JP2008106290A - Electrical contact member - Google Patents

Abstract

An electrical contact member capable of suppressing the generation of spontaneously generated whiskers and external stress-type whiskers even when Sn plating is performed.
An electric contact member in which a Sn or Sn alloy plating film is coated on the surface of a metal substrate 3 by plating, and is a substance different from a matrix metal, in which internal stress is relaxed and Sn atoms are precipitated in the plating film. The hollow portions 8 formed by the above are dispersed.
[Selection] Figure 7

Description

  The present invention relates to an electrical contact member applied to terminals, connectors, switches, relays, lead frames and the like, and more particularly to an electrical contact member having a structure in which a surface of a metal substrate is coated with a Sn or Sn alloy plating film.

  In various recent electronic devices on which a plurality of printed wiring boards are mounted, flexible flat cables (hereinafter referred to as FFC) are widely used for connection between the printed wiring boards, and each terminal of the FFC is a printed wiring board. Connected to the side connector.

  The terminal (fine conductor) of the FFC is formed of a metal such as copper, but eutectic solder plating is applied to the surface of the terminal in order to ensure corrosion resistance, low electrical contact resistance characteristics, and the like. By the way, since eutectic solder contains harmful lead, in recent years, Sn (including Sn-based alloy) plating treatment that does not contain lead has been performed due to problems such as environmental pollution.

  However, when lead-free Sn plating is performed, whisker-like single crystals called whiskers are easily generated on the plating film surface. For this reason, when a whisker is generated at the terminal of the FFC, a problem occurs in which adjacent terminals are short-circuited.

For this reason, a technique has been proposed in which the generation of whiskers can be suppressed even when Sn plating is performed (for example, Patent Document 1). In the invention of Patent Document 1, a copper alloy containing 0.1 to 10 mass% of Zn has a plating thickness of 0.5 to 2 μm, a surface reflectance of 30% or more, and a C amount during plating of 0.05. The occurrence of whiskers is suppressed by applying electro-gloss Sn plating in which ~ 1 mass%, the crystal grain size of plating is 0.1 to 1 µm, and the orientation index of the (101) plane of plating is 2.0 or less.
JP 2002-266095 A

  By the way, the whisker includes a so-called spontaneously generated whisker caused by volume expansion accompanying the growth of an intermetallic compound layer during Sn plating, and, for example, a mechanical connection (insertion, pressing, insertion, caulking, etc.) between a terminal and a connector. There are two types of so-called external stress type whiskers that are caused by the above-mentioned phenomenon. In the invention of the above-mentioned Patent Document 1, there is an effect of suppressing the generation of spontaneously generated whiskers, but external stress type whiskers having different generation mechanisms. The effect of suppressing the occurrence cannot be obtained.

  According to the study by the present inventors, it has been found that the external stress type whiskers are generated by the following mechanism. That is, for example, when an external stress is applied to the surface layer of the terminal by mechanical joining in which the terminal is inserted into the connector, Sn atoms pushed into the valleys of the uneven intermetallic compound layer by the compressive stress induced by the external stress. However, whisker is generated by diffusing along the unevenness gradient and growing in a single crystal form from cracks or pinholes in the oxide film (SnOx) in an amorphous state.

  Therefore, an object of the present invention is to provide an electrical contact member that can suppress the occurrence of spontaneously generated whiskers and external stress-type whiskers even when Sn plating is performed.

  In order to achieve the above object, according to the first aspect of the present invention, the cavity formed by a substance different from the matrix metal, which relieves internal stress and precipitates Sn atoms, is dispersed in the plating film. Features.

  The invention described in claim 2 is characterized in that particles formed of a substance different from the matrix metal that relieves internal stress and precipitates Sn atoms are dispersed in the plating film.

  The invention according to claim 3 is characterized in that the substance is a gas.

  The invention according to claim 4 is characterized in that the substance is a polymer material.

  According to the present invention, by compressing internal stress and compressing stress caused by the intermetallic compound layer by dispersing cavities or particles formed of a material different from the matrix metal on which Sn atoms are deposited, in the plating film. Alternatively, it is possible to relieve the compressive stress caused by external stress and to avoid the specific induction path of the diffused Sn atoms even when Sn atoms diffuse from the intermetallic compound layer due to the compressive stress induced by the external stress. . Therefore, it is possible to satisfactorily suppress the generation of spontaneously generated whiskers and external stress type whiskers.

  Hereinafter, the present invention will be described based on the illustrated embodiments. FIG. 1 is a plan view showing the vicinity of a terminal of an FFC (flexible flat cable) as an electrical contact member according to the present invention, and FIG. 2 is a cross-sectional view showing the terminal of the FFC.

  As shown in FIG. 1, a plurality of strip-like terminals 2 to be inserted into connectors (not shown) on the printed wiring board side are arranged at predetermined intervals at the end of the FFC 1. As shown in FIG. 2, the FFC terminal 2 is provided with an intermediate layer 4 on a metal substrate 3 by plating or the like, and further Sn or Sn-based alloy (Sn—Ag, Sn—Bi) on the intermediate layer 4. The surface layer 5 formed by plating (matrix metal such as Sn-Zn) is laminated. The intermediate layer 4 is formed by Sn or Sn-based alloy plating in which minute cavities (hereinafter also including voids and cracks) are dispersed, or eutectoid plating containing soft fine particles.

  The metal substrate 3 is made of a metal such as Cu, Fe, or Al or an alloy thereof. Note that there are no clear interfaces in the vicinity of the interfaces between the metal substrate 3 and the intermediate layer 4 and between the intermediate layer 4 and the surface layer 5 because mutual electrons are in a diffused state.

  The intermediate layer 4 is mainly made of Sn or an Sn alloy by plating. In the intermediate layer 4, a plurality of soft fine particles 4 a and fine cavities (FIG. 4) formed of a material different from the matrix metal. Are distributed. A polymer material can be used as the substance that forms the soft microparticles 4a, and a gas can be used as the substance that forms the fine cavity. The polymer material forming the soft microparticles 4a can secure the electrical conductivity of the entire terminal 2.

FIG. 3 is a diagram showing the distribution of hardness with respect to the distance (thickness direction) from the surface of the metal base 3, the intermediate layer 4, and the surface layer 5 of the terminal 2. As shown in this figure, the hardness of the metal substrate 3 is greater than that of the intermediate layer 4. In the present embodiment, the intermediate layer 4 has a distribution in the thickness direction such as the hardness distribution A1 or the hardness distribution A2.
That is, conventionally, a hardness distribution such as AB-C1-DE is shown with the horizontal axis representing hardness and the vertical axis representing distance from the surface. In the embodiment of the present invention, however, AB-C2 A hardness distribution such as -DE or AB-C3-DE is shown.

  As the intermediate layer 4 having the hardness distribution A1, all polymer materials such as polystyrene and polyacryl can be employed. The intermediate layer 4 having the hardness distribution A2 has substantially the same hardness as that of the surface layer 5, and has fine cavities (including voids and cracks) in the uneven portions of the hardness distribution A2. This uneven portion (hollow portion) is more easily deformed than the periphery of the intermediate layer 4.

  As shown in FIG. 4, the intermediate layer 4 of the terminal 2a has a fine cavity 6 as described above, for example. The hollow portion 6 of the intermediate layer 4 is softer and less dense than its surroundings. For this reason, as shown in FIG. 5, when an external stress F is applied to the surface layer 5 of the terminal 2 a by mechanical joining in which the terminal 2 a is inserted into the connector, the external stress F is relaxed by the cavity 6 of the intermediate layer 4. The Further, the internal stress due to volume expansion accompanying the growth of the intermetallic compound layer at the interface between the metal substrate 3 and the intermediate layer 4 is also relieved by the cavity 6 of the intermediate layer 4.

  Furthermore, as described in the generation mechanism of the external stress type whisker described above, Sn atoms pushed into the valleys of the concavo-convex intermetallic compound layer by the compressive stress induced by the external stress F become the concavo-convex gradient. The diffused Sn atoms are confined in the cavity 6 before growing in a single crystal form from cracks or pinholes in the oxide film (SnOx) in an amorphous state. By these, generation | occurrence | production of a naturally generated type whisker and an external stress type whisker can be suppressed favorably.

(Example 1)
FIG. 6 is a cross-sectional view illustrating the terminal 2b of the FFC as the electrical contact member according to the first embodiment. In this example, the intermediate layer 4 was produced on the metal substrate 3 by porous plating.

  In the porous plating in this embodiment, an anti-pit agent such as sodium lauryl sulfate is not added to the electrolytic solution for plating, but a required amount of a lower alcohol having a valence of 2 or less such as ethylene glycol or propylene glycol is added. Thereby, the particle | grains which the alcohol molecule aggregated in the environment of high metal concentration of electrolyte solution adheres to the metal base | substrate 3, and the intermediate | middle layer 4 which has several crack 7 (cavity part) is produced. Another method is to dispose hydrophobic fine particles such as resin on the surface of the metal substrate in advance so that overvoltage is generated at the boundary between the conductive part and the insulating part, and fine hydrogen gas bubbles are generated. A stress relaxation layer having a plurality of bubbles can be produced by generating a large number and precipitating by Sn or Sn alloy plating so as to take in these bubbles.

In this case, a surfactant such as formic acid, acetic acid and acrylic acid can be used to obtain a high surface tension. The plating solution composition was prepared by mixing 50 to 100 g / L of stannous sulfate and 140 g / L of sulfuric acid, and appropriately adding a desired surfactant. The electrodeposition conditions were a temperature of 35 ° C. and a current density of ² to 10 A / dm ² .

(Example 2)
FIG. 7 is a cross-sectional view illustrating an FFC terminal 2c as an electrical contact member according to the second embodiment. In this example, the intermediate layer 4 was produced on the metal substrate 3 by plating containing nanobubbles.

  In this example, nanobubbles 8 (hollow part) having a diameter of 100 to 500 nm were taken into the intermediate layer 4 and were performed in two stages of a first plating bath and a second plating bath. In the present embodiment, as a first step, microbubbles having a diameter of 10 to 30 μm (fine bubbles) are given a sufficient swirl speed (200 rps) to the gas-liquid two-phase fluid in the first plating bath. A liquid spreads around by the centrifugal separation action, and this swirling gas cavity is cut and pulverized.

  Then, as a second step, a physical stimulus is applied to the microbubbles by an ultrasonic method or an ultrahigh-speed fluid method, thereby instantaneously adiabatically compressing the microbubbles. Since the obtained nanobubbles have a bubble diameter reduced to 100 to 500 nm, the charge density on the bubble surface is increased and an electrostatic repulsive force is generated. For this reason, even when the bubbles are dense, they exist independently and do not grow into large bubbles.

And the hydroxyl radical (OH ^{< - >} ) and hydrogen radical (H ^<+> ) which the water molecule ionized adsorb | sucks to the surface of a bubble. When an electric field is applied in this state, nanobubbles are taken into the cathode part together with metal ions. And the intermediate | middle layer 4 which has several nanobubble 8 (cavity part) is produced by performing Sn plating continuously in a 2nd plating bath. The plating solution composition was prepared by mixing 50 to 100 g / L of stannous sulfate and 140 g / L of sulfuric acid and adding a desired surfactant as appropriate. The electrodeposition conditions were a temperature of 35 ° C. and a current density of ² to 10 A / dm ² .

(Example 3)
FIG. 8A is a cross-sectional view showing an FFC terminal 2d as an electrical contact member according to the third embodiment, and FIG. 8B is a partially enlarged view showing the intermediate layer 4. FIG. In this example, the stress relaxation layer 4 was formed on the metal substrate 3 by Sn eutectoid plating containing soft fine particles.

  Soft microparticles made of a polymer material such as polyethylene, polystyrene, and polyamide may be dispersed in a predetermined surfactant and dispersed in the Sn plating solution, or the surface of the microparticles may be activated in advance. Alternatively, particles subjected to metal film treatment in complex metal ions may be used. If a positive charge is given to these fine particles, Sn eutectoid plating can be easily performed.

As the surfactant, a 28% aqueous solution of stearyltrimethylammonium chloride or a 20% aqueous solution of polydiallyldimethylammonium chloride is used, and polyamide fine particles having a diameter of 0.05 to 0.1 μm are weight (%) of 10 to 70 g / L. After being dispersed in the Sn plating solution, the intermediate layer 4 was prepared by electrodeposition at a current density of ² to 10 A / dm ² . As a result, as shown in FIG. 8B, the ultrathin Sn film 10 is coated on the surface of the microparticles 9. And the surface layer 5 is produced on the intermediate | middle layer 4 by performing Sn plating in the plating bath which does not contain the said microparticle.

  In the present embodiment, the Sn film 10 is coated on the surface of the microparticles 9 so that the diffusion of the metal substrate 3 can be prevented. Thereby, formation of the intermetallic compound layer at the interface between the metal substrate 3 and the intermediate layer 4 can be significantly suppressed.

  Also in Examples 1 to 3, generation of the above-described spontaneously generated whiskers and external stress-type whiskers could be satisfactorily suppressed.

  The present invention can be similarly applied to electrical contact members such as connectors, switches, relays, lead frames and the like in addition to the terminals described above.

The top view which shows the terminal vicinity of FFC as an electrical contact member which concerns on embodiment of this invention. Sectional drawing which shows the terminal of FFC which concerns on embodiment of this invention. The figure which showed distribution of the hardness with respect to the thickness direction in the metal base | substrate of a terminal, an intermediate | middle layer, and a surface layer. Sectional drawing which shows the terminal with which the cavity part was formed in the intermediate | middle layer. The figure which shows the state by which the Sn atom diffused in the cavity part of an intermediate | middle layer is confined when external stress is added. Sectional drawing which shows the terminal of FFC as an electrical contact member which concerns on Example 1. FIG. Sectional drawing which shows the terminal of FFC as an electrical contact member which concerns on Example 2. FIG. (A) is sectional drawing which shows the terminal of FFC as an electrical contact member which concerns on Example 3, (b) is a partially expanded view which shows a stress relaxation layer.

Explanation of symbols

1 FFC
2, 2a, 2b, 2c, 2d terminal (electrical contact member)
3 Metal substrate 4 Intermediate layer 4a, 9 Fine particles 5 Surface layer (plating film)
6, 8 Cavity 7 Crack 10 Sn film (plating film)

Claims (4)

In the electrical contact member having a configuration in which the surface of the metal substrate is coated with a Sn or Sn alloy plating film,
An electrical contact member characterized in that in the plating film, a cavity formed by a substance different from a matrix metal, which relieves internal stress and precipitates Sn atoms, is dispersed. In the electrical contact member having a configuration in which the surface of the metal substrate is coated with a Sn or Sn alloy plating film,
An electrical contact member characterized in that particles formed of a substance different from a matrix metal, which relieves internal stress and precipitates Sn atoms, are dispersed in the plating film.   The electrical contact member according to claim 1, wherein the substance is a gas.   The electrical contact member according to claim 2, wherein the substance is a polymer material.

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