JP4878735B2 - Flat cable - Google Patents

Description

The present invention relates to a flat cable having electrical connecting portion.

  As electronic devices become smaller and lighter, electronic components and wiring components mounted on these devices are becoming smaller. In particular, wiring members for electrical wiring are required to be capable of high-density wiring in a limited space. Examples of such wiring members include a flexible circuit board, a flat cable using a flat conductor, and an electrical connector used for connecting these. These wiring members are required to have a large number of electrical conductors arranged at high density and electrically insulated from each other and to ensure good electrical connection.

  For the electrical conductors of these wiring members, copper is usually used which has good electrical conductivity, is rich in ductility, has an appropriate strength, and can be easily coated with other metals. The wiring member using copper is generally tin-plated for the purpose of corrosion resistance and solderability. Tin plating is usually formed by electroplating, and it is known that acicular crystals (hereinafter referred to as whiskers) are generated on the surface of the electrotin plating.

  In particular, when tin plating is performed on a copper-based metal material, copper atoms diffuse into the tin plating film to form a copper-tin intermetallic compound. Since this intermetallic compound has a crystal structure different from that of tin and can be distorted in the interstitial distance, compressive stress is generated in the tin plating film. Since this compressive stress is a driving force for whisker growth, it is said that whisker is likely to occur when tin plating is applied to a copper-based material. Since this whisker causes electrical short-circuiting between conductors, various improvement measures have been proposed so far.

For example, Patent Document 1 discloses a flat conductor on which tin plating having crystal grains having a major axis / minor axis ratio of 3 or more is disclosed, and Patent Document 2 discloses that the conductor surface is tin-plated at 232 ° C. to It is disclosed that heat treatment is performed at a temperature of 350 ° C. for 0.5 seconds to 3 seconds. In addition to this, it is well known that a tin plating with a smaller thickness tends to generate whiskers, and that a small amount of lead can be added to tin to suppress whisker generation.
JP 2000-173364 A JP 2001-73186 A

  It is well known that whisker generation can be suppressed by adding lead to tin plating. However, lead compounds are toxic and are not preferred due to environmental problems. A wiring member is desired. It is possible to suppress the generation of whiskers to some extent by applying lead-free tin plating to the electrical connection portion and, for example, heat-treating the tin plating as disclosed in Patent Document 2. However, it can be seen that whiskers are likely to be generated specifically at the contact portion using the insertion / extraction type electrical connector, and a defect due to a short of the whisker occurs at this portion. This is considered to be caused by the external stress applied to the surface of the tin plating by the contact piece of the electrical connector.

But the present invention has been made in view of the above, Oite flat cable subjected to tin plating lead-free, the occurrence of whiskers at a portion receiving external stress pressure due contact piece of an electrical connector anti The problem is to provide a flat cable .

In the flat cable according to the present invention, tin plating having a thickness of 0.5 μm or more and less than 1.0 μm is applied to at least a copper-based electrical connection portion that receives pressure stress of an electrical connector contact piece of the flat cable . It is characterized by being recrystallized by heating above its melting point.

It is said that whisker is likely to occur when an extremely thin tin plating with a plating thickness of less than 1.0 μm is applied. However, as in the present invention, the tin plating is heated above the melting point of tin and recrystallized. Thereby, generation | occurrence | production of a whisker can be suppressed. As a result, it is possible to obtain a flat cable excellent in reliability on which lead-free thin tin plating is applied, and it is possible to prevent occurrence of a short-circuit accident with an insertion / extraction type high-density connector.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows an example of a flat cable composed of a plurality of flat conductors, FIG. 1 (A) is a front view thereof, FIG. 1 (B) is a side view thereof, and FIG. 1 (C) is an example of connection to an electrical connector. FIG. In the figure, 1 is an electrical conductor, 1a is an electrical connection part, 2 is an insulation coating, 2a and 2b are insulation films, 2c is an adhesive layer, 3 is a reinforcing plate, 4 is a flat cable, 5 is an elastic contact piece, 6 is An electrical connector is shown.

The present invention is, for example, subject the insulating film 2a at a high density array number of the electrical conductor 1 comprising a flat conductive as shown in FIG. 1, sandwiched by 2b, and the flat cable formed by integrally bonded by an adhesive layer 2c And The electric conductor 1 of the flat cable 4 is formed, for example, by subjecting a round copper wire to tin plating by electroplating, and then extending and rolling to form an electric conductor 1 having a tin plating thickness of 0.5 μm to less than 1.0 μm. .

  The rolled flat electrical conductor 1 is recrystallized by, for example, heat-treating the conductor to a melting point of tin (232 ° C.) or higher by passing an electric current through the conductor. The plurality of heat-treated electric conductors 1 are arranged in an electrically insulated state at a predetermined interval, sandwiched between insulating films 2a and 2b, and bonded and integrated with an electrically insulating adhesive layer 2c. The flat cable 4 is covered with the insulating coating 2.

  At least one end of the flat cable 4 has a shape that can be easily connected to, for example, an insertion / extraction type electrical connector 6 as the electrical connection portion 1a. For this reason, at the end portion of the flat cable 4, the one insulating film 2b side of the cable end portion is removed to expose the surface of the electric conductor 1, and the reinforcing plate 3 is adhered to the outside of the other insulating film 2a side. Increase the strength and stiffness of the end. As shown in FIG. 1C, the electrical connection portion 1a formed in this way is inserted into the elastic contact piece 5 disposed at a high density in the electrical connector 6, and is pressed by its elasticity. An electrical connection is formed.

  As an example of the present invention, after applying tin plating to a round copper wire by electroplating, this round copper wire was drawn and rolled to obtain a width of 0.3 mm, a thickness of 35 μm, and a tin plating thickness of 0.9 μm. A flat conductor was produced. An electric current was passed through the flat conductor and heated above the melting point of tin to recrystallize the tin plating. Using this flat conductor, a flat cable as shown in FIGS. 1A and 1B was produced with an inter-conductor pitch of 0.5 mm (a gap between the conductors was 0.2 mm). The electrical connection portion of the flat cable was inserted into a lead-free electrical connector as shown in FIG. 1C and left at room temperature (22 ° C.) for 120 hours, and then the occurrence of whiskers was observed. Although 70 points of the contact portion were observed with an electron microscope, there were no places where whiskers were generated.

  As a comparative example of the present invention, a round copper wire was electroplated and tin-plated in the same manner as in the examples, and then this round copper wire was drawn and rolled to obtain a flat type having a width of 0.3 mm and a thickness of 35 μm. A conductor was produced. However, the tin plating thickness was 2.5 μm. Then, in the same manner as in the example, an electric current was passed through the flat conductor and heated above the melting point of tin to recrystallize the tin plating. Using this flat conductor, a flat cable as shown in FIGS. 1A and 1B was produced with an inter-conductor pitch of 0.5 mm (a gap between the conductors was 0.2 mm). The electrical connection portion of the flat cable was inserted into a lead-free electrical connector as shown in FIG. 1C and left at room temperature (22 ° C.) for 120 hours, and then the occurrence of whiskers was observed. Although 70 points of the contact portion were observed with an electron microscope, whiskers having a length of 20 μm were generated at three points.

  From the above results, when an extremely thin tin plating having a plating thickness of less than 1.0 μm was applied, the generation of whiskers could be suppressed by heating the tin plating to a temperature higher than the melting point of tin and recrystallization. . It has also been found that if the thickness of the tin plating is 1 μm or more, whisker generation cannot be completely suppressed even if the tin plating is heated above the melting point of tin and recrystallized. Further, in the case of tin plating with a thin thickness of less than 0.5 μm, there is a possibility that an unplated portion may occur, and solder wettability and corrosion resistance may be impaired.

It is a figure explaining embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Electric conductor, 1a ... Electrical connection part, 2 ... Insulation coating, 2a, 2b ... Insulating film, 2c ... Adhesive layer, 3 ... Reinforcement plate, 4 ... Flat cable, 5 ... Elastic contact piece, 6 ... Electrical connector.

Claims (1)

A copper-based electrical connection portion that receives pressure stress of at least the electrical connector contact piece of the flat cable is subjected to tin plating with a thickness of 0.5 μm or more and less than 1.0 μm, and the tin plating is heated to a melting point of tin or higher. A flat cable characterized by being recrystallized.

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