can sufficiently reduce contact resistance under low contact pressure (<2N).

[0017] They then discovered that by using a structure having a base material, an underlayer, a Cu—Sn alloy layer, and an Sn layer in that order, and further evaluating the skewness (hereinafter also referred to as “Ssk”) of −3.50 to 3.50 in a 250 μm square region on the surface of the Sn layer side, where the Sn layer comprises 50% or more by area, with a cutoff value of 25 μm at very narrow intervals (i.e., excluding the effects of irregularities with wider intervals than 25 μm, such as surface irregularities of the base material), it is possible to impart sufficient heat resistance while sufficiently reducing contact resistance even under low contact pressure (<2 N). Here, Ssk is a parameter expressed by the following formula (1), and indicates the degree of bias (skewness) of the histogram of the height distribution on the contoured surface.

[0018]

[0019] In formula (1), Sq represents the root mean square height (i.e., the standard deviation of the height distribution), A represents the evaluation area (i.e., a 250 μm square area), Z represents the height in the evaluation area, and x and y represent each coordinate in the xy plane perpendicular to the Z (height) direction in the evaluation area.

[0020] Ssk is also described in ISO25178-2:2012, and indicates that when Ssk<0, the unevenness is biased toward the convex side (upward) (i.e., the area of ​​the convex portions is larger than the area of ​​the concave portions when viewed from above), and when Ssk>0, the unevenness is biased toward the concave side (downward) (i.e., the area of ​​the concave portions is larger than the area of ​​the convex portions when viewed from above).

[0021] The reason why the above configuration can reduce contact resistance is thought to be as follows: When Ssk is biased toward a positive value, the contact area becomes too small, resulting in an increase in contact resistance. On the other hand, when Ssk is biased toward a negative value, the contact area becomes too large, resulting in an increase in contact resistance without destroying the oxide film that increases contact resistance. Therefore, by controlling Ssk within the above specified range so that the contact area is appropriate, neither too large nor too small, it is thought that contact resistance can be sufficiently reduced even under low contact pressure (<2 N).

[0022] The inventors also discovered that a method for producing the conductive material includes a reflow treatment to form a Cu—Sn alloy layer, followed by cooling to 50°C or below, and then performing a matte Sn plating treatment to form a matte Sn plating layer with a plating thickness of 0.01 to 0.50 μm. The matte plating treatment can be performed within 30 seconds after cooling to 50°C, or after pickling treatment using sulfuric acid at a temperature of 40 to 60°C and a concentration of 30% by mass or higher. This has resulted in the realization of a conductive material that has sufficient heat resistance and can achieve sufficiently low contact resistance under low contact pressure (<2 N). Note that the above mechanism does not limit the technical scope of the embodiments of the present invention.

[0023] The following provides details of each requirement stipulated by the embodiment of the present invention.

[0024] A conductive material according to an embodiment of the present invention comprises, in this order, a base material made of copper or a copper alloy, one or more underlayers composed of at least one element selected from the group consisting of Ni, Co, and Fe, a Cu—Sn alloy layer, and a Sn layer, wherein the skewness, evaluated with a cutoff value of 25 μm, is −3.50 to 3.50 in a 250 μm square region of the surface of the conductive material facing the Sn layer, which region contains at least 50 area percent of the Sn layer. As a result, the conductive material has sufficient heat resistance while achieving sufficiently low contact resistance under low contact pressure (<2 N). Each layer is described in detail below.

[0025] In an embodiment of the present invention, the base material is made of copper or a copper alloy. Examples of materials that can be used for the base material include pure copper, as well as various copper alloys such as Cu-Ni-Si, Cu-Ni-Sn-P, Cu-Fe-P, Cu-Zn, Cu-Cr-Ti-Si, and Cu-Mg.

[0026] The shape of the base material is not particularly limited, and may be, for example, plate-like, strip-like, or processed into the shape of a terminal. The surface shape of the base material is also not particularly limited, and may be flat or uneven. In the manufacturing method of a conductive material according to an embodiment of the present invention described below, from the viewpoint of easily exposing a portion of the Cu—Sn alloy layer from the surface, it is preferable that the arithmetic mean roughness (hereinafter also referred to as "Ra") of the base material's surface on the underlayer side is 0.15 μm or more in at least one direction and 3.0 μm or less in all directions, when the cutoff value is 0.8 μm. The Ra of the base material's surface on the underlayer side can be measured, for example, using a surface roughness meter in accordance with JIS B0601:2001. The Ra of the base material's surface on the underlayer side can also be determined from a cross-sectional image parallel to the stacking direction of each layer of the conductive material. When the base material is plate-like, the thickness is not particularly limited, and may be, for example, 0.05 mm or more and 2 mm or less.

[0027] <Underlayer> The underlayer is one or more layers composed of any one pure metal selected from the group consisting of Ni, Co, and Fe, or two or more alloys. This underlayer prevents Cu and alloy elements in the base material from diffusing to the material surface, suppressing an increase in initial contact resistance and also suppressing an increase in contact resistance after prolonged use at high temperatures. The diffusion of the underlayer itself to the material surface can be suppressed by a Cu—Sn alloy layer, etc., as described below. Furthermore, the formation of the underlayer improves the sulfur dioxide corrosion resistance of the material.

[0028] The average thickness of the underlayer (if there are two underlayers, the average of the total thickness of the two layers) is preferably 0.1 μm or more in order to suppress pit defects in the underlayer and to further demonstrate the above-mentioned effects. On the other hand, in order to reduce costs and ensure moldability, the average thickness of the underlayer is preferably 3.0 μm or less. The average thickness of the underlayer can be measured, for example, using a fluorescent X-ray film thickness meter. The surface shape of the underlayer is not particularly limited, and can reflect, for example, the shape of the surface of the base material on the underlayer side.

[0029] <Cu—Sn alloy layer> The Cu—Sn alloy layer is made of an alloy of Cu and Sn. The Cu—Sn alloy layer is made of, for example, an η phase (Cu ６ Sn５ ) only or ε phase (Cu ３ The Cu—Sn alloy layer may be composed of an ε phase and an η phase. When the Cu—Sn alloy layer is composed of an ε phase and an η phase, the ε phase may be formed between the underlayer and the η phase and may be in contact with the underlayer. Since the ε phase is harder than the η phase, the presence of the ε phase is preferable because it makes the Cu—Sn alloy layer harder and further reduces the friction coefficient. Also, in the case of a Ni underlayer, for example, after reflow treatment, the (Cu, Ni) ６ Sn ５ An alloy layer may be formed.

[0030] Preferably, a portion of the Cu—Sn alloy layer is exposed from the surface of the Sn layer side of the conductive material according to an embodiment of the present invention; that is, the entire surface is not covered by the Sn layer. This reduces the coefficient of dynamic friction of the conductive material. The exposure of a portion of the Cu—Sn alloy layer from the surface of the Sn layer side of the conductive material can be confirmed, for example, by obtaining a backscattered electron (BSE) image of the surface of the Sn layer side of the conductive material using a scanning electron microscope (SEM). Figure 2B shows an example of a BSE image of a conductive material according to an embodiment of the present invention (Test No. 1). In Figure 2B, a relatively dark region due to the Cu—Sn alloy layer 1 and a relatively bright region due to the Sn layer (here, the matte plating layer 2b) are observed. Thus, when a relatively dark region due to the Cu—Sn alloy layer 1 is observed in the BSE image of the surface of the Sn layer side of the conductive material, it can be determined that a portion of the Cu—Sn alloy layer is exposed from the surface of the Sn layer side of the conductive material.

[0031] The area ratio of the exposed portion of the Cu—Sn alloy layer on the surface of the conductive material facing the Sn layer is preferably 10% or more. This allows for a further reduction in the dynamic friction coefficient of the conductive material. On the other hand, the area ratio of the exposed portion of the Cu—Sn alloy layer on the surface of the conductive material facing the Sn layer is preferably 50% or less, more preferably 40% or less. This allows for a further reduction in the contact resistance of the conductive material. The area ratio of the exposed portion of the Cu—Sn alloy layer on the surface of the conductive material facing the Sn layer can be calculated by acquiring the above-mentioned BSE image, binarizing the obtained BSE image using image analysis software (free software ImageJ 1.49), and then analyzing the image.

[0032] The Cu—Sn alloy layer preferably has an average crystal grain size of 30 μm or less when analyzed by electron backscatter diffraction (EBSD). This facilitates reducing unevenness of the surface irregularities. The Cu—Sn alloy layer more preferably has an average crystal grain size of 20 μm or less, even more preferably 10 μm or less, and particularly preferably 5 μm or less, when evaluated by EBSD. The lower limit of the average crystal grain size is not particularly limited, but may be, for example, 0.1 μm or more. Here, the average crystal grain size refers to the average diameter of the circle-equivalent area of ​​the region surrounded by crystal grain boundaries, where the boundary is determined by EBSD analysis to have a crystal orientation misorientation (oblique angle) of more than 15° (also known as a high-angle grain boundary).

[0033] The average thickness of the Cu—Sn alloy layer is preferably 3.0 μm or less in order to reduce costs and ensure formability. On the other hand, from the viewpoint of suppressing diffusion of the materials (Ni, Co, and Fe) of the underlayer into the Sn layer, the average thickness of the Cu—Sn alloy layer is preferably 0.2 μm or more. The average thickness of the Cu—Sn alloy layer can be measured, for example, using a fluorescent X-ray film thickness meter. Specifically, the value of the Sn component in the Cu—Sn alloy layer can be measured using fluorescent X-rays, and the average of the obtained Sn plating thicknesses can be used as the average thickness of the Cu—Sn alloy layer.

[0034] The Cu—Sn alloy layer may be continuous or discontinuous in a plane perpendicular to the stacking direction. If the Cu—Sn alloy layer is discontinuous, the Sn layer as the upper layer may be in direct contact with the lower layer (e.g., the underlayer) in a region where the Cu—Sn alloy layer is not present.

[0035] <Sn Layer> The Sn layer is disposed on the surface of the conductive material. The Cu—Sn alloy layer may be partially exposed from the surface. That is, the Sn layer may be discontinuous on the surface of the conductive material (in a plane perpendicular to the stacking direction).

[0036] The skewness (Ssk) of the conductive material is -3.50 to 3.50 when evaluated in a 250 μm square region containing 50% or more of the Sn layer on the surface of the Sn layer side, with a cutoff value of 25 μm (i.e., excluding the effects of irregularities with wide spacing of more than 25 μm, such as surface irregularities of the base material). This allows for reduced contact resistance under low contact pressure (<2 N). Ssk is preferably -2.50 to 2.50, more preferably -2.00 to 2.00, even more preferably -1.50 to 1.50, even more preferably -1.20 to 1.20, and particularly preferably -1.00 to 1.00. The Ssk can be determined by measurement using a laser microscope in accordance with ISO 25178. Whether the Sn layer comprises 50% or more by area can be confirmed from a BSE image of the surface of the Sn layer side of the conductive material, in the same manner as in the method for determining the area ratio of the exposed portion of the Cu-Sn alloy layer described above.

[0037] In a 250 μm square region on the surface of the Sn layer side of the conductive material, which includes 50% or more of the Sn layer by area, the arithmetic mean height (Sa) evaluated with a cutoff value of 25 μm (i.e., excluding the influence of widely spaced irregularities greater than 25 μm, such as surface irregularities of the base material) is preferably 0.03 μm or greater. This allows for a further reduction in the contact resistance of the conductive material due to factors such as the susceptibility of the oxide film that may form on the surface to destruction. Sa is more preferably 0.05 μm or greater, and even more preferably 0.07 μm or greater. For example, when evaluated with a cutoff value greater than 25 μm, the widely spaced irregularities that are less likely to destroy the oxide film are also reflected in the arithmetic mean height. Therefore, when evaluated with a cutoff value greater than 25 μm, even if the arithmetic mean height is 0.03 μm or greater, the contact resistance of the conductive material may not be fully reduced. The Sa can be determined by measurement using a laser microscope in accordance with ISO 25178. Whether the Sn layer comprises 50% or more by area can be confirmed from a BSE image of the surface of the Sn layer side of the conductive material, in the same manner as in the method for determining the area ratio of the exposed portion of the Cu-Sn alloy layer described above.

[0038] The Sn layer preferably has, in at least one cross section parallel to the stacking direction, convex portions on its surface where H / W is ≧0.15, where H is the height and W is the bottom width. This further reduces the contact resistance of the conductive material. Here, the bottom width W is the width along the surface of the Sn layer, and the height H is the height perpendicular to the surface. The average value of H is preferably 0.1 μm or more. The upper limit of the average value of H is not particularly limited, but may be, for example, 1 μm or less. The average value of W is preferably 5 μm or less, more preferably 3 μm or less. The lower limit of the average value of W is not particularly limited, but may be, for example, 0.1 μm or more. By satisfying these ranges, the contact resistance of the conductive material can be further reduced. For example, if the convex portions of the Sn layer are linearly formed in the surface image of the Sn layer side of the conductive material, a cross-sectional sample can be prepared near the center of the linear convex portions in a direction perpendicular to the linear convex portions and observed horizontally to confirm whether the convex portions have the above-mentioned shape.

[0039] The conductive material according to the embodiment of the present invention may include layers other than the base material, underlayer, Cu—Sn alloy layer, and Sn layer, as long as the object thereof is achieved.

[0040] A method for producing a conductive material according to an embodiment of the present invention includes forming the underlayer on the base material, forming a Cu layer and an Sn layer on the underlayer in this order, and then performing a reflow treatment to obtain the Cu-Sn alloy layer, cooling the base material to 50°C or below after the reflow treatment, and performing a matte Sn plating treatment after the cooling to form a matte Sn plating layer having a plating thickness of 0.01 to 0.50 μm, wherein the matte plating treatment is performed within 30 seconds after the base material is cooled to 50°C, or is performed after pickling treatment using sulfuric acid at a temperature of 40 to 60°C and a concentration of 30% by mass or more. Each step is described in detail below.

[0041] <Forming an Underlayer on a Base Material> A base material made of copper or a copper alloy as described above is prepared, and an underlayer is formed thereon, which is one or more layers composed of at least one element selected from the group consisting of Ni, Co, and Fe. From the perspective of ease of manufacturing, the underlayer is preferably one or two layers selected from the group consisting of a Ni layer, a Co layer, and an Fe layer. The method for forming the underlayer is not particularly limited, and it may be formed by a known method such as plating. In this case, it is preferable to form the underlayer on a base material surface roughened so that the arithmetic mean roughness in at least one direction is 0.15 μm or more and the arithmetic mean roughness in all directions is 3.0 μm or less. This allows the Cu—Sn alloy layer to be exposed from the surface of the conductive material facing the Sn layer, as described below. The base material can be roughened by, for example, rolling (using a work roll roughened by polishing or shot blasting, etc.) or mechanical methods such as polishing or shot blasting. Physical methods such as ion etching or chemical methods such as etching or electropolishing can also be used.

[0042] <Forming a Cu layer and a Sn layer in this order on an underlayer, followed by a reflow treatment to obtain a Cu—Sn alloy layer> The Cu layer and the Sn layer may be formed by a known method, such as plating. By forming the Cu layer and the Sn layer in this order and then performing a reflow treatment, the Cu in the Cu layer and the Sn in the Sn layer interdiffuse to form a Cu—Sn alloy layer. At this time, the Cu layer may disappear entirely, or a portion may remain between the Cu—Sn alloy layer and the underlayer. Similarly, the Sn layer may disappear entirely, or a portion may remain. Note that when the thickness of the Sn plating layer before the reflow treatment (ts) is defined as the thickness of the Cu plating layer (tc), if ts / tc = 2, both the Cu layer and the Sn layer are likely to disappear. Furthermore, when ts / tc < 2, the Cu layer is likely to remain, and when ts / tc > 2, the Sn layer is likely to remain (the remaining Sn layer may be referred to as the "residual Sn layer"). Furthermore, when ts / tc>2, only the η phase can be formed in an equilibrium state, but depending on the reflow treatment conditions, the ε phase, a non-equilibrium phase, can also be formed. It is preferable that ts and tc satisfy ts / tc>2, with tc being 0.1 μm or more and 1.5 μm or less, and ts being 0.35 μm or more and 3.15 μm or less. This allows the Cu—Sn alloy layer to be adjusted to a preferred average thickness (0.2 to 3.0 μm) while forming the ε phase. The reflow treatment conditions are preferably a temperature between the melting point of the Sn plating layer and 600°C for 3 to 30 seconds.

[0043] Here, when a base layer is formed on the above-mentioned roughened base material, the base layer and the Cu—Sn alloy layer may also have irregularities reflecting the shape of the base layer. Furthermore, if a portion of the Sn layer remains after the reflow treatment, the remaining Sn layer may be unevenly distributed in areas corresponding to the recesses of the roughened base material due to molten Sn flowing into the recesses during reflow. In other words, the layer structure in areas corresponding to the protrusions of the roughened base material may be base material / base layer / Cu—Sn alloy layer, and the layer structure in areas corresponding to the recesses of the roughened base material may be base material / base layer / Cu—Sn alloy layer / residual Sn layer.

[0044] <Cooling to 50°C or less after reflow treatment> After reflow treatment, the temperature is cooled to 50°C or less. If the temperature exceeds 50°C, the desired matte plating layer cannot be formed in the next step. The cooling method is preferably to immediately water-cool from the reflow treatment temperature range to 50°C or less, preferably 40°C or less, using cooling water. This allows the crystal grains of the residual Sn layer to be uniformly refined throughout the product. The lower limit of the water temperature is not particularly limited, but can be, for example, 5°C or more.

[0045] <After cooling, a matte plating treatment is performed to form a matte Sn plating layer with a plating thickness of 0.01 to 0.50 μm.> The matte plating treatment is performed within 30 seconds after cooling to 50°C, or after pickling treatment using sulfuric acid at a temperature of 40 to 60°C and a concentration of 30% by mass or more. If the time interval between cooling to 50°C and formation of the matte plating layer is long, oxides and the like may form on the surface, which may hinder the formation of the matte Sn plating layer, and as a result, Ssk within the above range may not be obtained. To obtain Ssk within the above range, the matte Sn plating layer must be formed immediately (specifically, within 30 seconds) after cooling to 50°C, or the matte Sn plating layer must be formed after pickling treatment using sulfuric acid at a temperature of 40 to 60°C and a concentration of 30% by mass or more to sufficiently remove the oxides and the like. Here, the matte Sn plating layer refers to a Sn plating layer formed using a plating bath that does not contain additives such as a brightener that refines crystal grains, and the plating bath is, for example, SnSO ４ and H ２ SO ４ The plating thickness of the matte Sn plating layer is 0.01 to 0.50 μm. If it is less than 0.01 μm, the amount of convex portions formed in the Sn layer will be small, and Ssk may become less than −3.50. On the other hand, if it exceeds 0.50 μm, the convex portions of the Sn layer will bond together to form large, gentle convex portions, and Ssk may become more than 3.50.

[0046] The plating thickness of the matte Sn plating layer is preferably 0.025 μm or more, more preferably 0.05 μm or more, and even more preferably 0.07 μm or more. The plating thickness of the matte Sn plating layer is preferably 0.25 μm or less, more preferably 0.20 μm or less, and even more preferably 0.15 μm or less. This allows a Sn layer having fine convex portions to be obtained, and in a 250 μm square region containing 50 area% or more of the Sn layer, the arithmetic mean height evaluated with a cutoff value of 25 μm can be 0.03 μm or more, more preferably 0.05 μm or more, and even more preferably 0.07 μm or more.

[0047] Here, when an underlayer is formed on the above-mentioned roughened base material, the matte Sn plating layer can grow on the residual Sn layer that can be present in the areas corresponding to the recesses of the roughened base material. In other words, the layer structure in the areas corresponding to the protrusions of the roughened base material can be base material / underlayer / Cu-Sn alloy layer, and the layer structure in the areas corresponding to the recesses of the roughened base material can be base material / underlayer / Cu-Sn alloy layer / Sn layer (i.e., residual Sn layer + matte Sn plating layer).

[0048] <Exposing a Portion of the Cu—Sn Alloy Layer on the Sn Layer-Side Surface of the Conductive Material> The method for manufacturing a conductive material according to an embodiment of the present invention preferably further includes exposing a portion of the Cu—Sn alloy layer on the Sn layer-side surface of the conductive material. Various methods can be used to expose a portion of the Cu—Sn alloy layer on the Sn layer-side surface of the conductive material. For example, by forming an underlayer on the roughened base material described above, a portion of the Cu—Sn alloy layer can be exposed on the Sn layer-side surface of the conductive material at locations corresponding to the convex portions of the roughened base material. Even when the underlayer is formed on a flat base material, a portion of the Cu—Sn alloy layer can be exposed by roughening the underlayer in the same manner as the base material. Furthermore, as described in, for example, JP 2013-174006 A, a Cu—Sn alloy layer with a steep profile can be formed by adjusting the elements of the underlayer and the reflow treatment. Alternatively, after forming the base material / underlayer / Cu--Sn alloy layer / Sn layer, part of the Sn layer can be mechanically removed to expose part of the Cu--Sn alloy layer.

[0049] The method for producing a conductive material according to an embodiment of the present invention may include other steps within the scope in which the object of the method is achieved.

[0050] The following examples are provided to more specifically describe the embodiments of the present invention. The embodiments of the present invention are not limited to the following examples, and may be modified as appropriate within the scope of the above-described and below-described aims, and all such modifications are within the technical scope of the embodiments of the present invention.

[0051] The base material used was a copper alloy plate (Cu-Ni-Sn-P system) with a thickness of 0.25 mm, which had been rolled with a roughened roll. The average arithmetic roughness Ra of the base material rolled with the roughened roll was 0.48 μm in one direction and 0.49 μm in all directions. The surface roughness (arithmetic mean roughness Ra) of the base material was measured in accordance with JIS B0601:2001 using a contact surface roughness meter (Tokyo Seimitsu Co., Ltd.; Surfcom 1400). The surface roughness measurement conditions were a cutoff value of 0.8 mm, a reference length of 0.8 mm, an evaluation length of 4.0 mm, a measurement speed of 0.3 mm / s, and a stylus tip radius of 5 μmR. The surface roughness measurement direction was perpendicular to the rolling direction (which may be the direction in which the surface roughness is calculated to be greatest).

[0052] Next, a Ni plating layer was formed as a base layer on the surface of the roughened base material by a known method so as to have an average thickness of 0.3 μm. Specifically, the Ni plating solution used before the reflow treatment was that described in JP-A-2004-68026, and the plating conditions were a current density of 5 A / dm ２ The bath temperature was set to 60°C.

[0053] Furthermore, on the underlayer, a Cu plating layer and a Sn plating layer were formed in this order by a known method. Here, the thickness (tc) of the Cu plating layer and the thickness (ts) of the Sn plating layer before the reflow treatment were set to tc: 0.15 μm and ts: 0.9 μm so that the Cu—Sn alloy layer would have a preferred average thickness (0.2 to 3.0 μm) while forming the ε phase. Specifically, the Cu plating solution and the Sn plating solution used were those described in JP 2004-68026 A, and the plating conditions were as follows: for Cu plating, a current density of 3.5 A / dm ２ The bath temperature was 35°C, and the current density for Sn plating was 3.0 A / dm ２ The bath temperature was set to 35° C. Thereafter, a reflow treatment was carried out at a temperature of 232° C. or higher (substantial temperature) at which Sn melts.

[0054] After the reflow treatment, the material was water-cooled to 10 to 30°C. After water-cooling, the material was pickled using sulfuric acid at a temperature of 50°C and a concentration of 30 mass% or more. After the pickling treatment, a matte Sn plating treatment was performed to form a matte Sn plating layer, and a conductive material of Test No. 1 was obtained. Specifically, the matte Sn plating layer was formed using SnSO ４ (80 g / L) and H ２ SO ４ (80 g / L) was prepared, and the current density was 3 A / dm ２ The conductive material of Test No. 1 was subjected to electrical conduction for 2 to 8 seconds at a temperature of 1000 K. Here, the average Sn layer thickness before and after matte Sn plating was measured using a fluorescent X-ray film thickness meter, and the increase in the average thickness of the matte Sn plating layer was determined to be 0.10 μm.

[0055] The conductive material of Test No. 1 was prepared by changing the thickness of the matte Sn plating layer to prepare Test No. 2 (matte Sn plating layer 0.05 μm) and Test No. 3 (matte Sn plating layer 0.025 μm).

[0056] The base material of the conductive material of Test No. 1 was changed to a material obtained by rolling a 0.20 mmt copper alloy plate (Cu—Ni—Sn system) with a flat roll, to prepare the conductive material of Test No. 4.

[0057] The base material of the conductive material of Test No. 1 was changed to a material obtained by rolling a 0.80 mmt copper alloy plate (Cu—Fe—P system) with a flat roll, thereby producing a conductive material of Test No. 5.

[0058] The conductive material of Test No. 6 was produced from the conductive material of Test No. 1 by changing the process so that the pickling treatment was not performed, but rather the matte Sn plating treatment was performed within 30 seconds after the reflow treatment and cooling to 50°C.

[0059] Test No. 7 was prepared by removing the matte Sn plating layer from the conductive material of Test No. 1.

[0060] The conductive material of Test No. 8 was produced by changing the base material of the conductive material of Test No. 1 to a material obtained by rolling a 0.20 mmt copper alloy plate (Cu—Ni—Sn system) with a flat roll, and by not forming an underlayer.

[0061] The matte Sn plating layer of the conductive material of Test No. 1 was changed to a bright Sn plating layer to prepare a conductive material of Test No. 9. The bright Sn plating layer was made of SnSO ４ (80 g / L) and H ２ SO ４ A plating bath containing 80 g / L of copper and 35 g / L of brightener was prepared at a current density of 3 A / dm ２ The film was formed by applying current for 2 to 8 seconds.

[0062] The base material of the conductive material of Test No. 1 was changed to a material obtained by rolling a 0.20 mmt copper alloy plate (Cu—Ni—Sn system) with a flat roll, and no underlayer was formed, thereby producing the conductive material of Test No. 10.

[0063] When the surfaces of Test Nos. 1 to 10 after reflow treatment (before the formation of the Sn plating layer) were analyzed by EBSD, the average crystal grain size was all 5 μm or less (and 0.1 μm or more).

[0064] The surfaces of the Sn layer side of the conductive materials of Test Nos. 1 to 10 were observed using an optical microscope. As an example, FIGS. 1A to 1C show optical microscope images of the conductive materials of Test Nos. 1 to 3, respectively, on which a matte Sn plating layer was formed, and FIG. 1D shows an optical microscope image of the conductive material of Test No. 7, which does not have a matte Sn plating layer. As shown in FIG. 1D, the conductive material of Test No. 7 has a flat surface due to the absence of a matte Sn plating layer. Note that in FIG. 1D, the relatively dark areas are the Cu—Sn alloy layer 1, and the relatively light areas are the residual Sn layer 2a. Compared to the conductive material of Test No. 7 in FIG. 1D, the conductive materials of Test Nos. 1 to 3 in FIGS. 1A to 1C have a matte Sn plating layer 2b, indicated by the darker areas, and are therefore found to have greater surface roughness.

[0065] The surfaces of the Sn layer side of the conductive materials of Test Nos. 1 to 10 were observed using an SEM. As an example, FIG. 2A shows a secondary electron (SE) image of the surface of the Sn layer side of the conductive material of Test No. 1, and FIG. 2B shows a backscattered electron (BSE) image. The SE image in FIG. 2A reveals that the surface of the Sn layer side of the conductive material has at least a matte Sn plating layer 2b with fine protrusions and a flat Cu—Sn alloy layer 1. The BSE image in FIG. 2B more clearly reveals that a portion of the Cu—Sn alloy layer 1 (a relatively dark region) is exposed on the surface of the Sn layer side of the conductive material. Similarly, Test Nos. 2 to 4, 6 to 7, and 9 also confirmed that a portion of the Cu—Sn alloy layer 1 was exposed on the surface of the Sn layer side of the conductive material, but no exposure of the Cu—Sn alloy layer 1 was confirmed in Test Nos. 5, 8, and 10. This is consistent with Test Nos. 2 to 4, 6 to 7, and 9. This is thought to be because the manufacturing methods of the conductive materials of Test Nos. 5, 8, and 10 did not include a means for exposing a portion of the Cu—Sn alloy layer 1, such as roughening the base material, unlike Test Nos. 1 to 4, 6 to 7, and 9. In addition, the entire surface of the Sn layer side of the conductive materials of Test Nos. 1 to 10 was observed using an SEM, and it was confirmed that the area ratio of the Sn layer was 50 area % or more throughout the entire surface (i.e., in any 250 μm square region).

[0066] Cross sections parallel to the lamination direction of each layer of the conductive materials of Test Nos. 1 to 10 were observed using an SEM. As an example, FIG. 3A shows a cross-sectional SEM image of the conductive material of Test No. 1, FIG. 3B shows a cross-sectional SEM image of a portion of the conductive material of Test No. 7 where a portion of the Cu—Sn alloy layer 1 is exposed from the surface, and FIG. 3C shows a cross-sectional SEM image of a portion of the conductive material of Test No. 7 where the Cu—Sn alloy layer 1 is not exposed from the surface. As shown in FIGS. 3B and 3C, the conductive material of Test No. 7 has a surface on the Sn layer side that is composed of the Cu—Sn alloy layer 1 and the residual Sn layer 2a, or the residual Sn layer 2a, and it can be seen that the surface is flat. On the other hand, as shown in FIG. 3A, in Test No. It can be seen that the conductive material No. 1 has a matte Sn plating layer 2b on its surface and forms convex portions with a ratio H / W≧0.15, where H is the height and W is the width at the bottom (as an example, FIG. 3A shows one convex portion with H (≈0.53) and W (≈1.33), and it can be seen that the H / W ratio for this convex portion is ≈0.40). It can also be seen from FIG. 3A that the convex portions resulting from the matte Sn plating layer 2b grow not on the Cu—Sn alloy layer 1 but on the residual Sn layer 2a. Furthermore, in FIGS. 3A to 3C, a base layer 3 is confirmed below the Cu—Sn alloy layer 1. At least for the conductive material No. 1, the average value of H was within the range of 0.1 to 1 μm and the average value of W was within the range of 0.1 to 3 μm.

[0067] The conductive materials of Test Nos. 1 to 10 were further evaluated as follows.

[0068] <Evaluation of Skewness (Ssk)> Using a laser microscope (Olympus Corporation; OLS-4100), Ssk was measured on the surface on the Sn layer side of the conductive materials of Test Nos. 1 to 10 based on ISO 25178. The surface roughness measurement conditions were a cutoff value of 25 μm and an evaluation area of ​​250 μm square.

[0069] <Evaluation of arithmetic surface height (Sa)> Using a laser microscope (Olympus Corporation; OLS-4100), Sa of the surface on the Sn layer side of the conductive materials of Test Nos. 1 to 10 was measured based on ISO 25178. The surface roughness measurement conditions were a cutoff value of 25 μm and an evaluation area of ​​250 μm square.

[0070] <Contact Resistance Evaluation> The contact resistance of the conductive materials in Tests No. 1 to 6 was evaluated using an electrical contact simulator (manufactured by Yamazaki Seiki Kenkyusho). The contact resistance measurement probe in this device is a gold wire. However, in this test, a separate probe material (Cu-Ni-based base material (0.2 mm) / Cu-Sn alloy layer (0.3 μm) / Sn layer (0.7 μm)) was prepared, and a jig was created to attach a female test piece, a hemispherical processed material (outer diameter: 0.55 mm). The female test piece was used as the contact resistance measurement probe, allowing it to contact the surface of the Sn layer side of the male test piece, which was a plate material cut from each conductive material. Three measurements were then performed using the four-terminal method under conditions of an open circuit voltage of 20 mV, a current of 10 mA, and a load of 1 N, and the average value was used as the contact resistance value. The male test piece was also heated in air at 160°C for 120 hours, and the contact resistance was measured before and after heating. The evaluation criteria for initial contact resistance were as follows: AA (very excellent): 2.3 mΩ or less, A (excellent): 2.4 to 6.3 mΩ, B (sufficient): 6.4 to 6.8 mΩ, C (insufficient): 6.9 mΩ or more; and for contact resistance after heating: AA (very excellent): 2.9 mΩ or less, A (excellent): 3.0 to 6.9 mΩ, B (sufficient): 7.0 to 9.9 mΩ, C (insufficient): 10.0 mΩ or more.

[0071] <Evaluation of Dynamic Friction Coefficient> The shape of the indented portion of an electrical contact in a mating connection component was simulated, and the dynamic friction coefficients of the conductive materials of Test Nos. 1 to 6 were evaluated using an apparatus such as that shown in FIG. 4. As shown in FIG. 4, male test pieces 10 made of plate material cut from each conductive material were fixed to a horizontal table 11, and female test pieces 12 made of hemispherical processed material (outer diameter: 0.55 mm) of the above-mentioned probe material were placed on top of them, with the Sn layer sides of each test piece brought into contact with each other. Next, a 1.0 N load (weight 13) was applied to the female test piece 12 to hold down the male test piece 10, and the maximum friction force (unit: N) was measured when the male test piece 10 was slid horizontally once using a horizontal load measuring device (Aiko Engineering Co., Ltd.; Model-2152). The sliding distance was 5 mm, and the sliding speed was 80 mm / min. In Figure 4, 14 indicates a load cell, the arrow indicates the sliding direction, and for both the male test piece 10 and the female test piece 12, the rolling perpendicular direction was parallel to the sliding direction. When the normal force (load) is P (= 1.0 N) and the maximum friction force is F, the dynamic friction coefficient μ' is expressed as μ' = F / P. Measurement tests were conducted four times for each conductive material, and the average value of the dynamic friction coefficients was calculated and used as the dynamic friction coefficient of the conductive material. The evaluation criteria for the dynamic friction coefficient were AA (very excellent): 0.50 or less, A (excellent): 0.51 to 0.64, B (sufficient): 0.65 to 0.74, and C (insufficient): 0.75 or more.

[0072] The above results are summarized in Table 1.

[0073]

[0074] The results in Table 1 can be considered as follows: All of the conductive materials in Test Nos. 1 to 6 satisfied the requirements defined in the embodiment of the present invention, possessing sufficient heat resistance (contact resistance after heating of 9.9 mΩ or less), while also achieving sufficiently low contact resistance (initial contact resistance of 6.8 mΩ or less) under low contact pressure (<2 N), and also possessing a sufficiently low dynamic friction coefficient (0.74 or less). On the other hand, all of the conductive materials in Test Nos. 7 to 10 did not satisfy the requirements defined in the embodiment of the present invention, and the initial contact resistance and / or contact resistance after heating were insufficient.

[0075] The conductive material of Test No. 7 did not have a matte Sn plating layer formed thereon, and the Ssk was greater than 3.50, resulting in insufficient initial contact resistance.

[0076] The conductive material of Test No. 8 did not have an underlayer formed thereon, and the contact resistance after heating was insufficient.

[0077] The conductive material of Test No. 9 had a bright Sn plating layer formed instead of a matte Sn plating layer, and Sska exceeded 3.50, resulting in insufficient initial contact resistance. Perhaps due to this influence, the contact resistance after heating was also insufficient.

[0078] The conductive material of Test No. 10 did not have an underlayer formed thereon, and the initial contact resistance and the contact resistance after heating were insufficient.

[0079] The results of the initial contact resistance and the contact resistance after heating will be further considered. Test Nos. 1, 2, and 4 to 6 satisfied the preferable conditions of -1.20≦Ssk≦1.20, or -2.50≦Ssk≦2.50 and Sa≧0.05, and therefore the contact resistance could be further reduced, and specifically, both the initial contact resistance and the contact resistance after heating were excellent. Test Nos. 1 and 4 to 6 satisfied the more preferable conditions of -1.00≦Ssk≦1.00, or -2.00≦Ssk≦2.00 and Sa≧0.07, and therefore the contact resistance could be further reduced, and specifically, either one of the initial contact resistance or the contact resistance after heating was extremely excellent. Test No. Nos. 1 and 4 satisfied the more preferable conditions of -1.00≦Ssk≦1.00 and Sa≧0.07, and therefore the contact resistance could be further reduced, and specifically, both the initial contact resistance and the contact resistance after heating were very excellent.

[0080] The results of the dynamic friction coefficient will be further considered. Tests Nos. 1 to 3 and 6 satisfied the preferable requirement that a portion of the Cu—Sn alloy layer be exposed on the surface of the Sn layer side of the conductive material, and therefore had very excellent dynamic friction coefficients.

[0081] This application claims priority from Japanese Patent Application No. 2024-028951, filed February 28, 2024. Japanese Patent Application No. 2024-028951 is incorporated herein by reference.

[0082] REFERENCE SIGNS LIST 1 Cu—Sn alloy layer 2 Sn layer 2a Residual Sn layer 2b Matte Sn plating layer 3 Undercoat layer 10 Male test piece 11 Horizontal base 12 Female test piece 13 Weight 14 Load cell

Claims

1. A conductive material comprising, in this order, a base material made of copper or a copper alloy, one or more underlayers composed of at least one type selected from the group consisting of Ni, Co, and Fe, a Cu-Sn alloy layer, and a Sn layer, wherein the skewness, evaluated with a cutoff value of 25 μm, is -3.50 to 3.50 in a 250 μm square region of the surface of the conductive material on the Sn layer side, which region contains at least 50 area% of the Sn layer.

2. The conductive material according to claim 1, wherein the arithmetic mean height, evaluated with a cutoff value of 25 μm, of the surface of the conductive material on the Sn layer side is 0.03 μm or more in a 250 μm square region containing 50 area % or more of the Sn layer.

3. The conductive material according to claim 1 or 2, wherein a portion of the Cu-Sn alloy layer is exposed on the surface of the conductive material on the Sn layer side.

4. The conductive material according to claim 1 or 2, wherein the surface of the base material facing the undercoat layer has an arithmetic mean roughness of 0.15 μm or more in at least one direction and an arithmetic mean roughness of 3.0 μm or less in all directions.

5. A method for producing a conductive material, comprising: forming, on a base material made of copper or a copper alloy, one or more underlayers composed of at least one type selected from the group consisting of Ni, Co, and Fe; forming a Cu layer and an Sn layer on the underlayer in this order, and then performing a reflow treatment to obtain a Cu-Sn alloy layer; cooling to 50°C or less after the reflow treatment; and performing a matte Sn plating treatment after the cooling to form a matte Sn plating layer having a plating thickness of 0.01 to 0.50 μm, wherein the matte plating treatment is performed within 30 seconds after cooling to 50°C, or is performed after a pickling treatment using sulfuric acid at a temperature of 40 to 60°C and a concentration of 30 mass% or more.

6. The method for producing a conductive material according to claim 5, further comprising exposing a portion of the Cu-Sn alloy layer on the surface of the matte Sn plating layer.

7. A manufacturing method according to claim 5 or 6, wherein the formation of the underlayer is carried out by forming the underlayer on the surface of the base material that has been roughened so that the arithmetic mean roughness in at least one direction is 0.15 μm or more and the arithmetic mean roughness in all directions is 3.0 μm or less.

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